In `S_(N)1` reaction of `(S)-2` bromobutane with water, two substitution products are formed one has the same relative configuration as the reactant and the other has the inverted configuration. This as in most `S_(N)1` reactions, the racemic product is non-50:50 mixture i.e, racemization takes place, but the inverted product is more than 50% . The formation of different composition of the product is due to following steps involved in `S_(N)` reactions.
`R-X " " to R^(+) X^(-) to R^(+) " " "solvent" X^(-) " " to R^(+)+X^(-)`
`{:("Undissociated Molecule", "Intimate ion pair", "solvent-separated ion pair", "dissociated ions"),(I,II,III,IV):}`
50 : 50 racemic mixture is formed due to the attack of nucleophile on which intermediate species of the reaction ?

Options

A. I
B. II
C. III
D. IV

Answer

To solve the question regarding the formation of a racemic mixture in the \( S_N1 \) reaction of \( (S)-2 \) bromobutane with water, we will break down the process step by step.

### Step-by-Step Solution:

1. **Understanding the Reaction Mechanism**:
- The \( S_N1 \) reaction involves two main steps: the formation of a carbocation intermediate and the nucleophilic attack by the solvent (water in this case).
- The first step is the departure of the leaving group (bromide ion, \( Br^- \)), which leads to the formation of a carbocation.

2. **Formation of the Carbocation**:
- When \( (S)-2 \) bromobutane loses the bromide ion, it forms a secondary carbocation. This carbocation is planar due to sp² hybridization, which means that the nucleophile can attack from either side of the plane.

3. **Nucleophilic Attack**:
- The nucleophile (water) can attack the planar carbocation from either the top or the bottom. This results in two possible products: one that retains the original configuration (same relative configuration) and one that has the inverted configuration (opposite relative configuration).

4. **Formation of Racemic Mixture**:
- Since the attack can occur from either side with equal probability, the products formed will be a racemic mixture (equal amounts of both enantiomers). However, in this specific case, the mixture is not a perfect 50:50 due to the dynamics of the reaction, where the inverted product is favored.

5. **Identifying the Intermediate**:
- The 50:50 racemic mixture is formed due to the attack of the nucleophile on the **dissociated ions**, specifically the carbocation intermediate that is formed after the leaving group departs.

### Final Answer:
The 50:50 racemic mixture is formed due to the attack of the nucleophile on the **dissociated ions**, which corresponds to the carbocation intermediate.

---