NEET 2024 · ChemistryPrevious Year Question

The most stable carbocation among the following is:

Answer: (C) $CH X_{3} - CH X^{+} - CH X_{2} - CH X_{2} - CH X_{3}$ . \textbf{Answer:} (c) \textbf{Solution:} Carbocation stability increases with the number of $α$ -hydrogens available for hyperconjugation (and with the alkyl $+ I$ effect).

A. $(CH X_{3}) X_{3} C - CH X^{+} - CH X_{3}$
B. $CH X_{3} - CH X_{2} - CH X^{+} - CH X_{2} - CH X_{3}$
C. $CH X_{3} - CH X^{+} - CH X_{2} - CH X_{2} - CH X_{3}$ ✓
D. $CH X_{3} - CH X_{2} - CH X_{2} X^{+}$

Correct Answer

Solution & Explanation

\textbf{Answer:} (c) \textbf{Solution:} Carbocation stability increases with the number of $α$ -hydrogens available for hyperconjugation (and with the alkyl $+ I$ effect). Options (A), (B) and (C) are secondary carbocations while (D) is a primary carbocation (least stable). Among the secondary cations, the pentan-2-yl cation in (C) is flanked by a $CH X_{3}$ group and a $CH X_{2} - CH X_{2} - CH X_{3}$ chain, giving the greatest number of $α$ C-H bonds (9 $α$ -H) for hyperconjugative stabilisation. Hence (C) is the most stable.

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