Give the following reactions.
(i) Benzoin condensation
(ii) Knoevenagal reaction.

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Solution

(i) Benzoin condensation:
When benzaldehyde is refluxed with aqueous alcoholic potassium cyanide - $α$ - hydroxy ketone called benzoin is formed.
$C_{6} H_{5} C H = O + H - O ∥ C - C_{6} H_{5} a l c . - -- \to K C N C_{6} H_{5} C H O O H - O ∥ C - C_{6} H_{5} B e n z o i n$
(ii) Knoevenagal reaction:
Benzaldehyde condenses with malonic acid in presence of pyridine forming cinnamic acid, pyridine is the basic catalyst here. This mechanism is quite similar to the previous reaction, involving carbanion, from the malonic acid.
$C_{6} H_{5} C H = O + H_{2} C (C O O H)_{2} P y r i d i n e - ---- \to C_{6} H_{5} C H M a l o n i c a c i d = C (C O O H)_{2} C O_{2} - - \to Δ C_{6} H_{5} C H = C H C O O H C i n n a m i c a c i d$

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