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Which of the following statements about hyperconjugation are correct?

AIt requires at least one C-H bond on an sp3 carbon attached to a pi system or a cationic centre.
BThe ethyl cation $CH X_{3} CH X_{2} X^{+}$ has 3 hyperconjugative structures.
CThe tert-butyl cation $(CH X_{3}) X_{3} C X^{+}$ has 9 hyperconjugative structures.
DThe methyl cation $CH X_{3} X^{+}$ is stabilised by hyperconjugation.

Answer: A, B, C

Hyperconjugation needs alpha C-H bonds, so statement 1 is true. The number of structures equals the number of alpha hydrogens: 3 for

CH X_{3} CH X_{2} X^{+}

and 9 for

(CH X_{3}) X_{3} C X^{+}

.

CH X_{3} X^{+}

has no alpha carbon, so statement 4 is false.

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