Which is correct for the above reaction $:-$

The correct order for increasing reactivity towards $S_N1$ substitution for the following compounds is:
$1. CH_3CH_2Br$
$2. CH_2=CHCH(Br)CH_3$
$3. CH_2=CHBr$
$4. CH_3CH(Br)CH_3$

An unknown compound $A$ has the molecular formula $C_5H_9Cl$. It does not react with $Br_2 / CCl_4$. When treated with a strong base,it gives a compound $B$ with the molecular formula $C_5H_8$,which reacts with $Br_2 / CCl_4$. Ozonolysis of $B$ followed by treatment with dimethyl sulfide gives a compound with the molecular formula $C_5H_8O_2$. What is the structure of $A$?

Which of the following cannot undergo $E_2$ reaction?

Explain the stereochemistry of $S_{N}2$ reaction with a suitable example.

Consider the reactions.
$(i) \ (CH_3)_2CH-CH_2Br \xrightarrow{C_2H_5OH} (CH_3)_2CH-CH_2OC_2H_5 + HBr$
$(ii) \ (CH_3)_2CH-CH_2Br \xrightarrow{C_2H_5O^-} (CH_3)_2CH-CH_2OC_2H_5 + Br^-$
The mechanisms of reactions $(i)$ and $(ii)$ are respectively: