Increasing order of reactivity of the following compounds for $S_N1$ substitution is:
$(a)$ $(CH_3)_2CH-CH_2Cl$
$(b)$ $CH_3CH_2Cl$
$(c)$ $CH_3O-C_6H_4-CH_2Cl$
$(d)$ $C_6H_5-CH_2Cl$

In the dehydrohalogenation of $2$-bromobutane,which conformation leads to the formation of $cis$-but-$2$-ene?

The $C-X$ bond is strongest among the following:

Identify reagent $B$ in the following reaction: $R - X \xrightarrow{B} \text{Nitroalkane}$

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

The correct order of the rate of reaction of the following reactants with a nucleophile by $S_{N}1$ mechanism is:
(Given: Structure $I$ and $II$ are rigid)