nucleophilic substitution.docx

8/14/2019 NUCLEOPHILIC SUBSTITUTION.docx

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UCLEOPHILIC SUBSTITUTION

nding in the halogenoalkanes

ogenoalkanes (also known as haloalkanes or alkyl halides) are compounds containing a halogen atomorine, chlorine, bromine or iodine) joined to one or more carbon atoms in a chain.

e interesting thing about these compounds is the carbon-halogen bond, and all the nucleophilic substitutio

ctions of the halogenoalkanes involve breaking that bond.

e polar i ty of the carbon-halogen bond s

h the exception of iodine, all of the halogens are more electronegative than carbon.

Electronegativi ty values (Pauling scale)

C 2.5 F 4.0Cl 3.0Br

2.8

I 2.5

at means that the electron pair in the carbon-halogen bond will be dragged towards the halogen end, leavhalogen slightly negative ( -) and the carbon slightly positive ( +) - except in the carbon-iodine case.

hough the carbon-iodine bond doesn't have a permanent dipole, the bond is very easily polarised by anytproaching it. Imagine a negative ion approaching the bond from the far side of the carbon atom:

e fairly small polarity of the carbon-bromine bond will be increased by the same effect.

e strengths o f the carbon -halogen bon ds

Note: If you haven't done any work on bond strengths, or are a bit rusty, it doesn't matter. Just realise that the bigger the


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number, the stronger the bond. And don't worry if you have found slightly different numbers in a different data source - therelot of variability in the quoted values, but the overall pattern is still the same.

ok at the strengths of various bonds (all values in kJ mol - ).

C-H 413 C-F 467C-Cl 346C-Br 290C-I 228

all of these nucleophilic substitution reactions, the carbon-halogen bond has to be broken at some pointing the reaction. The harder it is to break, the slower the reaction will be.

e carbon-fluorine bond is very strong (stronger than C-H) and isn't easily broken. It doesn't matter that the

bon-fluorine bond has the greatest polarity - the strength of the bond is much more important in determinreactivity. You might therefore expect fluoroalkanes to be very unreactive - and they are! We shall simplyore them from now on.

he other halogenoalkanes, the bonds get weaker as you go from chlorine to bromine to iodine.

at means that chloroalkanes react most slowly, bromoalkanes react faster, and iodoalkanes react faster s

Rates of reaction: RCl < RBr < RI

ere "


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cleophiles

ucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge inmething else.

cleophiles are either fully negative ions, or else have a strongly - charge somewhere on a molecule.mmon nucleophiles are hydroxide ions, cyanide ions, water and ammonia.

tice that each of these contains at least one lone pair of electrons, either on an atom carrying a full negatarge, or on a very electronegative atom carrying a substantial - charge.

e nucleophilic substitution reaction - an SN2 reaction

'll talk this mechanism through using an ion as a nucleophile, because it's slightly easier. The water andmonia mechanisms involve an extra step which you can read about on the pages describing those particchanisms.

'll take bromoethane as a typical primary halogenoalkane. The bromoethane has a polar bondween the carbon and the bromine.

'll look at its reaction with a general purpose nucleophilic ion which we'll call Nu-. This will have

east one lone pair of electrons. Nu

-

could, for example, be OH

-

or CN

-

.

e lone pair on the Nu-ion will be strongly attracted to the + carbon, and will move towards it, beginning ke a co-ordinate (dative covalent) bond. In the process the electrons in the C-Br bond will be pushed eve

ser towards the bromine, making it increasingly negative.

Note: A co-ordinate bond is a covalent bond in which both electrons come from one of the atoms.

e movement goes on until the -Nu is firmly attached to the carbon, and the bromine has been expelled asion.


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Note: We haven't shown all the lone pairs on the bromine. These other lone pairs aren't involved in the reaction, and includthem simply clutters the diagram to no purpose.

ings to not ice

e Nu-ion approaches the + carbon from the side away from the bromine atom. The large bromine atomders attack from its side and, being -, would repel the incoming Nu-anyway. This attack from the back iportant if you need to understand why tertiary halogenoalkanes have a different mechanism. We'll discuss later on this page.

ere is obviously a point in which the Nu -is half attached to the carbon, and the C-Br bond is half way to bken. This is called a t ransi t ion state. It isn't an intermediate. You can't isolate it - even for a very short tijust the mid-point of a smooth attack by one group and the departure of another.

w to w r i te the mechanism

e simplest way is:

Note: In exam you mus tshow the lone pair of electrons on the nucleophile (in this case, the Nu-ion). It probably doesn't ma

whether you show them on the departing Br-ion or not.

If you aren't happy about the use ofcurly arrowsin mechanisms, follow this link before you go on. Use the BACK button on ybrowser to return to this page.

chnically, this is known as an SN2reaction. Sstands for substitution, Nfor nucleophilic, and the 2is becainitial stage of the reaction involves two species - the bromoethane and the Nu-ion. If your syllabus doe

er to SN2 reactions by name, you can just call it nuc leophi li c subst i tu t ion.

me examiners like you to show the transition state in the mechanism, in which case you need to write it inmore detail - showing how everything is arranged in space.
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very careful when you draw the transition state to make a clear difference between the dotted lines show

half-made and half-broken bonds, and those showing the bonds going back into the paper.

tice that the molecule has been inverted during the reaction - rather like an umbrella being blown inside-o

Note: If you aren't happy about the various ways ofdrawing bonds,it is important to follow this link to find out exactly what tvarious symbols mean.

It is also important to know which of these ways of drawing the mechanism your particular examiners want you to use. If youhaven't already checked yoursyllabus, recent exam papers and mark schemes,you must do so! At the time of writing, Edexfor example, wanted the transition state included, and that isn't obvious from their syllabus. You haveto check mark schemeexaminers reports.

Use the BACK button on your browser to return to this page.

cleophilic substitution in tertiary halogenoalkanes

Warning! Check your syllabus, past papers and any support material published by your examiners to find out whether you nthis. If there's no mention of tertiary halogenoalkanes or SN1 reactions, then you probably don't need it.

member that a tertiary halogenoalkane has three alkyl groups attached to the carbon with theogen on it. These alkyl groups can be the same or different, but in this section, we shall justnsider a simple one, (CH3)3CBr - 2-bromo-2-methylpropane.

e nucleophilic substitution reaction - an SN1 reaction
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ce again, we'll talk this mechanism through using an ion as a nucleophile, because it's slightly easier, andain we'll look at the reaction of a general purpose nucleophilic ion which we'll call Nu-. This will have at lee lone pair of electrons.

y is a di f ferent mechanism n ecessary?

u will remember that when a nucleophile attacks a primary halogenoalkane, it approaches the + carbonm from the side away from the halogen atom.

h a tertiary halogenoalkane, this is impossible. The back of the molecule is completely cluttered with CHups.

ce any other approach is prevented by the bromine atom, the reaction has to go by an alternativechanism.

e alternative mechanism

Important! To understand this section, you need to know what acarbocation (carbonium ion)is, and about the relative stabof primary, secondary and tertiary carbocations.

If you follow this link, use the BACK button on your browser to return to this page.

e reaction happens in two stages. In the first, a small proportion of the halogenoalkane ionises to give abocation and a bromide ion.

s reaction is possible because tertiary carbocations are relatively stable compared with secondary or primes. Even so, the reaction is slow.

ce the carbocation is formed, however, it would react immediately it came into contact with a nucleophile. The lone pair on the nucleophile is strongly attracted towards the positive carbon, and moves towards i
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ate a new bond.

w fast the reaction happens is going to be governed by how fast the halogenoalkane ionises. Because thal slow step only involves one species, the mechanism is described as SN1 - substitution, nucleophilic, o

ecies taking part in the initial slow step.

y do n't pr imary halogenoalkanes use the SN1 mechanism?

primary halogenoalkane did use this mechanism, the first step would be, for example:

rimary carbocation would be formed, and this is much more energetically unstable than the tertiary onemed from tertiary halogenoalkanes - and therefore much more difficult to produce.

s instability means that there will be a very high activation energy for the reaction involving a primaryogenoalkane. The activation energy is much less if it undergoes an SN2 reaction - and so that's what it dotead.

cleophilic substitution in secondary halogenoalkanes

ere isn't anything new in this. Secondary halogenoalkanes will use both mechanisms - somelecules will react using the SN2 mechanism and others the SN1.

e SN2 mechanism is possible because the back of the molecule isn't completely cluttered by alkyl groupsthe approaching nucleophile can still get at the + carbon atom.

e SN1 mechanism is possible because the secondary carbocation formed in the slow step is more stable rimary one. It isn't as stable as a tertiary one though, and so the S N1 route isn't as effective as it is withiary halogenoalkanes.

E NUCLEOPHILIC SUBSTITUTION REACTIONS BETWEEN HALOGENOALKANES AND HYDROXIDENS


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s page gives you the facts and simple, uncluttered mechanisms for the nucleophilic substitution reactionsween halogenoalkanes and hydroxide ions (from, for example, sodium hydroxide). If you want thechanisms explained to you in detail, there is a link at the bottom of the page.

e reaction of primary halogenoalkanes with hydroxide ions

Important! If you aren't sure about the difference betweenprimary, secondary and tertiary halogenoalkanes,it is essentithat you follow this link before you go on.


e facts

halogenoalkane is heated under reflux with a solution of sodium or potassium hydroxide, the halogen islaced by -OH and an alcohol is produced. Heating under reflux means heating with a condenser placedtically in the flask to prevent loss of volatile substances from the mixture.

e solvent is usually a 50/50 mixture of ethanol and water, because everything will dissolve in that. Theogenoalkane is insoluble in water. If you used water alone as the solvent, the halogenoalkane and thedium hydroxide solution wouldn't mix and the reaction could only happen where the two layers met.

example, using 1-bromopropane as a typical primary halogenoalkane:

u could write the full equation rather than the ionic one, but it slightly obscures what's going on:

e bromine (or other halogen) in the halogenoalkane is simply replaced by an -OH group - hence abstitution reaction. In this example, propan-1-ol is formed.

e mechanism

re is the mechanism for the reaction involving bromoethane:
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s is an example ofnucleoph i l ic sub st i tut ion.

cause the mechanism involves collision between two species in the slow step (in this case, the only step)reaction, it is known as an SN2 reaction.

Note: Unless your syllabus specifically mentions SN2 by name, you can just call it nucleophilic substitution.

our examiners want you to show the transition state, draw the mechanism like this:

e reaction of tertiary halogenoalkanes with hydroxide ionse facts

e facts of the reaction are exactly the same as with primary halogenoalkanes. If the halogenoalkane is heder reflux with a solution of sodium or potassium hydroxide in a mixture of ethanol and water, the halogen

laced by -OH, and an alcohol is produced.

example:

if you want the full equation rather than the ionic one:


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e mechanism

s mechanism involves an initial ionisation of the halogenoalkane:

owed by a very rapid attack by the hydroxide ion on the carbocation (carbonium ion) formed:

s is again an example ofnucleoph i l ic subs t i tut ion.

s time the slow step of the reaction only involves one species - the halogenoalkane. It is known as an S Nction.

e reaction of secondary halogenoalkanes with hydroxide ionse facts

e facts of the reaction are exactly the same as with primary or tertiary halogenoalkanes. The halogenoalkeated under reflux with a solution of sodium or potassium hydroxide in a mixture of ethanol and water.

example:

e mechanism

condary halogenoalkanes use bothSN2 and SN1 mechanisms. For example, the SN2 mechanism is:


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ould you need it, the two stages of the SN1 mechanism are:

E NUCLEOPHILIC SUBSTITUTION REACTIONS BETWEEN HALOGENOALKANES AND WATER

s page gives you the facts and simple, uncluttered mechanisms for the nucleophilic substitutionctions between halogenoalkanes and water. If you want the mechanisms explained to you in detare is a link at the bottom of the page.

e reaction of primary halogenoalkanes with water

Important! If you aren't sure about the difference betweenprimary, secondary and tertiary halogenoalkanes,it is essthat you follow this link before you go on.


e facts

ere is only a slow reaction between a primary halogenoalkane and water even if they are heated. Theogen atom is replaced by -OH.

example, using 1-bromoethane as a typical primary halogenoalkane:


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alcohol is produced together with hydrobromic acid. Be careful not to call this hydrogen bromide. Hydrogmide is a gas. When it is dissolved it in water (as it will be here), it's called hydrobromic acid.

e mechanism

e mechanism involves two steps. The first is a simple nucleophilic substitution reaction:

cause the mechanism involves collision between two species in this slow step of the reaction, it is known SN2 reaction.


e nucleophilic substitution is very slow because water isn't a very good nucleophile. It lacks the full negatarge of, say, a hydroxide ion.

e second step of the reaction simply tidies up the product. A water molecule removes one of the hydrogenached to the oxygen to give an alcohol and a hydroxonium ion (also known as a hydronium ion or an oxon).

e hydroxonium ion and the bromide ion (from the nucleophilic substitution stage of the reaction) make updrobromic acid which is formed as well as the alcohol.

e reaction of tertiary halogenoalkanes with water

e facts

he halogenoalkane is heated under reflux with water, the halogen is replaced by -OH to give an alcohol.ating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatilebstances from the mixture. The reaction happens much faster than the corresponding one involving a prim


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ogenoalkane.

example:

e mechanism


owed by a very rapid attack by the water on the carbocation (carbonium ion) formed:



w there is a final stage in which the product is tidied up. A water molecule removes one of the hydrogensached to the oxygen to give an alcohol and a hydroxonium ion - exactly as happens with primaryogenoalkanes.

e rate of the overall reaction is governed entirely by how fast the halogenoalkane ionises. The fact that wt as good a nucleophile as, say, OH -doesn't make any difference. The water isn't involved in the slow stereaction.

e reaction of secondary halogenoalkanes with water


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very unlikely that any of the current UK-based syllabuses for 16 - 18 year olds will ask you about this. Inremely unlikely event that you will ever need it, secondary halogenoalkanes use bothan SN2 mechanismd an SN1.

ke sure you understand what happens with primary and tertiary halogenoalkanes, and then adapt it forcondary ones should ever need to.

ere would you like to go now?

Help! Talk me through these mechanisms . . .

To menu of nucleophilic substitution reactions. . .

To menu of other types of mechanism. . .

To Main Menu . . .

E NUCLEOPHILIC SUBSTITUTION REACTIONS BETWEEN HALOGENOALKANES AND CYANIDE IO

s page gives you the facts and simple, uncluttered mechanisms for the nucleophilic substitution reactionsween halogenoalkanes and cyanide ions (from, for example, potassium cyanide). If you want the

chanisms explained to you in detail, there is a link at the bottom of the page.

e reaction of primary halogenoalkanes with cyanide ions

Important! If you aren't sure about the difference betweenprimary, secondary and tertiary halogenoalkanes,it is essenthat you follow this link before you go on.


e facts

halogenoalkane is heated under reflux with a solution of sodium or potassium cyanide in ethanol, theogen is replaced by a -CN group and a nitrile is produced. Heating under reflux means heating with a
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ndenser placed vertically in the flask to prevent loss of volatile substances from the mixture.

e solvent is important. If water is present you tend to get substitution by -OH instead of -CN.

Note: A solution of potassium cyanide in water is quite alkaline, and contains significant amounts of hydroxide ions. The

react with the halogenoalkane.

example, using 1-bromopropane as a typical primary halogenoalkane:

u could write the full equation rather than the ionic one, but it slightly obscures what's going on:

e bromine (or other halogen) in the halogenoalkane is simply replaced by a -CN group - hence a substituction. In this example, butanenitrile is formed.

Note: When you are naming nitriles, you have to remember to include the carbon in the -CN group when you count thelongest chain. In this example, there are 4 carbons in the longest chain - hence butanenitrile.

e mechanism

re is the mechanism for the reaction involving bromoethane:

s is an example ofnucleoph i l ic sub st i tut ion.

cause the mechanism involves collision between two species in the slow step (in this case, the only step)reaction, it is known as an SN2 reaction.


HE NUCLEOPHILIC SUBSTITUTION REACTIONS BETWEEN


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ALOGENOALKANES AND AMMONIA

s page gives you the facts and simple, uncluttered mechanisms for the nucleophilic substitution reactionsween halogenoalkanes and ammonia to produce primary amines. If you want the mechanisms explained

u in detail, there is a link at the bottom of the page. If you are interested in further substitution reactions, yalso find a link to a separate page dealing with these.

e reaction of primary halogenoalkanes with ammonia

mportant! If you aren't sure about the difference betweenprimary, secondary and tertiary halogenoalkanes,it is essentialth

ou follow this link before you go on.


e facts

e halogenoalkane is heated with a concentrated solution of ammonia in ethanol. The reaction is carried o

ealed tube. You couldn't heat this mixture under reflux, because the ammonia would simply escape up thndenser as a gas.

'll talk about the reaction using 1-bromoethane as a typical primary halogenoalkane.

e reaction happens in two stages. In the first stage, a salt is formed - in this case, ethylammonium bromids is just like ammonium bromide, except that one of the hydrogens in the ammonium ion is replaced by ayl group.

ere is then the possibility of a reversible reaction between this salt and excess ammonia in the mixture.

e ammonia removes a hydrogen ion from the ethylammonium ion to leave a primary amine - ethylamine.

e more ammonia there is in the mixture, the more the forward reaction is favoured.


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Note: You will find considerable disagreement in textbooks and other sources about the exact nature of the products in this

eaction. Some of the information you'll come across is simply wrong!

You can read the arguments aboutthe products of this reactionby following this link.

e mechanism

e mechanism involves two steps. The first is a simple nucleophilic substitution reaction:

cause the mechanism involves collision between two species in this slow step of the reaction, it is known SN2 reaction.


he second step of the reaction an ammonia molecule may remove one of the hydrogens on the -NH 3+. A

monium ion is formed, together with a primary amine - in this case, ethylamine.

s reaction is, however, reversible. Your product will therefore contain a mixture of ethylammonium ions,monia, ethylamine and ammonium ions. Your major product will only be ethylamine if the ammonia issent in very large excess.

fortunately the reaction doesn't stop here. Ethylamine is a good nucleophile, and goes on to attack unusemoethane. This gets so complicated that it is dealt with on a separate page. You will find a link at the bot
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his page.

e reaction of tertiary halogenoalkanes with ammonia

e facts

e facts of the reactions are exactly the same as with primary halogenoalkanes. The halogenoalkane is hea sealed tube with a solution of ammonia in ethanol.

example:

lowed by:

e mechanism


owed by a very rapid attack by the ammonia on the carbocation (carbonium ion) formed:



ere is a second stage exactly as with primary halogenoalkanes. An ammonia molecule removes a hydrogfrom the -NH3

+group in a reversible reaction. An ammonium ion is formed, together with an amine.


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e reaction of secondary halogenoalkanes with ammonia

very unlikely that any of the current UK-based syllabuses for 16 - 18 year olds will ask you about this. Inremely unlikely event that you will ever need it, secondary halogenoalkanes use bothan SN2 mechanismd an SN1.

ke sure you understand what happens with primary and tertiary halogenoalkanes, and then adapt it forcondary ones should ever need to.



To look at further substitution in these reactions . . .



To Main Menu . . .

m Clark 2000

HE NUCLEOPHILIC ADDITION OF HYDROGEN CYANIDE TO

LDEHYDES AND KETONES

s page gives you the facts and simple, uncluttered mechanisms for the nucleophilic addition reactionsween carbonyl compounds (specifically aldehydes and ketones) and hydrogen cyanide, HCN. If you wan
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ehydes and ketones behave identically in their reaction with hydrogen cyanide, and so will be consideredether - although equations and mechanisms will be given for both types of compounds for the sake of

mpleteness.

e reaction of aldehydes and ketones with hydrogen cyanidee facts

drogen cyanide adds across the carbon-oxygen double bond in aldehydes and ketones to producempounds known as hydroxynitriles.

example, with ethanal (an aldehyde) you get 2-hydroxypropanenitrile:

h propanone (a ketone) you get 2-hydroxy-2-methylpropanenitrile:

Note: When you are naming these compounds, don't forget that the longest carbon chain has to include the carbo

the -CN group. In both of the above examples, the longest carbon chain is 3 carbons - hence the "prop" in both nam

The carbon with the nitrogen attached is always counted as the number 1 carbon in the chain.

e reaction isn't normally done using hydrogen cyanide itself, because this is an extremely poisonous gas.tead, the aldehyde or ketone is mixed with a solution of sodium or potassium cyanide in water to which a

phuric acid has been added. The pH of the solution is adjusted to about 4 - 5, because this gives the fastction.

e solution will contain hydrogen cyanide (from the reaction between the sodium or potassium cyanide andphuric acid), but still contains some free cyanide ions. This is important for the mechanism.


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Note: If the reaction is done using hydrogen cyanide itself, a little sodium hydroxide solution is added to produce s

cyanide ions from the weakly acidic HCN. Again the pH of the solution is adjusted to around pH 5 - in other words,

sodium hydroxide is not added to excess. The rate of the reaction falls if the pH is any higher.

Whichever set of reagents you use, the reaction contains the same mixture of hydrogen cyanide and cyanide ions.

e mechanisms

ese are examples of nucleoph i l ic addi t ion.

e carbon-oxygen double bond is highly polar, and the slightly positive carbon atom isacked by the cyanide ion acting as a nucleophile.

Nucleophile: A species (molecule or ion) which attacks a positive site in something else. Nucleophiles are either f

negative ions or contain a fairly negative region somewhere in a molecule. All nucleophiles have at least one active

pair of electrons. When you write mechanisms for reactions involving nucleophiles, you mus tshow that lone pair.

e mechanism for the addi t ion of HCN to propanon e

he first stage, there is a nucleophilic attack by the cyanide ion on the slightly positive carbon atom.

e negative ion formed then picks up a hydrogen ion from somewhere - for example, from a hydrogen cya

lecule.

e hydrogen ion could also come from the water or the H3O+ions present in the slightly acidic solution. Yo


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n't need to remember all of these. One equation is perfectly adequate.

Note: The product molecule here has been drawn differently from the one in the equation further up this page. It h

been rotated through 90. There is no reason why you can't do that if it makes the appearance of the mechanism e

to follow.

e mechanism fo r the addi t ion of HCN to ethanal

before, the reaction starts with a nucleophilic attack by the cyanide ion on the slightly positive carbon ato

s completed by the addition of a hydrogen ion from, for example, a hydrogen cyanide molecule.

Note: Again, the product molecule looks different from the one in the equation further up this page. The central ca

atom still has the same four groups attached, but to make the mechanism easier to follow, they are simply arranged

differently. That's not a problem - we're stil l talking about the same substance.

tical isomerism in 2-hydroxypropanenitrile

en 2-hydroxypropanenitrile is made in this last mechanism, it occurs as a racemic mixture- a 50/50 mixwo optical isomers. It is possible that you might be exected to explain how this arises.


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Note: You almost certainly won't be able to tell whether or not you need to know this from the syllabus. You need t

refer to recent exam papers and mark schemes. If you haven't already got these, you can obtain them from your Ex

Board via links on thesyllabusespage.

tical isomerism occurs in compounds which have four different groups attached to a single carbon atom. case, the product molecule contains a CH3, a CN, an H and an OH all attached to the central carbon ato

e reason for the formation of equal amounts of two isomers lies in the way the ethanal getsacked.

anal is a planar molecule, and attack by a cyanide ion will either be from above the planehe molecule, or from below. There is an equal chance of either happening.

ack from one side will lead to one of the two isomers, and attack from the other side will lead to the other

Note: This is probably as much as you need to know for exam purposes, but a full explanation of this is given on th

"talk through" page. Follow the link below.

aldehydes will form a racemic mixture in this way. Unsymmetrical ketones will as well. (A ketone can besymmetrical in the sense that there is a different alkyl group either side of the carbonyl group.) What matthat the product molecule must have four different groups attached to the carbon which was originally par

carbon-oxygen double bond

HE REDUCTION OF ALDEHYDES AND KETONES

s page gives you the facts and mechanisms for the reduction of carbonyl compounds (specifically aldehy
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d ketones) using sodium tetrahydridoborate (sodium borohydride) as the reducing agent.

y one UK A level Exam Board (AQA) is likely to ask for these mechanisms, and they are happy with aplified version of what is quite a complex mechanism. Because of that simplification, these reactions are

alt with entirely on this page - without the "talk through" page that you will find for other mechanisms on th

.

e reduction of aldehydes and ketones by sodium tetrahydridoboratee facts

dium tetrahydridoborate (previously known as sodium borohydride) has the formula NaBH4, and contains

4-ion. That ion acts as the reducing agent.

ere are several quite different ways of carrying out this reaction. Two possible variants (there are several

ers!) are:

The reaction is carried out in solution in water to which some sodium hydroxide has been added to mit alkaline. The reaction produces an intermediate which is converted into the final product by additiona dilute acid like sulphuric acid.

The reaction is carried out in solution in an alcohol like methanol, ethanol or propan-2-ol. This producan intermediate which can be converted into the final product by boiling it with water.

each case, reduction essentially involves the addition of a hydrogen atom to each end of the carbon-oxyguble bond to form an alcohol. Reduction of aldehydes and ketones lead to two different sorts of alcohol.

e reduc t ion of an aldehyde

example, with ethanal you get ethanol:

tice that this is a simplified equation - perfectly acceptable to examiners. The H in square brackets meandrogen from a reducing agent".

general terms, reduction of an aldehyde leads to a pr imary alcoho l. A primary alcohol is one which only e alkyl group attached to the carbon with the -OH group on it. They all contain the grouping -CH 2OH.

Note: There is one exception to this. Methanol CH3OH is also a primary alcohol. Think of this as H-CH2OH.


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e reduc t ion of a ketone

example, with propanone you get propan-2-ol:

duction of a ketone leads to a second ary alcohol. A secondary alcohol is one which has two alkyl groupached to the carbon with the -OH group on it. They all contain the grouping -CHOH.

Beware! The following mechanisms are simplified for UK A level purposes to the point that they are wrong! If you

working outside the UK A level system, please don't read any further!

e simplified mechanisms

e BH4-ion is essentially a source of hydride ions, H -. The simplification used is to write H -instead of BH4

-

ng this not only makes the initial attack easier to write, but avoids you getting involved with some quitemplicated boron compounds that are formed as intermediates.

e reduction is an example of nucleop hi l ic addi t ion.

e carbon-oxygen double bond is highly polar, and the slightly positive carbon atom isacked by the hydride ion acting as a nucleophile. A hydride ion is a hydrogen atom with anra electron - hence the lone pair.

Nucleophile: A species (molecule or ion) which attacks a positive site in something else. Nucleophiles are eithe

negative ions or contain a fairly negative region somewhere in a molecule. All nucleophiles have at least one activ

lone pair of electrons. When you write mechanisms for reactions involving nucleophiles, you mus tshow that lone


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e mechanism for the reduct ion o f ethanal

he first stage, there is a nucleophilic attack by the hydride ion on the slightly positive carbon atom. The lor of electrons on the hydride ion forms a bond with the carbon, and the electrons in one of the carbon-oxynds are repelled entirely onto the oxygen, giving it a negative charge.

at happens now depends on whether you add an acid or water to complete the reaction.

ding an acid:

en the acid is added, the negative ion formed picks up a hydrogen ion to give an alcohol.

Note: You may find that other sources write the hydrogen ion simply as H+. That's not good practice, because it

suggests a free hydrogen ion. The hydrogen ion is actually attached to a water molecule as H3O+. Writing that ma

the equation look more complicated. H+

(aq)is a happy compromise.

ding water:

s time, the negative ion takes a hydrogen ion from a water molecule.


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e mechanism fo r the reduct ion of propano ne

before, the reaction starts with a nucleophilic attack by the hydride ion on the slightly positive carbon ato

ain, what happens next depends on whether you add an acid or water to complete the reaction.

ding an acid:

e negative ion reacts with a hydrogen ion from the acid added in the second stage of the reaction.

ding water:

s time, the negative ion takes a hydrogen ion from a water molecule.

portant!

member that the equations and mechanisms given on this page are not the truth - they are merelyplifications to suit the demands of a particular A level syllabus.

LECTROPHILIC ADDITION


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ckgroundctrophilic addition happens in many of the reactions of compounds containing carbon-carbon double bonalkenes.

e structure of ethene

are going to start by looking at ethene, because it is the simplest molecule containing a carbon-carbonuble bond. What is true of C=C in ethene will be equally true of C=C in more complicatedenes.

ene, C2H4, is often modelled as shown on the right. The double bond between thebon atoms is, of course, two pairs of shared electrons. What the diagram doesn't show ist the two pairs aren't the same as each other.

e of the pairs of electrons is held on the line between the two carbon nuclei as you would expect, but theer is held in a molecular orbital above and below the plane of the molecule. A molecular orbital is a region

space within the molecule where there is a high probability of finding a particular pair oelectrons.

In this diagram, the line between the two carbon atoms represents a normal bond - thpair of shared electrons lies in a molecular orbital on the line between the two nucleiwhere you would expect them to be. This sort of bond is called a sigma bond.

The other pair of electrons is found somewhere in the shaded part above and below thplane of the molecule. This bond is called a pi bond. The electrons in the pi bond are f

move around anywherein this shaded region and can move freely from one half to the other.

Note: This diagram shows a side view of an ethene molecule. The dotted lines to two of the hydrogens show b

going back into the screen or paper away from you. The wedge shapes show bonds coming out towards you.

e pi electrons are not as fully under the control of the carbon nuclei as the electrons in the sigma bond ancause they lie exposed above and below the rest of the molecule, they are relatively open to attack by othngs.

Note: Check yoursyllabusto see if you need to know how a pi bond is formed. Haven't got a syllabus? If you a

working towards a UK-based exam, find out how to get one by following this link.


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If you doneed to know about thebonding in ethenein detail, follow this link as well.

ctrophiles

electrophile is something which is attracted to electron-rich regions in other molecules or ions. Because acted to a negative region, an electrophile must be something which carries either a full positive charge,

s a slight positive charge on it somewhere.

Note: The ending ". . phile" means a liking for. For example, a francophile is someone who likes the French; an

anglophile is someone who likes the English.

ene and the other alkenes are attacked by electrophiles. The electrophile is normally thehtly positive ( +) end of a molecule like hydrogen bromide, HBr.

Note: If you aren't sure about why some bonds are polar, read the page onelectronegativity.


ctrophiles are strongly attracted to the exposed electrons in the pi bond and reactions happen because ot initial attraction - as you will see shortly.

u might wonder why fully positive ions like sodium, Na+, don't react with ethene. Although these ions mayattracted to the pi bond, there is no possibility of the process going any further to form bonds between

dium and carbon, because sodium forms ionic bonds, whereas carbon normally forms covalent ones.

dition reactions

a sense, the pi bond is an unnecessary bond. The structure would hold together perfectly well with a singnd rather than a double bond. The pi bond often breaks and the electrons in it are used to join other atom
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ups of atoms) onto the ethene molecule. In other words, ethene undergoes addition reactions.

r example, using a general molecule X-Y . . .

mmary: electrophilic addition reactions

addition reaction is a reaction in which two molecules join together to make a bigger one. Nothing is lost in the

cess. All the atoms in the original molecules are found in the bigger one.

electrophilic addition reaction is an addition reaction which happens because what we think of as theportant" molecule is attacked by an electrophile. The "important" molecule has a region of high electron

nsity which is attacked by something carrying some degree of positive charge.

Note: When we talk about reactions of alkenes like ethene, we think of the ethene as being attacked by other

molecules such as hydrogen bromide. Because ethene is the molecule we are focusing on, we quite arbitrarily t

of it as the central molecule and hydrogen bromide as its attacker.

There's no real justification for this, of course, apart from the fact that it helps to put things in some sort of logicapattern. In reality, the molecules just collide and may react if they have enough energy and if they are lined upcorrectly.

derstanding the electrophilic addition mechanism

e mechanism for the reaction between ethene and a molecule X-Y

s very unlikely that any two different atoms joined together will have the samectronegativity. We are going to assume that Y is more electronegative than X, so thatpair of electrons is pulled slightly towards the Y end of the bond. That means that the X

m carries a slight positive charge.

Note: Once again, if you aren't sure aboutelectronegativity and bond polarityfollow this link before you read on

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e slightly positive X atom is an electrophile and is attracted to the exposed pi bond in the ethene. Nowagine what happens as they approach each other.

u are now much more likely to find the electrons in the half of the pi bond nearest the XY. As the processntinues, the two electrons in the pi bond move even further towards the X until a covalent bond is made.

e electrons in the X-Y bond are pushed entirely onto the Y to give a negative Y-ion.

Help! Why does the carbon atom have a positive charge? The pi bond was originally made using an electron fr

each carbon atom, but both of these electrons have now been used to make a bond to the X atom. This leaves right-hand carbon atom an electron short - hence positively charged.

Note also that we are only showing one of the pairs of electrons around the Y-ion. There will be other lone pairs

well, but we are only actually interested in the one we've drawn.


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wn in the previous diagrams.

HE REACTION BETWEEN SYMMETRICAL ALKENES AND THEYDROGEN HALIDES

s page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactionsween the hydrogen halides and alkenes like ethene and cyclohexene. Hydrogen halides include hydrogeoride and hydrogen bromide. If you want the mechanisms explained to you in detail, there is a link at thetom of the page.

ectrophilic addition reactions involving hydrogen bromide

e facts

enes react with hydrogen bromide in the cold. The double bond breaks and a hydrogen atom ends upached to one of the carbons and a bromine atom to the other.

he case of ethene, bromoethane is formed.

Note: Be careful when you write the names of the addition products that you change the en eending in the or

alkene (showing the C=C) into an an eending (showing that it has been replaced by C-C).

h cyclohexene you get bromocyclohexane.

e structures of the cyclohexene and the bromocyclohexane are often simplified:


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Note: Each corner in one of these diagrams represents a carbon atom. Each carbon atom has enough hydro

attached to make the total number of bonds up to 4.

In the case of the bromocyclohexane, it isn't necessary to write the new hydrogen into the diagram, but it is heto put it there to emphasise that addition has happened.

sure that you understand the relationship between these simplified diagrams and the full structures.

e mechanisms

e reactions are examples of electroph i l ic addi t ion.

h ethene and HBr:

d with cyclohexene:

ectrophilic addition reactions involving the other hydrogen halidese facts

drogen chloride and the other hydrogen halides add on in exactly the same way. For example, hydrogenoride adds to ethene to make chloroethane:

e only difference is in how fast the reactions happen with the different hydrogen halides. The rate of reacreases as you go from HF to HCl to HBr to HI.


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F slowest reaction

Cl

Br

fastest reaction

e reason for this is that as the halogen atoms get bigger, the strength of the hydrogen-halogen bond fallsnd strengths (measured in kilojoules per mole) are:

F 568

Cl 432

Br 366

I 298

Note: You may find slightly different values depending on which data source you use. It doesn't matter - the

differences are minor and the pattern is always the same.

you have seen in the HBr case, in the first step of the mechanism the hydrogen-halogen bond gets brokebond is weaker, it will break more readily and so the reaction is more likely to happen.

e mechanisms

e reactions are still examples of electrophi l ic add i t ion.

h ethene and HCl, for example:

s is exactly the same as the mechanism for the reaction between ethene and HBr, except that we'velaced Br by Cl.

the other mechanisms for symmetrical alkenes and the hydrogen halides would be done in the same wa


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Suggestion: Find out if your syllabus mentions a particular hydrogen halide, and learn that mechanism. You

simply swap the halogen atom if a different hydrogen halide comes up in an exam.

Haven't got asyllabus?If you are working towards a UK-based exam, follow this link to find out how to get one

HE REACTION BETWEEN SYMMETRICAL ALKENES ANDULPHURIC ACID

s page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactionsween sulphuric acid and alkenes like ethene and cyclohexene. If you want the mechanisms explained to

detail, there is a link at the bottom of the page.

e electrophilic addition reaction between ethene and sulphuric acide facts

enes react with concentrated sulphuric acid in the cold to produce alkyl hydrogensulphates. Ethene reace ethyl hydrogensulphate.

e structure of the product molecule is sometimes written as CH3CH2HSO4, but the version in the equation

ter because it shows how all the atoms are linked up. You may also find it written as3CH2OSO3H.

nfused by all this? Don't be!

you need to do is to learn the structure of sulphuric acid, and after that the mechanism is exactlysame as the one with hydrogen bromide. As you will find out, the formula of the product follows

m the mechanism in an inevitable way.

Important! Learn this structure for sulphuric acid. Sketch it over and over again until you can't possibly get

wrong.


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e mechanism for the reaction between ethene and sulphuric acid

lphur ic acid as an electroph i le

e hydrogen atoms are attached to very electronegative oxygen atoms which means that the hydrogens w

ve a slight positive charge while the oxygens will be slightly negative. In the mechanism, we just focus onhe hydrogen to oxygen bonds, because the other one is too far from the carbon-carbon double bond to bolved in any way.

e mechanism

ok carefully at the structure of the product so that you can see how it relates to the various formulae givenlier (CH3CH2OSO2OH etc).

e electrophilic addition reaction between cyclohexene and sulphuric acids time we are going straight for the mechanism without producing an initial equation. This is to show that

n work out the structure of obscure products provided you can write the mechanism.

e mechanism for the reaction between cyclohexene and sulphuric acid


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ving worked out the structure of the product, you could then write a simple equation for the reaction if younted to.

our examiners want you to show the transition state, draw the mechanism like this:

e reaction of tertiary halogenoalkanes with cyanide ionse facts

e facts of the reaction are exactly the same as with primary halogenoalkanes. If the halogenoalkane is heder reflux with a solution of sodium or potassium cyanide in ethanol, the halogen is replaced by -CN, andile is produced.

example:

if you want the full equation rather than the ionic one:

e mechanism


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owed by a very rapid attack by the cyanide ion on the carbocation (carbonium ion) formed:



e reaction of secondary halogenoalkanes with cyanide ionse facts

e facts of the reaction are exactly the same as with primary or tertiary halogenoalkanes. The halogenoalkeated under reflux with a solution of sodium or potassium cyanide in ethanol.

example:

e mechanism

condary halogenoalkanes use bothSN2 and SN1 mechanisms. For example, the SN2 mechanism is:

ould you need it, the two stages of the SN1 mechanism are:


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To Main Menu . . .

E REACTION BETWEEN SYMMETRICAL ALKENES AND BROMINE

s page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactions

ween bromine (and the other halogens) and alkenes like ethene and cyclohexene. If you want the


e electrophilic addition of bromine to ethene

e facts

enes react in the cold with pure liquid bromine, or with a solution of bromine in an organic solvent like

achloromethane. The double bond breaks, and a bromine atom becomes attached to each carbon. The

mine loses its original red-brown colour to give a colourless liquid. In the case of the reaction with ethene

-dibromoethane is formed.
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s decolourisation of bromine is often used as a test for a carbon-carbon double bond. If an aqueous solu

bromine is used ("bromine water"), you get a mixture of products. The presence of the water complicates

chanism beyond what is required by current UK A level (or equivalent) syllabuses.

e other halogens, apart from fluorine, behave similarly. (Fluorine reacts explosively with all hydrocarbons

uding alkenes - to give carbon and hydrogen fluoride.)

ou are interested in the reaction with, say, chlorine, all you have to do is to replace Br by Cl in all the

uations on this page.

e mechanism for the reaction between ethene and bromine

e reaction is an example of electrophilic addition.

Warning! There are two versions of the ethene / bromine mechanism in common use, and you mus tknow which yo

examiners will accept.

One version is simplified to bring it into line with the other alkene electrophilic addition mechanisms. You will probablthat your examiners will accept this one, but you must find out to be sure.

You almost certainly won't be able to tell this from your syllabus. You need to refer to recent mark schemes, or to anysupport material that your examiners provide. If you still aren't sure, contact your examiners direct. If you are working

towards a UK-based exam, you can find out how to do this by using the link to your Board's web site on thesyllabusepage.

The person you need to contact will probably have the title Subject Officer for Chemistryor something similar. Askwhether they want the mechanism for the reaction between bromine and alkenes which proceeds via a carbocation oa bromonium ion intermediate.

om ine as an electrophi le

e bromine is a very "polarisable" molecule and the approaching pi bond in the ethene induces a dipole in mine molecule. If you draw this mechanism in an exam, write the words "induced dipole" next to the bromlecule - to show that you understand what's going on.

e simpl i f ied version of the mechanism
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Note: Use this version unless your examiners insist on the more accurate one.

e more accurate version o f the mechanism

Note: Don't learn this unless you have to. There is a real risk of getting confused. If your examiners are happy to ac

the simple version, there's no point in making life difficult for yourself.

he first stage of the reaction, one of the bromine atoms becomes attached to both carbon atoms, with thesitive charge being found on the bromine atom. A bromonium ion is formed.

e bromonium ion is then attacked from the back by a bromide ion formed in a nearby reaction.

e electrophilic addition of bromine to cyclohexene


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e facts

clohexene reacts with bromine in the same way and under the same conditions as any other alkene. 1,2-romocyclohexane is formed.

e mechanism for the reaction between cyclohexene and bromine

e reaction is an example of electroph i l ic addi t ion.

Warning! Again, there are two versions of this mechanism in common use, and you mus tknow which your examine

will accept.

om ine as an electrophi le

ain, the bromine is polarised by the approaching pi bond in the cyclohexene. Don't forget to write the worduced dipole" next to the bromine molecule.

e simpl i f ied version of the mechanism

Note: Use this version unless your examiners insist on the more accurate one.

e al ternat ive version of th e mechanism


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Note: Don't learn this unless you have to. If your examiners are happy to accept the simple version, there's no point

making life difficult for yourself.

he first stage of the reaction, one of the bromine atoms becomes attached to both carbon atoms, with thesitive charge being found on the bromine atom. A bromonium ion is formed.

e bromonium ion is then attacked from the back by a bromide ion formed in a nearby reaction.

nucleophilic substitution.docx

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