Question: Rank the following dienes in order of increasing heat of hydrogenation.

Heat of hydrogenation is the measure of the enthalpy of the reaction which involves the conversion of alkene to alkane via catalytic hydrogenation. More the stability of the alkene, lesser will be its heat of hydrogenation.

For substituted alkenes, the order of the stability of the alkenes is:

tetrasubstituted > trisubstituted> disubstituted > monosubstituted

The given compounds are:

Compound 1

One of the alkenes is tetrasubstituted and the other is disubstituted. The alkenes are also involved in conjugation. Thus, leading to higher stability of alkene.

Compound 2

One of the alkenes is trisubstituted and the other alkene is disubstituted. The alkenes are however isolated alkenes which do not take part in any conjugation. Thus the stability will be comparatively low.

Compound 3

One of the double bonds is disubstituted and the other is trisubstituted. The alkenes are also conjugated. Therefore, stability will be higher than compound 2, but comparatively lower than compound 1.

Compound 4

Here both the double bonds are disubstituted. However, they are isolated alkenes having no conjugation. Thus, owing to lower stability.

The most substituted diene with conjugation is compound 1. Thus, it will have the least heat of hydrogenation.

Compound 4 is the least substituted and are isolated dienes. Thus the heat of hydrogenation will be highest.

Thus, increasing order heat of hydrogenation:

Compound 4 < Compound 3 < Compound 2 < Compound 1