What are ambident nucleophiles? Explain with an example.

Which compound in each of the following pair will react faster in ${\mathrm{S}}_{\mathrm{N}}2$ reaction with ${\mathrm{OH}}^{-}$?

$({\mathrm{CH}}_3{)}_3\mathrm{CCl} \mathrm{or} {\mathrm{CH}}_3\mathrm{Cl}$

What is the stereochemical result of ${\mathrm{S}}_{\mathrm{N}}^1$ and ${\mathrm{S}}_{\mathrm{N}}^2$ reactions?