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

Concept: This question combines two highly important organic chemistry reactions in sequence:

• Friedel-Crafts Acylation: Benzene reacts with an acyl halide (such as acetyl chloride, $\text{CH}_3\text{COCl}$) in the presence of a Lewis acid catalyst ($\text{AlCl}_3$) to form an aromatic ketone (acetophenone).

• Haloform Reaction: A methyl ketone reacts with a sodium hypohalite solution (NaOCl, sodium hypochlorite) to produce a haloform ($\text{CHCl}_3$, chloroform) along with the sodium salt of the corresponding aromatic carboxylic acid.

Step 1: First reaction stage — Friedel-Crafts Acylation.
When benzene is treated with acetyl chloride ($\text{CH}_3\text{COCl}$) in the presence of anhydrous aluminum chloride ($\text{AlCl}_3$), an electrophilic aromatic substitution reaction occurs. The Lewis acid acts as a catalyst by extracting the chloride ion to generate a highly reactive acylium electrophile: \[ \text{CH}_3\text{COCl} + \text{AlCl}_3 \rightarrow \text{CH}_3\text{C}^+=\text{O} + \text{AlCl}_4^- \] This acylium ion ($\text{CH}_3\text{CO}^+$) attacks the electron-rich $\pi$-system of the benzene ring. After subsequent deprotonation to restore aromaticity, the intermediate formed is Acetophenone ($\text{C}_6\text{H}_5\text{COCH}_3$), which contains a methyl ketone group attached to the phenyl ring.

Step 2: Second reaction stage — Haloform reaction with NaOCl.
Acetophenone ($\text{C}_6\text{H}_5\text{COCH}_3$) contains the essential $-\text{COCH}_3$ (methyl ketone) functional assembly required to undergo a classic haloform reaction. When treated with sodium hypochlorite (NaOCl), the three alpha-hydrogen atoms on the methyl carbon are sequentially substituted by chlorine atoms due to the alkaline halogenating environment, yielding a trichloro intermediate ($\text{C}_6\text{H}_5\text{COCCl}_3$). The hydroxide ion ($\text{OH}^-$) present in the medium then attacks the carbonyl carbon, inducing carbon-carbon bond cleavage because $-\text{CCl}_3$ serves as an efficient leaving group: \[ \text{C}_6\text{H}_5\text{COCH}_3 \xrightarrow{\text{NaOCl}} \text{C}_6\text{H}_5\text{COO}^-\text{Na}^+ (\mathbf{P}) + \text{CHCl}_3 (\mathbf{Q}) \] Thus, the products of this sequence are:

• $\mathbf{P} = \text{C}_6\text{H}_5\text{COONa}$ (Sodium benzoate, which is the sodium salt of benzoic acid).

• $\mathbf{Q} = \text{CHCl}_3$ (Chloroform, a haloform compound).

Step 3: Analyzing the given options based on $\mathbf{P}$ and $\mathbf{Q}$.
Let us evaluate each statement critically to establish the valid option:

• Option (1): "Both $\mathbf{P}$ and $\mathbf{Q}$ are carbonyl compounds." — Incorrect. $\mathbf{P}$ is a carboxylate salt and $\mathbf{Q}$ is an alkyl halide ($\text{CHCl}_3$), which contains no carbonyl ($\text{C}=\text{O}$) double bond.

• Option (2): "If $\mathbf{P}$ is the sodium salt of a carboxylic acid, $\mathbf{Q}$ is a primary alcohol." — Incorrect. As proven above, $\mathbf{Q}$ is chloroform ($\text{CHCl}_3$), not an alcohol.

• Option (3): "$\mathbf{P}$ and $\mathbf{Q}$ are aromatic compounds." — Incorrect. While $\mathbf{P}$ (sodium benzoate) preserves the phenyl ring and is aromatic, $\mathbf{Q}$ ($\text{CHCl}_3$) is an aliphatic trihalomethane and is completely non-aromatic.

• Option (4): "If $\mathbf{P}$ gives a carboxylic acid on acidification, $\mathbf{Q}$ gives a poisonous gas on exposure to air and light." — Correct. Acidification of the salt $\mathbf{P}$ ($\text{C}_6\text{H}_5\text{COONa}$) gives benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$). Product $\mathbf{Q}$ is chloroform ($\text{CHCl}_3$). When chloroform is exposed to atmospheric oxygen ($\text{O}_2$) in the presence of sunlight, it undergoes slow oxidation to produce a highly toxic, poisonous gas called carbonyl chloride, commonly known as

phosgene ($\text{COCl}_2$): \[ 2\text{CHCl}_3 + \text{O}_2 \xrightarrow{\text{light}} 2\text{COCl}_2 + 2\text{HCl} \]
This accurately confirms that statement (4) is completely correct.