In which pairs, the first ion is more stable than the second?

Solution

Chemistry ma’am · 2 steps

In each pair, ask what feeds or soaks up the charge: lone pairs and π bonds beat plain alkyl groups.

1. The rules. A carbocation is stabilised by groups that push electrons in: a lone pair by resonance (+M) is strongest, then a C=C by resonance, then alkyl groups by hyperconjugation. A carbanion is stabilised by groups that pull electrons out, such as an aryl ring carrying NO₂.

   Pair	First ion	Second ion	First more stable?
   (A)	Cation with OMe on C⁺: the O lone pair gives +M, so every atom gets a full octet	Cation with Me on C⁺: only hyperconjugation	Yes
   (B)	Benzylic carbanion: the charge spreads into the ring and onto the o-NO₂ oxygens	Benzylic carbocation: the o-NO₂ pulls electrons away (−I, −M) and destabilises it	Yes
   (C)	Primary cyclohexylmethyl cation: weak hyperconjugation only	Allylic cation: the ring C=C delocalises the + charge	No
   (D)	tert-Butyl cation: hyperconjugation only	Cation with OMe on C⁺: +M from O	No

2. So. The first ion wins only in (A) and (B), so the answer is (a).

The idea to keep. Cations want electron donors: +M lone pairs first, then π bonds, then alkyl hyperconjugation. Anions want electron acceptors like NO₂. The same group can stabilise one ion and destabilise the other.

Answer from the NTA final answer key. Solution by PrepWiser.