The product obtained from the reaction is:

The correct decreasing order of $SN^2$ reactivity of the following alkyl halides is:
$(I)$ $CH_2=CH-Br$
$(II)$ $CH_3-Br$
$(III)$ $CH_3-CH(Br)-CH_3$
$(IV)$ $CH_3-CH_2-Br$

$A$ 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 compound added to prevent chloroform from forming phosgene gas is

Which one of the following is likely to give a precipitate with $AgNO_3$ solution?

Which of the following reactions is expected to take place by the $S_N2$ mechanism?