1.Addition of Unsymmetrical Reagents

In these reactions, one part of the molecule attaches itself to one carbon atom whereas the other part to the second carbon atom of the double bond. For example, 

However, if the alkene is unsymmetrical, then two products are possible. For example, when propene is treated with HBr, both 1-bromopropane and 2-bromopropane may be formed as the products.

In such cases, the addition is governed by Markovnikov's Rule proposed by a Russian chemist Markovnikov in 1869. It states that:

During the addition of an unsymmetrical reagent across unsymmetrical multiple bond, the negative part of the attacking reagent joins with that carbon atom of the multiple bond which carries lesser number of hydrogen atoms. For example, in case of addition of HBr to propene, 2-bromopropane is the major product.