Question

Consider the following carbocations: (I) $(CH_3)_2CH-CH_2^+$, (II) $CH_2=CH-C^+H-CH_3$, (III) $(CH_3)_2CH-C^+H-CH_3$. The correct order of stabilities of these is:

• A. I < III < II
• B. I < II < III
• C. III < II < I
• D. III < I < II

Hint:

Resonance-stabilized carbocations are always more stable than simple alkyl carbocations.

Answer 1

The Correct Option is A

Step 1: Recall what stabilises a carbocation.
A positive carbon is happier when nearby groups push or share electron density into it. The strength goes: resonance (best) > more alkyl groups (hyperconjugation and induction) > fewer groups.

Step 2: Look at cation I.
$(CH_3)_2CH-CH_2^+$ carries the charge on a primary ($1^{\circ}$) carbon, with the least support. So it is the least stable.

Step 3: Look at cation III.
$(CH_3)_2CH-C^+H-CH_3$ is a secondary ($2^{\circ}$) cation, flanked by alkyl groups that feed in electron density. It is more stable than I.

Step 4: Look at cation II.
$CH_2=CH-C^+H-CH_3$ is allylic; the double bond next door spreads the positive charge over two carbons by resonance. This is the strongest stabilisation, so it is the most stable.

Step 5: Rank them.
From least to most stable: I (primary) < III (secondary) < II (allylic).

Step 6: Conclusion.
So the stability order is $I < III < II$. \[ \boxed{I < III < II} \]