The mass spectra of some N‐substituted barbitals and their trimethylsilyl derivatives

The mass spectra of N‐propyl and N‐butyl barbitals show the loss of olefin radical (CnH2n‐1) in analogy to structurally similar molecules such as N‐alkyl succinimides and 3‐alkyl uracils. Trimethylsilylation of the N‐substituted barbitals suppresses this fragmentation and loss of olefin via apparent McLafferty rearrangement from the even‐electron ion, [M – 15]+, becomes significant. The trimethylsilyl derivatives of N‐allyl barbital and N‐phenyl barbital show an unusually facile elimination of the appropriate isocyanate from the molecular ion.