Which of the following compounds is most reactive towards $S_N1$ reaction?

The order of reactivity of the following compounds towards dilute aqueous $KOH$ in $SN_1$ reaction is
$I$. $CH_3CH_2CH_2CH_2Br$
$II$. $CH_3CH_2CHBrCH_3$
$III$. $(CH_3)_2CHCH_2Br$
$IV$. $(CH_3)_3CBr$

In the following reaction sequence,the correct structure$(s)$ of $X$ is (are)
$X$ $\xrightarrow[2) NaI, Me_2CO]{1) PBr_3, Et_2O}$ $\xrightarrow{3) NaN_3, HCONMe_2} \text{enantiomerically pure product}$

Chlorination of toluene in the presence of light and heat followed by treatment with aqueous $NaOH$ gives :

Amongst the following compounds, which one$(s)$ readily react with ethanolic $KCN$?

The rate of $S_{N}2$ reaction is maximum when the solvent is