Question

Rank the following compounds in order of increasing heat of hydrogenation. A B D E OB<C<E<A<D OD A<E<C<B OB«E<C<A<D O E<B<C<A<D O DA<C<<

Answer 1

Ans- B < E < C < A < D

Explanation-

Heat of Hydrogenation is the change in Enthalpy for hydrogenation of alkene. We know in hydrogenation process, an unstaturated Hydrocarbon (here Alkene) is converted into a saturated hydrocarbon (an Alkane). Now generally heat is released in this process.

Now more stronger an alkene is, less is the amount of heat released. Or we can the strength of alkene and Energy of hydrogenation is inversely proportional.

Thus stronger the alkene (here diene), lesser is the Heat of Hydrogenation and vice versa.

Now the rules followed in determineing the stability of diene is-

i) Conjugated dienes (double bonds are present alternate to each other) are more stable than other dienes

So here conjugated dines (in B and E) are more stsable than the rest

ii) Now more substituted dines (more groups attached at the douby boned C-atoms) are more stable than less substitued alkenes.

So here between B and E, (B is more stable) as the last end of B us more substituted.

And between A and C, (C is more stable)

iii) Between regular diene and allene (compound D) allenes are the least stable.

So here the compound D is the least stable among all.

Thus the decreasing order of diene stablity in the given compounds will be-

B > E > C > A > D

The the increasig order of Heat of Hydrogenation wiil be as (less stable, more Heat of Hydrogenation)

B < E < C < A < D