2-Butanone is reduced by hydride ion donors, such as sodium...

2-Butanone is reduced by hydride ion donors, such as sodium borohydride $\left(\mathrm{NaBH}_{4}\right),$ to the alcohol 2 -butanol. Even though the alcohol has a chiral center, the product isolated from the redox reaction is not optically active. Explain.

Key Concepts

Optical Activity and Chirality

Optical activity is a property of chiral molecules where they can rotate plane-polarized light. However, when two enantiomers are present in equal amounts, their optical activities cancel each other out, rendering the overall mixture optically inactive. This explains why, despite forming a chiral alcohol, the product of the reduction reaction is not optically active.

Racemic Mixture Formation

A racemic mixture is a 1:1 combination of two enantiomers that are mirror images of each other. Since the attacking hydride ion can approach the planar carbonyl from either side with equal likelihood, both enantiomers are formed in equal amounts. This equimolar formation leads to a racemic mixture, inherently lacking optical activity.

Planar Carbonyl Intermediate

In the reduction reaction, the hydride ion attacks the carbonyl carbon, which is sp2-hybridized and planar. This geometry allows the hydride to approach from either of the two faces of the carbonyl group with equal probability, leading to the formation of two different stereoisomers.

Nucleophilic Addition Mechanism

The reaction between the hydride donor and the carbonyl group is a classic nucleophilic addition mechanism. Here, the hydride ion acts as a nucleophile, attacking the electrophilic carbonyl carbon. Due to the planar nature of the carbonyl, there is no inherent bias in the direction of attack, which results in the formation of a 1:1 mixture of enantiomers.

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