Rank the alkyl halides in order of increasing E2 reactivity. Then do the same for E1 reactivity

Structure A and C are tertiary halides, whereas B is a secondary halide.

Representation of different types of alkyl halide

Tertiary halides give a trisubstituted product, whereas secondary halides give a disubstituted product. With the increase of substitution of the alkyl, the +I effect of the alkyl groups increases the stability of the system.

As a result, tertiary halides are found to be more reactive towards elimination(E1 and E2) than secondary halides.

Hence, structures C and A are found to be more reactive than B.

Bromine has a large size and more conjugate basicity than that of chloride. Hence bromide is considered to be a better leaving group than chloride. Hence, structure C is found to be more reactive than A.

The reactivity order will be the same for both E1 and E2 as both carry a similar explanation. The alkyl halides are ranked in the following based on E2 reactivity:-

Representation of the E2 reactivity order

The same trend is followed for E1 too.

Representation of the E1 reactivity order