Question

Which compound would you expect to have the greater heat of hydrogenation: 1,2-pentadiene or 1,4-pentadiene?

Short answer

Both compounds from the same product when they are hydrogenated, so the difference in the heats of hydrogenation will depend only on the difference in the stabilities of the reactants because 1,2 pentadiene has cumulated double bonds and 1,4 pentadiene has isolated double bonds, 1,2 Pentadiene is less stable and therefore will have a greater heat of hydrogenation.

Step-by-step solution

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Some definitions

Conjugated dienes =Conjugated dienes are those where two 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.