Which of the following is an example of an $S_N2$ reaction?

When $CH_3CH_2CHCl_2$ is treated with $NaNH_2$,the product formed is

Number of moles of $CH_3MgBr$ consumed in the above reaction is:

$A$ primary alkyl halide $(a)$ with molecular formula $C_{4}H_{9}Br$ reacts with alcoholic $KOH$ to give compound $(b)$. Compound $(b)$ reacts with $HBr$ to give compound $(c)$,which is an isomer of $(a)$. When $(a)$ reacts with sodium metal,it gives compound $(d)$ with molecular formula $C_{8}H_{18}$,which is different from the compound formed when $n$-butyl bromide reacts with sodium. Identify the structural formula of $(a)$ and write the equations for all the reactions.

The order of relative reactivity of the given halides towards $S_N1$ reaction is:

$2-$bromopentane is heated with potassium ethoxide in ethanol. The major product is