Which reagent is used to prepare primary alkyl halides?

To prepare primary alkyl halides, thionyl chloride is a highly effective reagent. The reaction between a primary alcohol and thionyl chloride results in the formation of an alkyl chloride. This process involves the conversion of the alcohol group (-OH) into a leaving group, which is then substituted by chloride ion to form the primary alkyl halide.

Thionyl chloride acts as a reagent that facilitates this transformation by forming an intermediate that allows for the efficient substitution of the hydroxyl group, leading to a more favorable reaction pathway. Moreover, thionyl chloride is ideal for this reaction because it produces byproducts that are gases (such as sulfur dioxide and hydrochloric acid), making the reaction easier to drive to completion.

In contrast, hydrogen bromide is generally used to prepare alkyl bromides and is not specifically geared towards primary alcohols. Bromine is a halogen that can lead to various reactions not limited to primary alkyl halide formation, and phosphorus tribromide, while it can convert alcohols to alkyl bromides, is less commonly used compared to thionyl chloride for the preparation of primary alkyl halides.