(a) Aldehydes containing α-hydrogen undergo self- addition in presence of dilute alkali to give β-hydroxy aldehyde (this is generally called as aldol).
Step 1: Abstraction of α-hydrogen from aldehyde by base to give carbanion.
Step 2: The carbanion formed acts as nucleophile and attacks the carbonyl carbon.
Step 3: The electron density(negative charge) on oxygen pulls a proton from water(solvent) to give β-hydroxy aldehyde.
(b) Aldehydes which do not have α-hydrogen(formaldehyde, benzaldehyde etc.) undergo disproportionation reaction(self -oxidation and reduction) in presence of concentrated alkali to give alcohol and salt of corresponding carboxylic acid.
Step 1: Hydroxide ion attacks the carbonyl carbon to form anion
Step 2: This anion formed shifts a hydride ion(H-) to carbonyl carbon of another molecule to form alcohol and acid salt.
(c) Catalytical reduction(hydrogen in presence of palladium and barium sulphate) of acid chlorides to aldehydes is known as rosenmund reduction.
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