In which of the following can hyperconjugation NOT operate?

A $C X_{6} H X_{5} - CH X_{3}$ (toluene)
B $CH X_{2} = CH - CH = CH X_{2}$ (buta-1,3-diene)
C $CH X_{3} - CH = CH X_{2}$ (propene)
D $CH X_{3} - CH X_{2} X^{+}$

Answer: B

Hyperconjugation requires an sp3 carbon bearing at least one C-H bond attached to a pi bond or a cationic/radical centre. Propene, toluene and the ethyl cation each have such a carbon. Buta-1,3-diene has no sp3 carbon, so it has none.

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In which of the following can hyperconjugation NOT operate?

Answer: B

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