Which of the following compounds reacts with $KCN$ to form a cyanide?

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

The product $(C)$ in the following reaction sequence is:
$4-\text{Bromoethylbenzene}$ $\xrightarrow[\Delta]{Br_2} B$ $\xrightarrow{\text{aq. } KOH} C$

Which of the following is the most reactive towards $S_{N}1$ mechanism?

When ethanol is reacted with $PCl_5$,it gives $A$,$POCl_3$,and $HCl$. Compound $A$ reacts with silver nitrite to give the major product $B$ and $AgCl$. What are $A$ and $B$ respectively?

The major product of the following reaction is: