- Reactivity of the radical anion OCC-
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The characteristic reactivity of the radical anion OCC- has been investigated in the gas phase at 298 K through determination of rate coefficients, products, and branching fractions for each of 29 ion-molecule reactions. A wide variety of reactions is observed including abstraction of H, H+, and H2+, nucleophilic displacement, charge transfer, and reactions involving electron detachment. Many of the reactions involve cleavage of the C--CO bond, consistent with the relatively small C--CO bond energy and the proposed1 electronic structure of the ground state anion in which both radical and charge are centered on the terminal carbon. Similarities are noted between the chemistry of OCC- and its neutral analogue OCC and between the chemistry of OCC- and the radical anions O- and o-C6H4-. Most reaction products observed are consistent with reaction mechanisms involving initial attack of the terminal carbon in OCC- on the neutral reaction partner. The gas-phase acidity of HCCO is bracketed between those of CH3NO2 and CH3CHO, yielding 1502 ± 8 > ΔGoacid(HCCO) ≥ 1463 ± 8 kJ mol-1 and 1531 ± 12 > ΔHoacid(HCCO) ≥ 1491 ± 12 kJ mol-1. Observation of H atom transfer from CH2Cl2 to OCC- indicates that ΔHof(OCC-) ≥ 148 ± 12 kJ mol-1 and gives a larger lower limit of ΔHoacid ≥ 1507 ± 15 kJ mol-1. These and related thermochemical values, including the hydrogen bond dissociation energy in HCCO, are compared with literature values.
- Van Doren, Jane M.,Miller, Thomas M.,Stevens Miller, Amy E.,Viggiano,Morris, Robert A.,Paulson, John F.
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p. 7407 - 7414
(2007/10/02)
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- Kinetics of Hydrolysis of Substituted β-Nitrostyrenes. Transition-State Imbalances and Intrinsic Rate Constants for Three Different Types of Nitronate Ion Forming Processes. Relevance to the Nitroalkane Anomaly
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A kinetic study of the hydrolysis of β-nitrostyrene and of the 4-chloro and 3-nitro derivatives in 50percent Me2SO-50percent water (v/v) at 20 deg C is reported.The mechanism involves four steps: nucleophilic addition of OH(-) or water to form ArCH(OH)CHNO2(-) (T-OH); carbon protonation of T-OH by water, H(+), and buffer acids to form ArCH(OH)CH2NO2 (T0OH); deprotonation of the OH group in T0OH to form ArCH(O(-))CH2NO2 (T-O); collapse of T-O into ArCHO and CH2NO2(-).In strongly acidic solution the aci form of T0OH, PhCH(OH)CH=NO2H (T0OH,aci), which is generated by protonation of T-OH on the nitro group, could also be detected.All steps are reversible, and the rate and equilibrium constats of most of them could be determined by a combination of kinetic experiments starting with the substrate, the products, the independently synthesized T0OH, and T-OH (generated at high pH and then subjected to a pH jump) and by measuring product ratios spectrophotometrically or by HPLC analysis.The intrinsic rate constants of the proton transfer, T-OH T0OH, are close to those for the deprotonation of nitromethane and 2-nitroethanol, and the Brnsted coefficients show a transition-state imbalance (αCH >> βB(-)) that is typical for the deprotonation of nitroalkanes.The other two processes that lead to the formation of a nitronate ion, i.e., nucleophilic addition to the olefin to form T-OH and collapse of T-O to form CH2NO2(-), show similar structure-reactivity behavior as the proton transfer, such as transition-state imbalances as manifested by αnnuc > βnnuc for nucleophilic addition and depressed intrinsic rate constants.However, these imbalances are smaller compared to the one in the proton transfers, especially so for the nucleophilic addition to the olefin.It is suggested that there is an inherent tendency for reactions leading to resonance-stabilized ions to have imbalanced transition states, but in proton transfers there are two imbalance enhancing factors, namely sp3 hybridization of the carbon in the protonated form and hydrogen bonding in the transition state, while in the collapse of T-O there is one such factor (sp3 hybridization of the carbon in T-O).
- Bernasconi, Claude F.,Paschalis, Peter
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p. 5893 - 5902
(2007/10/02)
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