The Addition of Borane to an Alkene Hydroboration-Oxidation

Another way to convert an alkene to an alcohol is by two successive reactions known as hydroboration-oxidation. 

An atom or molecule does not have to have a positive charge to be an electrophile. Borane (BH3) is an electrophile because boron has an incon tlete octet. Boron uses orbitals to form bonds with hydrogen, so has an en tty p orbital that can accept a share in a pair of electrons (see page 43).Thus, BH3 is the electrophile that adds to the alkene and the nucleophile 

Borane exists primarily as a colorless gas called diborane. Diborane is a dimer—a molecule formed by joining two identical molecules. Because boron does not have a complete octet—it is surrounded by only six electrons—it has a strong tendency to acquire an additional electron pair. In the dimer, therefore, two boron atoms share the two electrons in a hydrogen-boron bond by means of unusual half-bonds. These hydrogen-boron bonds are shown as dotted lines to indicate that they consist of fewer than the normal two electrons. 

Diborane (B2Hg) is a flammable, toxic, and explosive gas. A complex—prepared by dissolving diborane in THF—is a more convenient and less dangerous reagent. One of the lone pairs of the O atom in THF provides the two electrons that boron needs to complete its octet. The borane-THF complex is used as the source of BH3 for hydroboration. 

As the following reactions show, the alcohol formed from hydroboration-oxidation of an alkene has the H and OH groups switched compared to the alcohol formed from the acid-catalyzed addition of water. 

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