- Mechanism of reaction of hydroxide ion with dinitrochlorobenzenes
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Formation of dinitrophenoxide ion from 2,4- and 2,6-dinitrochlorobenzene (2,4- and 2,6-DNCB) and OH- (OD-) in 70:30 and 80:20 (v/v) DMSO-H2O (D2O) is accompanied by extensive 1H NMR line broadening of unreacted substrate and exchange of arene hydrogen with D2O, which is quantitative at the 3-position of 2,4-DNCB. Unproductive Meisenheimer complexes are detected spectrophotometrically in the course of reaction. For reaction of 2,4-DNCB, the Meisenheimer 3-complex is formed first and then the more stable 5-complex can be detected and characterized by NMR spectrometry. There is no hydrogen exchange of Meisenheimer complexes or dinitrophenoxide ions and their 1H NMR signals are not broadened. These results do not fit the classical mechanism of single-step nucleophilic addition, but they, and the kinetic results, are fitted by a reaction scheme involving single-electron transfer from OH- to give a charge-transfer complex of OH? and a radical anion which collapses to give Meisenheimer complexes and aryl oxide ion. This scheme is consistent with MO calculations by the AM1 method.
- Bacaloglu, Radu,Blaskó, Andrei,Bunton, Clifford,Dorwin, Ellen,Ortega, Francisco,Zucco, César
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p. 238 - 246
(2007/10/02)
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- Concerted Acetyl Group Transfer between Substituted Phenolate Ion Nucleophiles: Variation of Transition-State Structure as a Function of Substituent
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Second-order rate constants (kArO) have been measured for the concerted displacement of aryl oxide from aryl acetates in aqueous solution by substituted phenoxide ions.Values of kArO obey linear Bronsted correlations when either the leaving group or the attacking phenolate ion structures are varied.The Bronsted coefficients obey the equations βnuc = 0.20pK1g - 0.68 and β1g = 0.15pKnuc - 1.73 to a good degree of precision, and the variation indicates that the structure of the transition-state changes within the range of phenolate ions studied; this alsoprovides confirmation that a concerted mechanism operates.The equations for βnuc and β1g predict the equation (log kii = 0.17pKa2 - 2.41pKa + C) for kii, the rate constant for the reaction of aryl oxide ion with acetates bearing identical aryl oxide leaving groups.The identity rate constants may be interpolated from the observed rate constants (kArO) and exhibit excellent fit to the above equation with the single disposable parameter, C, set at 6.5.This is the first report of curvature in a Bronsted plot of identity rate constants.Effective charge development and loss on leaving and attacking oxyanions is fully balanced in the transition state when entering and leaving nucleophiles have a pKa of 7.1.Tetrahedral or acylium ion-like transition-state structures are predicted for hypothetical phenols with pKa's of 11.7 and 2.0, respectively.
- Ba-Saif, Salem,Luthra, Ajay K.,Williams, Andrew
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p. 2647 - 2652
(2007/10/02)
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- Reactions of Nitroanisoles. Part 2. Reactions of 2,4- and 2,6-Dinitroanisole with Piperidines in Benzene
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The reactions of 2,4- (I) and 2,6-dinitroanisole (VIII) with piperidine (II) and N-methylpiperidine (NMP) in benzene have been studied.It was found that (I) and (VIII) react with (II) to give not only the expected aromatic nucleophilic substitution (a.n.s.) products, but also the corresponding dinitrophenols, generated by an SN2 reaction.The rate for each reaction and the overall rates were measured and it was observed that compound (I) has similar rate coefficients for the a.n.s. as well as for the SN2 reaction.Mild acceleration by base was found for the a.n.s.reaction.When (VIII) reacts with (II) the rate of reaction to form 2,6-dinitrophenol is greater than the rate for the a.n.s.In the reaction with NMP only an SN2 reaction was observed.Furthermore, the rates of reaction of (VIII) with (II) and with NMP are ca. 1000 and 300 times, respectively, the rate of reaction of (I) with these amines.A field effect is proposed for the increase in rate.
- Nudelman, N. Sbarbati,Palleros, Daniel
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p. 995 - 999
(2007/10/02)
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