Q.Write the chemical reaction for the conversion of methanal to ethanol. OR Write a chemical reaction for the conversion of acetic acid to ethene.

A Grignard reagent adds across the C=O of formaldehyde; acidic hydrolysis of the resulting magnesium alkoxide gives a primary alcohol with one more carbon than the Grignard's alkyl group — here, ethanol.

Methanal (formaldehyde, HCHO) has only 1 carbon, while ethanol (CH$_3$CH$_2$OH) has 2 carbons, so this conversion requires extending the carbon chain by one carbon — the classic tool for this is a Grignard reaction.

Step 1 — Grignard addition:

Methylmagnesium bromide (CH$_3$MgBr, prepared from CH$_3$Br + Mg in dry ether) adds across the polar C=O bond of methanal. The nucleophilic methyl carbanion-like carbon attacks the electrophilic carbonyl carbon:

$HCHO + CH_3\text{-}MgBr \longrightarrow CH_3\text{-}CH_2\text{-}O\text{-}MgBr$

(a magnesium alkoxide)

Step 2 — Hydrolysis:

The magnesium alkoxide is hydrolysed with dilute acid (H$_3$O$^+$) to liberate the alcohol: …