Question

What is the correct order of heat of hydrogenation???

Answer 1

HELLO THERE!

Note: Heat of hydrogenation is inversely proportional to stability of a compound, hence more stable is the compound, less is it's heat of hydrogenation.

So, let's check the stability of the compounds.

There is a phenomena called Hyperconjugation, which determines the stability. According to this factor, if there are more number of Alpha hydrogens, (hydrogens attached to the carbon adjacent to the carbon directly attached with double bond) there will be more no-bond resonating structures, and hence, the compound will be more stable.

However, in all the three options there are six alpha hydrogens, so we need to find out a different factor that determines the stability of the compounds.

Consider Compound (II): Trans-2-butene: here, the two methyl (bulky) groups are present on opposite side, so there is no steric hindrance (repulsion between bulky groups that decrease the stability). Hence, this compound is the most stable, having the least Heat of hydrogenation. So, (2) will be at last. Options (1) and (3) get eliminated.

Consider Compound (III): Cis-2-butene: here, the two bulky methyl groups are present in the same side, so there is steric hindrance. So, this is less stable than Option (2).

Consider Compound (I): In this case, the two bulky methyl groups are present more close, so there is more steric hindrance, which makes Option (1), the least stable, having the greatest heat of hydrogenation.

So, the correct order is (I) > (III) > (II).

Option (2) is correct.

THANKS!