Infra Red Spectrum - 7

Infrared spectrum 7: identifying an unknown, C₄H₈OAn unknown compound has the molecular formula C₄H₈O. Possible structures include butanal, butanone and but-3-en-1-ol. Its infrared spectrum shows:• a strong, sharp absorption at 1715 cm^-1• absorptions at 2880 to 2980 cm^-1• no absorptions at 2720 cm^-1, 1650 cm^-1 or 3200 to 3550 cm^-1Method: work through the functional groups one at a time.1. C=O? Yes. The strong absorption at 1715 cm^-1 shows a C=O group. So the compound is an aldehyde or a ketone.2. O-H? No broad absorption at 3200 to 3550 cm^-1, so it is not an alcohol. This rules out but-3-en-1-ol.3. C=C? No absorption at about 1650 cm^-1, so there is no C=C bond. This also rules out but-3-en-1-ol.4. Aldehyde C-H? No absorption at about 2720 cm^-1, so there is no CHO group. This rules out butanal.The absorptions at 2880 to 2980 cm^-1 are ordinary C-H bonds. The C=O wavenumber, 1715 cm^-1, is in the ketone range (1700 to 1720 cm^-1).Answer: the compound is the ketone butanone, CH₃COCH₂CH₃.Examiner tip: use every absorption that is absent as well as every one that is present, and link each one to a specific bond and a wavenumber from the data booklet.