What is the principal product of the following reaction?
$CH_3-CH(Br)-CH_2-CH_2-CH(Cl)-CH_3 + NaN_3 \rightarrow \text{Product}$

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

Explain the factors favouring $S_{N}2$ reaction.

Find the reactants which on heating with alcoholic $KOH$ produce the compound $CH_3-CH_2-CH_2-CH=CH_2$.
$(i) \ CH_3-CH(Br)-CH_2-CH_2-CH_3$
$(ii) \ CH_3-CH_2-CH(Br)-CH_2-CH_3$
$(iii) \ CH_3-CH_2-CH_2-CH_2-CH_2Br$
$(iv) \ CH_3-CH_2-CH_2-CH(Br)-CH_3$

The correct order of reactivity of the following benzyl halides towards reaction with $KCN$ is:

Which of the following statements is invalid?