Which reagents can be used for the substitution of alcohols to form halogenoalkanes?

The formation of halogenoalkanes from alcohols typically involves a substitution reaction where the -OH group of the alcohol is replaced by a halogen atom. The correct choice involves reagents that can effectively facilitate this substitution process.

Using HCl (hydrochloric acid) provides a source of chloride ions, which can directly substitute the hydroxyl group of the alcohol to form a halogenoalkane. This reaction generally occurs under acidic conditions, helping to protonate the alcohol, making the -OH group a better leaving group, and facilitating the formation of the halogenoalkane.

Thionyl chloride (SOCl2) is another reagent that is particularly effective for this conversion. Thionyl chloride reacts with alcohols to create an intermediate (an alkyl chloride) while producing byproducts that are gases (SO2 and HCl), making it a clean reaction. This mechanism is often favored because it proceeds via an efficient pathway resulting in high yields of the desired halogenoalkane.

In contrast, the other options listed either do not provide adequate halogen sources for substitution reactions or are used for different types of reactions involving oxidation or other functional group transformations, rather than direct halogenation of alcohols.