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JEE Chemistry 2025 Question with Solution

Reaction scheme showing a dibromonitrobenzene ring reacting with sodium ethoxide in ethanol to form product P.Text below the reaction scheme stating that product P formed is to be identified from the given structures.Option structure 1 showing bromonitrobenzene with one ethoxy substituent replacing one bromine atom.Option structure 2 showing nitrobenzene ring with two ethoxy substituents and no bromine substituent remaining.Option structures 3 and 4 showing alternative bromonitroethoxybenzene arrangements with different substitution positions.
  • A
    Benzene ring with nitro group at one position, bromine at the top, and ethoxy group at the adjacent upper-right position.
  • B
    Benzene ring with nitro group at one position and ethoxy groups at the top and upper-right positions, with no bromine left.
  • C
    Benzene ring option showing nitro group, ethoxy group at the top, and bromine at the upper-right position.
  • D
    Benzene ring with bromines at the top and upper-right positions and ethoxy group opposite the top substituent, representing another possible product.

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: A dibromonitrobenzene reacts with sodium ethoxide in ethanol. Find: the correct product P formed in this substitution reaction.

Identify the principle: This is a nucleophilic aromatic substitution reaction. The NO2\text{NO}_2 group is an electron-withdrawing group and activates the aromatic ring toward substitution at positions ortho and para to it.

Apply the idea: Ethoxide ion from sodium ethoxide acts as the nucleophile and replaces the bromine atom that is activated by the nitro group. The reaction proceeds through a Meisenheimer-type intermediate.

The solution states that one bromine atom is replaced by OC2H5\text{OC}_2\text{H}_5, giving 2-Bromo-4-nitrophenetole.

Therefore, the correct option is A.

Detailed Explanation

Given: The substrate is 1,2-dibromo-4-nitrobenzene and the reagent is sodium ethoxide in ethanol.

Find: Which product is formed.

Step 1: Understand the reaction type. This is not SN1\text{S}_\text{N}1 or SN2\text{S}_\text{N}2 at an aliphatic carbon. Since the substrate is an aromatic halide containing a strongly electron-withdrawing NO2\text{NO}_2 group, the relevant mechanism is nucleophilic aromatic substitution.

Step 2: Role of the nitro group. The NO2\text{NO}_2 group stabilizes the intermediate formed after nucleophilic attack. Therefore, a bromine atom activated by the nitro group can be displaced by the ethoxide nucleophile.

Step 3: Nucleophile involved. Sodium ethoxide provides OC2H5−\text{OC}_2\text{H}_5^-, which attacks the activated aromatic carbon and ultimately replaces one bromine atom.

Step 4: Product identification. The extracted solution explicitly concludes that the product formed is 2-Bromo-4-nitrophenetole and also marks Option A as correct.

Therefore, the correct option is A.

Common mistakes

  • Choosing a product with two ethoxy substitutions is incorrect because the provided solution describes replacement of only one bromine atom under the given conditions. Keep track of the extent of substitution shown in the solution.

  • Treating the reaction as ordinary SN1\text{S}_\text{N}1 or SN2\text{S}_\text{N}2 on an aromatic carbon is wrong because aryl halides do not undergo these mechanisms easily. Use nucleophilic aromatic substitution when an electron-withdrawing group such as NO2\text{NO}_2 is present.

  • Ignoring the activating effect of the NO2\text{NO}_2 group leads to the wrong substitution position. The nitro group makes certain ring positions more susceptible to nucleophilic attack, so identify the bromine activated by NO2\text{NO}_2 before selecting the product.

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