= Solution
All comparisons assume the same temperature and comparable reagent thermodynamic activities in <methanol>.
* \b[Iodomethane:] faster, because iodide is a better <leaving group> and the C-I bond is more readily broken than C-Cl.
* \b[1-Chloropropane:] slower than methyl chloride for <SN2>, because its ethyl substituent crowds backside approach and raises the substitution barrier. It is still a primary substrate, so <SN2> remains plausible.
* \b[MeSNa:] generally faster substitution with methanethiolate than methoxide in this <protic solvent>. Sulfur is more polarizable and its larger anion is less strongly hydrogen-bond-solvated, making it a better <nucleophile> here. This must not be inferred simply from basicity: methanethiolate is the weaker base.
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