Question

For the following reaction sequence, choose the correct option

• A. Both $\mathbf{P}$ and $\mathbf{Q}$ are carbonyl compounds.
• B. If $\mathbf{P}$ is the sodium salt of a carboxylic acid, $\mathbf{Q}$ is a primary alcohol.
• C. $\mathbf{P}$ and $\mathbf{Q}$ are aromatic compounds.
• D. If $\mathbf{P}$ gives a carboxylic acid on acidification, $\mathbf{Q}$ gives a poisonous gas on exposure to air and light.

Hint:

To remember the chemical safety of haloforms: chloroform ($\text{CHCl}_3$) is always stored in dark, amber-colored tightly closed bottles filled up to the brim. This is specifically done to exclude light and air, preventing its conversion into the deadly poisonous gas phosgene ($\text{COCl}_2$) detailed in option (4)!

Answer 1

The Correct Option is D

Step 1: Recognise the two-step sequence.
The scheme couples a Friedel-Crafts acylation with a haloform reaction.
Step 2: Friedel-Crafts acylation.
Benzene with \(CH_3COCl\) and \(AlCl_3\) forms an acylium ion that attacks the ring, giving acetophenone, \(C_6H_5COCH_3\), a methyl ketone.
Step 3: Haloform reaction.
Acetophenone has the \(-COCH_3\) group, so with \(NaOCl\) the methyl carbon is fully chlorinated and the C-C bond cleaves, giving sodium benzoate \((P = C_6H_5COONa)\) and chloroform \((Q = CHCl_3)\).
Step 4: Test the wrong options.
P is a carboxylate salt and Q is an alkyl halide, so they are not both carbonyl compounds; Q is chloroform, not an alcohol; and Q is aliphatic, so they are not both aromatic.
Step 5: Test option (4).
Acidifying P gives benzoic acid \((C_6H_5COOH)\), confirming the carboxylic-acid part.
Step 6: Check Q in air and light.
Chloroform slowly oxidises in air and sunlight to the poisonous gas phosgene: \[ 2CHCl_3 + O_2 \xrightarrow{light} 2COCl_2 + 2HCl \] So option (4) is correct.
\[ \boxed{\text{If P gives a carboxylic acid on acidification, Q gives a poisonous gas on exposure to air and light.}} \]