Question

Which of the following compounds show cis-trans isomerism? Draw the cis and trans isomers of those that do.

1. $\mathbf{CHF}\mathbf{=}\mathbf{CHF}$
2. ${\mathbf{F}}_2\mathbf{C}\mathbf{=}{\mathbf{CH}}_2$
3. ${\mathbf{CH}}_2\mathbf{=}\mathbf{CH}\mathbf{-}{\mathbf{CH}}_2\mathbf{-}{\mathbf{CH}}_3$
4. 
   

e.

f.

Short answer

Only compound (a) will show cis-trans isomerism.

Step-by-step solution

Cis-trans isomerism

Cis-trans isomerism is observed in alkenes and alicyclic compounds. For an alkene to show cis-trans isomerism, it must have two different substituents. Cis-trans isomers are geometric isomers that are the subset of stereoisomers.

Rules for displaying cis-trans isomerism

The compounds with the following substitution pattern would not show cis-trans isomerism.

Cis-trans isomerism would not be observed for alkenes with this substitution pattern.

Determination of compounds that show cis-trans isomerism

Compounds (b), (c), and (d) would not show cis-trans isomerism.

Cis-trans notation cannot be used for alkenes with more than two different substituents. Hence, the compound (e) and (f) would not show cis-trans isomerism. For these compounds E/Z notation system is appropriate.

The only compound that will exhibit cis-trans isomerism is compound (a). Cis-trans isomers of compound (a) are,

Cis-trans isomerism of compound (a).