Question

Why must the crude alkyl halide product be dried carefully with anhydrous `CaCl_2` before distillation?

Answer 1

Because any water contaminating the alkyl halide, will likely also contaminate the distillate.

Explanation:

Water often azeotropes with organic compounds, sometimes at much lower temperatures than `100` `""^@C`. Thus, it makes sense to dry the alkyl halide before distillation, especially if the halide is going to be used for say a Grignard or lithium reagent reaction. If you are looking for practical advice, typically basic alumina is thrown into the bottle, which will mop up the water, and also bind any hydrogen chloride.