THE IONIZATION BEHAVIOR OF AMIDES IN CONCENTRATED SULFURIC ACIDS: II. APPLICATIONS OF THE <i>H</i><sub>A</sub> FUNCTION TO RATES AND EQUILIBRIA

mentioning

confidence: 99%

Protonation of conjugated carbonyl groups in sulfuric acid solution. I. Adaptation of the amide acidity function H_A for protonation of the carbonyl group in non-Hammett bases

Zalewski¹,

Dunn²

1968

“…They can be ignored for large but not for small values of R . Equations [I] and [2] apply to reactions or equilibria in solutions of zero ionic strength (13 (16). In these solutions the dielectric constant D, will be reduced (l8), and to a lesser extent D,,; this might account for the larger ApK (-2.7).…”

Section: ~T R~d ~~mentioning

confidence: 99%

Basicity of the thionamide group of α-aminothionamides. Attenuation of electrostatic effects by solvation

Paventi¹,

Heirtzler²,

Edward³

1986

. 64,23 10 (1986). The first dissociation constants pK1 of four diprotonated cx-aminothionamides, determined spectrophotometrically, are between -8.1 to -8.8; the second dissociation constants pK2, of three of these compounds, determined titrimetrically, are between 7.3 and 7.7. Both pK1 and pK2 values are close to those calculated from current electrostatic theory for equilibria in solutions of zero ionic strength, in spite of the fact that data for pK1 values were obtained in solutions of high ionic strength. Results indicate a much larger effect of the charged ammonium group on the basicity of the thionamide group than on the basicity of the amide group. A possible explanation based on the greater hydration of the protonated amide group is advanced. MARTINO PAVENTI, FENTON HEIRTZLER et JOHN T. EDWARD. Can. J. Chem. 64, 2310Chem. 64, (1986. Utilisant des mCthodes spectrophotomCtriques on a determine les premieres constantes de dissociation (pK,) de quatre a-aminothioamides diprotones; comme Ctant entre -8,l et -8,8; faisant appel B des mCthodes titrimetriques, on a aussi determink les deuxikmes constantes de dissociation (pK2) de trois de ces composks comme etant entre 7,3 et 7,7. Les valeurs tant des pK1 que des pK2 sont proches des valeurs calculies a partir de la theorie ilectrostatique actuelle sur les Cquilibres dans des solutions de force ionique Cgale a zero, mCme si les donnees relatives aux valeurs de pKl sont obtenues dans des solutions de forces ioniques ClevCes. Les risultats indiquent que le groupement ammonium charge provoque un effet beaucoup plus important sur la basicit6 du groupement thionamide que sur la basicite du groupement amide. On propose une explication plausible qui est bas& sur une plus grande hydratation du groupement amide proton-6.[Traduit par la revue]Starting more than 60 years ago with Bjerruln ( I ) , a long succession of investigations has elaborated an electrostatic theory of substitutent effects which is gradually winning general acceptance (2-9). However, the electrostatic effect can be profoundly altered by the presence of a solvent, and although "emphasis has shifted from continuum electrostatics to interactions between specific functionalities in the ions and appropriate sites in a few solvent molecules" (lo), a discontinuous solvation model (1 1) of quantitative (as opposed to qualitative) predictive power has been slow to emerge.The older continuum solvent model was applied to electrostatic effects by Kirkwood and Westheimer (4) in 1938. As an example, we may consider the fact that the dissociation constant K2 of monoprotonated glycine 2 would be expected to be higher than KAH of the parent acid, acetic acid, because the substituted acetate ion 3 is stabilized by electrostatic interaction between NH3+ and the negatively charged carboxylate group.' On the other hand, further dissociation of 3 should be made more difficult than the dissociation of the parent methylammonium ion (KBH+) by the same electrostatic stabilization, the substituent now being C 0 2 -. Classic...

“…In practice, it is found that slopes often vary from unity, but their closeness to unity is an indication of the applicability of the acidity function to the particular compounds. Values in the range 0.95-1.05 are desirable, but those between 0.9-1.1 are considered acceptable (11). Table 2 lists the 20 thiocarbonyl compounds investigated, along with the slopes of log I us.…”

Section: Applicability Of H Functionmentioning

confidence: 99%

Applicability of the H_T acidity function to the protonation of thioamides, thioureas, and thionesters

Edward¹,

Derdall²,

Wong³

1977

The protonation of eleven thioamides, five thioureas, and four thionbenzoates in aqueous sulfuric acid has been found to follow the HT acidity function with acceptable accuracy. Protonation constants pKTH+ obtained by use of HT agreed fairly well with pKTH+ values obtained by the Bunnett–Olsen method, but less well with those obtained by the Marziano–Cimino–Passerini procedure. Linear free-energy relationships of pKTH+ values are discussed.