Chapter 12, Problem 12.21P

Interpretation Introduction

Interpretation:

The detailed mechanism for the formation of the monoalkylborane in the given reaction is to be drawn. Also, the structure of trialkylborane that is ultimately produced is to be drawn.

Concept introduction:

Hydroboration is the addition of $\text{H}$ and ${\text{BH}}_2$ groups across the carbon-carbon double bond in an alkene. These are stereospecific reactions in which the two said groups are added in a syn fashion. Hydroboration is also regioselective, where the hydrogen atom adds to the carbon atom with the greater number of alkyl groups (i.e., the most substituted carbon atom in the double bond), whereas the ${\text{BH}}_2$ group adds to the carbon atom with the fewer number of alkyl groups (i.e., the least substituted carbon atom in the double bond). Thus, the addition is said to occur according to anti-Markovnikov’s rule.

As with the addition of ${\text{BH}}_3$, each of these is a syn addition, and the bulkier B-containing portion adds preferentially to the less sterically hindered doubly bonded carbon atom. Once the trialkylborane is formed, it is then treated with a basic solution of hydrogen peroxide to convert it into an alcohol.