Ch 6: Markovnikov's rule

Chapter 6: Reactions of Alkenes: Addition Reactions

Markovnikov's Rule

Markovnikov's rule (1870)

This is an empirical rule based on Markovnikov's experimental observations on the addition of hydrogen halides to alkenes.

The rule states that :

"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 is illustrated by the following example:
example of Markovnikov's rule
Look at the position of the H and the Br in relation to the statement of Markovnikovs rule given above. Note that the major product, which is often referred to as the Markovnikov product, is the more highly substituted alkyl halide. In the example above, the secondary bromide is formed in preference to the primary bromide.

Modern mechanistic knowledge indicates reaction occurs via protonation of the alkene to give the more stable carbocation intermediate:

protonation of propene can give two carbocations
Here we see that in principle, propene can protonate to give two different carbocations, one 2^o and the other 1^o.
Formation of the more stable 2^o carbocation is preferred.

The carbocation then reacts with the nucleophile (e.g. the halide ion) to give the alkyl bromide and hence 2-bromopropane is the major product.

bromide reactions with the carbocation

Although Markovnikov's rule was developed for and is specifically applied to the addition of hydrogen halides to alkenes, many other additions are also described as Markovnikov or anti-Markovnikov depending on the regioselectivity of the addition reaction, e.g. the acid catalysed hydration of alkenes (Markovnikov) and the hydroboration / oxidation of alkenes (anti-Markovnikov).

In more general terms, Markovnikov's rule can be "modernised" to cover other addition reactions by considering that the electrophile adds to the least substituted end of the alkene giving rise to the more stable intermediate. So let's rephrase our statement of Markovnikov's rule:

"when an unsymmetrical alkene undergoes addition with E-Nu, then the electrophile, E, adds to the carbon of the alkene that has the greater number of hydrogen substituents, and the nucleophile, Nu, to the carbon of the alkene with the fewer number of hydrogen substituents"

Therefore, the key is to recognise the electrophilic portion of the reagent as it adds to the π bond first so as to give the more stable intermediate.