Hammett Acidity Function Research Articles

Breakdown of the Hammett acidity function concept

Breakdown of the Hammett acidity function concept

A General Relation Between H0, Acid Concentration, and Water Activity for Strong Acids at Moderate Concentrations

Values of the Hammett acidity function H0 can be correlated for strong acid solutions up to −H0 values of 3.5 via a Bascombe and Bell type equation:[Formula: see text] taking into account the degree of dissociation of the acids. This can be interpreted according to[Formula: see text]and[Formula: see text]Alternatively, an approach based on the treatment of equilibrium constants in terms of mole-fractions yields[Formula: see text]valid for the same acids (H2SO4, HClO4, HCl, and HNO3) and again involving a tri-dehydration of the tetra-hydrated proton. The log of the ratio [Formula: see text] has been found to be proportional to −H0 itself and to be otherwise independent of the nature of the acid anion.

Electrochemical behaviour and glass formation in molten bisulphates

Molten bisulphates form an interesting class of low-temperature fused salts which quench to form glasses; some technical and theoretical applications of these systems are discussed. The NaHSO4KHSO-4 eutectic (46.5% KHSO4, m.p. 125°C,Tg 25°C) is easy to study since at lower temperatures decomposition of the melt is retarded and supercooling readily occurs. The solvent properties of this eutectic towards both inorganic and organic solutes have been investigated. The oxidation states available to transition and post-transition metals were studied spectrophotometrically and voltametrically; the results show a very restricted pattern of redox chemistry. The (abbreviated) electrochemical series is V(V)/V(IV)>Mn(III)/Mn(II)>H(I)/H2(O)>Ag(I)/Ag(O)>Mo(VI)/Mo(V)> Hg(I)/Hg(O)>Cu(II)/Cu(O)>Zn(II)/Zn(O). The Hammett acidity function,H0, was measured as in the range −1 to −2. Various factors influencing redox reactions in the melt are discussed.

Hammett acidity function in concentrated aqueous solutions of hydrochloric acid–lithium chloride

The Hammett acidity function in hydrochloric acid–lithium chloride aqueous solutions, at 25·0 °C has been measured at various ionic strengths (µ). The H0-value is very sensitive to the ionic strength. H0+ log CH+ is not a constant but a plot thereof against CH+(at constant ionic strength) is linear and a set of such plots, for different µ-values, are parallel. At a given acid concentration: H0=Lµ+ constant (the constant is almost zero). Other relationships are presented and discussed.

Reduction of cerium(IV) by hydrogen peroxide. Dependence of reaction rate on Hammett's acidity function

Reduction of cerium(IV) by hydrogen peroxide. Dependence of reaction rate on Hammett's acidity function

The acid-catalysed hydrolysis of nitro- and nitrito-pentammine complexes of cobalt(III), rhodium(III),and iridium(III)

The kinetics of the replacement of nitro- and nitrito-ligands from the complexes M(NH3)5X2+(where X = NO2 or ONO and M = Co, Rh, or Ir) at various temperatures have been studied. The nitro-complexes are hydrolysed in concentrated acid and the logarithm of the corresponding first-order rate constant is proportional to Hammett's acidity function. The nitrito-complexes are hydrolysed in relatively dilute solutions of strong acids. The logarithm of first-order rate constant is proportional to log [H+]2 or 2H0. The Arrhenius parameters and kinetics are interpreted as follows: (a) The nitro-complexes are hydrolysed by a loss of NO+ ion from the transition state; water is not involved in the transition state, and (b) the nitrito-complexes react via a mechanism involving mono- and diprotonated forms. The equilibria are very sensitive to ionic strength. It is difficult to ascertain from the kinetics whether or not water is involved in the rate-determining step. The similarity of Arrhenius parameters, irrespective of central metal ion, suggests that oxygen–nitrogen bond breaking occurs rather than metal–oxygen.

Hammett acidity functions (H–) of lithium, sodium, and potassium glycoxide solutions in ethylene glycol at 25 °C

H – scales have been set up for solutions of lithium, sodium, and potassium glycoxides in anhydrous ethylene glycol at 25 °C by use of nitro-substituted anilines and diphenylamines as indicators. The order of increase of H– with alkali concentration is Li < Na < K. This may be due to ion-pair formation which will be in the order Li > Na > K, and is reflected in the H– being in the reverse order, as expected. The H– scales for the three systems (i) NaOH in water, (ii) NaOMe in methanol, and (iii) Na glycoxide in ethylene glycol are compared. A comparison is also made of the aqueous and glycolic systems on a mole-fraction scale, which seems to indicate that, like the autoprotolysis constant, the H– scale in any amphiprotic solvent is largely a composite function of bulk dielectric constant of the solvent, as well as the intrinsic acidity and basicity of the solvent molecules.

Destruction of polyacetals in acid media

1. The kinetics of the hydrolysis of methylal in aqueous solutions of sulfuric acid was investigated by the method of gas-liquid chromatography. 2. A linear dependence of the logarithm of the effective rate constant of the reaction, calculated according to first order, on the Hammett acidity function H0 was demonstrated. 3. The kinetic isotopic effect in solutions of 10% and 20% H2SO4 at 25° is equal to three. 4. The hydrolysis of methylal proceeds according to an A-1 mechanism.

HO data and other properties of some hydrogen fluoride based solvents

AbstractThe properties of a new acidic solvent system consisting of equimolar amounts of anhydrous hydrogen fluoride and oxygen‐containing components, such as alcohols, ether or acetic acid, are described. Through the addition of BF3 Hammett's acidity function, HO, can be varied between ‐ 1.4 and ‐ 7. These acids proved to be particularly useful for the study of the photochemistry of aromatic hydrocarbons.Measurements of the Hammett function, HO, are reported for hydrogen fluoride containing sodium fluoride, water, ethanol and BF3.

Étude des effets de sels dans les solvants organiques—I : Mécanisme de l'addition de l'acide acétique au cyclohexène en solvant acide acetique

The mechanism of acetic acid addition to cyclohexene has been studied with acetic arid as solvent. Study of the rate gives a linear correlation between log k exp and H o (Hammett acidity function). This correlation depends on the protonic acid nature (H 2SO 4, HClO 4, HSO 3C 6CH 3(p) and HSO 3CH 3). It has also been demonstrated that general acid catalysis takes place, and solvent isotope effects show that the mechanism implies a slow proton transfer, analogous to that observed for olefinic hydration. This work has resulted in a general mechanism for solvents less basic than water, such as acetic arid.

Evidence for a cyclic A A11 mechanism in the hydrolysis of benzylidene diacetates

The acid-catalysed hydrolyses of a series of benzylidene diacetates have been studied. The rates of hydrolysis were found to depend on the Hammett acidity function H0, and plots of log kobs+Ho against log aw had negative slopes. The hydrolyses therefore occur by an A1 mechanism, and the correlation of the rates of reaction with σ+ establishes that the mechanism is AA11. The entropies of activation obtained are consistent with the formation of a cyclic transition state, in which a protonated acetoxy-group acts as a Lewis acid in assisting the loss of the second-acetoxy-group.

The protonation of weak bases. A study of the protonation of some nitroaniline indicators and the determination of the H 0 values of aqueous sulphuric acid

The protonation of some nitroaniline indicators has been studied in aqueous sulphuric acid over the temperature range 15–55°. The log l values of these indicators decrease with increasing temperature and the pKBH+ values vary linearly with temperature. These log l and pKBH+ values have also been used to calculate the Hammett acidity function (H0) of the acid media over the temperature range 15–55°.

Nucleophilic catalysis. Part III. Acid-catalysed hydrolyses of 3-phenylsydnone and 3-m-nitrophenylsydnone

The effects of added acids and salts on the hydrolyses of 3-phenylsydnone and 3-m-nitrophenylsydnone have been studied in both water and aqueous dioxan as solvents. The mechanism of hydrolysis is shown to involve specific hydrogen ion catalysis and nucleophilic catalysis by the anions of the acids involved. Values of the Hammett acidity function H0 are reported for hydrochloric and hydrobromic acids in 40% dioxan–water.

Correlation of composition of acid solutions with activity and acidity functions

Presents correlation tables of percentage composition with molarity, molality, activity coefficient, water activity, and Hammett Acidity function for aqueous solutions of hydrochloric, perchloric, phosphoric, and sulfuric acids.

Studies in nuclear magnetic resonance—IX. Rotational barriers in substituted N,N‐dimethylbenzamides

AbstractThe barriers to rotation about the CN bond in eighteen substituted N,N‐dimethylbenzamides have been determined by complete line shape analysis of the NMR spectra of the N,N‐dimethyl protons. The barriers have been correlated with the substituent constants σ and σ+. It has been shown that polar solvents increase the barrier in N,N‐dimethylbenzamide.Acid catalysis of rotation about the amide CN bond in N‐(p‐N,N‐dimethylcarboxamidobenzyl)‐pyridinium bromide has been investigated. 18O exchange studies show that catalysis is due to N‐protonation rather than the formation of a tetrahedral intermediate. The rate of rotation is a function of the Hammett acidity function, H0, and the water activity, and it is shown that proton exchange between the N‐ and O‐protonated species involves the intermediacy of a water molecule.The differences in chemical shifts for the non‐equivalent N, N‐dimethyl groups of the benzamides are also a function of the substituents. Possible explanations of this phenomenon are discussed.

The kinetics of hydrogen isotope exchange reactions. Part XV. Acid-catalysed tritiation of mesitylene and correlation of exchange rates with Hammett's acidity function

Download options Please wait... Article information DOI https://doi.org/10.1039/J29690000950 Article type Paper Download Citation J. Chem. Soc. B, 1969, 950-956 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions The kinetics of hydrogen isotope exchange reactions. Part XV. Acid-catalysed tritiation of mesitylene and correlation of exchange rates with Hammett's acidity function J. R. Adsetts and V. Gold, J. Chem. Soc. B, 1969, 950 DOI: 10.1039/J29690000950 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author J. R. Adsetts V. Gold

Rates of formation of isoperthiocyanic acid (5-Amino-1,2,4-dithiazole-3-thione) from thiocyanate in aqueous solution

The rates of formation of 5-amino-1,2,4-dithiazole-3-thione from potassium thiocyanate in aqueous sulphuric acid have been studied spectrophotometrically. They show close to third-order dependence on thiocyanate concentration and linear dependence on the Hammett acidity function, h-. Between 25 and 45� the rate constant is given by the Arrhenius expression k = 9.27 x 10-5exp(-8800/T) 1.2 mole-2 sec-1 A mechanism is proposed.

Studies of the Pinacol Rearrangement. IV. The Kinetics and Mechanism of the Rearrangement of meso- and dl-2,2′ Dimethoxybenzopinacol

Abstract The first-order rate constants of the perchloric acid-catalyzed rearrangements of the meso-and dl-2,2′-dimethoxybenzopinacols were measured in anhydrous acetic acid. A linear correlation of the logarithm of the first-order rate constants (logk) with Hammett acidity function (H0) was obtained in each case. The large positive values of the entropy of activation support the A-l mechanism, which is also supported by the linearities of logk with H0, suggesting that the classical carbonium ion is an intermediate in these processes. The migration aptitude of the meso-isomer decreases with time, while that of dl increases. On the basis of this information, the mechanism of the rearrangement was discussed.

Concurrent Oxygen Exchange and Racemization Reactions of Diaryl Sulfoxides in Concentrated Sulfuric Acid

Abstract When diaryl sulfoxides are dissolved in concentrated sulfuric acid at 0°C, characteristic bluish colored solutions are obtained, from which the starting sulfoxides can be recovered quantitatively upon quenching with a large amount of water. When 18O-labeled and optically active sulfoxides are used, one observes the concurrent oxygen exchange and racemization reactions of the sulfoxides. The kinetic behaviors of these reactions have been studied in detail using various substituted diaryl sulfoxides. The rate of the oxygen exchange was found to be identical to that of the racemization while the rates of the racemization were nicely correlated with the Hammett acidity functions (H0) (Slope=1.07). Meanwhile the polar effect of subsituents on the rates of both oxygen exchange and racemization was found to be very small. From these results possible mechanisms for these reactions have been discussed.

Substituent effects of positive poles in aromatic substitution. Part VI. The nitration of substituted anilinium ions through proton loss from the –NH3+group

The rate profile for the nitration of 2-chloro-4-nitroaniline in 85–100% sulphuric acid accords with a reaction of the free amine, and this interpretation is consistent with the rate of nitration relative to other aniline derivatives and with the solvent isotope effect for reaction in deuteriosulphuric acid. Similar evidence indicates that the rate-determining step in the nitration of p-nitroaniline to picramide involves the nitration of the p-nitroaniline molecule. When the concentrations of these amine molecules are calculated from the Hammett acidity function, the relevant second order rate coefficients for nitration are ∼108 l. mole–1 sec.–1 in accord with reaction on encounter with nitronium ions. However, the activation energies are unexpectedly high for such encounter reactions; from this discrepancy and from related evidence with other substrates, it is suggested that the lifetime of the free amines may be too short to permit the formation of the free amine and the attack of the nitronium ion to be regarded as discrete steps.