Which of the following is boiled with ethyl chloride to form ethyl alcohol?

The correct order of boiling points of alkyl halides is:

$KI$ in acetone undergoes $S_N2$ reaction with each $P, Q, R$ and $S$. The rates of the reaction vary as:
$P: CH_3-Cl$
$Q: (CH_3)_2CH-Cl$
$R: CH_2=CH-CH_2-Cl$
$S: C_6H_5-CO-CH_2-Cl$

Predict the order of reactivity of the following compounds in $S_{N}1$ and $S_{N}2$ reactions:
$(i)$ The four isomeric bromobutanes
$(ii)$ $C_{6}H_{5}CH_{2}Br$,$C_{6}H_{5}CH(C_{6}H_{5})Br$,$C_{6}H_{5}CH(CH_{3})Br$,$C_{6}H_{5}C(CH_{3})(C_{6}H_{5})Br$

$A$ bromoalkane $X$ reacts with magnesium in dry ether to form compound $Y$. The reaction of $Y$ with methanal followed by hydrolysis yields an alcohol having molecular formula $C_{4}H_{10}O$. The compound $X$ is

An organic halide is shaken with aqueous $NaOH$,followed by the addition of $dil. HNO_3$ and silver nitrate solution,which gives a white precipitate. The substance can be: