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?

Write a note on $S_{N}1$ reaction.

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$ :
Assertion $(A)$ : $S_N2$ reaction of $C_6H_5CH_2Br$ occurs more readily than the $S_N2$ reaction of $CH_3CH_2Br$.
Reason $(R)$ : The partially bonded unhybridized $p$-orbital that develops in the trigonal bipyramidal transition state is stabilized by conjugation with the phenyl ring.
In the light of the above statements,choose the most appropriate answer from the options given below:

The major product of the following reaction is $CH_{2}=CH-CH_{2}-OH \xrightarrow[\text{Excess}]{HBr} \text{Product}$

The major product is:

The most reactive molecules towards $SN^1$ reaction are: