$CH_3-CH_2-Cl + NaI \xrightarrow{\text{Acetone}} CH_3-CH_2-I + NaCl$ The above reaction is known as $:-$

Which is the weakest nucleophile in an aprotic solvent?

Explain the mechanism of a unimolecular nucleophilic substitution $(S_N1)$ reaction with a suitable example.

Predict the order of reactivity of the following compounds in $S_N1$ reaction :
$(I)$ $CH_3-CH_2-CH(Br)-CH_3$
$(II)$ $(CH_3)_2CH-CH_2-Br$
$(III)$ $(CH_3)_3CBr$

The relative reactivity of the following halides toward $S_{N}2$ reaction follows the order:
$(P)$ $CH_2=CH-Cl$
$(Q)$ $CH_3-CH_2-Cl$
$(R)$ $CH_3-O-CH_2-CH_2-Cl$
$(S)$ $C_6H_5-CH_2-Cl$

The major product of the following reaction is: