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Why is the Pd catalyst in Rosenmund's reduction *poisoned* with $BaSO_4$ / quinoline / sulphur?
ATo prevent over-reduction of $RCHO$ to $RCH_2OH$
BTo prevent the acyl chloride from hydrolysing
CTo favour the reverse reaction
DTo make the catalyst water-soluble
Answer & Solution
Correct answer: A. To prevent over-reduction of $RCHO$ to $RCH_2OH$
Plain Pd would happily reduce $RCHO$ further to $RCH_2OH$. Poisoning lowers the activity just enough to stop at the aldehyde stage — a classic example of selective catalysis.
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