Which one of the following halogen compounds is not useful as a substrate in an $S_N2$ reaction?

$1$-methyl ethylene oxide (propylene oxide) when treated with an excess of $HBr$ produces:

Which of the following,when undergoing addition of $HBr$,will form $ONLY$ a pair of diastereomers?

Predict the order of reactivity of the following four isomers towards $S_{N}2$ reaction.
$(I)$ $CH_{3}CH_{2}CH_{2}CH_{2}Cl$
$(II)$ $CH_{3}CH_{2}CH(Cl)CH_{3}$
$(III)$ $(CH_{3})_{2}CHCH_{2}Cl$
$(IV)$ $(CH_{3})_{3}CCl$

Which of the following compounds has the lower boiling point?

The major product obtained in the following reaction is: