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 inversion of configuration (chirality) that occurs during a nucleophilic substitution reaction.
- This phenomenon is characteristic of SN2 bimolecular nucleophilic substitution reactions, where the nucleophile attacks the carbon from the side opposite to the leaving group.
- As a result, the spatial arrangement of atoms around the chiral center gets inverted — this is what we call a Walden inversion.
- SN1 reactions proceed through a planar carbocation intermediate and often lead to racemization rather than inversion.
- Elimination reactions (E1 or E2) do not involve inversion of configuration, as they lead to the formation of alkenes.
- Therefore, only SN2 reactions exhibit Walden inversion.

Answer 2

Walden inversion is the change in the spatial arrangement of atoms at a chiral carbon that happens when a nucleophile displaces a leaving group attached to that same carbon. Let's check each option against this definition.

1. SN1: An SN1 reaction breaks the carbon-leaving group bond first, forming a flat carbocation. The incoming nucleophile can then attack from either face with roughly equal chance, so the product is usually a mix of both configurations (racemization), not a clean, single inversion.
2. SN2: In an SN2 reaction, the nucleophile attacks the carbon from the side directly opposite the leaving group, since that is the only approach not blocked by the group leaving. As the new bond forms and the old one breaks at the same time, the three other groups on that carbon flip through the transition state, much like an umbrella turning inside out in the wind. This single-step, back-side attack is exactly the event Walden originally observed and named after himself.
3. Both SN1 and SN2: Since SN1 tends to give racemization rather than a defined inversion, lumping both mechanisms together as the source of Walden inversion is not accurate.
4. Elimination: E1 and E2 reactions remove a proton and a leaving group to form a double bond. There is no new bond forming at the original stereocenter that could flip its configuration, so elimination does not produce Walden inversion.

Only the back-side attack mechanism of SN2 forces the stereocenter through an inversion every time, which matches Walden's original observation.

So the correct answer is SN2.