Which of the following is most reactive toward $S_N2$ reaction?

For the reactions given below:
$(I)$ Cyclohexyl chloride $\rightarrow$ Cyclohexyl cation + $Cl^-$,$\Delta H_1^{\circ}$
$(II)$ Cyclohex$-2-$enyl chloride $\rightarrow$ Cyclohex$-2-$enyl cation + $Cl^-$,$\Delta H_2^{\circ}$
$(III)$ Benzyl chloride $\rightarrow$ Benzyl cation + $Cl^-$,$\Delta H_3^{\circ}$
$(IV)$ Chlorobenzene $\rightarrow$ Phenyl cation + $Cl^-$,$\Delta H_4^{\circ}$
The correct decreasing order of enthalpies of reaction for producing the carbocation is:

Identify the reagent $R$ used in the following conversion: $tert$-butyl bromide $\xrightarrow{R}$ Isobutylene

$tert$-Butylbromide reacts with $aq.$ $NaOH$ by $S_N1$ mechanism while $n$-butylbromide reacts by $S_N2$ mechanism. Why?

Which of the following is liquid at room temperature?

An isomer of $C_3H_6Cl_2$ on boiling with aqueous $KOH$ gives acetone. Hence,the isomer is