Q11P - Organic Chemistry (Mcmurry)

Question

The following cycloalkene gives a mixture of two alcohols on hydroboration followed by oxidation. Draw the structure of both, and explain the result.

Step-by-Step Solution

Answer

Step 1: Hydroboration-oxidation

Hydroboration-oxidation is a process that produce alcohols, involving two steps. Anti-Markonikov rule is followed in which hydrogen from   attaches itself to the more substituted carbon and boron attaches to the least substituted carbon in alkene double bond. Boron accepts two electrons from electron rich alkene,  in its empty p orbital. This Anti-Markonikov’s hydroboration mechanism is stereospecific (syn addition), in which the hydroboration takes place on the same face of the double bond leading to syn product.

Step 2: The given molecule

This structure given in the question is of 1,4-dimethyl cyclopentene. In the hydroboration-oxidation of this molecule boron and hydrogen add from the same face of the double bond as stated above (Syn addition). Boron attaches itself to less substituted carbon atom and during oxidation, it is replace by hydroxyl group

Structures of two alcohols obtained from hydroboration-oxidation are-

 

Step 3: Explanation

As we already know that the addition of borane to the double bond is stereospecific which means, Boron and Hydrogen attach from the same face of the double bond, one where the attack is from above the plane of 1,4-dimethyl cyclopentene and another from below. This is the reason we get racemic mixture of alcohols which can be shown by the following reaction-