| Chemistry
3719 - Dr.
Peter Norris Radical Addition to Alkenes |
| Radical Addition reactions are transformations in which reagents such as HBr are added to compounds containing multiple (double or triple) bonds in the presence of a radical initiator such as a peroxide. The mechanism goes via radical intermediates, the formation of which is dictated again by relative stability (tertiary>secondary>primary>methyl). The peroxide is broken into radicals (initiation) which then abstract an H atom from HBr and generate Br radical (propagation). The Br radicals add to the alkene double bond to give carbon radicals (propagation), which in turn abstract an H atom from HBr to give the addition product. |
|
Free Radical Addition of HBr
to Alkenes using Peroxides - Overall reaction
|
| The
reaction is initiated by the
peroxide
molecule breaking into two radicals (homolytic cleavage of the O-O
bond).
These oxygen radicals, which are very unstable, then abstract an H atom
from HBr to generate Br radicals (propagation step).
|
|
The
reactive Br radical then adds to the
double bond of the alkene to form a new C-Br bond and a carbon radical.
When different radicals are possible, the most stable is favoured
(terttiary>secondary>primary>CH3)
and leads to the major product (Markovnikoff
rule).
|
| In
the final step the C radical abstracts
an H atom from HBr and forms the addition product.
|
| The
following animation puts these events
in sequence:
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|
When
unsymmetrical substrates are employed, Markovnikoff
addition
occurs. Essentially, the major product of addition
will be the one that results from the most stable radical intermediate.
For example in the above reaction:
|
| Peter
Norris, Ph.D. |
