Q24E - Organic Chemistry (Mcmurry)

Question

Name and assign R or S stereochemistry to the product (s) you would obtain by reaction of the following substance with ethylmagnesium bromide. Is the product chiral? Is it optically active? Explain.

Step-by-Step Solution

Answer

A mixture of diastereomers (3R,4S)-3,4-Dimethyl-3-hexanol and (3S,4S)-3,4-Dimethyl-3-hexanol are formed that are chiral and are optically active compounds.

Step 1: Grignard addition to carbonyl compounds

When Grignard reagents (RMgX) are added to ketones and esters, a tertiary alcohol product is formed. With aldehydes, the product obtained is secondary alcohols, and with formaldehyde, a primary alcohol product is observed.

Step 2: The reaction of the substance with ethylmagnesium bromide and its stereochemistry

When 3-methylpentan-2-one reacts with ethylmagnesium bromide, a mixture of the product of (3R,4S) and (3S,4S) diastereomers are formed. These are formed in unequal amounts and are optically active. So, it is difficult to predict which diastereomer will predominate.

                 The reaction of the substance with ethylmagnesium bromide