Chemistry, Third edition

17 · ORGANIC CHEMISTRY: HYDROCARBONS

Styrene

n(CH 2 =CH)···CH 2 —CH—CH 2 —CH···

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C 6 H 5 C 6 H 5 C 6 H 5

styrene polystyrene

Vinyl chloride

n(CH 2 =CH)···CH 2 —CH—CH 2 —CH···

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Cl Cl Cl

vinyl chloride poly(vinyl chloride) (PVC)

Note that the starting molecules (monomers) are named by their common, older

names so that you can see how the names of the polymers arose.

320

BOX 17.6

Better model for bonding in alkenes

Although Lewis structures can be used to

describe bonding in alkenes, they do not really

explain why alkenes are so much more

chemically reactive than alkanes. In order to

explain the reactivity of alkenes, a more

sophisticated model of bonding must be used.

In the Lewis structure model for ethene, two

electron pairs are shared by the carbon

atoms:

The Lewis structure suggests that both bonds

in this double bond are of the same type.

However, in our ‘improved’ model for the

bonding in ethene, you will see that they are

not the same.

First, we need to consider that the bonding

electrons in the atoms concerned are in

orbitals. Covalent bonds are formed by overlap

of these orbitals. Each carbon in ethene forms

three ‘normal’ covalent bonds by overlap with

the orbitals containing electrons on the other

carbon and two hydrogen atoms. The electron

cloud between the nuclei of the atoms in each

bond ‘glues’ the atoms together. Normal

covalent bonds of this type are called 

(sigma) bonds (Fig. 17.3).

One electron is left over on each carbon atom.

The ‘left over’ electrons are in porbitals, as

shown in Fig. 17.4.

A(pi) bond is formed by sideways overlap

of these porbitals on the carbon atoms. In

this type of bond, the electron cloud is

concentrated above and below the horizontal

plane joining the two carbon atoms. The

double bond between the carbon atoms in

ethene therefore consists of a bond and a

bond. The exposed negatively charged

electron cloud in the bond is open to attack

by positively charged species and this is why

Fig. 17.3bonds in ethene. alkenes are so reactive.

Fig. 17.4Formation of a bond in ethene.