Complete the following analogy:

$S_N2$: Inversion of configuration;  $S_N1$: ______

Racemisation

The correct answer is Option (1) → Racemisation

$S_N2$ and $S_N1$ reactions differ fundamentally in their reaction mechanism and stereochemical outcome.

$S_N2$ Reaction

$S_N2$ proceeds through a single-step backside attack.

The nucleophile attacks from the opposite side of the leaving group, causing:

Inversion of configuration

Also known as Walden inversion

Thus, $S_N2$ gives 100% inversion.

$S_N1$ Reaction

$S_N1$ proceeds through a two-step mechanism:

Step 1: Formation of carbocation intermediate

Step 2: Nucleophile attacks the planar carbocation

The carbocation formed is planar (sp² hybridised).

Since it is planar, the nucleophile can attack from:

Front side

Back side

This results in:

Retention of configuration (from one side attack)

Inversion of configuration (from the opposite side attack)

Because both occur, the final product is a mixture of enantiomers.

This leads to racemisation.                 

Why other options are incorrect:

Option 2 Retention

$S_N1$ does not give pure retention; it gives a mix of retention and inversion.

Option 3 Optical rotation

This is a property, not a stereochemical outcome.

Option 4 Chiral

Chirality is not the stereochemical result of $S_N1$ mechanism.

Thus, the stereochemical outcome of $S_N1$ reaction is racemisation.

Final Answer: Racemisation