Question

Walden inversion includes:

• A. SN1
• B. SN2
• C. Both SN1 and SN2
• D. Elimination

Hint:

In SN2 reactions, the backside attack by the nucleophile causes Walden inversion — key for chirality-based GPAT questions!

Answer 1

The Correct Option is B

Walden inversion refers to the flip in configuration at a stereocenter during a substitution reaction, first noticed by Paul Walden when he converted one optically active malic acid into its mirror image through a series of substitution steps. We need to find which mechanism causes this flip.

1. SN1: goes through a planar carbocation intermediate, so the nucleophile can add from either side. The result is a mixture of both configurations, not one clean inverted product.
2. SN2: is a one-step process where the nucleophile pushes in from the back of the leaving group. This back-side push is what forces the other three groups to swing to the opposite side, exactly the kind of clean inversion Walden recorded.
3. Both SN1 and SN2: cannot be correct together because SN1 gives racemization, not a reliable inversion.
4. Elimination: removes a beta-hydrogen and the leaving group to build a pi bond, so there is no substitution event at the stereocenter to invert in the first place.

The correct choice is SN2, since it is the only mechanism among these that always inverts the configuration at the reacting carbon.