NEET 2020 · ChemistryPrevious Year Question

A tertiary butyl carbocation is more stable than the secondary butyl carbocation because of which one of the following?

Answer: (B) Hyperconjugation. \textbf{Answer:} (b) Hyperconjugation \textbf{Solution:} A tertiary carbocation is stabilised relative to a secondary one by a greater number of hyperconjugating ( $α$ C-H) bonds from the three attached methyl groups, together with their electron-donating $+ I$ effect.

A. $- R$ effect of $- CH X_{3}$ group
B.Hyperconjugation✓
C. $- I$ effect of $- CH X_{3}$ groups
D. $+ R$ effect of $- CH X_{3}$ groups

Correct Answer

(B) Hyperconjugation

Solution & Explanation

\textbf{Answer:} (b) Hyperconjugation \textbf{Solution:} A tertiary carbocation is stabilised relative to a secondary one by a greater number of hyperconjugating ( $α$ C-H) bonds from the three attached methyl groups, together with their electron-donating $+ I$ effect. Among the given options, the operative stabilising factor is hyperconjugation. The $- R$ , $+ R$ and $- I$ descriptions as stated are not the correct cause, so the answer is (b).

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