When (S)-1-bromo-1-fluoroethane reacts with sodium methoxide, an S_N2 reaction takes place in which the bromine atom is replaced by a methoxy group (OMe). The product of this reaction is (S)-1-fluoro-1-methoxyethane. How can it be that the starting material and the product both have the S configuration? Shouldn’t S_N2 involve a change in the configuration? Draw the starting material and the product of inversion, and then explain the anomaly.