During S_N1 reaction which of the following statement is correct

The correct answer is Almost racemization.

To understand why this is correct, let us analyze the step-by-step mechanism of an S_N1 (Substitution Nucleophilic Unimolecular) reaction:

1. Formation of the Carbocation Intermediate:
In the first rate-determining step, the bond between the carbon atom and the leaving group (X) breaks, leading to the formation of a carbocation. The carbocation intermediate has an sp² hybridized carbon with a planar geometry.

2. Nucleophilic Attack:
Because the carbocation is flat and planar, a nucleophile (Nu⁻) can theoretically attack the carbon from either the front side (the side from which the leaving group departed) or the back side (the opposite side) with equal probability.
• Attack from the front side leads to retention of configuration.
• Attack from the back side leads to inversion of configuration.
If the carbocation were completely free and isolated, this equal probability of attack would result in exactly 100% racemization (an equimolar mixture of both enantiomers).

Why is it "Almost Racemization"?
In reality, the leaving group does not immediately drift far away from the carbocation. Instead, it remains in close proximity for a brief moment, forming what is known as an intimate ion pair:

$R^{+}···X^{-}$

Because the negatively charged leaving group (X⁻) is still hovering close to the front side of the carbocation, it partially shields or blocks that side. This makes front-side attack by the incoming nucleophile slightly hindered. As a result, back-side attack (inversion) occurs more frequently than front-side attack (retention).

Consequently, the reaction yields a mixture containing both enantiomers, but with a slight excess of the inverted product. Therefore, instead of complete 100% racemization, the process is described as almost racemization (or racemization accompanied by partial inversion).