Which of the following is most reactive towards $S_{N}1$ reaction?

The major product obtained when this substrate is subjected to $E_2$ reaction under the treatment of potassium $tert$-butoxide is:

For each of the following pairs of $E_2$ reactions,select the one that occurs with the greater rate constant:
$(1)$ $CH_3-CH_2-CH_2-Cl + CH_3-C(CH_3)_2-O^-$
$(2)$ $CH_3-CH_2-CH_2-I + CH_3-C(CH_3)_2-O^-$
$(3)$ $CH_3-CH(Br)-CH_3 + CH_3-O^-$
$(4)$ $CH_3-CH(Br)-CH_3 + CH_3-S^-$
$(5)$ $Ph-CH_2-CH(Br)-CH_2-CH_3 + OH^-$
$(6)$ $Ph-CH_2-CH_2-CH(Br)-CH_3 + OH^-$

Dehydrohalogenation of an alkyl halide is

Compare the relative rate of reaction with $H_2O$ for the following compounds:

What is the order of reactivity of alkyl halides with ammonia?