Question

Identify the product formed when ethyl benzene reacts with nitric acid.

• A. o-Nitro ethylbenzene
• B. Ethoxybenzene
• C. p-Nitro ethylbenzene
• D. Benzoic acid

Hint:

Remember the golden rule of benzylic oxidation: as long as the alkyl side chain has at least one hydrogen atom on the carbon attached directly to the ring, the entire side chain will always get chewed down to form exactly one benzoic acid group (-COOH)!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the principal product formed during the chemical reaction of ethylbenzene with nitric acid under typical oxidative conditions.

Step 2: Key Formula or Approach:
Alkyl side chains attached to aromatic benzene rings are highly susceptible to strong oxidative cleavage at the benzylic position when heated with robust mineral oxidants such as concentrated nitric acid ($\text{HNO}_3$) or potassium permanganate ($\text{KMnO}_4$). Regardless of the length of the alkyl side chain, the entire chain is oxidized down to a single carboxylic acid group attached directly to the ring.

Step 3: Detailed Explanation:
Let's analyze the structural behavior of ethylbenzene ($\text{C}_6\text{H}_5\text{-CH}_2\text{-CH}_3$):
The carbon atom directly bound to the aromatic ring is the benzylic carbon and holds two benzylic hydrogens.
When treated with hot, concentrated nitric acid ($\text{HNO}_3$), it acts as a strong oxidizing agent rather than a nitrating mixture (since sulfuric acid is absent here).
The oxidative cleavage cleaves the C-C bond of the ethyl group, converting the benzylic carbon into a carboxyl group ($-\text{COOH}$), while the terminal methyl carbon escapes as carbon dioxide ($\text{CO}_2$).
The total transformation can be written as:
$$\text{C}_6\text{H}_5\text{-CH}_2\text{-CH}_3 \xrightarrow{\text{HNO}_3,\, \Delta} \text{C}_6\text{H}_5\text{-COOH}$$ The final aromatic product is benzoic acid, which matches option (D).

Step 4: Final Answer:
The product formed when ethylbenzene reacts with nitric acid under these conditions is Benzoic acid, which corresponds to option (D).