Preparation of Alcohols via Addition Reactions

Markovnikov's rule states that during addition reactions, the hydrogen adds to the less-substituted carbon of the double bond, while the hydroxyl group adds to the more-substituted carbon. This regioselective preference occurs because the proton bonds to the less-substituted end first, creating a more stable carbocation intermediate that is subsequently attacked by water or hydroxide.

Acid-catalyzed hydration produces a racemic mixture because the carbocation intermediate is trigonal planar. Water can attack this planar carbocation from either face of the molecule with equal probability, generating both enantiomers of the alcohol product in equal amounts, resulting in a racemic mixture.

Oxymercuration-demercuration avoids forming a conventional carbocation intermediate. Instead, mercury acetate forms a three-membered mercurinium ion, which water attacks anti to the mercury addition. Sodium borohydride then replaces mercury with hydrogen. This mechanism prevents carbocation rearrangement, yielding better product yields than acid-catalyzed hydration.

Hydroboration-oxidation is anti-Markovnikov because borane adds to the less-substituted carbon of the double bond due to steric effects, while hydrogen adds to the more-substituted carbon. This reverses the typical Markovnikov pattern. The boron is then oxidized and replaced with a hydroxyl group, yielding the anti-Markovnikov alcohol product.

A mercurinium ion is a three-membered ring intermediate formed when the electrophilic mercury in mercury acetate attacks the double bond of an alkene. This intermediate can be considered a resonance hybrid with minor carbocation character. It forms because the positively-charged mercury creates a strong electrophile that the π electrons of the alkene attack, stabilizing the reaction pathway.

Borane initially adds to one alkene molecule through a four-membered transition state, forming an alkylborane. This intermediate retains reactive B-H bonds that can sequentially add to two additional alkene molecules, ultimately producing a trialkyl borane. This trialkyl borane is then oxidized with hydrogen peroxide and sodium hydroxide to yield the final alcohol product.

Carbocation rearrangement occurs when a more-substituted carbon is adjacent to the double bond. The positive charge in the carbocation intermediate can shift via 1,2 hydride shift or 1,2 methyl shift to form a more stable carbocation. This rearrangement produces a mixture of constitutional isomers as products, reducing the selectivity of acid-catalyzed dehydration of alcohols to alkenes.