Chemistry, Third edition

18 · COMMON CLASSES OF ORGANIC COMPOUNDS

Halogenoalkanes (or alkyl halides)

We have already met these compounds in Chapter 17. They are formed by replacing

the hydrogen atom of a hydrocarbon by a halogen (F, Cl, Br or I) and have the general

formula R–X where X = a halogen. Thus:

CH 3 F fluoromethane

CH 3 Cl chloromethane

CH 3 Br bromomethane

CH 3 I iodomethane

Halogenoalkanes are insoluble in water. A mixture of halogenoalkane (if liquid) and

water forms two layers and the halogenoalkanes tend to form the bottom layer,

because they are quite dense.

Important halogenoalkanes

Trichloromethane (or chloroform, CHCl 3 ) is non-flammable and was used as an

anaesthetic, before it was found to cause liver damage. Tetrachloromethane (or car-

bon tetrachloride, CCl 4 ) also has anaesthetic properties, but it is even more toxic.

Bromochlorotrifluoroethane (common name halothane) is now widely used. It also

has anaesthetic properties and is much safer to use.

F

\ /

H

F—C—C—Br

F

/ \

Cl

halothane

Substitution of the halogen in halogenoalkanes

A carbon–halogen bond is polar (C–X–) and the carbon can be attacked by

groups that carry an unshared pair of electrons. The result is a substitution reaction,

in which one atom, ion or group is substituted for another. If some halogenoalkanes

are heated with aqueous sodium hydroxide, the halogen is substituted by an –OH

group, producing an alcohol:

CH 3 CH 2 CH 2 Br OH– CH 3 CH 2 CH 2 OHBr–

1-bromopropane hydroxide ionpropan-1-ol bromide ion

Box 14.1 gives more detail on how this reaction occurs.

Alcohols

The formulae of these compounds can be represented by R–OH. Their names end in

‘ol’. The ending is sometimes prefixed by a number to indicate the carbon atom to

which the –OH group is attached. Examples of alcohols are shown in Table 18.2.

Alcohols with more than one –OH group are named using diol, triol etc., depending

on how many such groups are present. A 1,2 diol is often also referred to as a glycol.

18.2

18.1

330

Halogenoalkanes

(i) Write the structural

formulae of the

following alkyl halides:

(a) chloroethane

(b) 2-bromopropane

(c) 1,2-dichloro-3-

fluorobutane

(d) 1,3,5-tribromobenzene.

(ii) Name the following:

Br

(a) CH 3 CH

Cl

(b)

(c)

(d)

Exercise 18A