antiMarkownikov

The term antiMarkownikov refers to a regiochemical outcome in addition reactions, specifically the addition of a protic acid (like HBr) or a halogen (like Br2) across an alkene or alkyne. This outcome is the opposite of Markovnikov's rule, which states that the hydrogen atom of the protic acid will attach to the carbon atom of the double or triple bond that already has the greater number of hydrogen atoms. In an antiMarkownikov addition, the hydrogen atom attaches to the carbon atom with fewer hydrogen atoms, and the other atom (e.g., the halogen or the hydroxyl group in hydroboration-oxidation) attaches to the carbon atom with more hydrogen atoms.

AntiMarkownikov additions are often observed in radical addition reactions. For instance, the addition of HBr to an alkene in the presence of peroxides proceeds via a radical mechanism, resulting in the antiMarkownikov product. This is because the radical initiator abstracts a hydrogen atom from HBr, forming a bromine radical. This bromine radical then attacks the alkene at the less substituted carbon, forming a more stable secondary or tertiary radical intermediate. This intermediate then abstracts a hydrogen atom from another HBr molecule, propagating the chain and yielding the antiMarkownikov product.

Another important example of antiMarkownikov addition is hydroboration-oxidation. In this reaction, borane (BH3) adds across the double or triple bond. The boron atom, being less electronegative than hydrogen, attaches to the less substituted carbon, and the hydrogen atom attaches to the more substituted carbon. Subsequent oxidation with hydrogen peroxide in a basic solution replaces the boron atom with a hydroxyl group, leading to an alcohol with antiMarkownikov regiochemistry.