In the synthesis of alkyl halides from alcohols:?

Question

Why is an excess of concentrated HCl necessary?
And whys is it necessary for the concentrated HCl to be cold?
Why is solid NaHCO3 used instead of liquid NaHCO3?
Why should the alkyl halide be dried carefully with anhydrous CaCl2 before distillation?

Answer 1

You don't say, but apparently you're considering the synthesis of tert-butyl chloride from tert-butyl alcohol and conc. HCl. You use excess HCl, because you want to drive the equilibrium to maximize the formation of halide. You consider the alcohol and halide to be more valuable than HCl. Warm HCl risks dehydrating the alcohol to isobutylene, (CH3)2C=CH2. Solid NaHCO3 neutralizes excess HCl adequately. What you do not want is to have the fresh tert-butyl chloride exposed to a lot of water, which would hydrolyze a lot of it back to alcohol. The same with drying with CaCl2 before distillation. Without drying, distillation would "cook" the halide with moisture, hydrolyzing a lot of it.

Answer 2

Synthesis Of Alkyl Halides