A primary alkyl halide would prefer to undergo :-

Options

A. `S_(N^1)` reaction
B. `S_(N^2)` reaction
C. `prop`-elimination
D. racemisation

Explanation

To determine the preferred reaction pathway for a primary alkyl halide, we will analyze the options provided: SN1, SN2, alpha elimination, and racemization.

### Step-by-Step Solution:

1. **Understanding SN1 and SN2 Reactions**:
- **SN1 Reaction**: This is a first-order nucleophilic substitution reaction. It involves the formation of a carbocation intermediate. The rate of the reaction depends only on the concentration of the alkyl halide. The order of reactivity for alkyl halides in SN1 reactions is tertiary > secondary > primary. Therefore, primary alkyl halides do not favor SN1 reactions due to the instability of the primary carbocation.
- **SN2 Reaction**: This is a second-order nucleophilic substitution reaction. It involves a direct attack by the nucleophile on the carbon atom bonded to the leaving group (halide) in a single step. The order of reactivity for alkyl halides in SN2 reactions is primary > secondary > tertiary. Primary alkyl halides react rapidly in SN2 reactions because there is less steric hindrance.

2. **Analyzing the Options**:
- **SN1 Reaction**: As established, primary alkyl halides do not undergo SN1 reactions due to the instability of primary carbocations.
- **SN2 Reaction**: Primary alkyl halides prefer SN2 reactions because they can easily undergo backside attack by the nucleophile, leading to an inversion of configuration.
- **Alpha Elimination**: This reaction involves the removal of both a hydrogen atom and a halide from the same carbon atom. This process is not favored for primary alkyl halides due to the formation of unstable intermediates.
- **Racemization**: This occurs in SN1 reactions where a racemic mixture is formed due to the planar nature of the carbocation intermediate. Since primary alkyl halides do not undergo SN1 reactions, racemization is not applicable.

3. **Conclusion**:
- Based on the analysis, the preferred reaction pathway for a primary alkyl halide is the **SN2 reaction**.

### Final Answer:
A primary alkyl halide would prefer to undergo **SN2 reaction**.