Predict the order of reactivity of the following compounds in $S_{N}1$ and $S_{N}2$ reactions:
$(i)$ The four isomeric bromobutanes
$(ii)$ $C_{6}H_{5}CH_{2}Br$,$C_{6}H_{5}CH(C_{6}H_{5})Br$,$C_{6}H_{5}CH(CH_{3})Br$,$C_{6}H_{5}C(CH_{3})(C_{6}H_{5})Br$

(N/A) $(i)$ $S_{N}1$ order: $CH_{3}CH_{2}CH_{2}CH_{2}Br < (CH_{3})_{2}CHCH_{2}Br < CH_{3}CH_{2}CH(Br)CH_{3} < (CH_{3})_{3}CBr$
$S_{N}2$ order: $CH_{3}CH_{2}CH_{2}CH_{2}Br > (CH_{3})_{2}CHCH_{2}Br > CH_{3}CH_{2}CH(Br)CH_{3} > (CH_{3})_{3}CBr$
In $S_{N}1$,reactivity depends on carbocation stability. Tertiary > Secondary > Primary. Among primary,$(CH_{3})_{2}CHCH_{2}Br$ is more reactive than $CH_{3}CH_{2}CH_{2}CH_{2}Br$ due to the inductive effect. In $S_{N}2$,reactivity depends on steric hindrance. Primary > Secondary > Tertiary.
$(ii)$ $S_{N}1$ order: $C_{6}H_{5}CH_{2}Br < C_{6}H_{5}CH(CH_{3})Br < C_{6}H_{5}CH(C_{6}H_{5})Br < C_{6}H_{5}C(CH_{3})(C_{6}H_{5})Br$
$S_{N}2$ order: $C_{6}H_{5}C(CH_{3})(C_{6}H_{5})Br < C_{6}H_{5}CH(C_{6}H_{5})Br < C_{6}H_{5}CH(CH_{3})Br < C_{6}H_{5}CH_{2}Br$
In $S_{N}1$,stability of the carbocation is enhanced by resonance from phenyl groups. $C_{6}H_{5}C(CH_{3})(C_{6}H_{5})Br$ forms a tertiary benzylic carbocation,which is most stable. In $S_{N}2$,reactivity decreases as steric hindrance increases.