[al]

When (2S,3S)-3-bromo-2-methoxybutane undergoes an Sn2 reaction with methoxide ion, the product is not optically active. Why is that??

[akami]

MeO- ion attaks bromine's alpha proton, and maybe form an alkene by expulsion of the methoxyde group. The product is 2-bromo-2-butene, no optical activity. It seem weird, since I think that Br- is a better leaving group than MeO-.

A more possible reaction is a deprotonation to give 3-methoxybut-1-ene, which undergoes a sigmatropic rearrangement

[akami]

no sigmatropic rearrangement because no prototropy, sorry

[dexangeles]

what's it going to replace when the Substitution occurs?
what happens to the confirmation of the chirality center attached to the substituent being substituted?
what's the rule for a substance to be optically active?

[AWK]

Optically active compounds during SN2 substitution form optically active compounds.

[dexangeles]

Optically active compounds during SN2 substitution form optically active compounds.

but the 3S after substitution will become a 3R
doesn't that make it meso; thus, optically inactive?

[AWK]

My fault, I did not read AL question exactly.
Starting reagent is (2S,3S)-3-bromo-2-methoxybutane (not dibromo- compound), hence during SN2 substitution one bromine is exchanged with methoxy group with inversion of configuration, and meso
2R,3S compound is formed.