Mass spectra of substituted uracils

The mechanisms for the major fragmentations obtained with selected substituted uracils are discussed. Interpretation of data was facilitated by use of metastable peaks, high‐resolution data, and low‐voltage spectra. The major fragmentation obtained with N‐alkyl substituted uracils, when the alkyl group contains 2 or more carbons, is due to cleavage of the alkyl substituent. This cleavage is accompanied by a rearrangement of 1 or 2 hydrogens from the alkyl group to the uracil ring. Possible mechanisms for the rearrangements are discussed. It was found that the molecular ion of 1‐ and 3‐alkyl substituted uracils (where the alkyl group has 2 or more carbons) does not undergo the expected ‘retro Diels‐Alder Reaction’. Instead, the odd‐electron ion formed by loss of the alkyl substituent with a single hydrogen rearrangement undergoes this reaction (loses HNCO). Since it is formed as a secondary reaction product, the relative abundance of the ‘retro Diels‐Alder’ fragment is low compared to what is obtained in the spectra of the simple uracils. The ‘retro Diels‐Alder Reaction’ can be used to differentiate between 2‐ and 4‐thiouracils, and between 1‐ and 3‐methyl and phenyl substituted uracils. It was found that 1‐ and 3‐alkyl substituted uracils (alkyl group of 2 or more carbons) can be differentiated by the mass of the M‐alkyl fragment since the 3‐substituted compounds give predominantly a double hydrogen rearrangement and the 1‐substituted compound gives mainly a single hydrogen rearrangement. In addition the intensity of the molecular ion, relative to the M‐alkyl ion, is considerably stronger in the 1‐substituted uracils.