1. Aims

In this experiment, the aim is to prepare 1-bromobutane using purifying methods such as reflux and distillation. We are then required to determine the percent yield and density of the sample prepared.

2. Introduction

Reflux and distillation is required for this experiment.

Reflux is used in chemical reactions, it is to apply heat to the chemicals without losing any of it in the process. The vapour is immediately condensed in the condenser therefore there is no need to worry about the solution boiling itself dry.

In reflux, the reactants is heated to provide sufficient energy for them to react. In the process, the products might evaporate out. The columns has water entering and exiting throught it. The water is to cool down the vapours that has evaporated and condenses it back into the flask.

Taken from: http://en.wikipedia.org/wiki/Reflux

Distillation is also used in this experiment. Distillation is a widely used method to separate mixtures in liquids, base on their differences in conditions required to change the phase of substances in the mixture.

The different substances in the mixture that has different boiling points, thus this method enables us to heat the liquid to a certain temperature and force the substance required to change into gas phase. The gas is then condensed back into liquid form and collected. Repeating the process on the collected liquid improves the purity of the product. This is then called ‘double distillation’.

Taken from: http://www.docbrown.info/page01/ElCpdMix/EleCmdMix2.htm

In this experiment, we extracted the solution from the start of the heating till the temperature of the vapours reach 105 °C. Even though the boiling point of 1-bromobutane is 101°C, the yield of the product is too low. Thus we extracted the solution from the start of heating till 105 °C.

Alkyl halide is formed in this experiment. There are a few ways to in which alkyl halides can be formed:

From:Alkanes: R-H + X2 R-X + HX

Alkenes: (i) CH2= CH2 + X2 CH2X CH2 X

(ii) CH2= CH2 + HX CH3 CH2 X (Markovnikov’s Rule)

Alcohol: (i) R-OH + HX R-X + H2O

(ii) 3 R-OH + PX3 R-X + H3PO3

R-OH + PX5 R-X + HX + POX3

Alkyl halides are used as flame retardants, refrigerants, propellants and solvents. They are classified by primary (1°), secondary (2°) and tertiary (3°) alkyl halides. Alkyl halides are normally colourless, relatively odourless, and hydrophobic. They also have a higher boiling point than alkanes and are slightly polar.

3. Procedures

Approximately 30 mL of deionised water was added in a 250 mL round-bottom flask and placed in an ice-bath. 35 mL (65g, 0.65 mol) of concentrated H2SO4 were added in small portions followed by 25 mL (18.5g, 0.25 mol) of n-butyl alcohol which were added slowly while mixing the solution thoroughly and cooling. 35.7g (0.3 mol) of powdered potassium bromide crystals were then added in small portions while shaking the flask to prevent formation of lumps. These were all done with the round-bottom flask in the ice-bath. Bumping chips were added and mixture is set for reflux for 30 mins.

After 30 mins, flask is allowed to cool slightly while we arranged the apparatus for distillation. 15 mL of deionised water were added to rinse down the inner walls of the condenser. The mixture is then distilled and the distillate was collected from the start to 105°C for 30 mins.

The distillate was then transferred to a separatory funnel 25 mL of deionised water was added and the mixture was shake gently. The organic and aqueous layer was identified and noted down. The organic

and aqueous layers were drawn out into a small beaker and the organic layer was added back to the separatory funnel. 12.5 mL of 80% H2SO4 was poured into the organic layer in the separatory funnel. Mixture was then shake gently and allowed to settle. The organic and aqueous layers were identified and noted down. The organic and aqueous layers were then drawn off again and the organic layer added back to the separatory funnel.

Sodium hydrogencarbonate was now added and the solution was swirled gently without the stopper until effervescence was no more seen. The stopper was then placed back and the funnel was then shake vigorously. The pressure that built up was frequently released for every 2-3 shakes. The organic and aqueous layers were again identified and noted down. The organic layer was added back again into the separatory funnel.

25 mL of deionised water was now added into the separatory funnel to wash the final product. The organic and aqueous layers were again identified and noted down.

Approximately 4g of anhydrous calcium chloride was now added into the 1-bromobutane and stirred gently to mix. The solid was then allowed to settle. The dried product was filtered into a 25 mL of dry and weighed measuring cylinder. The product, together with the measuring cylinder was then weighed again and its volume is measured.

4. Results

4.1 Density of product

Mass of product = mass of product & measuring cylinder – mass of measuring cylinder = 76.477g – 55.154g = 21.323g

Volume of product = 15 mL

Density = Mass

Volume

= 21.323 g

15 mL

= 1.4215 g/mL

4.2 Percent yield of product

Number of moles of n-butyl alcohol used = 0.25 molExpected number of moles of 1-bromobutane = 0.25 molExpected mass of 1-bromobutane = 18.5gMass of 1-bromobutane = 21.323g

Percent yield = mass of 1−bromobutane

expected mass of 1−bromobutane x 100%

= 21.323 g18.5 g x 100%

= 115%

CH3CH2CH2CH2

OH + HBr

H+

C

H H

CH2CH2CH3

H2O Br -

C

HH

CH2CH2CH3

Br H2Oδ -δ +

C

HH

CH3CH2CH2

Br + H2O

slow

rds

5. Discussion

Mechanism involved in the reaction:

Suggest reasons to explain why the concentrated H2SO4 needs to be cold before 1-butanol is

added. Write balanced chemical equations, including the necessary conditions to explain your answers, if necessary.

H2SO4 is used to as a catalyst. Other than that, H2SO4 is used to make the alcohol a better leaving group. If the alcohol is not protonated, it will become a bad leaving group in the mechanism as it is not stable. Since it does not have a high stability, the energy of the transition state is higher, thus making it slower. H2SO4 needs to be cold as it is a volatile.

Write balanced equations for the reaction(a) KBr + H2SO4

KBr + H2SO4 2HBr + K2SO4

(b) HBr + H2SO4

HBr + H2SO4 Br2 + SO4 + 2H2O

Which layer, top or bottom is the organic layer in(a) Step 9 – bottom(b) Step 11 – top(c) Step 15 – bottom(d) Step 16 – bottom

The roles of sodium hydrogen carbonate and anhydrous calcium chloride.

Sodium hydrogen carbonate is used to wash away the organic substances and anhydrous calcium chloride is used as a drying agent to remove any excess water and unreacted alcohol.

Precautions taken

The pressure in the separatory funnel due to the liberated carbon dioxide and vapour built inside must be released after a few shakes of the funnel to avoid pressure building up inside that could cause the stopper to be pushed out and product lost when they’re sprayed out.

The mixings were all to be done in the fume hood as the some of the chemicals are corrosive. The vapours emitted from the chemicals could also be harmful.

6. Conclusion

Theoretically, the density of 1-bromobutane is 1.276 g/mL at 25°C and the density that we obtained from the experiment is 1.4215 g/mL. I think this may be due to impurities present in the product, which brings to the percent yield calculated. My percent yield is 115%, exceeding the 100%. This confirms that there were impurities present in our product.

Other than that, I consider this experiment a success as the aims of this experiment were met. I am familiar and understand with the purifying methods of reflux and distillation in preparing 1-bromobutane. 7. References

2010 About.com, What Is Distillation? [online] Available from: http://chemistry.about.com/cs/5/f/bldistillation.htm[Accessed on 18 July 2010]

Reflux [online] Available from: http://en.wikipedia.org/wiki/Reflux[Accessed on 18 July 2010]

Appendix

CH3CH2CH2

CH H

BrNA + OH - +

C

HH

CH2CH2CH3

Br HOδ -

slow

rds

C

H H

CH2CH2CH3

HO + NaBr

1-Butanol

δ +

Questions and Answers

1. Write a balanced chemical equation to show an alternative method you would use to prepare 1-bromobutane from n-butanol.

3 CH3CH2CH2CH2 OH + PBr3 3 CH3CH2CH2CH2 Br + H3PO3

2. Illustrate, with equation and reaction conditions, how you would prepare 1-butene from 1-bromobutane. Propose the mechanisms for this reaction.