Which is the most reactive halide towards $S_{N^1}$ reaction?

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

The reaction of ethyl bromide with lead-sodium alloy gives ......... .

Arrange the following alkyl halides in the increasing order of their reactivity towards nucleophilic substitution reaction:
$(I)$ $CH_3CH_2CH(Cl)CH_3$
$(II)$ $CH_2=CHCH(Br)CH_3$
$(III)$ $CH_2=CHCH(Cl)CH_3$
$(IV)$ $CH_3CH=C(Br)CH_3$

Which of the following pairs is/are correctly matched?

Reaction	Product
$I$. $RX + AgCN$	$RNC$
$II$. $RX + KCN$	$RCN$
$III$. $RX + KNO_2$	$RONO$
$IV$. $RX + AgNO_2$	$RNO_2$

Which is a true statement concerning the transition state of an $S_N2$ reaction?