The product obtained by reaction of 1-ethyl-4-nitro benzene with $Br_{2}$ is presence of heat or U.V. light is:

Question

Accepted Answer

The reaction of 1-ethyl-4-nitrobenzene with $ Br_2 $ in the presence of heat or U.V. light is a free radical bromination reaction. This type of reaction typically occurs at the benzylic position, which is the carbon atom directly attached to the benzene ring in an alkyl side chain.

1. **Explanation for Option 3 (Correct Answer):**
   - In 1-ethyl-4-nitrobenzene, the ethyl group attached to the benzene ring has a benzylic hydrogen. Under the conditions of heat or U.V. light, a free radical mechanism is initiated, leading to the abstraction of a hydrogen atom from the benzylic position. This results in the formation of a benzylic radical.
   - The benzylic radical is highly reactive and will readily react with $ Br_2 $ to form the brominated product. In this case, the bromine atom will replace the hydrogen atom at the benzylic position, resulting in the formation of 1-(1-bromoethyl)-4-nitrobenzene.
   - Therefore, the correct product of this reaction is 1-(1-bromoethyl)-4-nitrobenzene, which corresponds to Option 3.

2. **Explanation for why other options are incorrect:**
   - **Option 1: 2-bromo-1-ethyl-4-nitro-benzene**
     - This option suggests bromination at the ortho position of the benzene ring relative to the ethyl group. However, under free radical conditions, bromination occurs at the benzylic position, not on the aromatic ring itself. Therefore, this option is incorrect.
   
   - **Option 2: 1-(2-bromo ethyl)-4-nitro-benzene**
     - This option implies bromination at the terminal carbon of the ethyl group, which is not favored in free radical bromination. The reaction preferentially occurs at the benzylic position due to the stability of the benzylic radical. Thus, this option is incorrect.
   
   - **Option 4: 3-bromo-1-ethyl-4-nitro-benzene**
     - Similar to Option 1, this option suggests bromination at the meta position of the benzene ring relative to the ethyl group. Again, free radical bromination targets the benzylic position, not the aromatic ring. Hence, this option is incorrect.

In conclusion, the correct product of the reaction is 1-(1-bromoethyl)-4-nitrobenzene, which is accurately represented by Option 3.