A reaction such as the above that leads to preferential formation of one of several possible structural isomers is said to be regioselective. The addition of proton acids to alkenes, which follows an ionic mechanism, is an example of a regioselective reaction.
FORMATION OF ANTI-MARKOVNIKOV PRODUCTS VIA FREE RADICAL MECHANISMS.
Ionic mechanisms favor formation of the Markovnikov product through formation of the most stable carbocation. However, one can manipulate conditions to favor formation of the anti-Markovnikov product through a different mechanism, or a different series of steps. This is the case in the addition of HBr to alkenes in the presence of peroxides. Peroxides are well known free radical initiators. This promotes a reaction where the reactants add in a different order and by a different mechanism, in this case with formation of the most stable free radical.
2o bromide, anti-Markovnikov product.
Refer to section 8-3B of the Wade textbook (p. 319 of the 5th ed.) and remember the following points.
a) The anti-Markovnikov addition of proton acids to alkenes works only with HBr.
b) It requires the presence of peroxides, which are free radical initiators.
c) The reaction follows a free radical mechanism, where the bromine is first to add to the alkene with formation of the most stable free radical, which eventually leads to the anti-Markovnikov product.
Other proton acids that follow Markovnikov’s rule include HCl. HI, and H2O (acid-catalyzed) to form alcohols. We now summarize all the important addition reactions of alkenes, including the markovnikov addition of proton acids and water. Then we address some concepts and mechanistic considerations relevant to these reactions.