Mass Spectrum Example 2

Example 2: propanal and propanonePropanal, CH₃CH₂CHO, and propanone, CH₃COCH₃, are structural isomers with the formula C₃H₆O. Both have M_r = 58, so both show a molecular ion peak at m/z 58. The molecular ion alone cannot tell them apart, but their fragment peaks can.PropanoneBreaking a C-C bond next to the C=O group gives CH₃CO⁺ by loss of a methyl radical. This is the base peak at m/z 43 (58 – 15 = 43).CH₃COCH₃⁺ → CH₃CO⁺ + CH₃•There is also a peak at m/z 15 from CH₃⁺. Propanone has no ethyl group, so there is no significant peak at m/z 29.PropanalBreaking the C-C bond next to the C=O group gives C₂H₅⁺ and CHO⁺, which both have m/z 29, so propanal has a large peak at 29.CH₃CH₂CHO⁺ → C₂H₅⁺ + CHO•A peak at m/z 57 shows loss of a hydrogen atom (58 – 1) from the CHO group. Propanal has no CH₃CO group, so there is no large peak at m/z 43.Conclusion: a C₃H₆O compound with a base peak at m/z 43 and no peak at 29 is propanone. One with a large peak at 29 and a peak at 57 is propanal.