Question

What is the order of reactivity from fastest to slowest in an SN2 reaction for the following substrates?

a. t-butyl bromide
b. isopropyl bromide
c. neopentyl bromide
d. ethyl bromide
e. methyl bromide

So far have methyl bromide> ethyl bromide> neopentyl bromide> isopropyl bromide > t-butyl bromide
I know t-butyl is on a 3 prim carbon so that would be last
isopropyl is on a 2 prime carbon so thats second
but the methyl, ethyl, and neopentyl is where i am confused at since they are both primary

Answer 1

Your initial listing is correct... The rate depends upon the least stable carbon attached to the halogen for Sn2 reaction mechanisms.

MtBr with methyl carbon is the fastest substitution
EtBr and neopentyl-Br both have primary carbons attached to the Bromide
Isopropyl Br => secondary carbon and...
t-BuBr => tertiary carbon (prefers to follow the Sn1 pathway via ionization; i.e., is a 2-step process).