Appearance of Maxima on Conductometric Titration Curves of Sulfonic Acids and the Evidence of Strong Homoconjugation Reactions in Benzonitrile

Abstract

The conductometric titrations of acids with bases usually give decreasing or increasing straight lines from the beginning to the equivalence point in protic solvents. However, in aprotic solvents, such as acetone 1 and acetonitrile 2 , maxima at the half equivalence point on titration curves have been observed for carboxylic acids with amine bases. The cause of the maxima at the half equivalence point has been attributed to homoconjugation reactions 2 between an acid (HA) and the conjugate base anion (A -) on the neutralization: B+2HA BH + ... H 2 A -. In a poorly solvating solvent, nitrobenzene, the maxima could be observed even at the one-third, and not at the half equivalence point, suggesting the formation of an A -(HA) 2 -type homoconjugate species. 3 We have reported maxima on conductometric titration curves of phosphoric, phosphonic, and related organophosphorus acids with triethylamine in DMF. 4 The stability of the homoconjugated species was found to depend much upon the number of OH groups involved homoconjugation reactions, e.g., not (PhO) 2 PO(OH) but PhOPO(OH) 2 gave a maximum in DMF, regardless of the acidities of the acids.Gunduz et al. 5 stressed an important role of the base strength of a conjugate anion upon the appearance of maxima in conductometric titrations of substitutedacetic and benzoic acids. In acetonitrile, the equilibrium constants of the homoconjugation reactions of acetate and benzoate ions with two acid moleclues (A -+2HA A -(HA) 2 , K homo ) have been determined to be 5.7×10 7 and ca. 1×10 7 , respectively. 6 Some phenols, especially 3,5-dinitrophenol etc.1,7 can form the homoconjugate species in aprotic solvents. The homoconjugation reaction or the appearance of a maximum on conductometric curve of sulfuric acid (SO 2 (OH) 2 ) has been observed in acetonitrile 2,8 and acetone. 9 However, maxima on conductometric curves for sulfonic acids (RSO 2 (OH)) have not been reported so far, even though fairly large homoconjugation constants of sulfonic acids were obtained in acetonitrile. 10 An sulfonic acid has only a single OH group, while sulfuric acid has two OH groups in the molecule. Therefore, the stability of the homoconjugated species from an sulfonic acid must have been too small to give a maximum on the titration curve; this point encouraged us to perform the present study.Sulfonic acids are rather strong acids (e.g., pK a =0.7 for benzenesulfonic acid) 11 in aqueous solution, however, the acidities of the acids are extremely weakened in aporic solvents. 10,12 The dissociation and homoconjugation constants of methanesulfonic and related acids have been determined in propylene carbonate, and compared with those in acetonitrile. 13 We have reported 14 on the formation constants of "triple ions" of trialkylammonium sulfonates and nitrate through hydrogen bonding forces as well as Coulombic forces in benzonitrile. Judging from the 1:1 association and "triple ion" formation constants of R 3 NH + A -in benzonitrile, the basicity of the conjugate anions were found to increase as follo...

Specific coordination phenomena of alkaline earth metal ions with aromatic sulfonate ions in alcohols and binary solvents of acetonitrile–alcohols

“…Some sulfonic acids, such as methanesulfonic and p-toluenesulfonic acids, have been observed to be weak acids in benzonitrile (DN = 11.9, AN = 15.5) to form the 1:2-type homoconjugated species by means of conductometry [20]. Meanwhile, conductometric titrations with Et 3 N of di-and trisulfonic acids in MeCN have revealed the strong homoconjugation for the di-and trisulfonic acids [21].…”

Evidence for Oxonium Ions in Ethereal “Hydrogen Chloride”

“…In specific cases, careful experimentation has yielded unexpected insights. Conductometric titration of sulfonic acids in benzonitrile has demonstrated that the active species is trimeric in acid; carefully conducted IR spectroscopic studies revealed that triflic acid protonates water in nonpolar organic solvents to form a hydronium triflate ion pair, but the stronger acid bis­(trifluoromethyl)­sulfonyl imine (HNTf 2 ) instead forms a hydrogen-bonded complex without proton transfer . Nonetheless, most acid/solvent combinations that find use in organic chemistry remain incompletely characterized.…”

Use of the Sulfide Mineral Pyrite as Electrochemical Sensor in Non-aqueous Solutions: Potentiometric Titration of Weak Acids in Acetonitrile, Propionitrile and Benzonitrile

“…8 Hojo and Chen investigated homoconjugation reactions in this solvent. 9 As can be seen from the literature data, nitriles are extensively used as a medium in potentiometric determinations of many acidic substances. A glass electrode is most frequently used as an indicator electrode, in both aqueous and non-aqueous media.…”