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NVAMediumJEE 2024Alkenes (Electrophilic Addition, Polymerisation)

JEE Chemistry 2024 Question with Solution

3-Methylhex-2-ene on reaction with HBr in presence of peroxide forms an addition product (A). The number of possible stereoisomers for 'A' is:

Answer

Correct answer:4

Step-by-step solution

Standard Method

Given: 3-Methylhex-2-ene reacts with HBr in presence of peroxide to form addition product A.

Find: The number of possible stereoisomers of A.

In the presence of peroxide, HBr adds to the alkene by the anti-Markovnikov radical mechanism.

From the provided solution, the product is identified as 1-bromo-3-methylhexane.

The solution states that this product has two chiral centers.

Therefore, the number of possible stereoisomers is

2n=22=42^n = 2^2 = 4

So the possible stereoisomers are RR, SS, RS, and SR.

Therefore, the number of possible stereoisomers for product A is 4.

Direct Stereoisomer Count

Given: HBr adds to 3-Methylhex-2-ene in presence of peroxide.

Find: Total stereoisomers of the product.

The provided solution uses the peroxide effect to infer anti-Markovnikov addition and then counts the stereogenic centers in the product.

Number of chiral centers,

n=2n = 2

Hence,

Number of stereoisomers=2n=22=4\text{Number of stereoisomers} = 2^n = 2^2 = 4

Thus, the final answer is 4.

Common mistakes

  • Assuming Markovnikov addition instead of peroxide-induced anti-Markovnikov addition is incorrect because HBr in presence of peroxide follows a radical pathway. First identify the reaction condition, then determine the product structure.

  • Counting the number of chiral centers incorrectly leads to a wrong stereoisomer count. After obtaining the product, inspect each carbon carefully and check whether it is attached to four different groups before using 2n2^n.

  • Using the answer key instead of the worked solution can give the wrong result here. The provided solution explicitly concludes that the product has two chiral centers and hence 4 stereoisomers, so the solution-derived answer must be used.

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