Iodoalkanes are readily prepared from the corresponding chloro compounds by SN2 reaction with sodium iodide in acetone. This particular procedure is especially useful because the inorganic by-product, sodium chloride, is insoluble in acetone; its precipitation drives the equilibrium in the desired direction. Thus, it is not necessary to use excess NaI, and the process goes to completion in a very short time. Because of its great convenience, this method is named after its developer (the Finkelstein reaction). In an attempt to synthesize optically pure (R)-2-iodoheptane, a student prepared a solution of (S)-2-chloroheptane in acetone. In order to ensure success, he added excess sodium iodide and allowed the mixture to stir over the weekend. His yield of 2-iodoheptane was high, but, to his dismay, he found that his product was racemic. Explain.