Question

The heat of hydrogenation of 2,3-pentadiene, a cumulated diene, is 70.5 kcal/mol. What are the relative stabilities of cumulated, conjugated, and isolated dienes?

Short answer

The smaller the heat of hydrogenation (the positive value of heat of hydrogenation) the more stable the compound. Therefore, the relative stabilities of dienes are:

Conjugated diene˃ isolated diene˃ cumulated diene

Step-by-step solution

Some definitions

Conjugated dienes =Conjugated dienes are those wheretwo double bonds separated by a single bond.

Isolated dienes =Isolated dienes are those wheretwo double bonds are separated by more than one single bond.

Cumulated dienes =Cumulated dienes are those where two double bonds connected to the same atom.

Final answer

The smaller the heat of hydrogenation (the positive value of heat of hydrogenation) the more stable the compound. Therefore, the relative stabilities of dienes are:

Conjugated diene˃ isolated diene˃ cumulated diene