Question 1

What role do peroxides play in anti-Markovnikov addition of hydrogen bromide to alkenes?

Accepted Answer

Peroxides or hydroperoxides initiate the radical chain reaction by undergoing homolytic cleavage of their weak O–O bond when heated, forming two alkoxy radicals. These alkoxy radicals then abstract a hydrogen atom from hydrogen bromide to generate bromine radicals, which start the propagation cycle and sustain the chain reaction.

Question 2

How does chain propagation work in radical anti-Markovnikov addition?

Accepted Answer

Chain propagation occurs in two repeating steps. First, a bromine radical adds to the alkene double bond, forming a more stable tertiary alkyl radical. Second, this alkyl radical abstracts a hydrogen atom from hydrogen bromide, regenerating the bromine radical and forming the final product. These steps repeat continuously to sustain the reaction.

Question 3

Why is a tertiary alkyl radical formed instead of a primary one during the addition step?

Accepted Answer

The bromine radical adds to the alkene double bond in a way that produces the more stable tertiary alkyl radical rather than a less stable primary alkyl radical. This regioselectivity occurs because the more substituted carbon can better stabilize the unpaired electron through hyperconjugation and inductive effects from surrounding alkyl groups.

Question 4

What is the initiation phase of the radical anti-Markovnikov mechanism?

Accepted Answer

Initiation consists of two steps. Heat triggers homolytic cleavage of the peroxide O–O bond, generating two alkoxy radicals. An alkoxy radical then abstracts a hydrogen atom from hydrogen bromide, producing a bromine radical that enters the propagation cycle and initiates the chain reaction.

Question 5

How does the radical chain reaction terminate in anti-Markovnikov addition?

Accepted Answer

Termination occurs when two bromine radicals couple together to form a stable bromine molecule. This coupling removes reactive radicals from the system, stopping the chain reaction and preventing further propagation cycles from continuing. The reaction ends when radical concentration becomes too low to sustain propagation.

Question 6

What makes anti-Markovnikov addition different from typical Markovnikov addition of HBr to alkenes?

Accepted Answer

Anti-Markovnikov addition occurs via a radical mechanism in the presence of peroxides, producing opposite regioselectivity compared to ionic Markovnikov addition. The bromine attaches to the less substituted carbon of the alkene, while hydrogen attaches to the more substituted carbon, yielding the anti-Markovnikov product.

Question 7

Why is the bromine radical considered the key propagating species in this mechanism?

Accepted Answer

The bromine radical is the key propagating species because it initiates each cycle by adding to the alkene double bond and is continuously regenerated at the end of each propagation cycle. This regeneration allows the bromine radical to repeat the two-step propagation process many times, sustaining the chain reaction efficiently.

Ajout radical anti-Markovnikov aux alcènes : mécanisme

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