Chemistry, Third edition

17 · ORGANIC CHEMISTRY: HYDROCARBONS

Chemical properties of alkenes

The physical properties of the alkenes are very similar to those of the alkanes. Chemic-

ally, however, they are much more reactive than the alkanes because they contain

double bonds. Alkenes are said to be unsaturatedhydrocarbons, because of the

double bond, their molecules can add on more atoms of hydrogen. The unsaturation

of alkenes gives rise to reactions with substances other than hydrogen. The chemical

properties of alkenes are as follows:

1. Combustion

Like the alkanes, they burn in a plentiful supply of oxygen to form carbon dioxide

and water:

C 2 H 4 (g) 3O 2 (g)2CO 2 (g)2H 2 O(l)

2. Addition

Because alkenes are unsaturated, they will undergo reactions in which the carbon–

carbon double bond becomes a single bond and other atoms bond to the carbons of

the original double bond. A reaction in which two molecules combine to yield a

single molecule of product is called an addition reaction. For ethene the general

reaction may be written as

XY

||

CH 2 =CH 2 X—YCH 2 —CH 2

Some examples of the reagents ‘X–Y’ are given below:

Bromine (Br 2 ) Br–Br

Alkenes react rapidly with bromine at room temperature to form colourless

dibromoalkanes. For example, if ethene gas is bubbled into bromine water(an

aqueous solution of bromine) the colour of the solution turns from reddish-brown

to colourless. This reaction is often used to test for unsaturation in a hydrocarbon:

Br Br

||

CH 2 =CH 2 Br—BrCH 2 —CH 2

Chlorine (Cl 2 ) Cl–Cl

A similar reaction occurs to that with bromine but without the distinctive colour

change:

Cl Cl

||

CH 2 =CH 2 Cl—ClCH 2 —CH 2

318

Structural and

geometric

isomerism

Below are two isomers

with the formula C 2 H 2 Br 2 :

(i) A third isomer exists.

Draw its structural

formula and label it ‘3’.

(ii)What type of isomerism

exists between the

isomers 1 and 2?

(iii)What type of isomerism

exists between isomers

2 and 3?

(iv)Name compounds 1, 2

and 3.

Exercise 17L