Question

What is the product formed when side chain chlorination of toluene is carried out followed by acid hydrolysis at 373 K ?

• A. Benzaldehyde
• B. Benzal chloride
• C. Chlorobenzene
• D. Benzoic acid

Hint:

Keep track of chlorination levels: Replacing 1 hydrogen of toluene gives Benzyl chloride $\rightarrow$ yields Benzyl alcohol. Replacing 2 hydrogens gives Benzal chloride $\rightarrow$ yields Benzaldehyde. Replacing all 3 hydrogens gives Benzotrichloride $\rightarrow$ yields Benzoic acid.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The problem outlines a two-step industrial preparation sequence: first, the side chain chlorination of toluene is performed, followed by high-temperature acid hydrolysis. We need to find the final organic compound produced.

Step 2: Key Formula or Approach:
1. Controlled side-chain radical chlorination of toluene ($\text{C}_6\text{H}_5\text{CH}_3$) in the presence of sunlight/UV light replaces two benzylic hydrogens with chlorine atoms, producing benzal chloride ($\text{C}_6\text{H}_5\text{CHCl}_2$).
2. Hydrolysis of a gem-dihalide using water under acidic conditions replaces the two chlorine atoms with two hydroxyl groups (-OH), forming an unstable gem-diol intermediate that spontaneously loses a water molecule to yield a carbonyl compound.

Step 3: Detailed Explanation:
Let's look at the step-by-step chemical transformations: 3.

Side-chain chlorination: Toluene is treated with chlorine gas in the presence of light to form benzal chloride: $$\text{C}_6\text{H}_5\text{CH}_3 + 2\text{Cl}_2 \xrightarrow{h\nu} \text{C}_6\text{H}_5\text{CHCl}_2 + 2\text{HCl}$$ 4.

Acid Hydrolysis: Benzal chloride is hydrolyzed at $373\text{ K}$ to substitute the chlorines with hydroxyl groups: $$\text{C}_6\text{H}_5\text{CHCl}_2 + 2\text{H}_2\text{O} \xrightarrow{\text{H}^+, 373\text{ K}} \left[\text{C}_6\text{H}_5\text{CH(OH)}_2\right] + 2\text{HCl}$$ 5. The intermediate species, benzylidene glycol $[\text{C}_6\text{H}_5\text{CH(OH)}_2]$, contains two hydroxyl groups attached to the same carbon atom (a gem-diol). Gem-diols are highly unstable due to steric crowding and electronic repulsion, and they immediately eliminate a water molecule: $$\left[\text{C}_6\text{H}_5\text{CH(OH)}_2\right] \xrightarrow{-\text{H}_2\text{O}} \text{C}_6\text{H}_5\text{CHO}\text{ (Benzaldehyde)}$$

Step 4: Final Answer:
The stable product formed at the end of the sequence is benzaldehyde, which corresponds to option (A).