- Aminolysis of Y-substituted-phenyl 2-methoxybenzoates in acetonitrile: Effect of the o-methoxy group on reactivity and reaction mechanism
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Second-order rate constants (kN) were measured for aminolyses of Y-substituted-phenyl 2-methoxybenzoates 2a-i and 4-nitrophenyl X-substituted-benzoates 3a-j in MeCN at 25.0 °C. The Bronsted-type plot for the reactions of 2a-i with piperidine curves downward, indicating that a change in rate-determining step (RDS) occurs. The Hammett plot for the reactions of 3a-j with piperidine consists of two intersecting straight lines, which might be taken as evidence for a change in RDS. However, the nonlinear Hammett plot has been suggested not to be due to a change in RDS but rather to the stabilization of the ground state of substrates possessing an electron-donating group (EDG) (e.g., 3a-c) through a resonance interaction, since the corresponding Yukawa-Tsuno plot exhibits an excellent linear correlation with ρ = 0.54 and r = 1.54. The ρ value found for the reactions of 3a-j in MeCN is much smaller than that reported previously for the corresponding reactions in H2O (i.e., ρ = 0.75). It is proposed that the reactions of 3a-j in MeCN proceed through a forced concerted mechanism due to instability of T± in the aprotic solvent, while the reactions of 2a-i proceed through a stepwise pathway with a stabilized T ± through an intramolecular H-bonding interaction.
- Um, Ik-Hwan,Bae, Ae Ri
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p. 7510 - 7515
(2011/11/13)
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- Conjugation and proton exchange equilibria. Heteroconjugation constants in substituted phenol-piperidine systems in acetonitrile
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It has been shown by mathematical transformations that the final equations and expressions required for determining equilibrium concentrations of major species in HA + B systems are analogous to those used previously for HA + A1- systems. Heteroconjugation constants K→AHB = [AHB]/([HA][B]) for eight substituted phenol (HA)-piperidine (B) systems in acetonitrile (AN) were determined from emf measurements. A fairly linear dependence between log K→AHB and ΔpKaAN = pKBH+AN - pKHAAN was observed with a slope of 0.52. The [KAHB2/(KAHA - KBHB+)]atδpKa=0 quotient appeared to be much greater than [KAHA1-2/(KAHA - KA1HA1-)]atΔpKa=0 calculated from results obtained previously for HA + A1- type systems. From this fact it has been concluded that the formation of AHB type complexes (relative to the AHA- and BHB+ type complexes) is likely to be favoured by their overall neutrality ensuring weaker peripheral interactions.
- Magonski, Jozef,Rajzer, Barbara
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p. 1181 - 1185
(2007/10/03)
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- STERIC FACTORS IN THE SHORT-RANGE SOLVATION OF SECONDARY AMINES
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Steric aspects of the short-range solvation of proton transfer complexes between 2,4-dinitrophenol and various secondary amines was studied in benzene-DMSO and benzene-dioxane mixed solvent systems.Diisopropylamine was hindered when compared with diethylamine, and 2,2,6,6-tetramethylpiperidine was hindered when compared with 2,6-dimethylpiperidine in forming the proton transfer complex.The smaller electron-donating solvent dimethylsulfoxide was capable of solvating the more hindered amines, but the the bulkier dioxane could only solvate diethylamine and dimethylpiperidine readily.Tetrahydropyran could solvate tetramethylpiperidine weakly, inferring that it did not solvate in precisely the same fashion as does dioxane.It is concluded that the solvation structure is sufficiently crowded as to be sensitive to small changes in the structures of the participants.
- Tayh, Jamal A.,Scott, Ronald M.
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p. 297 - 305
(2007/10/02)
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- Amine basicities in benzene and in water
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The constants for ion-pair formation with 2,4-dinitrophenol in benzene (KB), the pKa values in water, of thirty-three amines have been measured.According to the class of amine, two different situations can be observed: for primary amines and secundery cyclic amines, the effects of structural variations on basicity are higher in water than in benzene; on the other hand, for tertiary amines these effects are similar in the two solvents.KB Values of primary amines give a good correlation with ? *.The Tatf and Hancock equation allows a unifying treatment of KB values of the various classes of amines.
- Frenna, Vincenzo,Vivona, Nicolo,Consiglio, Giovanni,Spinelli, Domenico
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p. 1865 - 1868
(2007/10/02)
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