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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3o4217

3-Hydroxybutanal is dehydrated in basic medium to but-2-enal by an E1cB pathway. Which statements are correct?

AThe key intermediate is an enolate (carbanion) formed by removal of the alpha-hydrogen.
BThe poor leaving group $OH X^{-}$ can be expelled because the lone pair of the carbanion pushes it out.
CThe product is $CH X_{3} CH = CH - CHO$ .
DA carbocation forms first and may undergo rearrangement.

Answer: A, B, C

In E1cB, a proton acidified by the carbonyl is removed first to give an enolate. The enolate then expels the poor leaving group

OH X^{-}

to form the conjugated alkene

CH X_{3} CH = CHCHO

. No carbocation is involved, so statement 4 is wrong. Statements 1, 2 and 3 are correct.

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