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 (C_nH_2n-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.