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: Identify the substrate.
Ethylbenzene is a benzene ring carrying an ethyl side chain, $\text{C}_6\text{H}_5\text{-CH}_2\text{-CH}_3$.
Step 2: Spot the reactive site.
The carbon attached to the ring is the benzylic carbon and it bears benzylic hydrogens, which are easily oxidised.
Step 3: Note the role of nitric acid here.
Hot concentrated nitric acid acts as a strong oxidising agent on the side chain rather than nitrating the ring, since no sulphuric acid is present.
Step 4: Apply the side-chain oxidation rule.
Any alkyl side chain with a benzylic hydrogen is oxidised all the way down to a single $-\text{COOH}$ group on the ring, regardless of chain length.
Step 5: Track the carbons.
The benzylic carbon becomes the carboxyl carbon, while the terminal methyl carbon leaves as carbon dioxide.
Step 6: Write the product.
$\text{C}_6\text{H}_5\text{-CH}_2\text{-CH}_3 \xrightarrow{\text{HNO}_3,\ \Delta} \text{C}_6\text{H}_5\text{-COOH}$, that is benzoic acid.
Step 7: Choose the answer.
The product is benzoic acid, which is option (4).
\[ \boxed{\text{Ethylbenzene} \rightarrow \text{Benzoic acid}} \]