For each of the following reactions, give the substitution products; if the products can exist as stereoisomers, show what stereoisomers are obtained:
a. $(R)$ -2-bromopentane $+$ high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$
b. $(R)$ -2-bromopentane $+\mathrm{CH}_{3} \mathrm{OH}$
c. trans-1-chloro-2-methylcyclohexane + high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$
d. trans - 1 -chloro- 2 -methylcyclohexane $+\mathrm{CH}_{3} \mathrm{OH}$
e. 3 -bromo- 2 -methylpentane $+\mathrm{CH}_{3} \mathrm{OH}$
f. 3 -bromo- 3 -methylpentane $+\mathrm{CH}_{3} \mathrm{OH}$