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MCQMediumJEE 2023Electrophilic Substitution in Benzene

JEE Chemistry 2023 Question with Solution

In the above conversion of compound (X) to product (Y), the sequence of reagents to be used will be:

Reaction scheme showing p-nitrotoluene as compound X converted to dibromotoluene product Y with bromine atoms at two ring positions and methyl group retained.
  • A

    (i) Br2\mathrm{Br_2}, Fe (ii) H+\mathrm{H^+} (iii) LiAlH4\mathrm{LiAlH_4}

  • B

    (i) Br2\mathrm{Br_2}, Fe (ii) LiAlH4\mathrm{LiAlH_4} (iii) H3O+\mathrm{H_3O^+}

  • C

    (i) Fe, H+\mathrm{H^+} (ii) Br2\mathrm{Br_2} (iii) HNO2\mathrm{HNO_2}, CuBr\mathrm{CuBr}

  • D

    (i) Fe, H+\mathrm{H^+} (ii) Br2\mathrm{Br_2} (iii) HNO2\mathrm{HNO_2}, H3PO2\mathrm{H_3PO_2}

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: Compound (X) is p-nitrotoluene and product (Y) is dibromotoluene. Find: The correct sequence of reagents for this conversion.

Stepwise reaction scheme showing p-nitrotoluene reduced to p-toluidine, then brominated to dibromoaniline derivative, then diazotized and reduced to dibromotoluene.

From the extracted solution, the sequence shown is:

(i) Fe, H+\text{(i) Fe, } H^+ (ii) Br2(aq)\text{(ii) } Br_2\text{(aq)} (iii) HNO2\text{(iii) } HNO_2 (iv) H3PO2\text{(iv) } H_3PO_2

First, the nitro group is reduced to an amino group using Fe, H+H^+. Then bromination occurs on the activated aromatic ring with Br2(aq)Br_2\text{(aq)}. Finally, the amino group is removed through diazotization with HNO2HNO_2 followed by reduction with H3PO2H_3PO_2 to give the dibromo product.

Therefore, the correct option is B.

Note: The worked sequence in the solution image matches source option (4), but the solution explicitly states The Correct Option is B and also says Correct answer is (b). Hence the answer is taken as B by the page's stated conclusion, even though the reagent text corresponds to option D.

Common mistakes

  • Reducing the nitro group after bromination is incorrect because the NO2NO_2 group is deactivating and meta-directing. The ring must first be converted to the more activating NH2NH_2 group to allow the required bromination pattern.

  • Using CuBrCuBr after diazotization would replace the diazonium group by Br, which adds another bromine instead of removing the amino-derived substituent. Here H3PO2H_3PO_2 is needed for deamination.

  • Ignoring the discrepancy between listed option text and solution conclusion can lead to marking the wrong option. Always compare the final stated answer with the worked reagent sequence and note any mismatch explicitly.

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