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MCQMediumJEE 2025Alkenes (Electrophilic Addition, Polymerisation)

JEE Chemistry 2025 Question with Solution

Which compound would give 3-methyl-6-oxoheptanal upon ozonolysis ?

  • A
    Cyclohexene ring with one double bond on the upper right side and methyl substituents at the top and lower left positions.
  • B
    Cyclohexene ring with one double bond on the upper right side and methyl substituents at the top and bottom positions.
  • C
    Cyclohexene ring with one double bond on the upper left side and methyl substituents at the top and lower right positions.
  • D
    Cyclohexene ring with one double bond on the upper left side and methyl substituents at the top and upper left external carbon positions.

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: We need the compound that gives 3-methyl-6-oxoheptanal on ozonolysis.

Find: The correct precursor alkene.

Ozonolysis of alkenes cleaves the carbon-carbon double bond and converts the two alkene carbons into carbonyl groups. Therefore, we identify the alkene whose cleavage gives the required aldehyde-ketone product.

Reaction scheme showing a dimethyl-substituted cyclohexene undergoing ozonolysis with ozone and zinc-water to form 3-methyl-6-ketoheptanal.

From the provided solution, the structure that matches the required cleavage pattern is Option B. The solution explicitly states that ozonolysis of Option B forms the required product 3-methyl-6-ketoheptanal, corresponding to 3-methyl-6-oxoheptanal.

Therefore, the correct option is B.

Detailed Elimination of Options

Given: Ozonolysis cleaves an alkene into carbonyl compounds.

Find: Which option gives 3-methyl-6-oxoheptanal.

The extracted solution explains the selection by checking the ozonolysis products of each option:

  1. Option A: does not provide the correct aldehyde and ketone combination.
  2. Option B: gives fragments consistent with the target product.
  3. Option C: does not yield the required ozonolysis fragments.
  4. Option D: also does not match the required product.

Hence, the precursor alkene is Option B.

Common mistakes

  • Assuming ozonolysis removes the double bond without cleaving the molecule. This is wrong because ozonolysis breaks the C=C\text{C=C} bond into carbonyl groups. Always split the alkene at the double bond first.

  • Ignoring whether each alkene carbon becomes an aldehyde or a ketone. This is wrong because substitution on the double-bond carbon determines the carbonyl product. Check hydrogen substitution carefully before choosing the option.

  • Matching only the carbon count and not the position of the methyl group. This is wrong because the product is specifically 3-methyl-6-oxoheptanal. Track the substituent positions after cleavage, not just the total number of carbons.

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