Water can be added to an alkene in such a way that the major product is not that predicted by the Markovnikov rule. An example of such a reaction is the indirect addition of water to an alkene via a hydroboration‐oxidation reaction. In this reaction, a disubstituted boron hydride is added across the carbon‐carbon double bond of an alkene. The resulting organoborane compound is oxidized to an alcohol by reaction with hydrogen peroxide in a basic media, such as aqueous sodium hydroxide solution.
No carbocation intermediate forms during this reaction. Although the elements of water are added to an alkene, water is not a reactant; the hydrogen comes from a boron hydride molecule, and the hydroxide group comes from a peroxide molecule.
The first step in the hydroboration mechanism is the formation of the organoborane molecule from the alkene. This reaction occurs rapidly. The boron atom generally bonds to the less substituted, and thus less sterically hindered, carbon. This first step proceeds via a reaction between the disubstituted organoborane and the π bond of the alkene, followed by formation of a C−H bond via a four‐center interaction. A four‐center interaction is a reaction in which bonds between four atoms are created and broken simultaneously.
The alkylborane then undergoes a three‐stage oxidation reaction to form the alcohol. In the first step, a hydroperoxide anion, formed by the reaction of a hydroxide ion with a peroxide molecule, adds to the electron‐deficient boron atom.
This intermediate is unstable and rearranges, losing a hydroxide ion to form a borate ester.
The borate ester then reacts with alkaline hydrogen peroxide to produce a trialkyl borate.
Finally, the trialkyl borate is hydrolyzed (which means split by the elements of water) to alcohols and a borate ion by the aqueous hydroxide ion.