`S_N2` reaction leads to

Options

A. inversion of configuration
B. retention of configuration
C. partial racemisation
D. no racemisation

Explanation

To solve the question regarding what an S_N2 reaction leads to, we will follow these steps:

### Step 1: Understand the S_N2 Mechanism
The S_N2 (Substitution Nucleophilic Bimolecular) reaction is a type of nucleophilic substitution reaction where a nucleophile attacks an electrophile (usually an alkyl halide) from the opposite side of the leaving group.

**Hint:** Remember that S_N2 stands for bimolecular, indicating that both the nucleophile and the substrate are involved in the rate-determining step.

### Step 2: Identify the Reactants
In our example, we consider chloroethane (C2H5Cl) as the alkyl halide and hydroxide ion (OH⁻) as the nucleophile. The reaction can be represented as follows:

\[ \text{C}_2\text{H}_5\text{Cl} + \text{OH}^- \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{Cl}^- \]

**Hint:** Look at the structure of the alkyl halide and the nucleophile to visualize how they will interact.

### Step 3: Mechanism of the Reaction
In the S_N2 mechanism, the nucleophile attacks the carbon atom bonded to the leaving group (Cl) from the backside. This backside attack leads to the formation of a transition state where the carbon is simultaneously bonded to both the nucleophile and the leaving group.

**Hint:** Visualize the transition state as a temporary structure where the carbon is forming bonds with both the nucleophile and the leaving group.

### Step 4: Inversion of Configuration
As the nucleophile (OH⁻) attacks from the backside, the leaving group (Cl) departs from the front side. This results in the inversion of the stereochemistry at the carbon center. If the original configuration was R, it will become S, and vice versa.

**Hint:** Remember that inversion of configuration means that the spatial arrangement of the substituents around the carbon atom is flipped.

### Step 5: Conclusion
Thus, the S_N2 reaction leads to **inversion of configuration** at the carbon atom where the substitution occurs. This is a defining characteristic of the S_N2 mechanism.

**Final Answer:** The S_N2 reaction leads to **inversion of configuration**.

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