Question:
Which one of the following alkenes forms the same product with HBr in the absence as well as in the presence of peroxide?
Which one of the following alkenes forms the same product with HBr in the absence as well as in the presence of peroxide?
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Peroxide reverses the orientation of HBr addition, but this makes no difference for a symmetrical alkene.
Updated On: Nov 12, 2026
- 2-butene
- 1-butene
- propene
- 2-methyl propene
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Concept
Without peroxide, HBr adds by the Markovnikov rule (ionic path). With peroxide it adds by the anti-Markovnikov rule (free radical path).
Step 2: When products differ
The two products are different only when the alkene is unsymmetrical.
Step 3: Check the alkenes
2-Butene, \(\text{CH}_{3}\text{CH}=\text{CHCH}_{3}\), is symmetrical. Br on either alkene carbon gives 2-bromobutane in both cases.
1-Butene, propene and 2-methylpropene are unsymmetrical, so they give different products.
Final Answer:
2-Butene gives the same product in both conditions. \[ \boxed{\text{(A) }\text{2-butene}} \]
Without peroxide, HBr adds by the Markovnikov rule (ionic path). With peroxide it adds by the anti-Markovnikov rule (free radical path).
Step 2: When products differ
The two products are different only when the alkene is unsymmetrical.
Step 3: Check the alkenes
2-Butene, \(\text{CH}_{3}\text{CH}=\text{CHCH}_{3}\), is symmetrical. Br on either alkene carbon gives 2-bromobutane in both cases.
1-Butene, propene and 2-methylpropene are unsymmetrical, so they give different products.
Final Answer:
2-Butene gives the same product in both conditions. \[ \boxed{\text{(A) }\text{2-butene}} \]
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