Studies on hydantoin. Part 2. Substituent effects in 3-arylhydantoins on the formation of aryl isocyanate ions using the mass-analysed ion kinetic energy–direct analysis of daughter ions

Abstract

The unimolecular reaction of seven 3-arylhydantoins, relatively slow fragmentation in the free drift region, has been investigated by analysis of the mass-analysed ion kinetic energy (m.i.k.e.) spectrum using direct analysis of daughter ions (d.a.d.i.). Aryl isocyanate ion formation, which gives the base peaks or highly abundant ions, occurs through three pathways involving initial loss of CO, CH, and C₂H₂NO, respectively, and the substituent effects on the competing metastable transitions have been found. For electron-attracting groups in the para- or meta-positions, loss of CO· is the dominant process. The formation of the M⁺˙– C₂H₂NO˙ ions is the major pathway for electron-donating groups in the aromatic ring. The substituent effects on the formation of aryl isocyanates obey the Hammett equation. Correlation with σ and σ⁺ are related semiquantitively to the solution chemistry of hydantoin.