AntiMarkownikoff

Anti-Markovnikov addition refers to a regioselective chemical reaction where a hydrogen atom and a substituent add across a carbon-carbon double or triple bond in a way that is opposite to Markovnikov's rule. Markovnikov's rule states that in the addition of a protic acid HX to an alkene or alkyne, the hydrogen atom attaches to the carbon atom with the greater number of hydrogen substituents, and the halide or other substituent attaches to the carbon atom with fewer hydrogen substituents.

In contrast, anti-Markovnikov addition results in the hydrogen atom attaching to the carbon with fewer hydrogen atoms, and the substituent attaching to the carbon with more hydrogen atoms. This type of addition is typically observed in the hydrohalogenation of alkenes when carried out in the presence of peroxides. The mechanism involves a free radical chain reaction where the regioselectivity is dictated by the stability of the intermediate radical. The bromine radical, for example, adds to the less substituted carbon of the double bond to form the more stable secondary or tertiary radical.

Another common example of anti-Markovnikov addition is the hydroboration-oxidation reaction of alkenes. In this process, borane (BH3) adds across the double bond in an anti-Markovnikov fashion, followed by oxidation with hydrogen peroxide, which replaces the boron atom with a hydroxyl group, also resulting in anti-Markovnikov regiochemistry. This reaction is valuable for synthesizing primary alcohols from alkenes. The key distinction lies in the outcome of where the hydrogen and the additional atom or group attach to the unsaturated carbon chain.