Addition Reactions to Carbon/Carbon Multiple Bonds
Addition reactions represent one of the simplest types of organic mechanisms. In the one step addition of two molecules to each other, it is reasonable to suggest that such an addition occurs via a cyclic transition state, with the new bonds being formed simultaneously with the old bonds being broken. The addition of a small symmetrical molecule, such as bromine, to a symmetrical substrate, such as ethene, is the simplest type of two step addition reaction. Probably the most important reason is that the cyclic bromonium ion that contains a carbon/carbon double bond is more strained than the saturated version, and hence is more unstable, and so less readily formed. The addition of an unsymmetrical molecule to an unsymmetrical alkene is governed by Markovnikov’s rule, which states that the positive portion of the reagent goes to the side of the double or triple bond that has more hydrogens. Radical addition is usually syn in nature, and anti-Markovnikov in regiochemistry.