Markovnikov’s Rule55.2 Markovnikov’s Rule
Beforewecontinuediscussingreactions, weneedtotakeadetouranddiscussasubjectthat’s
very important in Alkene reactions, ”Markovnikov’s Rule.” This is a simple rule stated by
the Russian Vladmir Markovnikov in 1869, as he was showing the orientation of addition
of HBr to alkenes.
His rule states:”When an unsymmetrical alkene reacts with a hydrogen halide to give an
alkyl halide, the hydrogen adds to the carbon of the alkene that has the greater number of
hydrogen substituents, and the halogen to the carbon of the alkene with the fewer number
of hydrogen substituents” (This rule is often compared to the phrase: ”The rich get richer
and the poor get poorer.” Aka, the Carbon with the most Hydrogens gets another Hydrogen
and the one with the least Hydrogens gets the halogen)
This means that the nucleophile of the electophile-nucleophile pair is bonded to the position
most stable for a carbocation, or partial positive charge in the case of a transition state.
55.2.1 Examples
CH 2 =CH−CH 3 +H−Br−>CH 3 −CHBr−CH 3 Here the Br attaches to the mid-
dle carbon over the terminal carbon, because of Markovnikov’s rule, and this is called a
Markovnikov product.
55.2.2 Markovnikov product
The product of a reaction that follows Markovnikov’s rule is called a Markovnikov product.
55.2.3 Markovnikov addition
Markovnikov addition is an addition reaction which follows Markovnikov’s rule, producing
a Markovnikov product.
55.2.4 Anti-Markovnikov addition
Certain reactions produce the opposite of the Markovnikov product, yielding what is called
anti-Markovnikov product. That is, hydrogen ends up on the more substituted carbon of
the double bond. The hydroboration/oxidation reaction that we’ll discuss shortly, is an
example of this, as are reactions that are conducted in peroxides.
A modernized version of Markovnikov’s rule often explains the ”anti-Markovnikov” behavior.
The original Markovnikov rule predicts that the hydrogen (an electrophile) being added
across a double bond will end up on the carbon with more hydrogens. Generalizing to
all electrophiles, it is really the electrophile which ends up on the carbon with the greatest
number of hydrogens. Usually hydrogen plays the role of the electrophile; however, hydrogen
can also act as an nucleophile in some reactions. The following expansion of Markovnikov’s
rule is more versitile: