Chemistry

Chemistry

Problem

In the hydrolysis of a single isomer of an optically active halogenoalkane, the optical purity of the product will depend on whether the reaction happens by the SN1\text{S}_{\text{N}}1 or SN2\text{S}_{\text{N}}2 mechanism. a. Explain why the SN2\text{S}_{\text{N}}2 mechanism produces an optically pure product. b. Explain why the SN1\text{S}_{\text{N}}1 mechanism produces a racemic mixture.
Intuitive explanation

The key is understanding how each mechanism treats the stereochemistry at the chiral center. In SN2\text{S}_{\text{N}}2, the nucleophile attacks from the back, inverting configuration completely—so if you start pure, you end pure (opposite enantiomer). In SN1\text{S}_{\text{N}}1, the carbocation intermediate is planar and achiral, allowing attack from both faces; this gives a racemic or partially racemic mixture, drastically lowering optical purity. The mechanism you follow determines whether stereochemistry is preserved or scrambled.

Intuitive explanation

The key is understanding how each mechanism treats the stereochemistry at the chiral center. In SN2\text{S}_{\text{N}}2, the nucleophile attacks from the back, inverting configuration completely—so if you start pure, you end pure (opposite enantiomer). In SN1\text{S}_{\text{N}}1, the carbocation intermediate is planar and achiral, allowing attack from both faces; this gives a racemic or partially racemic mixture, drastically lowering optical purity. The mechanism you follow determines whether stereochemistry is preserved or scrambled.

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