Infra Red Spectrum - 8

Infrared spectrum 8: combining infrared and mass spectraAn unknown compound has the formula C₃H₈O. It could be propan-1-ol, CH₃CH₂CH₂OH, or propan-2-ol, CH₃CH(OH)CH₃.Step 1: infrared spectrumThe spectrum shows a broad absorption at about 3350 cm^-1 (O-H) and no absorption near 1700 cm^-1 (no C=O). The compound is an alcohol. Both isomers are alcohols, so the absorptions above 1500 cm^-1 cannot tell them apart. Their fingerprint regions are different, but a known spectrum would be needed to compare them.Step 2: mass spectrumThe molecular ion peak is at m/z 60, which matches M_r(C₃H₈O) = 36 + 8 + 16 = 60.The base peak is at m/z 31. This is CH₂OH⁺, formed when the molecular ion loses an ethyl radical (60 – 29 = 31):CH₃CH₂CH₂OH⁺ → CH₂OH⁺ + C₂H₅•Only a compound with a CH₂OH group can form this ion.Answer: the compound is propan-1-ol. Propan-2-ol has no CH₂OH group; it loses a methyl radical to give CH₃CHOH⁺, so its base peak is at m/z 45 (60 – 15).Examiner tip: infrared identifies the functional group, and mass spectrometry gives the M_r and, through fragments, the carbon skeleton. Use them together and quote the evidence from each.