46

APPENDIX 1 : Guide to common laboratory glassware

47

APPENDIX 2 : Standard apparatuses

Much of the glassware used in the laboratory comes with Quickfit ground glass joints. This makes glassware

expensive, but convenient to use and quick to assemble. There are two parts which make a Quickfit joint — the cone

and the socket which fit together:

Quickfit equipment comes with joints in a number of standard sizes to suit different scales of apparatuses. A letter is

used to indicate the type of the joint (usually a “B”) followed by a number which describes the diameter of the joint (in

mm). The joints are conical, and the two numbers refer to the smaller diameter and the larger diameter at the two ends.

Typical sizes are: 10/13, 14/23, 19/26, 24/29 and 29/32. The most frequently used joints are in the B series with

diameters of 14/23, 19/26, 24/29, 29/32 mm; for simplicity these are called B14, B19, B24 and B29 joints,

respectively. B14 and B29 joints are preferred in continental Europe, whereas most of your glassware (as those

elsewhere in Britain) is in the B19 and B24 size.

A. Apparatus for filtration

Apparatus 1 — Fluted filter paper (see following page) together with a filter funnel are used for filtration of solutions to

remove insoluble debris and dust. They are not suitable for highly volatile solvents such as diethyl ether and

dichloromethane.

48

Apparatus 2 & 3 — Vacuum filtration apparatus for collection of crystalline solids by vacuum filtration. The Hirsch

and Büchner (also spelled Buchner) funnels are made of a white porcelain material with small holes in their base plates.

The base plate needs to be covered by a filter paper disk of exactly the right diameter to lie flat on the base of the

funnel. It is good practice to clamp the filter flask / tube to a lab stand to prevent the heavy-walled vacuum tubing (that

connects the apparatus to the pump) from tipping it over. The main difference between Hirsch and Büchner funnels is

their size. Use the smaller Hirsch funnel for filtering small amounts of solid, the larger Büchner funnel for larger

amounts.

How to make fluted filter paper (http://chem-ilp.net/labTechniques/FlutedFilterPaper.htm)

49

B. Apparatus for reflux

A: Round-bottom flask containing reaction mixture and either anti-bumping granules or a magnetic stirrer bar

B: Liebig condenser set up for reflux

NB: When clamping glassware, be careful not to overtighten clamps, but also ensure that the apparatus cannot slip out

of the jaws. In general, rest the apparatus against the fixed jaw of the clamp and gently but firmly bring the moveable

jaw of the clamp into place. The metal jaw of the clamp should be protected by a piece of rubber (tubing) since glass

breaks easily and it is a good idea to avoid direct contact between bare metal and glass. Retort stands should be set up

so that the weight of the apparatus is above the base plate of the stand. The vertical bar of the retort stand is normally

positioned behind the apparatus so as not to obstruct access.

When two clamps are being used to hold the apparatus, ensure that there is no strain and that all of the glass joints

remain together — this is especially important for a distillation or reflux apparatus where a fire could result through

escaping solvent from a poorly fitted joint. Make sure that it is always possible to lift the apparatus up and away from

the heat source. For this reason, the clamp at the condenser should never be tightened. As a rule, the rubber tubing and

electrical leads should also lead away to the rear of the apparatus. Moisten rubber tubing to help fit it onto condenser

side arms and use a twist action as you do so. Use of too much force to put rubber tubing onto side arms leads to

breakages and is one of the commonest sources of injury. If the end of the tubing has perished, cut it off; ensure that

there are no small splits at the end of the tubing which could cause the tubing to come off the apparatus at some critical

point! The water outlet tube from condensers should be inserted well into the bench drain to prevent accidental floods.

It is a good idea to clamp the tubing to ensure that the waste water goes into the drains and not over the bench.

Water in

Water out

xxxxxHeat source

(isomantle or

hotplate stirrer + oil or

water bath)

This clamp carries

the full weight and

must be tightened at

the joint

Optional clamp

(you may want to place

a clamp here to prevent

the condenser from

falling over)

NEVER TIGHTEN

HERE

NO STOPPER !

50

C. Heat sources

Many reactions require heat in order to proceed at a reasonable rate. As a rough rule of thumb, a 10 °C increase in the

temperature at which a reaction is carried out doubles the rate of the reaction. For this reason many reactions are

carried out at elevated temperatures. As it is usually necessary to carry out a reaction in a solvent — so that reactants

can mix at the molecular level — heating a reaction may cause the solvent to boil. The vapour from the boiling solvent

leaving the reaction flask needs to be condensed and returned to the flask so that the solvent level does not drop. This is

called reflux and is carried out in practice using the reflux apparatus shown previously. In addition, refluxing ensures

that the reaction temperature is kept constant (and thus reproducible) at the boiling point of the solvent.

Isomantles (also called “heating mantles”) provide a convenient way of controlled heating of reaction vessels; in this

lab course they are used for heating reactions under reflux and for carrying out distillations. They consist of a heating

element enclosed within a knitted glass-fibre fabric which is usually protected with a safety earth. The heating unit is

enclosed within an outer rigid housing (often made of polypropylene or aluminium) which is thermally insulated so that

the isomantle may be handled at a low outer case temperature. The heating control uses nowadays a in-built regulator

(as illustrated above). The flask is often rested on a protective hemispherical metal gauze within the mantle cavity.

Fixed sizes for round-bottomed flasks having capacities from 50 mL to 5 litres are standard. You will find a variety of

isomantles in a bench cupboard located at the end of your bench row.

Most of our isomantles are designed for use with a 250 mL round bottom flask, although in practice this lab will

generally require their use with a 100 mL flask instead. Isomantles can deliver high temperatures up to 200 °C and you

may have to slightly raise a flask in a mantle to prevent over-heating its walls — particularly when the flask contents

get low. CAUTION: If you are carrying out a distillation with an isomantle never distil to dryness — some trace

residues that are left over when organic compounds are distilled can be explosive. This is especially the case with

ethers, which contain small amounts of explosive peroxides formed in the presence of air. Do not put a hot

isomantle back in the bench drawer at the end of the afternoon — leave it out on the bench top to cool.

Other heat sources frequently used are: steam baths (available on your bench top) for heating organic solvents up to

about 75 – 80 °C; and silicone oil or sand baths which can be heated by an electrical hot plate for boiling or distilling

solvents up to 180 °C.

51

D. Apparatus for simple distillation

A: Distillation flask containing sample and anti-bumping granules

B: Distillation (still) head

C: Thermometer pocket and thermometer

D: Condenser

E: Receiver adapter

F: Receiver flask

E. Apparatus for steam distillation

A: Distillation flask containing sample and anti-bumping granules

B: Distillation (still) head

C: Gas (steam) inlet tube

D: Condenser

E: Receiver adapter

F: Receiver flask

G: Steam generator

water out

water in

clamp

clamp

clamp

clamp

wire gauze

A

B

CD

E

F

G

52

APPENDIX 3 : Recrystallisation

Introduction

The products of organic reactions are seldom pure. They are usually contaminated with by-products arising from side

reactions and, if the reaction didn’t go to completion, some starting material, too. A chemist’s primary task after

finishing a reaction is to isolate the pure product. This often requires more effort than setting up the reaction itself!

Organic compounds which are solid at room temperature are usually purified by recrystallisation. The technique relies

for its effectiveness on the fact that most compounds are more soluble in hot than cold solvents. The general technique

involves dissolving the material to be crystallised in the minimum quantity of a hot (usually boiling) solvent or solvent

mixture and cooling the solution slowly. In most cases the product which crystallises out on cooling is purer than the

original solute, minor impurities being left behind in solution.

Consider the following example. A reaction product contains 20 g of A, the desired compound, and 2 g of B, an

impurity. Suppose the crude mixture will just dissolve in 100 mL of boiling ethanol. On cooling, crystals begin to

separate out. Suppose that A and B are equally soluble in ethanol at room temperature, dissolving to the extent of 2 g

per 100 mL. If we leave the solution at room temperature long enough for equilibrium to be reached, then the crystals

deposited should constitute 18 g of pure A. Since only 2 g of B is present, it should all remain in solution along with 2

g of A which will be lost. Thus in one operation we have obtained a 90% recovery of pure A from the original mixture.

If B is less soluble than A, say only l g per 100 mL of ethanol at room temperature, we cannot effect complete

purification in one step; a second recrystallisation will be required for that. The first crystallisation will deposit a

mixture of 18 g of A, as before, but l g of B will also crystallise. Thus we have effected only partial purification. If the

mixture obtained is recrystallised a second time from 100 mL boiling ethanol, 16 g of pure A should be obtained. In

this case we have obtained an 80% recovery of A after two crystallisations.

We can see that the effectiveness of recrystallisation as a means of purification decreases:

(1) as the impurity becomes increasingly less soluble than the desired product, and

(2) as the absolute amount of impurity increases.

In an extreme case, where the impurity is very much less soluble than the required compound, it can in fact be

concentrated by crystallisation. In such a situation another solvent with a more favourable solubility ratio should be

sought.

Although the main priority in carrying out a recrystallisation is to obtain pure material, it is also very important that the

percentage recovery should be as high as possible. Economic considerations apply to all syntheses, not just to industrial

preparations where a 1% difference in yield can turn a profit into a loss, or vice versa. Organic chemists routinely carry

out a series of reactions in which the product of one reaction is used as the starting material for another. Unless

particular attention is paid to obtaining the maximum yield at each stage it is easy to have insufficient material with

which to continue after a few stages. For example, after 5 successive reaction stages all with 60% yields, the overall

yield will be only 7.8% (NB: 0.65 = 0.078). If all the yields were 80%, then the overall yield would be 32.8%. In each

stage, of course, the amount of pure product depends on the degree of completion of the reaction as well as the

efficiency of purification. Bear in mind that a good recovery on recrystallisation depends on a relatively low solubility

in cold solvent and a relatively high solubility in boiling solvent.

The first problem in performing a crystallisation, then, is selecting a solvent in which the material to be crystallised

exhibits the desired solubility behaviour.

Ideally, the material should be sparingly soluble at room temperature and yet be quite soluble at the boiling point of the

solvent selected. The solubility curve should be quite steep, as can be seen in line A of the Figure below. A curve with

a low slope (line B) would not cause significant crystallisation when the temperature of the solution was lowered. A

solvent in which the material was very soluble at all temperatures (line C) would also be unsuitable. The basic problem

in performing a crystallisation is to select a solvent (or mixture of solvents) which exhibits a steep solubility versus

53

temperature curve for the material to be crystallised. That is, a solvent that gives the behaviour shown in line A is an

ideal recrystallisation solvent.

Note, however, that even in case A, the material has some solubility in the cold solvent, so to obtain the most efficient

recovery of material one should use the minimum volume of hot (boiling) solvent.

Normally, the finding of a suitable solvent (or solvent mixture) for recrystallisation is done by trial and error or by

analogy to similar compounds. It is sometimes useful to remember that ‘like dissolves like’, i.e. non-polar compounds

tend to be soluble in hydrocarbon solvents while polar compounds tend to be soluble in polar solvents such as methanol

or acetone. Remember also that:

i The solvent should not react chemically with the solute;

ii Solvents of b.p. 50 − 100 °C are generally preferred — high-boiling solvents are difficult to remove, while low-

boiling solvents cause manipulative problems due to their rapid evaporation;

iii Solvents which are non-flammable, non-toxic and cheap should be used whenever possible.

Crystallisation from solution does not necessarily occur quickly and spontaneously. In some cases it may prove very

difficult or impossible to obtain crystals; the presence of impurities often hinders crystallisation. If no material comes

out of solution on cooling, crystallisation may often be induced either by scratching the wall of the containing vessel

below the level of the solution with a glass rod or metal spatula (crystals tend to grow on the sharp edges of scratches)

or by seeding the solution with a very small amount of the pure solute. If this is not available, a little crude crystalline

material may suffice. Agitation of the solution or cooling below room temperature may also promote crystallisation. If

these methods fail, it is likely that the solution is not in fact saturated and needs to be concentrated. Make sure that the

solute has not come out of solution as a second liquid phase, i.e. as an oil. This appears at first as a cloudiness, and later

coagulation to droplets occurs. “Oiling-out” like this will certainly occur if the compound separates out of solution at a

temperature above its melting point, but may also sometimes occur below the compound’s melting point. This may be

due to a low tendency to crystallise, to cooling too rapidly, or to using too concentrated a solution. If the mixture is

reheated until solution occurs, subsequent scratching or seeding may avoid a recurrence; if it does not, the mixture

should be diluted with solvent and the oil redissolved before being allowed to cool slowly.

Quantity

dissolved

Temperaturesolvent

boiling

point

C

A

B

54

A. Recrystallisation of an organic compound from a single solvent

The steps are:

1 Find a solvent with a steep solubility vs. temperature characteristic. This is done by trial and error using small-

scale trials in test tubes or, if the compound is known, by consulting the literature or a handbook. In the Level 1

lab course the lab manual will suggest a suitable solvent when you are required to perform a recrystallisation.

2 If you anticipate that you need to do a hot filtration (see 6) as part of the recrystallisation, flute a filter paper†, fit it

into a conical glass filter funnel (preferably short-stemmed) and place both in a hot oven.

3 Weigh out* and transfer the sample to be recrystallised into a conical flask. A flask of capacity 100 mL is usually

suitable for 2 − 5 g of material unless the sample is of below average solubility, in which case transfer of the

contents to a larger flask may be necessary.

4 Place about 50 mL of the chosen solvent in a second conical flask. Add 3 − 4 anti-bumping granules to the flask

and bring the solvent to the boil (using a heat source§ appropriate to the solvent in use).

5 Add sufficient of the hot solvent to the first flask (containing the sample to be recrystallised) to cover the solid,

add 3 − 4 anti-bumping granules, and then bring the solvent to the boil. Slowly add the hot solvent from the

second flask, keeping the solution on the boil, until all the sample (except for insoluble impurities and the anti-

bumping granules) has dissolved. For the reasons explained earlier in this Appendix, you should aim to dissolve

the sample in the minimum volume of hot (boiling) solvent. Sometimes, a sample will be contaminated by

insoluble impurities (e.g. by-products, charcoal, filter paper residues). In such a case, you need to filter the

solution whilst it is still hot, before allowing your sample to crystallise.

6 If there is nothing in the first flask left undissolved apart from the anti-bumping granules, a hot filtration is

unnecessary; simply decant the solution from the anti-bumping granules into a new conical flask and proceed

with 7. If a hot filtration is necessary, remove the filter paper and funnel from the oven. Rest them on an empty

conical flask suitable to receive the volume of solvent used. Stand this conical flask on a steam bath to keep the

apparatus hot. Filter the hot solution as soon as possible through the fluted filter. If a flammable solvent is in use,

all flames in the vicinity must be extinguished. If any crystallisation occurs in the filter paper, wash through with

a little hot solvent, noting the additional volume used. If the funnel stem becomes plugged, reflux the filtrate in

the receiver conical flask until the plug dissolves before adding hot solvent to the funnel.

7 Allow the flask containing the clear solution to cool for a few minutes, remove the funnel, and loosely cork the

apparatus. Strictly speaking, you should let a crystallisation proceed for at least two hours; due to the constraints

on time in the Level 1 lab course, however, allow the mixture to stand for about 15 minutes after the hot solution

has cooled to room temperature.

Collecting and drying a recrystallised compound

1 Use a clean spatula to gently break up the crystalline mass if it has caked, and scrape free any material adhering to

the flask walls.

2 Fit a Büchner flask with a rubber tube to a vacuum tap,17 ensuring that the tap is initially closed. Moistening the

end of the rubber tube aids fitting it to the flask side-arm. Clamp the mouth of the flask to a retort stand to keep it

stable and prevent it from falling over.

† See p. 54 or http://chem-ilp.net/labTechniques/FlutedFilterPaper.htm for instructions on how to make fluted filter

paper.

* If the sample is supplied in a sample vial, weigh the vial plus contents and subtract the weight of the empty vial after

transfer of the contents to the flask.

§ A water bath is used for organic solvents with boiling points below 80 °C; an electric heating mantle is used for

water or organic solvents boiling above 80 °C.

55

3 Fit the Büchner funnel with a rubber gasket and place on top of the Büchner filter flask. Put a filter paper of the

appropriate diameter18 in the funnel and moisten with a little solvent. Open the pump tap and check that the paper

is seating well in the funnel — it may be necessary to wash a little solvent through before filtering your sample in

order to help the filter paper to seat well. It may also be necessary to apply a little pressure on the Büchner funnel

to make a seal between the gasket and the filter flask.19

4 Filter your suspension of crystals before the filter paper dries out. Press the crystals down on the filter with a

spatula to an even cake. Turn off the tap to the pump, and remove the Büchner funnel from the filter flask. Now

rinse the conical flask out with a little of the filtrate20 from the filter flask to transfer any residual crystals.

Reassemble the Büchner flask and funnel, open the tap to the pump, and filter off the remaining crystals. Finally,

wash the combined crystals in the Büchner flask with a little pure solvent. (Check at all stages that there is a good

seal by gently pressing down the Büchner funnel.)

5 For low-boiling solvents, a provisional drying of the product can be easily done by sucking air through the solid

while it is still on the filter paper. However, it is a good idea to empty the filter flask first to avoid unnecessary

evaporation of the filtrate liquid.

6 Turn off the pump tap, dismantle the Büchner assembly, and transfer the crystals to a weighed petri dish or watch

glass, discarding the filter paper (→ solid waste). If the crystals are still wet (particularly when you use water), it is

often better to leave the solid to air-dry on the filter paper overnight. Otherwise you run the risk of scraping off

some of the cellulose from filter paper together with your product.

7 Compounds that are recrystallised from water should ideally be dried in a vacuum desiccator over calcium chloride;

21 in the lab classes, it is often sufficient to air-dry them from one lab session to the next. If you use an organic

solvent in the recrystallisation, dry the product in an oven of appropriate temperature (usually at 60 °C). (Check a

little of your product to see that it does not melt in the oven you are going to use.) When the weight of your sample

has dropped to a constant value, it is dry. Place your recrystallised material in a clean labelled sample container

and label with your name, date, class, experiment number and chemical name.

8 Dispose of the filtrate in the Büchner flask. Organic solvents go into the appropriate waste solvent winchesters —

either for halogenated or non-halogenated solvents. Halogenated solvents are disposed by high-temperature

incineration, to prevent the formation of polychlorinated dioxins (these are extremely toxic and carcinogenic

compounds). For this reason, incineration of halogenated solvents needs to be done separately to the incineration

of non-halogenated waste, and is also much more espensive.

17 We will use a diaphragm pump in the laboratory, not a water pump, to generate vacuum that is distributed through

the manifold taps along each bench. Evaporated solvent vapours will be condensed at the exhaust of diaphragm pump.

If a water pump were used, any solvent vapour would contaminate the waste water. Not such a good idea if you keep in

mind that in many regions or countries waste water is recycled to provide drinking water. 18 One that covers the holes in the flat plate but is small enough not to overlap the edges. 19 Filtration under reduced pressure (= suction filtration) is used here to remove as much solvent as possible. 20 The filtrate will be a saturated solution of your product sample, so there is little danger of your solid redissolving

again (which would happen if you used fresh solvent). 21 A guard cage should always be used when a desiccator is under vacuum to avoid accidents due to an implosion. For

experiments in the Level 1 lab course where water has been used for recrystallisation drying in an oven at 60 – 80 °C is

generally acceptable.

56

B. Recrystallisation of an organic compound from mixed solvents

It is sometimes difficult to find a single solvent which gives a good recovery of material on recrystallising a compound.

In such cases a mixture of two solvents often proves more satisfactory. For such a two-solvent recrystallisation, you

choose one solvent (solvent #1) in which your compound is rather too soluble for it alone to be used, in the other

(solvent #2) too insoluble. Solvent #1 and solvent #2 should be completely miscible with each other. Typical mixtures

used are water−alcohol (methanol or ethanol) or ethyl acetate−petroleum ether.

The steps are:

1 For most compounds the best procedure is to dissolve the sample in the solvent in which it is rather too soluble and

then add the solvent in which it is insoluble. So, dissolve your crude product in solvent #1 and heat to boiling on a

steam bath or stir/hot plate.22 Filter off any insoluble impurities if necessary.

2 Add the second solvent (solvent #2) to a hot solution in solvent #1 until the hot mixture is saturated with the

sample. Again heat the solution to the boiling point and continue adding solvent #2. After each drop you will

notice a cloudiness that dissolves away. When saturation occurs, the first crystals begin to appear and the solution

can be left to cool and crystallise. If an emulsion is formed (the solution goes cloudy, and the cloudiness does not

disappear on swirling) add a few drops of the solvent #1 to clear the solution, and leave the mixture to stand.

Seeding or scratching with a glass rod may hasten the start of crystallisation.

Oiling out: Sometimes crystallisation will not occur readily from hot solution, particularly when dealing with

low-melting compounds which have a tendency to “oil out”. This is bad since the oil will drap impurities even if

it subsequently solidifies. In these cases it is necessary to add the second solvent to a cold solution in the first

solvent. Addition of the second solvent will usually have to be continued even after crystallisation starts in order

for a good recovery to be obtained. Or add more of solvent #1 to slightly change the solvent composition, then

heat again to redissolve the oil.

3 Allow to cool undisturbed to room temperature before placing in an ice bath. Collect the crystals on a Büchner or

Hirsch funnel using suction filtration.

4 Dry your product as usual.

22 For maximising yield the solution in solvent #1 must be saturated. This is easier said than done. However, if you

carry out your recrystallisation in a solvent mixture such as methanol–water and your compound is soluble in methanol

but insoluble in water, try concentrating your two-solvent mixture on a rotary evaporator as methanol (b.p. 65 °C) will

come off well before water evaporates, thus allowing you to remove excess solvent quite easily.

57

APPENDIX 4 : Measuring a melting point

The melting point of a crystalline solid is the temperature range over which the solid melts into a liquid at atmospheric

pressure. Strictly speaking, we should call it a melting range but no one does.

The melting point (m.p.) is the most frequently recorded physical property of solid organic compounds, and an organic

chemist will determine, as a matter of course, the m.p. of every solid compound he or she prepares. There is a great

deal of theory associated with the melting of organic solids, but it is well to concentrate for the moment on a few

important generalisations:

a For most pure organic solids the melting point is usually sharp (within a 0.5 – 1.0 °C range). However,

some compounds decompose on heating instead of melting and others melt over a temperature range of several

degrees.

b The addition of an impurity to a pure compound usually lowers its melting point and broadens the melting

range. In general, the addition of successive amounts of an impurity to a pure substance will cause its melting

point to decrease in proportion to the amount of impurity. This is a result of the fact that the freezing point of a

substance is lowered by the addition of a foreign substance — remember Experiment 2 from the Phys Chem lab.

The freezing point is simply the melting point (solid —> liquid) being approached from the opposite direction

(liquid —> solid). The melting point can, therefore, be useful as a criterion of purity. Comparison of the m.p. of

a recrystallised sample with that recorded in the literature for the compound is a simple means of assessing the

effectiveness of a purification process.

c Any number of pure samples of the same substance will all have the same melting point. The melting point

can, therefore, be used as an aid to the identification of a compound by comparison with literature values and by

carrying out mixed melting points with authentic samples. Suppose you have a compound X, m.p. 120 – 122 °C,

and you suspect it to be benzoic acid, which has a reported m.p. of 122 °C. You can mix X with an authentic

sample of benzoic acid and determine the mixed m.p. If the mixed m.p. is still 122 °C, then X is benzoic acid. If

X is not benzoic acid, it will behave as a contaminant; the m.p. of the mixture will be well below 122 °C, and it

will melt over a wider range.

Melting points are determined using an electrically-heated apparatus. The sample is placed in a capillary tube, and

observed via a magnifying eyepiece.

1 Preparing the sample

Make sure the compound is perfectly dry. Powder a small amount of the substance finely using a clean spatula on a

watch glass or glazed tile — never on filter paper, which is likely to break up and contaminate your compound with

cellulose fibres. Introduce the powdered compound into the capillary tube and gently tap the tube so that the powder

drops down into the sealed end (see above). Use enough powder to give a depth of 1 – 2 mm only. If you use too much

sample, the m.p. range recorded will be broadened.

After the sample has been

powdered on a watch

glass scoop up a little into

the open end of the tube...

... then turn the tube

upside down and tap the

sample down to its closed

end so that you have a

depth of 1-2 mm.

58

2 Take an approximate melting point

If you do not know the identity of the compound, you will have to take a rough m.p. first — unless you want to spend

ages sitting next to the melting point apparatus. Place the capillary tube in the heating block of an electric m.p.

apparatus, switch the apparatus on and raise the temperature fairly briskly using the controls on the apparatus until the

substance melts. Record the temperature at which the sample appears to melt. Dispose of the used capillary in the

dedicated waste glass bin, never in the bins at the end of the benches. The apparatus has a main heating control for

adjusting the final temperature of the heating block; the higher this control is set the faster will be the rate of

temperature rise.

3 Take an accurate melting point

Repeat the determination with a fresh sample of the substance, but this time control the rate of heating very carefully so

that the temperature approaches the expected melting point at a rate of about 1 °C per minute. If the rate of temperature

increase is much faster than this, the temperature of the thermometer will not accurately indicate that of the heating

block and a false m.p. will be obtained, often with a wider temperature range. Note both the temperature at which

liquefaction commences, and the temperature at which it is complete. These give the m.p. range. Quote your result as

say, m.p. 7l – 74 °C.

You should never re-use m.p. samples: on cooling the liquid will solidify to a solid plug, whose m.p. is less easy to see;

in addition, some decomposition of the sample may also have occurred.

4 Taking a mixed melting point

Mix together an approximately equal quantity of the finely powdered samples as intimately as possible. It is crucial for

the success of this technique that the samples used are really finely ground together — so spend a good few minutes

doing this. Use this mixture for the m.p. determination. If the samples are identical, no change in m.p. compared to the

isolated samples will be noted. If they are different compounds, even though they have similar melting points, the m.p.

of the mixture will be lower and over a wider range.

59

APPENDIX 5 : Extraction and washing guide

A. Solvent extraction

The products of organic reactions are rarely pure. Solvent extraction is another useful technique to effect a preliminary

separation of the organic product from inorganic salts or other water-soluble by-products, prior to further purification.

It involves dissolving the mixture in a pair of mutually insoluble (i.e. immiscible) solvents which are chosen so that the

desired product is concentrated predominantly in the organic phase while by-products end up predominantly in the

aqueous phase. Thus, the separation depends on there being a difference in solubility of the solutes in the two mutually

immiscible (or nearly immiscible) phases.

The removal of undesired water-soluble impurities from a product mixture is often accomplished by extracting a

solution of the substances (dissolved in a water-immiscible organic solvent) with water; this is known as washing.23

The required solute is recovered by separating the organic phase from the aqueous phase using a separating funnel (see

Figure 1 below), drying the organic layer to remove traces of water in it and then evaporating the solvent on a rotary

evaporator.

Note the difference between extracting and washing. In an extraction you pull out what you want and keep the extracted

phase. In a wash you pull out impurities from what you want, and you then discard the wash phase.

The verb used to describe the distributing of a mixture between two different liquid phases in order to effect a

separation is “to partition”. In a solvent extraction each solute will distribute itself between the two layers so that the

ratio of the solute weights in each solvent remains a constant (the distribution or partition coefficient) at constant

temperature. The smaller the distribution coefficient between the organic solvent and water, the larger the number of

extractions with the organic solvent that will be required in order to remove the solute. It can be readily shown that, for

a given volume of extractant, several extractions with small portions of solvent will give better recoveries of solute than

a single extraction with a large volume of solvent. Use is frequently made of the fact that the solubility of the organic

component in water is considerably decreased by the presence of dissolved inorganic salts (sodium chloride, ammonium

sulfate, etc.). This is called salting-out. A further advantage is that the solubility of partially miscible organic solvents,

such as ether, is considerably less in the salt solution, thus aiding separation of the organic phase from the aqueous layer

in the separating funnel.

23 Note that a shake means a “vigorous shake”, whereas a wash usually just involves inverting the separating funnel

several times, without shaking it much (to avoid the danger of forming an emulsion).

60

B. Using a separating funnel

1 Pick the right size of separating funnel. Select a separating funnel with a volume about twice that of the total

volume of liquid to be extracted. These funnels come in different shapes — you will be limited by the particular

funnel available in your bench kit.

2 Mount the separating funnel. Mount the separating funnel on a ring (preferably covered with rubber tubing)

attached to a retort stand if globular or pear shaped (types a and b), or use a boss and clamp if cylindrical (type c).

3 Check the tap. Check that the tap turns easily and is not loose (i.e. will not leak). If your separating funnel has a

glass tap, you may have to lubricate the stopcock lightly with silicone grease by placing small spots of grease on

the plug as indicated in Figure 2 (“x” marks the spot where the grease should be put). The plug should be worked

in the barrel so that a very thin film of grease covers the whole contact area. Teflon taps don’t need any grease, so

if your separating funnel possesses a Teflon tap, then there is no need of lubrication. Never put the Teflon plugs

and the plastic screws in a hot oven to dry.

4 Choose the extractant. The ideal extractant should possess:

i) a low solubility in water;

ii) a high solubility for the solutes concerned;

iii) a relatively low b.p., to facilitate eventual removal of the solvent.

Popular water-immiscible solvents used for solvent extraction are: diethyl ether, ethyl acetate, petroleum ether,

toluene, chloroform and dichloromethane. Diethyl ether was historically the most common extractant employed

but has the drawback of being extremely flammable and has a tendency of forming explosive peroxides. Ethyl

acetate works well but is a bit harder to remove. Toluene and dichloromethane have a nasty habit of forming

emulsions which makes them less attractive. Acetone, methylated spirits and methanol are miscible with water

and therefore cannot be used in solvent extractions.

5 Introduce your solution. Pour the solution to be extracted into the separating funnel with the help of a filter

funnel and glass rod to aid complete transfer, not forgetting to rinse the container with a little of the appropriate

solvent. Avoid getting the ground joint contaminated with product solution, as many organics act as a glue that

gets the stopper stuck.

6 Add extractant. Add the extractant, typically about one-third the volume of solution already present in the

funnel, using the technique described under 5, and stopper the funnel.

SAFETY NOTE

All naked flames in the vicinity should be extinguished the moment a potentially inflammable solvent is used.

Ether is particularly hazardous in this respect on account of its inflammability, low flash point, high vapour

pressure, and the fact that the density of its vapour is greater than that of air.

7 Shake the funnel. The funnel is inverted firmly some three dozen times to ensure proper equilibration of the two

phases — the technique should resemble that of a bar mixer. Release the excess pressure from time to time (every

15 – 30 seconds) by opening the tap whilst the funnel is in an inverted position. If a gas is formed during the

extraction or a highly volatile extraction solvent such as ether is used, then you have to release pressure after every

inversion of the separating funnel. Then return the funnel to the stand to enable the phases to separate.

Remove the stopper from the separating funnel. Wait for the layers to separate, then run off the lower layer

through the tap into a conical flask. This can be done fairly rapidly at first, but must be done more slowly as the

interface approaches the narrower part of the funnel, so that by closing the tap the interface can be trapped in the

bore of the barrel, thus bringing about a clean separation.

If you use chloroform or dichloromethane, then the organic phase will normally be the lower layer, whereas if

petroleum ether, toluene, diethyl ether or ethyl acetate are used, the organic layer will normally be the upper layer.

61

If you are not sure which layer is which,24 then add some water to each layer; the aqueous layer will remain

homogeneous, whereas the water will form a separate layer with the organic phase. Never, never, never, ever

discard any phases until you are absolutely sure that you have isolated your product. Many a student has

lost a valuable product by disposing of wrong phase prematurely.

8 Extract 3 times to maximise yield. The aqueous phase should be returned to the funnel and extracted with a

fresh portion of solvent. In general three extractions are required. [Obviously if multiple extractions are to be

performed on the top phase, it is only necessary to run out the lower phase and replace it with a fresh sample of

lower phase. If multiple extractions are to be performed on the lower phase, it must be returned to the funnel after

each separation]

Formation of emulsions: If you get an emulsion during the shaking of the separating funnel, try adding salt

(NaCl, Na2SO4) to saturate the aqueous phase with salt. You can further increase the density difference between

the two phases, e.g. by adding petroleum ether (density 0.67 g mL–1

) to ethyl acetate (which has a density of 0.902

g mL–1

quite close to water). Then wait! If necessary, give the phases time to separate overnight.

9 Finish with a brine wash. This helps to dry the organic layer by “extracting” the water that is dissolved in the

organic phase. Ether can contain up to 1.2% of water (ethyl acetate even up to 3.3%), so it is rather a good idea to

pre-dry the organic phase before moving on to a drying agent (desiccant), in step 10.

10 Dry the organic phase. Normally small amounts of drying agent (usually powdered anhydrous magnesium

sulfate or sodium sulfate, more below in the section on Common Drying Agents) are added to the organic phase

in the conical flask using a spatula and swirling the flask, until the next portion added no longer coagulates. The

amount of drying agent used should be kept to the minimum required for thorough drying. A wet organic phase is

often cloudy, a dry one is clear. The organic solution is left in contact with the drying agent until it becomes clear,

typically 15 minutes are enough. Occasional swirling, or stirring, speeds up the drying step.

11 Filter off the drying agent. The drying agent is removed from the dry solution using fluted filtration. The filter

paper should then be washed with a little of the pure dry solvent, see apparatus for fluted filtration in Appendix

2A. Some chemists prefer to use a Büchner funnel and suction filtration. The solvent can then be removed using

a rotary evaporator to give the organic product(s).

24 This can be an issue if you extract a high-density (e.g. brominated or chlorinated) product with a low-density solvent.

62

C. Common drying agents for organic solutions

The wet organic phases from an extraction experiment are usually dried by contact with a chemical drying agent. The

ideal drying agent should possess the following properties:

i) it must not combine chemically with the solute or solvent;

ii) it should have a rapid action and an effective drying capacity;

iii) it should not dissolve in the liquid phase;

iv) it should be as economical as possible.

The following list gives the more common drying agents with their practical limitations and important applications.

Powdered magnesium sulfate [MgSO4⋅H2O —> MgSO4⋅7H2O]

Excellent general purpose neutral desiccant, rapidly acting, high capacity, chemically inert. It can be used for most

solutes and solvents, including those for which calcium chloride is inapplicable.

Anhydrous calcium chloride [CaCl2 —> CaCl2⋅6H2O]

Widely employed desiccant, slowly acting, high capacity, cheap. Since CaCl2 is slightly basic it cannot be employed

for drying acids or acidic liquids. It combines with alcohols, phenols, amines, amides, ketones and some aldehydes and

esters, and thus cannot be used with these classes of compounds.

Anhydrous sodium sulfate [Na2SO4 —> Na2SO4⋅10H2O]

Neutral drying agent, slow acting, high capacity, cheap, chemically inert, but inefficient drying agent for toluene, and

cannot be used above 32.4 °C since the decahydrate begins to lose water of hydration at this temperature. Useful for

preliminary removal of large quantities of water.

63

D. Solvent extraction as a method of purifying acidic or basic products

Compounds that are weak acids or weak bases are frequently purified by extraction. Let’s consider the separation of a

mixture consisting of an acidic (HA), a neutral (N), and a basic component (B). The usually valid assumption is made

that organic salts will dissolve preferentially in the aqueous phase, whereas undissociated acids and bases will dissolve

almost entirely in the organic phase.

Remember that the pKa is the pH at which an acid and its conjugate base are present in equal molar concentration, i.e.

the pKa corresponds to the pH of an aqueous solution of acid which has been exactly half neutralised with alkali.

NOTE: The stronger the acid, the lower the pKa value. The stronger the base, the weaker the conjugate acid and hence

the higher the pKa value. Since there is a logarithmic relationship between hydrogen ion concentration and pH, it is

necessary in the case of an acid to work at a pH at least two units higher than the pKa to ensure that the acid is

completely dissociated and so water-soluble. In the case of a base, working at a pH two units lower than the pKa will

ensure that the base is completely in the salt form and so water-soluble.

Selected pKa values of some common acids and conjugate acids:

Extraction of an ether* phase containing an acid product (HA) and a neutral impurity (N) with aqueous alkali (pH = 12

– 14) causes the acid to be converted into its water-soluble salt form. The acid can be recovered by acidifying the

aqueous phase (which contains A–) to a pH below the pKa of the acid using aqueous mineral acid, then extracting the

undissociated form of the acid (HA) into ether.

Now consider that a base (B) and a neutral impurity (N) are equilibrated between dilute hydrochloric acid and ether.

Since the pH of the dilute HCl phase is about zero, the base will be in the form of its water-soluble hydrochloride

(BH+Cl–), whereas the the neutral substance (N) will remain in the ether phase. When the separated aqueous phase

(BH+Cl–) is made alkaline, the neutral base (B) will be freed and is either filtered off or extracted into ether.

B + N

Partition between ether and 2 M HCl

In aq. phaseIn ether phase

Basify (NaOH), then extract with ether

BH ClN

In aq. phaseIn ether phase

Na ClB

HA + N

Partition between ether and 2 M NaOH

In aq. phaseIn ether phase

Acidify (HCl), then extract with ether

Na AN

In aq. phaseIn ether phase

Na ClHA

* Other commonly used water-immiscible solvents for extractions are: ethyl acetate, petroleum ether (hexanes),

chloroform, dichloromethane.

64

APPENDIX 6 : Distillation

Purification of a liquid by simple distillation

If a liquid contains impurities, and these cannot be removed easily by chemical means, then purification is usually done

by distillation. Distillation is used extensively in industry. It is a process of purifying a liquid compound by

evaporation and recondensation. The basis of its effectiveness lies in the fact that the vapour in equilibrium with a

mixture of liquids (e.g. desired compound and impurity) will be richer in the component of lower boiling point (b.p.). If

that vapour is condensed separately from the original liquid mixture, the condensate or distillate obtained will show this

enrichment in the more volatile component because its composition must be the same as that of the vapour. In this lab

course you will encounter a few simple distillations, which are appropriate for purifying a liquid which contains only

non-volatile impurities or a mixture of components with boiling points that are separated by more than 30 °C. A related

technique, called fractional distillation, is used for mixtures of liquids with boiling points closer together than 30 °C.

For high-boiling liquids (b.p. > 200 °C) vacuum distillation is the norm.

Where the boiling points of two (miscible) components of a mixture are widely different, then a single evaporation and

recondensation can give virtually complete separation. In such a case we will obtain a change of b.p. with time for a

mixture of two components A and B of the form:

Note that the distillation temperature may drop a little during the mid-run (dotted line). This is because the amount of

vapour heating up the distillation head falls off. When compound B begins to boil and come over, the distillation

temperature rapidly rises to the boiling point of compound B. It remains at this temperature until all of B has distilled.

The temperature then falls again as no more vapour passes over to heat the distillation head.

If the boiling points of A and B are too close together, the distillate will only show a partial separation (fractionation) of

the components. Successive simple distillation will be necessary to complete separation.25

Where a simple distillation apparatus is too inefficient to separate a liquid mixture in one operation, it is quicker and

more convenient to use a more efficient apparatus rather than carry out several successive distillations. Increased

efficiency can be brought about by placing a vertical column (packed with some material which has a large surface area

but which does not hinder the vapour flow too much) between the container of boiling liquid and the condenser. In such

a column condensed liquid running down under gravity comes into close contact with ascending vapour, leading to

many stages of distillation within a small volume. This is called a fractional distillation or rectification.

25 In some cases mixtures of two liquids co-distil at a constant temperature in between the boiling points of the separate

pure compounds. In these instances, the distillate will contain a mixture of the components at a fixed composition.

These are called azeotropic mixtures and purification cannot be effected by standard distillation. Examples of

azeotropic mixtures are ethanol–water (b.p. 78 °C), or toluene–water (b.p. 84 °C).

b.p. measured at

distillation head

time

b.p. of

compound B

b.p. of

compound A collect

compound A

collect

compound B

forerun midrun end of

distillation

65

As a rough guide to the type of apparatus needed in any particular case we can say that a simple distillation apparatus

will suffice to separate liquids whose boiling points differ by more than 30 °C, or to separate a single liquid organic

compound from non-volatile material; if the difference is less than this, some type of distillation column must be used.

Simple distillation

The apparatus needed for a simple distillation is illustrated in Appendix 2D and Core Skill 4.

1 The glass apparatus should be of a size appropriate to the volume of liquid to be distilled. The distillation flask is

normally filled to about 50% of its capacity at the start of the distillation. If your chosen distillation glassware is

too large, the yield will be reduced because of the increased surface area to be wetted before the distillate reaches

the condenser. CAUTION: Always check the distilling flask against the light to make sure that it has no hairline

or star cracks that might cause it to fracture on heating.

2 Greasing of the joints is only necessary if you want to distil strongly alkaline solutions which can attack the glass

chemically or if you distil under vacuum.

3 Support the apparatus adequately; do not rely on the adhesive power of ground glass joints. You need at least 2

clamps and 2 lab stands. The reaction flask and the collection flask should usually be supported by a clamp and

boss secured to a retort stand. Stands should always be placed with the rod at the back of the apparatus and the base

protruding forwards to lie under the centre of gravity of the apparatus it is supporting.

4 In all simple distillations, you must add to the liquid some boiling aids to ensure even boiling. Boiling aids operate

by supplying a surface on which bubble nuclei can be formed, so that large bubbles can grow from these nuclei

smoothly. In the absence of suitable bubble nuclei, heated liquids often ‘superheat’ (= reach a temperature above

their b.p.) and this leads to intermittent violent eruptions of large quantities of vapour — a situation described as

‘bumping’. Porous chips (unglazed porcelain pieces) are commonly added boiling aids; alternatively, vigorous

stirring with a magnetic stirrer bar throughout the distillation will equally prevent bumping. Add a few anti-

bumping granules to the distilling flask before starting the distillation. Add more if the liquid goes off the boil

during the distillation, but never add them to liquids at their boiling point or the liquid may erupt violently out of

the flask; always cool the apparatus first.

5 A heating bath is generally preferable to using a high temperature source directly. For distillations below 75 °C

you can use a water/steam bath. Liquids boiling between 75 – 160 °C can be distilled using an oil bath heated by

an electric hotplate. Note that the bath itself should not be taken above 200 °C or it may ignite. CAUTION: If

drops of water enter an oil bath at 105 °C or above, violent bumping can ensue which could cause severe

burns. Distillations above 160 °C should be carried out using a sand bath or a Wood’s metal bath (Bi:Pb:Sn:Cd

4:2:1:1, m.p. 71 °C). Do not use an isomantle if you expect virtually all of the liquid to distil, since this can lead to

excessive heating and possible decomposition (or even an explosion) when the volume of liquid in the distillation

flask gets low.

6 The thermometer in the still head should ideally be placed with its bulb opposite the entrance to the side-arm. The

thermometer pocket should contain a small amount of high-boiling liquid (liquid paraffin or glycerol) so as to just

cover the bulb of the thermometer; it is used to provide thermal contact between the distilling liquid and the

thermometer bulb. When high-boiling liquids are being distilled, it may become necessary to insulate the still head

with aluminium foil against heat losses.

7 The use of a receiver adapter is essential to prevent liquid vapours escaping. For low-boiling liquids, the receiving

flask should be cooled in an ice/water bath.

8 Receiving vessels should be weighed before they are used, and preferably kept to hand on cork rings.

9 Distillation procedure:

i. Raise the temperature of the heating bath relatively quickly until the liquid to be distilled begins to boil.

ii. Control the heat source until the bath temperature has almost stopped climbing.

iii. Adjust the heat source so that the heating bath temperature increases only slowly.

66

iv. Note the still head temperature as the first drops of distillate come over.

v. Adjust rate of heating so that liquid distils at about 1 drop per second.

vi. Discontinue distillation when no more liquid is being collected. Never distil a flask to dryness because of

potential explosion hazards (see Appendix 2C on heating sources).

10 The collecting flask should be changed to collect the following fractions:

Forerun— b.p. rising

1st fraction— b.p. at steady value (3 – 5 °C range)

and, if more than one major component is present:

mid-fraction— b.p. unsteady, eventually rising

2nd fraction— b.p. at higher steady value (3 – 5 °C range)

end fraction— b.p. unsteady, eventually falling

You should keep a distillation protocol in your lab book with a record of still head and heating bath temperatures

throughout the distillation. A good distillation protocol also makes a note of the size of the flasks, the distillation

apparatus and the heat source used (e.g. “standard distillation apparatus with 100 mL RB distillation flask + isomantle,

setting: 4”).

67

APPENDIX 7 : How to use the rotary evaporator

GETTING READY

1 Check that the vapour tube inside the rotary evaporator is clean. You don’t want some impurity from another

student contaminating your precious product.

2 Get hold of a reduction adapter suitable for the rotary evaporator and your flask.

3 Check that the power is switched on for the rotary evaporator and the heating bath. Turn on the cooling water for

the rotary evaporator. Check that you have a gentle flow of cooling water, and not full power.

4 Attach your round-bottom flask and the adapter to the ground glass joint of the vapour tube of the rotary

evaporator. Hold the flask in place with your hand.

5 Turn on the vacuum with the check valve near the sink.

6 Close the stopcock at the top of the rotary evaporator condenser.

7 While you are still holding the flask, turn the dial for the rotary evaporator motor to rotate the flask, slowly at first,

then at a higher speed so that foaming of the solvent is kept at a safe limit. A good setting is about 5 − 7.

Efficient rotation is important to reduce bumping; it also reduces the time required for the solvent to evaporate, as

a thin film of fresh solvent is constantly exposed to the vacuum.

8 Release your hand holding the flask once the vacuum holds the flask in place. Note: This will only work for small

flasks and <150 mL of liquid. If you work on a larger scale, you will need to secure your flask and the reduction

adapter with a plastic Keck clip or, better still, a metal clip.

9 Now carefully lower the round-bottom flask into the water bath. If needed (because you have a high-boiling

solvent or evaporation is too sluggish), turn on the water bath to about 40 °C.

68

FINISHING WITH THE ROTARY EVAPORATOR

10 Lift the flask out of the water bath.

11 Turn off the vacuum at the check valve near the sink.

12 Support the flask with your hand while you vent the rotary evaporator by turning the stopcock at the top of the

rotary evaporator. If your flask contains a fine solid, you should vent slowly to prevent the solid from being

sprayed around the flask (or indeed the whole rotary evaporator).

13 Turn off the rotation and remove the flask. On rare occasions, the flask might get stuck to the adapter and you run

into danger of breaking the glass while turning it at the joint. Should this happen, seek advice from a

demonstrator.

14 Don’t forget to turn off the cooling water and the heat control of the water bath.

69

USEFUL TIPS

• The round-bottom flask should be no more than half full — otherwise you risk losing material through instant

(and excessive) splashing when applying the vacuum. Large amounts of solvents are better removed in smaller

portions.

• It is a good idea to weigh the empty flask prior to rotary evaporation. This gives you a chance of getting a

rough yield of your crude product.

• Check your flask before use. It should have no crack or star crack which could lead to an implosion under

vacuum. SAFETY FIRST!

• Your solvent should have (almost) quantitatively evaporated at the end unless you are using a solvent with a

high boiling point or a poor vacuum. Remember that the rotary evaporator evaporates solvent, but it is not an

efficient apparatus for drying solids. Similarly, if your residue in the flask is an oil, it will be almost

impossible to remove all solvents from it.

• The heating bath should be kept clean, so if you spill anything into it, ask a demonstrator on how best to clean

it. The “causer pays principle” applies also in our lab: If you mess it up, you clean it !

• Evaporated solvent will collect in the collection bulb underneath the condenser of the rotary evaporator, or

(more likely) at the exhaust of the diaphragm pump. The collection flasks have special joints and are quite

expensive. If they are getting full, ask a demonstrator to empty them for you.

• Ask a demonstrator before you evaporate a chlorinated solvent, as the collection flasks need to be emptied

before and after use since chlorinated solvent waste is disposed separately. No one else will know which

solvents are in the receiver flask!

• In the lab, the rotary evaporators are linked to a diaphragm pump. These pumps are more environmentally

friendly than the older water pumps which produced a lot of waste water contaminated with organic solvents.

Diaphragm pumps are capable of achieving a vacuum of 20 mbar (if you are the only one using the pump), but

if other valves along your bench are open at the same time the vacuum can be as bad as 100 mbar. So, check

who else has opened (or forgotten to close) the vacuum taps along your bench. Because you use a vacuum of,

say, 40 mbar on average during rotary evaporation, the boiling points of the solvents are going to be

significantly lowered compared to ambient pressure (see table below).26 This means that you will hardly have

to switch on the water bath unless you are using high-boiling solvents (water, toluene).

Solvent b.p. (1013 mbar) b.p. (40 mbar)

Acetonitrile 82 °C –4 °C

Diethyl ether 35 °C −42 °C

Ethanol 78 °C 7 °C

Ethyl acetate 77 °C –7 °C

Hexane 69 °C –23 °C

Methanol 65 °C –2 °C

Toluene 110 °C 36 °C

Water 100 °C 29 °C

26 For estimating boiling points under reduced pressure, try: http://www.trimen.pl/witek/calculators/wrzenie.html

70

APPENDIX 8 : Lab book note-taking

Good written communication is essential for chemists to transmit their work to the scientific community. The process

begins with a record kept in a laboratory notebook which is the source of information used to prepare scientific papers

published in journals or presented at meetings. For the industrial chemist, accurate laboratory note taking is especially

critical in obtaining patent coverage.

A laboratory notebook has several key components:

1. Date when experiment was conducted.

2. Title of experiment.

3. Aim: state the purpose for running the reaction.

4. Reaction scheme.

5. Table of reagents, quantities etc.

6. Details of procedure used.

7. Characteristics of the product(s): yield, appearance (e.g. yellow crystalline solid or colourless oil). Staple or glue

your IR spectrum into the lab book.

8. References to product or procedure (if any), i.e. if you followed a literature preparation.

In regard to point 5, quote your quantities to 3 significant figures unless you are dealing with solvent volumes in which

case the volumes are recorded to 2 significant figures, such as “... benzoic acid (1.22 g, 1.00 mmol) was dissolved in

methanol (15 mL) ...” You do not need to record the number of moles of solvent used (just give the volume) unless the

solvent is also a reactant. In a research lab it is also customary to record the source of the chemicals and starting

materials, be it a chemical supplier or an earlier experiment if it is a compound that you have made yourself. This is

because, if a reaction which normally works failed on a particular occasion, it will allow an investigator to check

whether anything was amiss with the reagents that were used in the experiment. A good way to find out physical

properties such as m.p. / b.p. and densities (if dealing with liquid reagents) is to look up the reagent in a chemical

supplier catalogue. Copies of the Aldrich Chemical Company and Alfa Aesar catalogue will be in the lab (or can be

accessed on the internet, e.g. http://www.sigmaaldrich.com/united-kingdom.html and

https://www.alfa.com/en/advanced-search/). Links are provided on Vision.

In regard to point 6, it is the obligation of the person doing the work to list the amounts of reagents, the experimental

conditions (how reagents were reacted together — order of addition, time of addition, reaction time, reaction

temperature, reflux etc.), and the method used to “work up” the reaction and isolate the product (distillation

temperatures or details of crystallisation, including approximate solvent volumes used). This is to be a record of what

YOU did in the lab including any mishaps, not a reiteration of the lab manual. Reaction times, colour or

temperature changes and any deviations from the original procedure should be carefully noted and recorded.

The reports should be written with care, be concise, and be clear. Even a bullet point list is better than no record at all

or one written up from memory after the lab. Although “brevity is the soul of wit”,27 as a rule sufficient detail should

be recorded that would enable a competent organic chemist to reproduce your experiment. Standard equipment is

not usually described at all, and there is therefore no need to write down the size of a flask or glassware you used. A

27 W. Shakespeare, Hamlet, Act 2, Scene 2.

71

trained chemist repeating your experiment will know how to do a standard reflux or suit the size of apparatus to the

reaction scale. However, if there are special reasons why a particular size or type of apparatus are critical to the success

of the experiment then these should be carefully recorded.

You must note any points where your procedure differed from that given in the lab manual; this includes especially any

mishaps and deviations in timing from your instructing manual. When giving quantities of reactants these should be put

in brackets after the name of the chemical and quoted to 3 significant figures or, in the case of liquids, 2 significant

figures. For example: “Phosphoric acid (85% w/w; 10 mL) was added slowly to cyclohexanol (24.3 g, 0.243 mol)...”.

In regard to point 7, you must record product yield, appearance (colour, crystal/needle/powder/oil) and m.p. (if a solid).

The yield is recorded as a weight in grams, the quantity in mol (or mmol) and as a percentage of the theoretical

maximum (rounded to full percent).

Here are a few additional points about the proper maintenance of a laboratory record:

• A hardbound, permanent notebook is preferred.

• Each page of the notebook should be numbered in consecutive order. For convenience, an index at the

beginning or end of the book is recommended and blank pages should be retained for this purpose.

• Always record your data in ink, not in pencil. If a mistake is made, draw a neat line through the word or words

so that they remain legible. Data are always recorded directly into the notebook, never on scrap paper! Use

the right hand pages of your lab book for writing-up your experiments and the left hand pages for rough

notes you need to make while doing the experiment, e.g. the weight of an empty flask, the weight of a

flask + reagent, calculation of yield etc. Your write-up should be done in the lab as you are doing the

experiment (for example, while a reflux reaction is proceeding).

• Make the record clear and unambiguous. Pay attention to grammar and spelling.

• In industrial research laboratories, your signature, as well as that of a witness, is required, because the

notebook may be used as a legal document in evidence that and when the experiment was done.

• Always write and organise your work so that someone else could come into the laboratory and repeat your

directions without confusion or uncertainty. Completeness and legibility are key factors.

A model lab report is included on the following pages, for illustration. Note that you DO NOT need to include the

postlab exercises for experiments 4 + 6 in the lab book, as they form part of your separate lab report submission.

As part of the Conclusions you should also comment on any difficulties you experienced, including any reasons why

you think your experiment did not proceed as well as it might have done and what could solve this.

72

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74

Bench Reagent Molarities

Sodium Hydroxide 2 M

Ammonia 2 M

Sodium Carbonate 1 M

Acetic Acid 2 M

Nitric Acid 2 M

Sulfuric Acid 2 M

Hydrochloric Acid 2 M

Concentrated Acids:

MW % w/w Density Molarity

Nitric Acid 63.0 70 1.42 g/mL 15.8

Sulfuric Acid 98.1 95 1.83 g/mL 17.7

Hydrochloric Acid 36.5 36 1.18 g/mL 11.7

Sample Calculation

How many moles of HNO3 are present in 50 mL of concentrated nitric acid?

The density of conc. nitric acid is 1.42 g mL–1

;

so 50 mL corresponds to (1.42 g mL–1

) × (50 mL) = 71 g

The composition of conc. nitric acid is 70% w/w;

so 71 g of conc. nitric acid contains to 0.70 × 71 g = 49.7 g of HNO3 (the other 30% is water).

The molecular weight of HNO3 is 63.0 g mol–1

;

so the number of moles of HNO3 present in 50 mL of conc. acid are 49.7 g/(63.0 g mol–1

) = 0.79 mol.

Since 50 mL has only two significant figures, the amount in mol should also have only two significant figures.

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APPENDIX 9 : IR and NMR

Characteristic 13

C NMR chemical shifts (δC):

13C NMR chemical shifts and multiplicities of common deuterated solvents

CDCl3

(Deuterated chloroform)

(CD3)2SO

(Deuterated DMSO)

CD3OD

(Deuterated methanol)

Residual

solvent

signal

77.00 39.43 49.05

80 75 70 45 40 35 55 50 45

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