Ross Stewart

Strongly basic systems—V : H-acidity scale based on the ionization of carbon acids

Abstract An investigation of the ionization of a number of carbon acids in strongly basic systems has been made. An H-acidity scale for ethanol-dimethyl sulfoxide solutions containing sodium ethoxide has been constructred and the pKa values of the acids have been measured. The values found range from 11·14 (malononitrile) to 21·0 (fluorene). The relation of the acidity scale to acid type and the effect of substituents on acidity are discussed.

The effects of high concentrations of aqueous tetramethylammonium chloride and other salts on the dissociation of phenylphosphonic acid and on the enolization of acetone

The dissociation constants of benzoic acid and phenylphosphonic acid have been determined in aqueous solutions containing tetramethylammonium chloride at concentrations up to 15 and 13 molal, respectively. The second dissociation constant of phenylphosphonic acid has also been determined in concentrated solutions of alkali halides. Whereas the organic electrolyte increases the observed pK values of the acids, the inorganic electrolyte idecrease them. The rate of enolization of acetone, catalyzed by phenylphosphonate dianion is increased by the presence of tetramethylammonium chloride, but is decreased by the presence of inorganic salts, in accordance with the changes in the apparent acidity constants of phenylphosphonate monoanion that the two different kinds of electrolyte bring about. The slope of the Brónsted plot for the inorganic salts is 0.76; that for the organic electrolyte is 0.46. Non-quaternary ammonium salts also increase the rate of the phenylphosphonate dianion catalyzed enolization of acetone, but this appears to be due to a considerable extent to the formation of reactive iminium ion intermediates.

The catalysis of proton exchange in methyl substituted 2-pyrimidones

Abstract The general-acid and general-base catalyzed exchange of the 4- or 6-Me groups in a series of Me substituted 2-pyrimidones has been studied. When the substrate is neutral the 6-Me group is 8–25 times more reactive than the 4-Me group; when the substrate is a cation (either a conjugate acid or a quaternary salt) there is little difference in reactivity. As a consequence the proton activating factor (paf) is larger for the 4-Me group than for the 6-Me group. It is shown that the reaction of hydroxide ion with neutral 1,6-dimethyl-2-pyrimidone is about a thousand times faster than the reaction of water with the compounds conjugate acid; that is, deprotonating the base is more effective at facilitating exchange than protonating the substrate.

Substituent effects in the reduction of trifluoroacetophenones by a dihydronicotinamide

Abstract Triflurooacetophenone reduction rates (by a dihydronicotinamide) and hydration equilibria are identically affected by ring substituents. High yields of alcohol are obtained when electron donating substituents are absent from the ring.

Effect of substituents at the 5-position on the first and second dissociation constants of isophthalic acid

The dissociation constants of benzoic acid and 20 of its meta- or para-substituted derivatives and of isophthalic acid and ten of its 5-substituted derivatives have been measured in 50 wt% aqueous methanol. The Hammett ρ value for benzoic acid is 1.28; for isophthalic acid the ρ values are 1.21 (pK1) and 1.20 (pK2). The σmeta values for hydroxy and acetoxy are –0.01 and +0.29, respectively, in this system. Values for σmeta for CO2H and CO2– are calculated to be +0.28 and –0.20, respectively; however there are indications that these values are not completely structure-independent.