Proton transfer reactions between ortho-methyl substituted derivatives of 4-nitrophenylphenylcyanomethane and nitrogen bases in acetonitrile solvent

Abstract

Equilibrium constants, rate constants, primary deuterium isotope effects, and activation parameters have been measured for the proton transfer reactions in acetonitrile solvent of 4-nitrophenylphenylcyanomethane and 2-methyl-4-nitrophenylphenyl-cyanomethane with tetramethylguanidine base and for the reactions of 2-methyl-4-nitrophenylphenylcyanomethane and 2,6-di-methyl-4-nitrophenylphenylcyanomethane with 1,5-diazabicyclo[5.4.0]undec-7-ene base. Introduction of the ortho-methyl groups in the substrate molecule caused significant reductions in the equilibrium and rate constants. The expected rise in the kinetic primary deuterium isotope effect was not observed when the first ortho-methyl group was introduced, but a 20% increase did accompany the introduction of the second ortho-methyl group. Enthalpy of activation measurements indicated that there was no increase in the proton tunnelling contribution to the isotope effect when the amount of steric hindrance is increased with ortho-methyl groups.

Study of the dissociation of the products of some proton transfer reactions in acetonitrile solvent

“…extensively studied for many years (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). The kinetics of ~h , kinetic studies of these reactions were in these systems of reactions were mostly carried out ~Pectro-a variety of solvents, mostly aprotic solvents, of different photometrically, taking advantage of the fact that the ionic polarity (2, 10-13), since Caldin and Mateo predicted that products formed after abstraction of the proton have large the value of KIE is strongly dependent on the polarity of the absorption coefficients in the visible range of the spectrum.…”

Proton transfer reactions between 4-nitrophenylphenylcyanomethanes and cyclic nitrogen bases in acetonitrile solvent

“…A good linear fit, correlation coefficients 0.997-0.9999, verified that the product was an ion-pair. The results are summarized in Table 1, together with those for TMG and DBU from earlier work (2).…”

“…The absorbance values for a series of base concentrations at a constant acid concentration were fitted to eq. [2] for an ion-pair product, where a and b are the initial concentrations of acid and base respectively and A is the equilibrium absorbance…”

“…[2], the values of K are derived from the intercept and the slope of the plot and are subject to larger errors than those indicated by the standard deviations. However, the logarithms of these equilibrium constants correlate with the pKa except for DBU, which gives an equilibrium constant for the abstraction of the proton from carbon acid 2 about 10 times larger than would be expected from the other equilibrium constants in Table 1.…”

“…Previous studies have investigated the effects of substituents in the carbon acid with respect to electronic effects (1) and steric effects (2). In this paper, structural changes in the nitrogen base are studied using two carbon acids, 4-nitrophenylphenylcyanomethane 1 and 2-methyl-4-nitrophenylphenylcyanomethane 2.…”