Which alkyl halide from the following pairs would you expect to react more rapidly by an S_N2 mechanism? Explain your answer.

(i)

(ii)

(iii)

In S_N2 Reaction, the product will be formed in a single step with no intermediates. Nucleophile attack will be easier for simple halides than hindered haloalkanes. Therefore,

Because of this in S_N2

i) Bromobutane (1°)reacts faster than 2-Bromobutane(2°)

ii) 2-Bromobutane(2°) reacts faster than 2-Bromo-2-methylpropane or tert-Butyl bromide (3°)

iii) 1-Bromo-3-methyl butane reacts faster than the 1-Bromo-2-methyl butane as the former is less hindered with respect to leaving halide than the later which is more hindered comparatively.