Final answer:
To rank the leaving groups in alkyl halides for SN1 and SN2 reaction rates, one must inverse their basicity. Iodide is the best leaving group followed by bromide, chloride, and fluoride. The correct option is 1.
Step-by-step explanation:
When ranking the leaving groups in alkyl halides used in SN1 and SN2 mechanisms according to their reaction rate, it's important to consider the basicity of the leaving groups. The trend is that good leaving groups are generally those that are weak bases. In halides, this means that the leaving group ability increases as we move down the group in the periodic table, since the halide's basicity decreases.
Therefore, the correct ranking of the leaving groups in order of reaction rate is: Iodide > Bromide > Chloride > Fluoride. This order reflects that iodide (I-) is the best leaving group, followed by bromide (Br-), chloride (Cl-), and finally fluoride (F-), which is the poorest leaving group among the halogens for SN1 and SN2 reactions.