Autoprotolysis Constants of Water–N-Methylacetamide Mixtures. A New Application of the Hammett Relationship to Acidity Constants

Abstract

The hydrogen electrode can be used to measure proton activities of both acid and basic solutions in water-N-methylacetamide (NMA) mixtures. and hence to calculate autoprotolysis constants for the different mixtures. The intermediate acidities between the acid and basic regions are established by the use of buffer solutions of vario~~sly substituted phenols, whose acidities in the different water-NMA mixtures have been established. The diminution o f the acidity constants with increasing concentrations of N-methylacetamide cannot be explained by the increased dielectric constants of the solution. Instead, we propose a new adaptation o f the Hammett relationship (pK,, -pKx = po,), which places N-methylacetamide between water and DMSO, and close to alcohols, which have a dielectric constant 5-fold smaller. Because of its precision, this method of analysis allows one to classify into one or more homogeneous families compounds having the sanie chemical function which w o~~l d be classified together by the classical treatment of Hammett.En plus de la mesure du produit ionique apparent, le bon fonctionnement de I'electrode d'hydrogene nous a permis la construction, dans chacun des melanges eau-NMA, d'une echelle d'acidite reliant les milieux acides aux milieux basiques. Pour cela, nous avons calibre des solutions tamponnees preparees a partir de phenols diversement s~~bstitues. Comme il s'avere que ['augmentation de la constante diilectriq~~e du milieu ne rend pas compte de la diminution d e la plupart des constantes d'acidite lorsque le melange s'enrichit en NMA, nous proposons une nouvelle adaptation de la relation de Hammett (pK, -pKx = pox) qui conduit a placer le NMA entre I'eau et le DMSO, tres pres des alcools dont la constante diilectrique est pourtant 5 fois plus faible. De par sa precision, la methode permet par ailleurs I'amelioration du classement, en une ou plusieurs familles homogenes, des composes de mCme fonction chimique, classement provenant du traitement plus classique de I'Cquation de Hammett.

Solvent effect on preferred protonation sites in nicotinate and isonicotinate anions

“…[6], which shows that plots of p~2 vs. pK$ must be linear with slopes For personal use only. and intercepts being characteristic of the class of acids studied and depending only upon the solvents (28,29,36,38,39). In agreement with this expectation, the acid-base behaviour of oxygen acids like carboxylic acids and phenols (28,38) and of nitrogen acids like anilinium (28) and imidazolium (29) cations was found to conform nicely to eq.…”

“…In agreement with this expectation, the acid-base behaviour of oxygen acids like carboxylic acids and phenols (28,38) and of nitrogen acids like anilinium (28) and imidazolium (29) cations was found to conform nicely to eq. [6] in H20/Me2S0 and H,O/N-methylacetamide mixtures (39). The similarity observed in the solvent dependence of the pKa, values for each of the two nicotinic acids studied and the pKa value of benzoic acid at Me2S0 contents 250450% suggests that the COOH functionality of AH0 forms behaves, not only qualitatively but quantitatively, as that of other benzoic acids.…”

Determination of Primary and Secondary Standards for pH Measurements in N-Methylacetamide and Its 0.50 Mass Fraction in Admixture with Water, with Characterization of Appropriate Salt Bridges

“…Indeed, p K AP can be determined by a thermodynamic method based on reversible cells, but so far the available p K AP data are scarce; , yet [Δ G ° W → Z ] H+ , a quantity concerning the single H + , falls outside the domain of exact thermodynamics , and thus requires extra-thermodynamic methods or assumptions, and the relevant data are even scarcer. , For the present case, p K AP data are available from the work by Asuero et al, which leads to 14.79 for pure N -methylformamide and 16.74 for its 0.5 mass fraction in water, not much different from the aqueous values. For the corresponding primary medium effects on H + , in terms of the ΔpH corrections, we have +2.3 and +1.2, respectively, from a recent nonreversible cell study based on the assumption of null liquid junction potential. , For N -methylacetamide at 0.5 mass fraction in water, we have p K AP = 14.48 at 25 °C from Asuero et al and p K AP = 14.85 at 20 °C from Halle et al., but no datum on primary medium effect, and the intersolvental systematization of the relevant pH scale in this solvent is, therefore, impossible.…”

Dependence of the Substituent Effect on Solvent Properties

“…26 ρ-Values for the above-mentioned reaction is 1.00 for water (by a definition), 1.14 for ethanol, 1.02 for methanol, 27 0.966 for n-butanol, 28 and 2.4 in 85.1 wt % Nmethylacetamide in water. 29 Moreover, the estimation of the reaction constant for gas phase data shows a tremendous increase of its sensitivity on the substituent effect, with ρ = 5.6. 30 Differences in ρ-values seem not to be related to electrostatic interactions described by electric permittivity constants: ε for water and methanol at 20 °C are 80.37 and 32.35, 31 respectively, whereas difference in ρ is insignificant.…”

“…Substituent effects, described traditionally by substituent constants (SC), also depend on a solvent, or more generally, on a kind of medium applied to their estimations. − An application of the Hammett substituent constants , σ to the reference reaction for SC, i.e., acid–base equilibria of substituted benzoic acids measured in various solvents, reveals that the reaction constant, ρ, describing a sensitivity of the reaction to the substituent effects (SE) strongly depends on the solvent used . ρ-Values for the above-mentioned reaction is 1.00 for water (by a definition), 1.14 for ethanol, 1.02 for methanol, 0.966 for n -butanol, and 2.4 in 85.1 wt % N -methylacetamide in water . Moreover, the estimation of the reaction constant for gas phase data shows a tremendous increase of its sensitivity on the substituent effect, with ρ = 5.6 .…”