Question

Among alkenes, alkynes and aromatic hydrocarbons, only alkenes exhibit cis-trans isomerism. Why don’t the others?

Short answer

The answer is,

There is only one substituent attached to the carbon atoms at the two ends of the triple bonds.

For aromatic compounds, the ring structure does not allow the alignment of the substituents in a cis and trans manner.

Step-by-step solution

 Cis-trans isomerism

The cis and trans are compounds of the alkene category. The cis alkenes are compounds with the same substituents on the same side of the double bonds, and trans alkenes are compounds with the same substituents on the different sides of the double bonds.

Explanation

The alkynes are compounds containing triple bonds and contain only one substituent attached to two carbon ends of the compounds. And hence cis and trans isomerism cannot be the old in the case of alkynes.

For aromatic compounds, the ring structure does not allow the alignment of the substituents in a cis and trans manner.

Alkenes are compounds containing double bonds containing two substituents attached to two carbon ends of the compounds. And hence cis and trans isomerism can be told in the case of alkynes.