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(-), CH(3)O(-) and HOO(-) very rapidly with rate coefficients of (2.5-4.3) x 10(-9) cm(3) s(-1) and reaction efficiencies of 60-100%, for example, k = 3.2 x 10(-9) cm(3) 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 NO(2)(-). 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 CH(3)I, CO(2), CS(2) and SO(2). Nucleophilic substitution (S(N)2) reaction occurs with CH(3)I while characteristic products CS(2)O(-) and SO(3)(-) are formed from CS(2) and SO(2), respectively, along with competing adduct formation. The SN(2) 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 [R(2)C(-) - NO2 <--> R(2)=NO(2)(-)] resulting in the greater basicity/nucleophilicy and the less negative charge on the oxygen site.

Fundamental Aspects of Gas Phase Ion Chemistry Studied Using the Selected Ion Flow Tube Technique

“…Anions and radicals of those species are also considered important intermediates in combustion. The SIFT technique has been employed to study, through an anionic approach, the structure and thermochemistry of diazocarbene (CNN) and the diazomethyl radical (HCNN), 106) small nitroalkanes, 107) and a series of five-membered azole compounds. [CNN], which are conjectured to be combustion intermediates that provide low energy paths to cleave N 2 to produce N atoms (Scheme 7).…”

“…157) We also demonstrated that negative ion chemistry with HO ῍ is an alternative tool to monitor plant emission of VOCs 158)ῐ160) and to detect small, volatile nitroalkane explosives. 107) These analytical applications and bio/ medico-chemical implications, as well as ionῐmolecule chemistry of interstellar relevance, will be discussed elsewhere. 161) A new reaction scheme of CIMS interest is given here.…”

“…We find the reactions to proceed very rapidly, k ῑ 2.5῎4.3ῐ10 ῌ9 cm 3 s ῌ1 , primarily producing [M῎H] ῌ anions via proton transfer (where an a-hydrogen is available) as well as NO 2 ῌ via the E2 pathway (where a b-hydrogen is available). 107) S N 2 formation of NO 2 ῌ is ruled out because NO 2 ῌ is not observed from CH 3 NO 2 . The acyclic nitroalkanes are similarly very acidic because of resonance stabilization involving the aci form 14 for the deprotonated anions, in addition to the strong inductive e#ect by the nitro group.…”

“…107) In the S N 2 reactions of [M῎H] ῌ anions with CH 3 I, the e$ciencies do not decrease but rather gradually increase with larger substituents R and R ῑ (Scheme 8).…”

“…The [M῎H] ῌ anion of nitrocyclopropane displays di#erent S N 2 and bimolecular reactivities than do acyclic anions, which is explained by the reduced contribution of the aci form in the cyclic anion. 107) Polynitrogen compounds have been of both fundamental and practical interests. 113) In theory those species can derive from isoelectronic substitution of N atoms for CH units; however, polynitrogen compounds are considerably less stable because of the nitrogen lone pair repulsion that is introduced.…”