An unknown compound gives a molecular ion of m / z70 in the mass spectrum. It reacts with semicarbazide hydrochloride to give a crystalline derivative, but it gives a negative Tollens test. The NMR and IR spectra follow. Propose a structure for this compound, and give peak assignments to account for the absorptions in the spectra. Explain why the signal at 1790 cm-1 in the IR spectrum appears at an unusual frequency.

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wavelength (am) 2.5 100 3.5 45 5 55 6 789 10 12 13 14 15 16 80 Problem 18-46 40 20 1790 4000 30002500 2000 1800 1600 1400 1200 000 800 600 180 Offset: 40 ppm 200 160 140 120 100 80 60 40 20 Problem 18-46 δ (ppm)