Question

Identify A and B in the following sequence of reaction.

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is B

In the reaction, toluene undergoes chlorination in the presence of light to form benzyl chloride (A). On further oxidation with water, it forms benzaldehyde (B).
Thus the correct answer is Option B.

Answer 2

The problem asks to identify the products A and B in a two-step reaction sequence starting from toluene.

Concept Used:

1. Free-Radical Halogenation: The reaction of an alkane or an alkylbenzene with a halogen (like Cl₂) in the presence of ultraviolet light (hν) or heat proceeds via a free-radical mechanism. This reaction results in the substitution of hydrogen atoms on the alkyl group (side-chain) with halogen atoms. For toluene, this substitution occurs at the benzylic carbon because the intermediate benzyl radical is highly stabilized by resonance.
2. Hydrolysis of Geminal Dihalides: A geminal dihalide is a compound with two halogen atoms attached to the same carbon atom (e.g., R-CHCl₂). When a geminal dihalide is hydrolyzed with water or an aqueous base, it first forms an unstable geminal diol (two -OH groups on the same carbon). This intermediate immediately loses a water molecule to form a stable carbonyl compound (an aldehyde or a ketone).

The general reaction for hydrolysis of a terminal geminal dihalide is:

\[ \text{R-CHCl}_2 + 2\text{H}_2\text{O} \longrightarrow [\text{R-CH(OH)}_2] \text{ (unstable)} \longrightarrow \text{R-CHO} + \text{H}_2\text{O} + 2\text{HCl} \]

Step-by-Step Solution:

Step 1: Identify the reaction of Toluene with Cl₂/hν to form A.

The starting material is toluene (\( \text{C}_6\text{H}_5\text{CH}_3 \)). The reaction is carried out with chlorine (\( \text{Cl}_2 \)) in the presence of UV light (\( h\nu \)). These conditions are characteristic of a free-radical substitution on the alkyl side-chain, not on the aromatic ring.

The hydrogen atoms of the methyl group are successively replaced by chlorine atoms. Depending on the stoichiometry, monochlorination, dichlorination, or trichlorination can occur.

• Monochlorination gives benzyl chloride (\( \text{C}_6\text{H}_5\text{CH}_2\text{Cl} \)).
• Dichlorination gives benzal chloride (\( \text{C}_6\text{H}_5\text{CHCl}_2 \)).
• Trichlorination gives benzotrichloride (\( \text{C}_6\text{H}_5\text{CCl}_3 \)).

We need to look at the next step to determine the structure of A.

Step 2: Identify the hydrolysis of A with H₂O to form B.

Product A is hydrolyzed with water at 373 K (100 °C). Let's consider the hydrolysis product for each possible structure of A:

• If A were benzyl chloride (\( \text{C}_6\text{H}_5\text{CH}_2\text{Cl} \)), hydrolysis would yield benzyl alcohol (\( \text{C}_6\text{H}_5\text{CH}_2\text{OH} \)).
• If A were benzal chloride (\( \text{C}_6\text{H}_5\text{CHCl}_2 \)), a geminal dihalide, hydrolysis would yield benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \)).
• If A were benzotrichloride (\( \text{C}_6\text{H}_5\text{CCl}_3 \)), hydrolysis would yield benzoic acid (\( \text{C}_6\text{H}_5\text{COOH} \)).

Looking at the options, the final product B is mostly shown as benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \)). This implies that the intermediate A must be benzal chloride (\( \text{C}_6\text{H}_5\text{CHCl}_2 \)).

The reaction sequence is as follows:

\[ \underset{\text{Toluene}}{\text{C}_6\text{H}_5\text{CH}_3} \xrightarrow[\text{h}\nu]{\text{Cl}_2} \underset{\text{(A) Benzal chloride}}{\text{C}_6\text{H}_5\text{CHCl}_2} \xrightarrow[\text{373K}]{\text{H}_2\text{O}} \underset{\text{(B) Benzaldehyde}}{\text{C}_6\text{H}_5\text{CHO}} \]

Final Result:

Based on the analysis, compound A is benzal chloride (\( \text{C}_6\text{H}_5\text{CHCl}_2 \)) and compound B is benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \)).

This corresponds to Option (2).