Question

In each of the following pairs of ions which ion is more stable?

• A. I,I,I,I
• B. I,I,II,II
• C. II,II,I,I
• D. II,II,II,II

Answer 1

The correct option is B

I,I,II,II

For part 1., we have to choose the more stable of benzyl and allyl
carbocations. The benzyl carbocation has more resonance contributors.
In theory, it can be considered to be more stable.

For part 2., we have to choose the more stable cation between an
$1^{\circ }$ alkyl and a vinyl cation. The latter is more unstable because
the positive charge rests on a carbon having a double bond - which has
more % s-character and is thus more electronegative than the alkane's
carbon bearing the positive charge. The primary alkyl carbocation is thus
more stable.

For part 3., we have a straightforward case where an allyl carbocation is
more stable than a primary alkyl carbocation.

For part 4., the lone pair on the adjacent nitrogen stabilizes the positive charge.
Hence the resonance stabilized carbocation is more stable!