When dissolved in CDCl3, a compound (K) with the molecular formula C4H8O2 gives a 1H NMR spectrum that consists of a doublet at d 1.35, a singlet at d 2.15, a broad singlet at d 3.75 (1H), and a quartet at d 4.25 (1H). When dissolved in D2O, the compound gives a similar 1H NMR spectrum, with the exception that the signal at d 3.75 has disappeared. The IR spectrum of the compound shows a strong absorption peak near 1720 cm-1.
(a) Propose a structure for compound K.
(b) Explain why the NMR signal at d 3.75 disappears when D2O is used as the solvent.