Gas-Phase Ion–Molecule Reactions of Small Nitroalkanes and Their Deprotonated Anions

Abstract

Gas-phase reactions of nitromethane (1), nitroethane (2), 2-nitropropane (3), 2-methyl-2-nitropropane (4) and nitrocyclopropane (5) were studied at 300 K using the flowing afterglow technique. These nitroalkanes react with gas-phase bases HO−, CH3O− and HOO− very rapidly with rate coefficients of (2.5–4.3) × 10−9 cm3 s−1 and reaction efficiencies of 60–100%, for example, k = 3.2 × 10−9 cm3 s−1 (86%) for 5 reacting with hydroperoxide anion. Proton transfer (PT) is the only reaction observed for 1 while elimination (E2) is the exclusive pathway for 4 yielding isobutene and NO2−. Both PT and E2 reactions are observed for 2, 3 and 5, the former being the major pathway. Deprotonated anions of 1, 2, 3 and 5 were subjected to reactivity studies with CH3I, CO2, CS2 and SO2. Nucleophilic substitution (SN2) reaction occurs with CH3I, while characteristic products CS2O− and SO3− are formed from CS2 and SO2, respectively, along with competing adduct formation. The SN2 rate is greater, whereas the reactivities with the triatomic reagents are smaller for deprotonated nitrocyclopropane than for the other acyclic anions. These observations strongly suggest that the reactions of nitroalkane [M – H]− anions occur through initial attack from the terminal oxygen; the nitrocyclopropane carbanion is more strained and, thus, less stabilized by resonance [R2C−−NO2 ↔ R2C=NO2−] resulting in the greater basicity/nucleophilicy and the less negative charge on the oxygen site.