Q.Acetic acid is prepared from methyl magnesium bromide and dry ice in presence of dry ether. Name the compound which serves not only as reagent but also as cooling agent in the reaction.

Since butanoic acid has the same number of carbons (4) as 1-chlorobutane, the conversion must NOT go through cyanide (which would add a carbon); instead, hydrolyse to the alcohol and then oxidise it fully to the acid.

1-Chlorobutane, $CH_3CH_2CH_2CH_2Cl$, already has four carbons — exactly the number needed for butanoic acid, $CH_3CH_2CH_2COOH$. So the terminal $-CH_2Cl$ carbon simply needs to become a $-COOH$ carbon, with no chain extension required (ruling out the usual $KCN$ chain-extension route used for acids with one extra carbon).

Step 1 — Hydrolysis (nucleophilic substitution by hydroxide):

$CH_3CH_2CH_2CH_2Cl + KOH \xrightarrow{aq.} CH_3CH_2CH_2CH_2OH\;(\text{butan-1-ol}) + KCl$

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Hydrolysis of benzamide ($C_6H_5CONH_2$) breaks the C–N bond to give benzoic acid ($C_6H_5COOH$) and ammonia.

Concept — hydrolysis of an amide. An amide is a carboxylic-acid derivative; on hydrolysis (acidic or basic) the $-CONH_2$ group is converted back to the parent carboxylic acid, releasing the nitrogen as ammonia (or ammonium salt).

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Complete acidic hydrolysis of a nitrile gives a carboxylic acid; CH3CN gives CH3COOH.

Nitriles undergo hydrolysis. On complete hydrolysis with aqueous acid (H3O+), the –CN group is converted, via the amide, all the way to the carboxylic acid:

CH3CN --H3O+--> CH3CONH2 --H3O+--> CH3COOH + NH4+

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