Textbook Question
Predict the product(s) that would result when the alkenes shown here are allowed to react under the following conditions: (iii) HBr, H2O2
(g)
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10. Addition Reactions
Hydrohalogenation
Multiple Choice
In the absence of peroxides, hydrohalogenation of alkenes with proceeds via carbocation formation. Which alkene reacts fastest with under these conditions?
A
ethene,
B
2-methylpropene (isobutene),
C
trans-2-butene,
D
1-butene,
Verified step by step guidance1
Recognize that hydrohalogenation of alkenes with HBr in the absence of peroxides proceeds via a carbocation intermediate, following Markovnikov's rule where the proton adds to the carbon with more hydrogens, and the halide attaches to the more substituted carbon.
Understand that the rate-determining step is the formation of the carbocation intermediate, so the stability of this carbocation greatly influences the reaction rate.
Recall the order of carbocation stability: tertiary > secondary > primary > methyl. More substituted carbocations are more stable due to hyperconjugation and inductive effects.
Analyze each alkene to determine the type of carbocation formed upon protonation: ethene forms a primary carbocation, 1-butene forms a primary carbocation, trans-2-butene forms a secondary carbocation, and 2-methylpropene forms a tertiary carbocation.
Conclude that the alkene that forms the most stable (tertiary) carbocation intermediate will react fastest with HBr under these conditions, which is 2-methylpropene (isobutene).
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