Chemistry, Third edition

ALKENES

Isomerism in alkenes

There are a number of possible structural arrangements for a compound of formula C 4 H 8 :

Experiments show that there are fourisomers of C 4 H 8. How can we account for

the fourth isomer? In fact, there are two types of but-2-ene – they have different boil-

ing points and melting points. In each butene, the atoms are arranged as follows:

Both molecules are flat (planar). In trans-but-2-ene the methyl groups are on

opposite sides of the double bond (transmeans ‘across’), whereas in cis-but-2-ene,

the methyl groups are on the same side of the double bond (cismeans ‘on the side’).

The two molecules are not the same because no free rotation can occur around the

carbon–carbon double bond. These two butenes only differ from one another in the

way that the atoms are orientated in space. They are geometricisomers.

trans-but-2-ene cis-but-2-ene

but-1-ene but-2-ene 2-methylpropene

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BOX 17.5

More about isomerism

There are two major types of isomerism,

structural isomerismandstereoisomerism.

Instructural isomerism, first met in the

discussion of alkanes and which exists

between but-1-ene, but-2-ene and 2-

methylpropene, the compounds have their

atoms bonded to different groups of atoms.

For example, consider the underlined carbon

atoms in the structural isomers of butene

(C 4 H 8 ):

Instereoisomerismall atoms in the

compounds are attached to the same

partners, but the partners are arranged

differently in space. Geometric isomerism,

the isomerism shown by some alkenes

because there is no free rotation about the

double bond, is one type of isomerism in this

class. The geometric isomers of 1,2-

dichloroethene are shown below:

Here, equivalent atoms in the two molecules

are attached to the same atoms, but

these atoms are arranged differently in

space.

Another type of stereoisomerism is optical

isomerism, which we will meet later.