An unknown compound gives the following mass, IR, and NMR spectra. Propose a structure, and show how it is consistent with the spectra. Show the fragmentations that give the prominent peaks at m/z 127 and 155 in the mass spectrum.
The mass spectrum shows a molecular ion at 170. This serves as the molecular weight for the compound. While (as is typical) the fragmentation is complex, there are two predominant peaks. One is at 155 m/z (M-15); this is most likely from the loss of a methyl group, as that is a common fragmentation and the -CH3 group weighs 15 grams per mole. The other prominent mass spectral peak is at 127 m/z (M-43). The most common organic fragment that has this weight is an acetyl group (CH3 CO) . This would make sense given that we already suspect the presence of a methyl group.
Looking next at the infrared spectrum, you can see some aromatic C-H peaks around 3000 cm-1 as well as a strong carbonyl stretch at 1680cm-1. Since carbonyls are usually found at 1720cm-1, something must be slightly weakening the strength of the carbonyl bond in order to shift it to this lower wavenumber; most likely, the carbonyl lies in conjugation with an aromatic ring. This confirms the C-H peaks seen at 3000cm-1.
Next, look at the 13C NMR spectrum. The small set of three peaks around 77 ppm belongs to the solvent (CDCI3) used to obtain the spectrum, and can be ignored. The other peaks mainly belong in the region from 120-140 ppm, which is the aromatic region. There is a single peak far downfield which is characteristic of a carbonyl carbon, which would reinforce the data found in the IR spectrum.
So far, we suspect that we have a heavily aromatic system with an acetyl group attached.