Equilibrium acidities of carbon acids. VI. Establishment of an absolute scale of acidities in dimethyl sulfoxide solution

An accurate spectrophotometric method of determining relative equilibrium acidities of carbon acids in DMSO has been developed. The pK scale in DMSO has been anchored by comparisons of values obtained by the spectrophotometric method with those obtained potentiometrically in the 8 to 11 pK range. As a result, the pK of fluorene, formerly arbitrarily taken as 20.5, has been raised to an absolute value of 22.6. The pK’s of other carbon acids previously reported, including nitromethane, acetophenone, acetone, phenylacetylene, dimethyl sulfone, acetonitrile, and the corresponding indicator pK‘s must also be raised. The pK’s have been found to be correlated with heats of deprotonation in DMSO by potassium dimsyl, and evidence is presented to show that pK measurements in DMSO are free from ion association effects. Data are presented which indicate a pK of 35.1 for DMSO. In the methane carbon acids, CH3EWG, the order of acidities is NO2 >> CH3CO > CN, CH3S.02. The differences amount to 12.2 and 6.8 kcal/mol, respectively, which are believed to be of a comparable magnitude to gas-phase substituent effects. Carbon acids wherein the charge on the anion resides mainly on oxygen, such as ketones and nitroalkanes, are found to be weaker acids in DMSO than in water by 5 .5 to 9.6 pK units. On the other hand, carbon acids wherein the charge on the anion is delocalized over a large hydrocarbon matrix, such as in the anion derived from 9-cyanofluorene, are stronger acids in DMSO than in water. Factors that may contribute to this reversal are discussed. The scale of pK’s for 9-substituted fluorenes in DMSO is shown to be expanded when compared to the earlier pK scale determined by the H method. A rationale is presented. The apparent relative acidities of fluorenes and phenylacetylene differ by 6 and 1 I pK units, respectively, for cyclohexylamine (CHA) vs. DMSO solvents and benzene vs. DMSO solvents. Similarly, in benzene, acetophenone is a stronger acid than fluorene by ca. 6 pK units, whereas in DMSO acetophenone is a weaker acid by 3.2 pK units. These differences result from ion association effects that occur in solvents of low dielectric constant (benzene, ether, CHA, etc.) causing relative acidities to be dependent on the reference base, as well as the solvent. This is not true in strongly dissociating solvents of high dielectric constant, such as DMSO. A list of 13 indicators covering the pK range 8.3 to 30.6 in DMSO is presented. Equilibrium acidities of weak (i.e., pK 1 15) carbon acids have been measured by a variety of methods3 in a variety of solvents including ether,4a benzene,4b d i g l ~ m e , ~ cyclohexylamine (CHA),6 mixtures of dimethyl sulfoxide (DMSO) with ethanol, methanol, or ~ a t e r , ’ . ~ . ~ and pure Journal of the American Chemical Society / 97:24 1 November 26,1975 DMSO.’O We have chosen DMSO for our studies because i t allows accurate measurements to be made spectrophotometrically for many different types of carbon acids over a wide range of pK (ca. 30 pK units) with apparently little or no interference from ion association effects.’ Furthermore,