Question

In which of the following pairs 1st is more stable than 2nd?
(A)
(B)
(C)
(D) $P{{h}_{3}}{{C}^{\bullet }},\text{ }{{(C{{H}_{3}})}_{3}}{{C}^{\bullet }}$

Answer 1

Hint: The compounds in the given options have carbocation. The tertiary carbocation is more stable than secondary and secondary is more stable than the primary. The carbocations are stabilized by either hyperconjugation effect or resonance effect.

Complete step by step answer:
All the compounds in the given options have carbocation.
-In the first option, the carbocation is attached to the carbon atom that has a double bond and the second compound has carbocation on the next carbon of the carbon atom having a double bond. So the resonance is possible in the second structure but not in the first structure. So the second is more stable than the first.
-In second option, in the first structure, the carbocation is on the secondary carbon atom and in the second structure, the carbocation is on the tertiary carbon atom. So the first structure has secondary carbocation and the second structure has tertiary carbocation. We know that tertiary carbocation is more stable than the secondary. Therefore, the second is more stable than the first.
-In third option, in the first structure, there is a ring of three carbon atoms and in the second structure, there is a ring of six carbon atoms. The ring of six carbon atoms is more stable than the ring of three carbon atoms. Therefore, the second is more stable than the first.
-In the last option, the first compound, the carbocation having carbon is attached to three phenyl groups that can be stabilized with resonance. But the second compound has carbocation having carbon attached to three methyl groups that is stabilized by hyperconjugation effect. The stability due to resonance is more prominent than the stability due to hyperconjugation. Hence, the first is more stable than the second.

Therefore, the correct answer is an option (d)-$P{{h}_{3}}{{C}^{\bullet }},\text{ }{{(C{{H}_{3}})}_{3}}{{C}^{\bullet }}$.

Note: The flow of electrons from one part of the conjugated system to the other creating centers of low and high electron density due to the phenomenon of resonance is called resonance effect or R-effect. The reverse of the inductive effect when the alkyl group is attached to the unsaturated system or carbocation is known as the hyperconjugation effect.