Final answer:
The reactivity trend for converting alcohols to alkyl halides increases from primary to secondary to tertiary alcohols, and the reactivity of HX used in the conversion follows the trend HI > HBr > HCl.
Step-by-step explanation:
The reactivity trend for the conversion of an alcohol to an alkyl halide using HX (where X is a halogen) involves evaluating the nature of the alcohol. Primary alcohols (attached to one alkyl group) are generally less reactive than secondary alcohols (attached to two alkyl groups), which are in turn less reactive than tertiary alcohols (attached to three alkyl groups). This trend is due to the stability of the carbocation intermediate that is formed during the reaction. Tertiary alcohols react the fastest because they can form the most stable carbocations. It's also important to note the role of the halogen acid used (HX). Generally, the reactivity of halogen acids follows the trend HI > HBr > HCl, with HI being the most reactive in converting alcohols to their corresponding alkyl halides. The strong acid acts as a catalyst in this reaction and facilitates the removal of an OH group from the alcohol and the subsequent addition of the halogen. In summary, the reactivity of alcohols in conversion to alkyl halides increases with the degree of substitution on the carbon bearing the hydroxy group (tertiary > secondary > primary) and with the reactivity of the hydrogen halide (HI > HBr > HCl).