Advices For Studying Organic Chemistry

A Mechanism for the Reaction

Anti-Markovnikov Addition
Chain Initiation

Step 1

RROO heat^2 RO

∆H° ≅ + 150 kJ mol–1

Heat brings about homolytic cleavage of the weak oxygen-oxygen bond.

Step 2

R O + H Br R O H + Br ∆H° ≅ – 96 kJ mol–1

Eact is low

The alkoxyl radical abstracts a H-atom from HBr, producing a Br-atom.

Step 3 Br +H^2 CCHCH^3 Br CH^2 CH CH^3
2° radical
A Br-atom adds to the double bond to produce the more stable 2° radical.

Step 4

Br CH 2 CH CH 3 +H Br Br CH 2 CH CH 3

H

+ Br

1-Bromopropane
The 2° radical abstracts a H-atom from HBr. This leads to the product and
regenerates a Br-atom. Then repetitions of steps 3 and 4 lead to a chain reaction.

(^)

10.9A SUMMARY OF MARKOVNIKOV VERSUS ANTI-MARKOVNIKOV

ADDITION OF HBr TO ALKENES

1. In the absence of peroxides, the reagent that attacks the double bond first is a
   proton.

1. Proton is small ⇒ steric effects are unimportant.


2. Proton attaches itself to a carbon atom by an ionic mechanism to form the more
   stable carbocation ⇒ Markovnikov addition.
   Ionic addition