Acid-base Equilibrium Constants Research Articles

Electronic Spectra of D-Ph-D' Systems. I. Aminophenol Cations as Isoelectronic Structures of Cresols

Abstract The electronic absorption spectra and luminescent properties in the first singlet π-π∗ excited states of aminophenol cations in aqueous solutions are studied from a theoretical and experimental point of view. Lifetime and quantum yield measurements are used in the electronic description of the first excited state. Cresols as isoelectronic species of these cationic compounds are used in order to stablish the nature of the main deactivation pathways in excited state. Acid—base equilibrium constants in excited state are determined according to Foster cycle. Paraaminophenol cation is found as an unusual acid species in excited state (pKa∗ = −4.8). This compound present a double fluorescence spectrum in aqueous solutions due to its high first order dissociation rate constant (log k(d) = 9.95). Non-radiative pathways in the first singlet π-π∗ excited state are found as the main deactivation channels in all these isoelectronic compounds.

Acid–base equilibria in aqueous micellar solutions. Part 2.—Sulphonephthalein indicators

The acid–base equilibria of three sulphonephthalein indictors, viz. bromocresol green, bromothymol blue and phenol red, in 1,4-dioxane–water mixtures and aqueous solutions of both non-ionic and cationic micelles have been studied. The difference between the acid–base equilibrium constant of a sulphonephthalein indicator in pure water, pKwa, and the apparent acid–base equilibrium constant of a sulphonephthalein located entirely within the interfacial microenvironment of a non-ionic micelle has been explained quantitatively in terms of the different intrinsic solvent properties of the two systems. A number of factors are considered to be primarily responsible for the difference between the apparent acid–base equilibrium constant of a sulphonephthalein residing with in the interfacial micro-environment of a cationic alkyl quaternary ammonium micelle and its pKwa value. These factors are the electrostatic micellar surface potential, the low effective interfacial dielectric constant and either an interfacial ‘salt-effect’ or the formation of an ion pair between the negatively charged sulphonate group of a sulphonephthalein and a cationic quaternary ammonium surfactant headgroup.

Spectra and Electronic structure in the excited state of carboxy-5-phenyltetrazoles

Absorption and luminescence spectra of ortho-, meta- and para-carboxy-5-phenyltetrazole in aqueous solutions at room and low temperature were measured. Investigations were carried out in super acidic to super basic media. Three dissociation forms were identified (anion, molecule and cation) and corresponding acid-base equilibrium constants in the ground state were determined spectrophotometrically, and in the first excited singlet state from titration curves. The latter ones for anion-molecule equilibrium are in agreement with those calculated by the Förster cycle. The formation of a twisted internal charge transfer (TICT) excited state of the anion is discussed.

ChemInform Abstract: METAL CHELATES OF AZO‐PYRIDAZINE DYES. III. CHELATING TENDENCIES OF PYRUVIC AND HIPPURIC MONOHYDRAZONE‐3‐HYDRAZINO‐4‐BENZYL‐6‐PHENYLPYRIDAZINE

AbstractQuantitative GIeichgewichtsuntersuchungen der wie aufgezeichnet synthetisierten, neuen Azopyrazin‐Liganden (I) und (III) werden detailliert beschrieben.

Metal chelates of azo-pyridazine dyes III chelating tendencies of pyruvic and hippuric monohydrazone-3-hydrazino-4-benzyl-6-phenylpyridazine

The synthesis and quantitative equilibrium studies of two new azo-pyridazine ligands, namely the pyruvic (PHP) and hippuric (HipHP) acid derivatives are described in detail. Their acid-base and metal-ligand equilibrium constants in 75% dioxan-water solvent at 30 °C have been calculated from potentiometric and spectrophotometric data. The probable structures are inferred from electronic and vibrational spectral results. Evidence is presented for the formation of protonated complexes withPHP.

Measurement of acid-base equilibrium constants in acetonitrile/18-crown-6 solutions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMeasurement of acid-base equilibrium constants in acetonitrile/18-crown-6 solutionsIldiko M. KovachCite this: Anal. Chem. 1983, 55, 13, 2184–2186Publication Date (Print):November 1, 1983Publication History Published online1 May 2002Published inissue 1 November 1983https://doi.org/10.1021/ac00263a041RIGHTS & PERMISSIONSArticle Views53Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (360 KB) Get e-Alerts Get e-Alerts

Hydrogen peroxide and sulfur (IV) in Los Angeles cloud water

Airborne collection and chemical analysis of cloud water samples in the Los Angeles Basin showed that the reaction between hydrogen peroxide and sulfur (IV) was inhibited in the collected samples, so these species reacted to form sulfate more slowly in these samples than would be expected from published laboratory data. The cloud water contained formaldehyde, which can react with sulfite to form hydroxymethanesulfonic acid (HMSA). This adduct contributed to measured S(IV) concentrations in the cloud water, which were more than 100 times greater than those calculated from ambient sulfur dioxide concentrations, water pH, and the Henry's Law and acid-base equilibrium constants. The average S(IV) concentration constituted 14% of the sulfate determined in the cloud water after oxidation. It is likely that a salt formed from HMSA contributed to prior observations of S(IV) and volatile sulfur in the Los Angeles aerosol.

Tautomerism in the ortho-aminomethylphenols

1. A tautomeric equilibrium is established between the neutral and zwitterion forms of the ortho-aminomethylphenols in aqueous solutions. 2. Determination has been made of the tautomeric equilibrium constants and the acid-base equilibrium constants for the various forms of the ortho-aminomethylphenols. 3. An equation for calculating the tautomeric equilibrium constant has been developed.

Reactions of carbonyl compounds in the presence of phase-transition catalysts. 4. Study of the reaction of acetoacetic ester with 1-chloro-3-methyl-2-butene in the presence of various alkaline agents

1. The conditions for the selective formation of prenylacetoacetic ester (I) in the presence of a 50% aqueous K2CO3 solution and triethylbenzylammonium chloride (TEBA chloride) and of diprenylacetoacetic ester (II) in the presence of a 50% aqueous KOH solution and TEBA chloride were found. The effect of the concentration of aqueous alkali on the direction of the reaction was studied. 2. The difference in the condensing activities of KOH and K2CO3 is due to the large difference in the acid-base equilibrium constants of their reactions with acetoacetic ester (AAE). The effect of the concentrations of aqueous solutions of KOH in the presence of TEBA chloride on the rate of monoalkylation was studied. 3. The kinetic basicity of a 50% aqueous solution of K2CO3 in the presence of a phase-transition catalyst was estimated, and it was assumed that the condensing activity of this system is determined exclusively by the catalytic action of TEBA hydroxide. 4. A scheme for the monoalkylation of AAE that presupposes several pathways is discussed. In the presence of a 50% aqueous solution of KOH and TEBA chloride the reaction proceeds through the potassium or triethylbenzylammonium derivative of AAE with KOH and the TEBA base, respectively. 5. Dialkylation proceeds only in the presence of concentrated aqueous solutions or solid KOH; this is associated with the difference in the acidities of AAE and ester I. 6. An induction period that does not depend on the presence of TEBA in the system and is due to the formation of water as a result of the reaction is observed during the formation of 2-methylhept-2-en-6-one in the presence of solid KOH.

Determination of acid-base equilibrium constants in the excited state

Determination of acid-base equilibrium constants in the excited state

Excited states of six-membered aza-aromatic rings. IX. 1,4- and 5,12-diazatriphenylenes

The two isomeric diazatriphenylenes, 1,4-DAT and 5,12-DAT, and the corresponding singly and doubly protonated forms were investigated in the ground state, phosphorescent state and fluorescent state by the methods of electronic spectroscopy, ESR spectroscopy and quantum chemistry. The following properties were measured: (π, π ∗) excitation energies, acid-base equilibrium constants, quantum yields of fluorescence, lifetimes and zero-field splitting parameters of the triplet states, for the bases and the conjugate acids. The results of semi-empirical calculations performed using the PPL LCI SCF method based on monoexcited configurations were compared with experimental data. The calculated electron densities on nitrogen were correlated with pK's for the two equilibria and the three electronic states. The unusually large increase of pK (by about six units) of the triplet 1,4-DAT, has been found and interpreted. A very good correlation of zero-field splitting parameter D 1 3 versus singlet-triplet splitting was obtained.

Neighbouring group participation by the carboxy-group in the solvolysis of the acylamino-acids in acetic acid at low water concentrations

The kinetics of solvolysis of some acylamino-acids have been studied in acetic acid solution containing 0·25M water. For the alkyl-substituted compounds the rates pass through a maximum at relatively low acid concentrations of ca. 0·05M. Increasing methylation at the carbon atoms adjacent to the amide bond produces increasing rates, which is a behaviour characteristic of ring closure reactions. The interpretation of the data is consistent with the rate-controlling step at the higher acid concentrations being a participation by the terminal carboxy-group to form an azlactone or its solvate which is followed by a rapid acid catalysed ring fission to form the products. When account is taken of the acid–base equilibria in the system, it is shown that the ring closure consists of simultaneous reactions by the protonated amide and the non-protonated amide through its zwitterion. The acid–base equilibrium constants which have been estimated from the kinetic data vary with structure in a similar manner to those of the corresponding acetamides which previously had been determined spectrophotometrically. Formylglycine has a faster rate than acetylglycine and its homologous acylglycines which is inconsistent with its lack of methyl groups to aid a ring closure reaction. These slower rates of the homologous acylglycines are due to the inductive effect of the α-carbon atom which is located at the point of ring closure. Methyl groups, therefore, are ambiguous in their behaviour and their position in the molecule determines whether they cause a rate increase or decrease. Strongly electronegative groups such as trifluoroacetyl and the positive terminal amino-group of a dipeptide have a reaction order of one with respect to the acid concentration and show no rate maximum even at the very high acid concentrations. It is suggested in these cases that the rate-controlling step is the acid catalysed ring fission for which the strongly electronegative groups will cause a decrease in the rate of ring fission and an acceleration in the ring closure.

Substituent effects in the acid solvolysis of para-substituted hippuric acids in acetic acid at low water concentrations. Neighbouring group participation by the carboxy-group

The rates of acid solvolysis of the para-substituted hippuric acids in acetic acid decrease with an increasing acid concentration for Cl, H, Me, and OMe indicating that the rate-controlling step is the ring closure reaction by the non-protonated amide to form an intermediate azlactone or its solvate. The acid–base equilibrium constants derived from the kinetic data vary in accordance with their substituent effects. Although the velocity constants show an irregular sequence, the products of the equilibrium and velocity constants vary as required by the substituent effects. This is in agreement with the reaction rate being dependent upon the zwitterion concentration through which the ring closure proceeds. The NO2 group behaves differently from the other groups and gives only increasing rates with increasing acid concentrations and this behaviour is consistent with that of other strongly electronegative groups such as the trifluoroacetyl and the positive terminal amino-group of a dipeptide. These groups favour ring closure so that the acid catalysed ring fission of the azlactone solvate becomes the slow reaction.

Excited states of six-membered N-heterocycles. Acid-base equilibria and ESR of 4, 10-diazachryzene in the lowest triplet state

The acid-base equilibrium constants of 4,10-diazachryzene in the ground state (pK G) and in the lowest triplet state (pK T) have been measured: pK G I = 1.44, pK G II = 3.22 and pK T I = 2.1, pK T II = 3.6. The triplet state data were obtained from T-T absorption, phosphorescence, and ESR spectroscopy. The Δ m = 2 and Δ m = 1 transitions have been identified for the free base and the two conjugate acids. The zero field splitting (zfs) parameters have been calculated, and phosphorescence lifetimes are reported.

The significance of an unique pH-profile for trypsin catalysis

The pH-profiles for the trypsin-catalyzed hydrolysis of non-N-acylated α-amino substrates were found to be complex. The profile for L-lysine methyl ester gave two rate constants and four acid-base equilibrium constants. The rate constants arise from two different ionic forms (structures) of the site which are reactive. The pKa values calculated from the profile are indicative of an ionizable amino, imidazoyl and carboxyl group at the site, and the free α-amino group in the substrate. A negative heat of ionization was obtained for the group with the smallest pKa, which is kinetic evidence for the existence of a free carboxyl at or near the site.

Catalysis in peptide synthesis with active esters. II. Effects of concentrated carboxylic acids on the aminolysis of benzyloxycarbonyl-L-phenylalanine p-nitrophenyl ester in dioxane.

Abstract The kinetics of the aminolysis reaction of benzyloxycarbonyl-l-phenylalanine p-nitrophenyI ester with glycine t-butyl ester in dioxane were studied in the presence of high concentrations of several carboxylic acids. Bell-shaped curves were obtained when the rate constants observed for the trimethylacetic acid-, acetic acid-, and monochloroacetic acid-catalyzed reactions were plotted against the logarithms of the reciprocals of the acid concentrations. Cyanoacetic acid and trichloroacetic acid did not accelerate, but, rather, retarded the reaction. In the mathematical derivation the catalyst apparently exhibits no accelerating effect, if k0K>kcat, where k0 and kcat are the uncatalyzed and the true catalyzed rate constants respectively, and where K is the acidbase equilibrium constant between the nucleophile and the catalyst in a given solvent. The true catalyzed rate constants, kcat, from which the effect of the protonation of the nucleophile had been eliminated were estimated for the trimethylacetic acid- and acetic acid-catalyzed reactions. The fact that the kcat value (63m−2 min−1) for trimethylacetic acid, the weaker acid, was greater than that (53m−2 min−1) for acetic acid strongly supports the bifunctional, concerted mechanism presented in the preceding paper.

Potentiometric acid-base titrations in molten salts: The acid character of group (VI) oxides as inferred from their reaction with molten KNO 3

The reaction between CrO 3, MoO 3, and WO 3, and fused KNO 3 at 350° has been studied by measuring the variation in the potential of an oxygen electrode relative to an Ag/Ag(I) reference electrode. When the reactions come to an end, the reaction products are determined by acid-base titrations in situ, using Na 2O 2 as titrant. The acid strength of the three oxides decreases in the order CrO 3 > MoO 3 > WO 3. This is evident from the variation of the potential of the oxygen electrode with time, and from the fact that, whereas dichromate is formed in case of CrO 3, only the triomolybdate is obtained from the reaction of MoO 3 with fused KNO 3. WO 3 is stable under the same experimental conditions. Neutralization of all the reaction products with Na 2O 2 leads to the formation of the most basic anion, XO 4 2−. Acid-base equilibrium constants for the reactions: 2 CrO 3 + O 2− → Cr 2O 7 2− CrO 3 + O 2− → CrO 4 2− MoO 3 + O 2− → MoO 4 2− Mo 3O 10 2− + 2O 2− → 3 MoO 4 2− WO 3 + O 2− → WO 4 2− have been calculated. The kinetics of the reaction: 2 CrO 3 + 2 NO 3- → Cr 2O 7 2- + N 2O 5 are established. The reaction is bimolecular with respect to CrO 3 and has an activation energy of 25.3 kcal/mole.

Equilibria in acetic acid: Ionic association constant of acetamidium perchlorate and acid-base equilibrium constant of acetamidium acetate at 105.7°

In acetic acid at 105.7� acetamidium perchlorate has an ionic association constant KA = 0.0491 x 106 l. mole-1, a limiting equivalent conductivity A, = 79.4 cm2 Ω-1 equiv.-1, and a centre-to-centre distance between the ions � = 4.67 Ǻ. The acid-base equilibrium constant for acetamidium acetate KB = [ΣNH3Ac+ OAc-]/ [NH2Ac] increases slightly with increasing dielectric constant.

Equilibria in acetic acid: ionic association constant of oxonium perchlorate, acid-base equilibrium constant of oxonium acetate, and autoprotolysisconstant of acetic acid at 105.7°

In aqueous acetic acid, 0.114-0.152M H2O, at 105.7�, oxonium perchlorate has limiting equivalent conductivity Λ0 90.7, ionic association constant KA = [H3O+ClO4-]/[H3O+][ClO4]f2 = 0.0310 x 106 and centre-to-centre distance between the ions (� = 4.84 Ǻ. The acid-base equilibrium constant for oxonium acetate KB = [ΣH3O+ OAc-]/[H2O] increases with the dielectric constant so that -log KB = 1.532+29.44/D. The autoprotolysis of acetic acid also increases with the dielectric constant.

Potentiometric acid—base titrations in molten salts—III. The use of a metal/ metal-oxide electrode as an indicator electrode and the determination of acid—base equilibria in fused KNO3

The feasibility of the Cu/CuO electrode for O2−-ion determination and for acid—base titrations in molten KNO3 at 350°C has been established. The potential of the electrode varies linearly with log [O2−] and the slope of the calibration curve is 2·303(RT/2F) at the working temperature. This allows the determination of the Cu/CuO/O2− standard potential. The CuO electrode is used for titrating the acid K2Cr2O7 with Na2O2 in molten KNO3. Sharp potential drops are recorded at the equivalence points. The equilibrium constant of the reaction Cr2O72− + O2− = 2CrO42− is calculated as ca 1·8 × 1012.The standard O2/O2− potential in fused KNO3 at 350°C has been calculated indirectly. With this value and the potentiometric titration curves obtained with the oxygen electrode, the first known acid—base equilibrium constants in fused salts are computed, as follows: ▪A correlation is drawn between the equilibrium constants calculated in the present study and the acidity numbers of the same oxy-anions previously proposed.