Final answer:
In SN1 reactions, reactivity of alkyl halides increases with more stable carbocations and better leaving groups, ranking from primary < secondary < tertiary alkyl halides, and with halide leaving groups in the order of F- < Cl- < Br- < I-.
Step-by-step explanation:
The SN1 reactivity of alkyl halides increases with the stability of the carbocation that is formed during the reaction. The stability of carbocations increases in the order of primary (RCH₂) < secondary (R₂CH) < tertiary (R₃C). It is also affected by the leaving group; the better the leaving group, the higher the reactivity in SN1 reactions.
In the SN1 reaction mechanism, the leaving group departs before the nucleophile attacks, forming a carbocation intermediate. Thus, the rate of reaction depends on the stability of this intermediate. Tertiary alkyl halides form the most stable carbocations and are thus more reactive in SN1 reactions compared to secondary and primary alkyl halides. Additionally, halides like iodide (I-) are better leaving groups compared to bromide (Br-), chloride (Cl-), and fluoride (F-).
Given the typical reactivity trends and without specific compounds to rank, you would expect that a tertiary iodide would be the most reactive, followed by a tertiary bromide, tertiary chloride, and so on down to a primary fluoride, which would be the least reactive in SN1 reactions. Always consider both the stability of the carbocation and the quality of the leaving group when ranking the reactivity of alkyl halides in SN1 reactions.