- Kinetics and mechanism of the aminolysis of phenyl and 4-nitrophenyl ethyl thionocarbonates
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The reactions of the title substrates (PTOC and NPTOC, respectively) with secondary alicyclic amines are subjected to a kinetic study in aqueous solution at 25.0°C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first-order rate coefficients (kobsd) are found throughout. The order in amine is one for the reactions of piperidine but is of intermediate order between 1 and 2 for the reactions of the other amines. The kinetic results can be accommodated by a reaction scheme with two hypothetical tetrahedral intermediates: a zwitterionic (T±) and an anionic (T-) one, whereby amine catalysis (deprotonation of T± to give T-) is kinetically important. Both the pKa of T± and the rate coefficient for proton transfer (k3 ca. 1010 s-1 M-1) are estimated. The values of the other rate microcoefficients of the scheme are found by a nonlinear least-squares fitting, and these values are compared with those exhibited in the aminolysis of phenyl thionoacetate (PTOA), and S-phenyl and S-(4-nitrophenyl) O-ethyl dithiocarbonates (PDTC and NPDTC, respectively). The Broensted type plots for amine basicity have slopes βN ca. 0.2 for rate-determining amine attack (k1) and βN ca. 0.8 for amine expulsion from T± (k-1), in accord with the βN values found in similar aminolyses. The general base catalysis by amine found in the aminolysis of NPTOC, in contrast with the lack of such catalysis in the aminolysis of 4-nitrophenyl methyl carbonate, is explained by a smaller rate coefficient for expulsion of 4-nitrophenoxide (k2) from T±(which competes with amine deprotonation of T±) relative to the same expulsion from the analogous oxy intermediate.
- Castro, Enrique A.,Cubillos, Maria,Santos, Jose G.
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p. 3501 - 3505
(2007/10/03)
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- Highly Sterically hindered Carbon Acids: The Intrinsic Reactivity of 5,5',5''-Trimethyl- and 3,3',3'',5,5',5''-Hexamethyl-2,2',2'',4,4',4''-Hexanitrotriphenylmethanes
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Rate constants (kpB,kpBH) for the reversible deprotonation of 5,5',5''-trimethyl- and 3,3',3'',5,5',5''-hexamethyl-2,2',2'',4,4',4''-hexanitrotriphenylmethanes (2 and 3) by primary aliphatic amines, piperidine and morpholine as well as by phenoxide anions and hydroxide anion have been measured in H2O-Me2SO (20:80) at 25 deg C.Comparison of the results obtained with those for 2,2',2'',4,4',4''-hexanitrotriphenylmethane (1a) shows that the introduction of methyl groups in positions adjacent to the nitro groups decreases markedly the thermodynamic acidity of theexocyclic CH group: ΔpK2a1a = 1.68; ΔpK3a1a = 6.48.It is suggested that these decreases are very likely the reflection of a twisting of the nitro groups out of their attached aromatic planes and therefore of a reduced resonance stabilization of the conjugated carbanions C-2 and C-3.Other important steric effects are operating in the ionization of 2 and 3.These arise from the accumulation of ortho-nitro groups in the triphenylmethane system which makes the approach of the base reagents from the exocyclic carbon of 2 and 3 very difficult.The finding of extremely low intrinsic reactivities for 2 and 3 and the observation of a much greater catalytic efficiency of primary amines than of secondary amines in assisting the proton transfers are the two most striking manifestations of these F-strain effects.
- Terrier, Francois,Xiao, Lan,Farrell, Patrick G.,Moskowitz, Danielle
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p. 1259 - 1263
(2007/10/02)
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- Nitrite Ion as a Nitrosating Reagent. Nitrosation of Morpholine and Diethylamine in the Presence of Formaldehyde
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The kinetics of the nitrosation of morpholine and diethylamine in the presence of formaldehyde has been studied at pH values between 6.5-8.2 and 6.9-8.7, respectively.The results are interpreted through a mechanism that implies the reaction between both the nitrite NO2(-) and iminium R2N(+)=CH2 ions.The latter ion results from the dehydration of the conjugated acid of the carbinolamine, R2NH(+)CH2OH, the initial product of the amine-formaldehyde reaction.The results allow the calculation of the equlibrium constants of the formation of carbinolamine, R2NCH2OH, and methanediamine, R2NCH2NR2 (only for the morpholine-formaldehyde system), and their conjugate acids.
- Casado, Julia,Mosquera, Manuel,Paz, L. Carlos,Prieto, M. Flor Rodriguez,Tato, Jose Vazquez
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p. 1963 - 1966
(2007/10/02)
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