Reactions
”When an alkene undergoes electrophilic addition, the electrophile adds to the carbon with
the greatest number of hydrogen substituents. The nucleophile adds to the more highly
substituated carbon.”
Or more simply:
”The species that adds first adds to the carbon with the greatest number of hydrogens.”
The fact that some reactions reliably produce anti-Markovnikov products is actually a pow-
erful tool in organic chemistry. For example, in the reactions we discuss below, we’ll show
two different ways of creating alcohols from alkenes: Oxymercuration-Reduction and Hy-
droboration/Oxidation. Oxymercuration produces a Markovnikov product while Hydrob-
oration produces an anti-Markovnikov product. This gives the organic chemist a choice
in products without having to be stuck with a single product that might not be the most
desired.
55.2.5 Why it works
Markovnikov’s rule works because of the stability of carbocation intermediates^2. Experi-
ments tend to reveal that carbocations are planar molecules, with a carbon that has three
substituents at 120° to each other and a vacantporbital that is perpendicular to it in the
3rd plane. Theporbital extends above and below the trisubstituent plane.
This leads to a stabilizing effect called hyperconjugation. Hyperconjugation is what happens
when there is an unfilled (antibonding or vacant) C-Cπorbital and a filled C-Hσbond
orbital next to each other. The result is that the filled C-Hσorbital interacts with the
unfilledC-Cπorbitalandstabilizesthemolecule. Themorehighlysubstitutedthemolecule,
the more chances there are for hyperconjugation and thus the more stable the molecule is.
Another stabilizing effect is an inductive effect^3.
55.2.6 Exceptions to the Rule
There are a few exceptions to the Markovnikov rule, and these are of tremendous importance
to organic synthesis.
- HBr in Hydrogen Peroxide: Due to formation of free radicals, and the mechanism
in which it reacts, the alkyl free radical forms at the middle atom, where it is most
stable, and a hydrogen attaches itself here. Note here hydrogen addition is the second
step, unlike in the above example.
2 Chapter 118 on page 381
3 https://en.wikibooks.org/wiki/Organic%20Chemistry%2FIntroduction%20to%20reactions%2FInductive%20effect