Equilibrium acidities of some carbon acids in acetonitrile

Abstract

The pK a values for C-acids (4-nitrophenylnitroalkanes, 4-nitrophenyl-(4-X-phenyl)cyanomethanes, 2,4,6-trinitrotoluene, and 9-cyanofluorene) in acetonitrile have been measured. These values were determined spectrophotometrically in buffer solutions consisting of a strong amine base and its picrate or perchlorate using known potentiometric pKa's of amines, and they range from 19 for bis(4-nitrophenyl)cyanomethane to 25.1 for ethyl bis(4-nitrophenyl)acetate. Keywords: C-acids, pKa values, equilibrium, reactivity, proton transfer reaction.

Potentiometric study of equilibrium acidities of some carbon acids in acetonitrile

“…In previous kinetic and equilibrium experiments, uncomplicated C-acids, such as 3, were used (14). At that time, the large primary deuterium kinetic isotope effect (KIE) values were attributed to three factors: steric hindrance of the reacting base (15,16), symmetry of the transition state (9,13,17), and polarity of the solvent used (13,18,19). To find the factors influencing the values of KIE, the series of C-acids with increasing R substituent bulk, and hence increasing steric hindrance, were synthesized and used (2,13,16,19,20).…”

“…For comparison, ten pK a s measured by the spectrophotometric method (9,30) are given in the sixth column of Table 1. In each case satisfactory agreement was achieved.…”

“…One could expect that the basicity of OHmight be strongly dependent on the amount of water present in the medium. The response of the electrode to the concentration of water added to a tetrabutylammonium hydroxide 9-Cyanofluorene 20.8 0 20.8 (9) a In this work, standard deviations are typically less than 0.1. b Spectrophotometric pK a refs. are given in parentheses.…”

“…A survey of spectrophotometric pK a values for C-acids in acetonitrile has already been published (9). The aim of this paper is to measure the pK a values for a set of 30 C-acids of the type CH·R 1 ·R 2 ·R 3 , where R 1 , R 2 , and R 3 are H, alkyl, aryl, and the most common electron withdrawing groups that activate C-acids, such as nitro, cyano, alkylsulfonyl, and carboxyalkyl (Table 1), all in acetonitrile, using the potentiometric method (10).…”

“…The spectrophotometric results reported in the previous paper (9) were measured in buffer solutions composed of a strong amine base of known potentiometric pK a value, and its perchlorate or picrate salt. This way, the pK a values for 11 of the C-acids most commonly used in kinetic and equilibrium experiments of proton-transfer reactions were found.…”