Final answer:
Hydroboration mechanism involves the addition of BH3 to an alkene via a four-membered transition state without carbocation intermediates, leading to an anti-Markovnikov organoborane, which can be oxidized to form an alcohol.
Step-by-step explanation:
The mechanistic explanation of hydroboration involves the addition of BH3 across the double bond of alkenes to form trialkylboranes. In this electrophilic addition reaction, the boron atom from BH3, which is electron-deficient due to sp2 hybridization, acts as a Lewis acid and forms a complex with the double bond of the alkene. Because boron has an empty p orbital and is electron-deficient, it seeks to accept a pair of electrons.
The reaction proceeds through a four-membered transition state in which both the boron and one of its attached hydrogen atoms are simultaneously added to the carbon atoms of the double bond. This process adds the elements of BH3 to the alkene without the formation of carbocation intermediates, thus avoiding the rearrangement that could occur in carbocation-based reactions like the Markovnikov-type additions. As a result, hydroboration leads to the formation of an organoborane product that is anti-Markovnikov. The organoborane can then be oxidized to produce an alcohol in a separate step known as oxidation.