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Answers15 Alcohols and halogenoalkanes

The preparation of 1-bromobutane from butan-1-ol

  1 a) Possible hazards: sulfuric acid is

corrosive; the bromine which forms

during the reaction is corrosive and

toxic; butan-1-ol is a flammableliquid; heating a liquid in a small

flask can lead to bumping; pressure

can build up in a tap funnel when

gases are evolved.

b) Wear eye protection. Carry out the

heating under reflux in a fume

cupboard. Heat with an electrical

heater instead of a flame. Release

the pressure in the tap funnel by

inverting it and opening the tap fromtime to time.

  2 NaBr(s) + H2SO4(l) → NaHSO4(s) +

HBr(g)

  3 Concentrated sulfuric acid rapidly

oxidises bromide ions or hydrogen

bromide to bromine (see Section 11.8

in the Student’s Book).

  4 Possible explanations: the reaction is

slow and more time is needed for it to

go to completion; some of the butan-1-ol forms by-products.

  5  The reflux condenser prevents the loss

of volatile liquids from the reaction

mixture during heating.

  6 1-bromobutane, unchanged butan-1-ol

and hydrogen bromide are volatile;

they distil over. The two ionic salts are

involatile and remain in the flask.

  7 1-bromobutane does not mix with

water.  8 Hydrochloric acid is an acid which can

transfer a hydrogen ion to the −OH

group in the alcohol

turning it into −OH2+. In this ionic form,

the compound is more soluble in water.

  9 Aqueous sodium hydrogencarbonate is

a weak alkali that neutralises acidswithout hydrolysing the product.

Sodium hydroxide is a strong alkali

which tends to hydrolyse

halogenoalkanes.

10 Fractional distillation separates liquids

according to their boiling

temperatures. Each fraction contains

the compounds that distil over in a

narrow range of boiling temperatures.

 The boiling temperature of the productis 102 °C.

11 1 mol butan-1-ol from 1 mol 1-

bromobutane.

Amount of butan-1-ol =

1

33

 

cm5.7cmg0.81−

−

×

= 0.08 mol

 Theoretical yield of 1-bromobutane

= 0.082 mol × 137 g mol–1 = 11.2 gPercentage yield = g11.2

g8.6 × 100% =

61%

12  There is a variety of explanations:

some unchanged butan-1-ol remains in

this preparation even with an excess of 

sodium bromide and concentrated

sulfuric acid; some of the butan-1-ol is

converted to by-products; some of the

product is lost during the process as

the chemicals are mixed, heated,distilled, transferred from one

container to another, washed, dried

and redistilled.

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 © G. Hill and A. Hunt 2008 Edexcel Chemistry for AS CD-ROM