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Geometric isomerism        

Geometric isomerism can arise when there is lack of free rotation around a bond, often a C = C bond.

Consider 1,2-dichloroethene. There are two geometric isomers (Table 3).

 

 

Table 3 - Dichloroethene isomers

 

 

They exist because rotation about the C = C bond is very difficult.

In Topic 2, the C = C bond was described as consisting of a bond and a bond (Figure 2).

Figure 2 - Carbon to carbon double bond

Rotation is restricted because this would involve breaking the bond. The two geometric isomers are not superimposable.Geometric isomers caused by restricted rotation around a bond are distinguished by

using the terms - 'cis' and 'trans':

• cis - both groups are on the same side of the double bond
• trans - the groups are on opposite sides of the double bond ('trans' means across)

Molecule(a) in Table 3 is trans-1,2-dichloroethene, while molecule(b) in Table 3 is cis-1,2-dichloroethene. A similar situation occurs with alkenes containing 4 or more carbon atoms, e.g.

Figure 3

Figure 4

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Geometric isomerism can also arise in disubstituted cycloalkanes, e.g.

Figure 5

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Sometimes geometric isomers can show differences in chemical properties although this is much less common than for structural isomers. A simple example involves the geometric isomers of butenedioic acid (Figure 6).

Figure 6 - Isomers of butenedioic acid

Cis-butenedioic acid readily eliminates water on heating. The product is known as a cyclic anhydride (Figure 7).

Figure 7 - Dehydration of cis-butenedioic acid

On the other hand, trans-butenedioic acid cannot eliminate water under the same conditions since the two carboxyl groups are on opposite sides of the double bond (Figure 6).