What is Markovnikov's rule in addition reactions?

Markovnikov's rule in addition reactions states that the hydrogen atom will add to the least substituted carbon of the alkene.

Markovnikov's rule is a principle in organic chemistry that is used to predict the product of an addition reaction. It was formulated by the Russian chemist Vladimir Markovnikov in 1869. The rule is particularly applicable to the addition of hydrogen halides to alkenes.

In an addition reaction, two or more molecules combine to form a larger one. Alkenes, which are hydrocarbons with a carbon-carbon double bond, are often involved in these reactions. When a hydrogen halide (HX, where X is a halogen) is added to an alkene, the hydrogen (H) and the halogen (X) can add across the double bond in two different ways, leading to two possible products.

Markovnikov's rule helps predict which of these products will be formed in greater quantity. It states that in such a reaction, the hydrogen atom will add to the carbon atom of the double bond that has more hydrogen atoms already attached to it (the least substituted carbon), while the halogen will add to the other carbon atom (the more substituted carbon).

This rule is based on the stability of carbocations, which are intermediates formed during the reaction. The more substituted a carbocation is, the more stable it is. Therefore, the reaction tends to proceed in a way that forms the more stable carbocation, leading to the product predicted by Markovnikov's rule.

However, it's important to note that there are exceptions to Markovnikov's rule. For example, in the presence of peroxides, the addition of hydrogen bromide (HBr) to alkenes can result in an 'anti-Markovnikov' product, where the bromine adds to the least substituted carbon. This is known as the peroxide effect or Kharasch effect.

In summary, Markovnikov's rule is a useful tool for predicting the major product of addition reactions involving alkenes and hydrogen halides. It's based on the stability of carbocation intermediates and reflects the fundamental principle that reactions tend to proceed in a way that lowers their energy.