Question

Arrange the alkyl bromides in order of reactivity in each reaction: 1-bromobutane, 2-bromobutane, cyclohexabromide, t-butylbromide

Answer 1

As the reaction conditions or mechanism is not given, I am assuming you are looking for the reactivity order of given alkyl bromides with respect to SN1 or SN2 reactions. Please let me know if you need an explanation in case of other types of reaction such as elimination.

SN1 reaction:

The SN1 reaction involves the formation of carbocation as an intermediate and hence the alkyl bromide which produces more stable carbocation will lead to the fast reaction. We know tert. carbocation is most stable which can be formed from tert. butyl bromide. And least reactivity can be seen in case of 1-bromobutane as it is a primary alkyl halide. Cyclohexylbromide has a more rigid structure and requires specific directional attack of nucleophile hence will have lower reactivity than that of 2-bromobutane. Thus in case of the SN1 reaction order of reactivity will be:

t-butyl bromide > 2-bromobutane > cyclohexylbromide > 1-bromobutane

SN2 reactions:

In the case of SN2 reactions, it involves crowded transition state where an attack of nucleophile and cleavage of C-Br bond happens simultaneously. Hence lower the crowded transition state faster will be the reaction. Thus primary alkyl halide often follows SN2 pathway as compared to the t-alkyl halide. Thus the order of reactivity will be:

t-butyl bromide < 2-bromobutane <  cyclohexylbromide < 1-bromobutane