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JEE Chemistry 2023 Question with Solution

Find out the major product from the following reaction.

Reaction scheme showing a cyclohexenone derivative reacting with (1) MeMgBr/CuI and (2) n-PrI, followed by four product options labeled 1 to 4.
  • A
    Option 1 shows a cyclohexene ring bearing an OH group and a methyl substituent, representing the first possible product structure.
  • B
    Option 2 shows a cyclohexanone ring with a propyl side chain and gem-dimethyl substitution, representing the second possible product structure.
  • C
    Option 3 shows a cyclohexanone ring with an n-propyl substituent and two methyl groups on the adjacent carbon, representing the third possible product structure.
  • D
    Option 4 shows a cyclohexane ring with a propyl substituent and three methyl substituents, representing the fourth possible product structure.

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: The substrate is an α,β\alpha,\beta-unsaturated ketone treated with MeMgBr/CuI followed by n-PrI.

Find: The major product formed in the reaction.

The reaction proceeds in two steps:

  1. 1,4-addition of the organocuprate: The organocuprate reagent (MeMgBr/CuI) adds to the α,β\alpha,\beta-unsaturated ketone in a 1,41,4-conjugate addition manner. This results in the formation of a ketone with a methyl group at the β\beta position.

  2. Alkylation of the ketone: The resulting ketone reacts with n-propyl iodide in an alkylation reaction. The enolate of the ketone attacks the alkyl halide, adding the n-propyl group to the α\alpha-carbon.

Mechanism showing conjugate addition of methyl cuprate to cyclohexenone, enolate formation, and subsequent alkylation with n-propyl iodide to give the final ketone product.

Therefore, the major product is option (3). Hence, the correct option is C.

Common mistakes

  • Choosing 1,21,2-addition instead of 1,41,4-addition is incorrect because organocuprate-type reagents formed with CuI preferentially undergo conjugate addition to α,β\alpha,\beta-unsaturated carbonyl compounds. Use 1,41,4-addition first.

  • Placing the n-propyl group at the wrong carbon is incorrect because alkylation occurs through the enolate, which reacts at the α\alpha-carbon. After conjugate addition, identify the enolate position before adding the alkyl halide.

  • Ignoring retention of the ketone functionality is incorrect because conjugate addition followed by alkylation does not reduce the carbonyl group. Keep the C=O group present in the final product.

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