Markovnikov's rule

In chemistry, Markovnikov's rule is an observation based on Zaitsev's rule. It was formulated by the Russian chemist Vladimir Vasilevich Markovnikov .It states that, in chemical reactions found particularly in organic chemistry, when a hydrogen halide reacts with the carbon-carbon double bond of an asymmetrical alkene, giving an alkyl halide, the hydrogen adds to the carbon of the alkene functional group that has the greater number of hydrogen substituents, and the halogen adds to the carbon on the other end of the double bond which has a smaller number of hydrogen substituents.

This illustrates Markovnikov's rule by the reaction of Propene with HBr, major product shown

The same is true when an alkene reacts with water in an addition reaction to form alcohol. The hydroxyl group (OH) bonds to the carbon that has the greater number of carbon-carbon bonds, while the hydrogen bonds to the carbon on the other end of the double bond, that has more carbon-hydrogen bonds.

However, the real reason that the hydrogen adds on to the carbon with the most hydrogens ("the rich get richer") is because the positive charge wants to be in the most stable form. The positive charge sits comfortably in the center of the molecule because of induction. This form of the molecule is known as the most stable carbocation (pronounced as 'carbo cat ion'). Therefore when a molecule of the form HX (where X is more electronegative than H) is added in an addition reaction to a carbon-carbon double bond, the H is added to the less substituted carbon atom, while the X is added to the more substituted. The stability of the carbocation reactive intermediate directly causes this rule. The product whose halide and hydrogen are added the opposite way around is produced as a minor reaction product because the intermediate carbocation is formed nonetheless, and (although unlikely) the electrophilic addition may produce the alternate product.

The rule may be summed up by quoting that the rich get richer and the poor get poorer, in that a carbon rich in substituents will get more substituents and the carbon with more hydrogens attached will get the hydrogen in case of many different organic addition reactions.

Mechanisms which avoid the carbocation intermediate may react through other mechanisms that are regioselective, against what Markovnikov's rule predicts: such as free radical addition. Such reactions are said to be anti-Markovnikov, since the halogen adds to the least substituted carbon. Again, like the positive charge, the radical prefers to be in the center of the molecule where it is most stable.