keonPebcl6eetaaaaneanetodesand Electronic Transitions of Acetone and 3-Buten-2-one agao2ad3g cHCCH at-opos [R-e m=4兰c R-c=6 M-C The symmetric ketone generatesasngacyllum peak.4 Conjugation of carbonyl groups with double bonds shifts the absorption to longer wavelengths: Mass spectral fragmentation of aldehydes and ketones provides structural information. 2-Pentanone, 3-pentanone and 3-methyl-2-butanone all fragment by α cleavage yielding the corresponding acylium cation and an alkyl radical. Note that the two acylium fragments identify the composition of the ketone. The peak at mass 58 results from a McLafferty rearrangement. This occurs in compounds having a hydrogen atom γ to the carbonyl oxygen and enough flexibility to allow them to get close together. This extra peak allows 2-pentanone to be distinguished from 3-methyl-2- butanone, which give otherwise identical fragments. The symmetric ketone generates a single acylium peak. Note the absence of a peak at 58, which is present in the mass spectrum of 2-pentanone