Textbook Question
(a) Based on what you know about the relative stabilities of alkyl cations and benzylic cations, predict the product of addition of HBr to 1-phenylpropene.
(b) Propose a mechanism for this reaction.
10. Addition Reactions
Hydrohalogenation
0:44 minutesProblem 10c
Textbook Question
Predict the product of the following haloalkane syntheses.
(c) HBr↗HOOH
Verified step by step guidance1
Identify the type of reaction: The presence of HBr and HOOH suggests a radical addition reaction, specifically anti-Markovnikov addition due to the presence of peroxide (HOOH).
Understand the mechanism: In the presence of peroxides, HBr undergoes a radical chain mechanism. The peroxide initiates the formation of bromine radicals.
Determine the regioselectivity: Anti-Markovnikov addition occurs because the bromine radical adds to the less substituted carbon of the alkene, leading to the formation of the more stable radical intermediate.
Predict the product: The bromine radical adds to the less substituted carbon, and the hydrogen from HBr adds to the more substituted carbon, resulting in the formation of a haloalkane.
Consider stereochemistry: Radical reactions often lead to racemic mixtures if the product has chiral centers, due to the planar nature of the radical intermediate.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Haloalkane Synthesis
Haloalkane synthesis involves the formation of alkyl halides through the reaction of alkanes with halogens. In this context, the reaction typically proceeds via a radical mechanism, where the halogen atom replaces a hydrogen atom in the alkane, resulting in the formation of a haloalkane.
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Peroxide Effect
The peroxide effect, also known as the anti-Markovnikov addition, occurs when peroxides are present in the reaction of alkenes with hydrogen halides. This effect leads to the addition of the halogen to the less substituted carbon atom, contrary to the usual Markovnikov rule, due to the formation of free radicals initiated by the peroxide.
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Radical Mechanism
A radical mechanism involves the formation and reaction of free radicals, which are highly reactive species with unpaired electrons. In the presence of peroxides, the reaction of HBr with alkenes proceeds via a radical chain mechanism, where the peroxide initiates the formation of bromine radicals, leading to the anti-Markovnikov addition of HBr.
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