Question

Draw the structures of two stereoisomeric alkenes that would give 3-hexanol as the only major product of hydroboration followed by treatment with alkaline H2O2:

Answer 1

Concepts and reason

The product of the hydroboration reaction of stereoisomeric alkenes that is the structure of an alcohol (3-hexanol) is given. The structure of the stereoisomeric alkenes needs to be drawn.

The alkenes display stereoisomerism in two form that is cis and trans. Hence, draw the cis and trans isomers to form the two alkenes.

Fundamentals

Hydroboration reaction of alkenes: The hydroboration of alkenes takes place in the presence of borane $\left( {{{\rm{B}}_2}{{\rm{H}}_6}} \right)$ and hydrogen peroxide in alkaline medium $\left( {{{\rm{H}}_2}{{\rm{O}}_2}/{\rm{O}}{{\rm{H}}^ - }} \right)$ to form alcohols. The reaction occurs in anti-Markovnikov’s fashion where the addition of hydroxyl group takes place on the carbon having more number of protons.

Geometrical isomerism: This is shown by the compounds having similar molecular formulae but different arrangement of atoms around the double bond. Hence, alkenes show geometrical isomerism which is cis (similar groups present on same side of double bond) and trans (similar groups present on opposite side of double bond).

The cis isomer of the given alcohol is as follows:

The trans isomer of the given alcohol is as follows:

Ans:

The structure of the two stereoisomeric alkenes is as follows:

Answer 2

ans. cis and trans alkenes