Question

Arrange the following in the increasing order of their heat of hydrogenation:

• A. $P<Q<R<S$
• B. $S<R<Q<P$
• C. $S<R<P<Q$
• D. $P<Q<S<R$

Answer 1

The correct option is A $P<Q<R<S$

$\text{Heat of hydrogenation}\propto \frac{1}{\text{stability of alkene}}$

The stability of alkenes depends upon two major factors:
i) Conjugation
ii) Number of $\alpha -$hygrogens

(P) being most stable has least heat of hydrogenation since the double bond is in conjugation.

Among (Q), (R) and (S), the number of $\alpha -$hygrogens are in the order: $Q>R>S$

Hence, the heat of hydrogenation follows the trend: $P<Q<R<S$