Claudio Tortato

Sulfuric Acid on Silica-gel: an Inexpensive Catalyst for Aromatic Nitration

Abstract Solid acidic catalysts made of sulfuric acid supported on silica-gel and their application to the nitration of aromatics with nitric acid and isopropyl nitrate are described. Substrates with very different levels of activation were investigated. Methods to overcome the poisoning produced by water and to tune the catalyst activity according to the reactivity of the substrate are outlined.

Acidity and reactivity of trifluoromethanesulfonic acid in liquid and solid acid catalysts

Abstract The acidic properties of CF 3 SO 3 H/SiO 2 acid catalysts have been investigated by the protonation of weak bases (B) (B+H + ⇌BH + ) and the proton-transfer process from (H + A − ) to (BH + A − ) has been analysed by a thermodynamic procedure used to account the variation of the activity coefficient terms of the species involved. Acid–base systems with different substituted pyridines as back-titrating agents of BH + (i.e. BH + →B) have also been studied and the changes observed in the acid–base interactions according to basicity of pyridines are discussed. The results in solid phase have been compared with those observed in concentrated aqueous solutions (i.e. CF 3 SO 3 H+H 2 O) where “acidity” and “protonating ability” have been distinguished as parameters of interest in the description of nonideal acid systems. Silica loaded with CF 3 SO 3 H, with H 2 SO 4 and with a mixture of both acids have been tested in acid catalysed reactions and their catalytic effectiveness has been explored towards substrates with high acid requirements for the conversion reagents–products. From the available observations in liquid and in solid phase, CF 3 SO 3 H has been proved to be a less effective acid catalyst than expected from the observed protonating ability of acidic medium. Strong interactions between ionic species and the involvement of ion-pairs in concentrated acid systems have been suggested.

On the acidity of liquid and solid acid catalysts. Part 2. A thermodynamic and kinetic study for acid-catalysed nitrations

Solid acids prepared by adding sulfuric acid on silica gel have been used as catalysts in the nitration of nitrobenzenes and their properties have been tested by kinetic studies at 25°C. Nitration rates in concentrated aqueous solutions of sulfuric acid were also analysed and the catalytic efficiencies of sulfuric acid in liquid and solid phase were compared by using kinetic data of analogous compounds. The results show that the solid acid samples exhibit nitrating properties very similar to those observed in concentrated aqueous solutions of sulfuric acid (range of 90 wt%). The relationship between nitration rates and effective concentration of electrophilic species [NO2+], determined by studying the protonation–dehydration equilibrium of nitric acid in strong acids (HNO3 + H+ ⇌ H2O + NO2+), was tested to better understand the acidity properties of medium.

On the acidity of liquid and solid acid catalysts. Part 3. Esterification of benzoic and mesitoic acids

Methyl esters of benzoic and mesitoic acid have been prepared with high yields (>98 wt%) from the corresponding carboxylic acids + methanol in aprotic solvents over samples of H2SO4/SiO2 at 60°C. The results show a high catalytic efficiency of the solids but also suggest an acid strength comparable to that observed in concentrated aqueous H2SO4 (range >90 wt%) when the acid requirements for the esterification of analogous compounds in aqueous acid solutions are taken into account. Indeed, different reacting species, i.e., ArC(OH)2+ from benzoic acid and 2,4,6‐triMe‐ArC=O+ from mesitoic acid are involved in the esterification, but the mesitoyl cation can be formed and esterified in the acidity ranges between 92 and 98 wt% H2SO4.

On the acidity of liquid and solid acid catalysts: Part 1. A thermodynamic point of view

The protonation equilibria of weak bases (B) in solid acids (HClO4/SiO2, CF3SO3H/SiO2, H2SO4/SiO2) were studied by UV spectroscopy and the results were compared to those obtained for analogous compounds in concentrated aqueous solutions of strong acids (HClO4, CF3SO3H, H2SO4). The behaviour of B in liquid (L) and solid (S) phase was analysed by titration curves, log[BH+]/[B] ratios and thermodynamic pKBH+ values. It has been shown that the proton transfer process acid → base (i.e., from (H+A-)(L,S) to (BH+A-)(L,S)} can be described by the relationship observed between the activity coefficient terms that are to be taken into account for acid–base equilibria occurring in nonideal systems ( – log(fBfB+/fBH+)(L,S)= -nBA log(fA–fH+/fHA)(L,S)) and can be estimated by the nBA values. Two “activity coefficient functions” (i.e., Mc(B) = – log(fBfB+/fBH+)and Mc(s) = – log(fA–fH+/fHA)) were used to describe, respectively, the equilibria of B and the equilibria of the acids in concentrated aqueous solutions and the meaning of terms “activity coefficient function” and “protonating ability of an acid” were discussed. The difference between “acidity functions”, determined for solutes (Ac(i)) and solvents (Ac(s)) in aqueous acids, and the Hx acidity functions, the latter developed for solutes in analogous media by the Hammett procedure, was also shown.

Quantum Chemical Investigation on Indole: Vibrational Force Field and Theoretical Determination of Its Aqueous pKa Value

Indole and its derivatives are molecules which play important roles in different fields, from biology to pharmacology. Here we report a thorough investigation on the anharmonic force fields of indole as well as the ab initio determinations of its gas phase basicity and aqueous pK(a) value. For the geometry optimizations, the calculations have been performed using both density functional (DFT) and second-order Møller-Plesset (MP2) levels of theory employing different basis sets. Anharmonic force fields have been obtained employing both the B3LYP and the B97-1 functionals and an hybrid approach: the best agreement to the experimental data has been determined employing the B3LYP functional combined with the recently developed N07D basis set (mean unsigned error, MUE, of 5.1 cm(-1) and a root-mean-square error, RMSE, of 7.2 cm(-1)). Gas phase basicity and proton affinity have been computed employing several computational schemes, namely the G3 and G4 Gaussian models, the complete basis set (CBS) extrapolation methods of Petersson and co-workers, several DFT calculations, and different hybrid extrapolation schemes based on combining single-point energy calculations performed at MP2 as well as at coupled cluster level of theory with single, double and perturbative triple excitations, CCSD(T). Regarding the aqueous pK(a) computations, two implicit solvation models (SMD and SM8) have been employed to determine the free energy of solvation and the corresponding pKa value.

Thermodynamic nitration rates of aromatic compounds. Part 3. Nitration of aromatic compounds in concentrated aqueous trifluoromethanesulphonic acid

The kinetics of nitration of mesitylene, toluene, benzene, bromobenzene and methyl phenyl sulphone, reacting as free bases in the range 50–100 wt% CF3SO3H are reported. The rates, related both to the stoichiometric concentration of nitric acid (rate =k2obs[Ar][HNO3]) and to the effective concentration of electrophilic species (rate =k2°[Ar][NO2+]) have been determined. Compared with k2obs, which exhibits a large solvent effect, the true nitration rates (k2°) were found to be almost independent of medium acidity and, for a given compound, rather similar in different acids. The k2° values were also found to be linearly related to the rate constants for nitration in the gas phase.The rates of reaction for methyl phenyl sulphone in 80–100 wt% acid were found to be lower in CF3SO3H than in H2SO4 by a factor of 8. A similar rate profile is observed, however, with an increase up to 90% acid and a decrease between 90 and 100%. The dependence of the rate profiles upon acidity and temperature in very concentrated acid solutions is discussed.

Equilibria and nitration of sulphonic acids in concentrated sulphuric acid

Protonation equilibria of alkane- and arene-sulphonic acids in concentrated aqueous solutions of sulphuric acid have been investigated, and the pKa values estimated using the Mc activity coefficient function. The reliability of the new values is discussed, since the results are found to disagree with the previous figures obtained by an acidity function procedure.The equilibrium data have been applied to nitration studies of arenesulphonic acids reacting in an analogous medium. The k2obs and k2° rate constants were estimated in connection, respectively, with the stoichiometric (rate =k2obs[Aromatic]st[HNO3]st) and with the effective concentration of solutes (rate =k2°[ Aromatic][NO2+]).In the range 40–90 wt% H2SO4, the k2° rates are found to be independent of solvent effects. Above 90 wt% H2SO4, a large decrease is observed in the rates of nitration of benzenesulphonic acid compared with that of other substrates, so that the involvement of PhSO3–·NO2+ ion-pair at high acidity is suggested.Equilibria and nitration of phenyl sulphoxides have also been discussed, and it is suggested that it is the unprotonated species (i.e. PhSO3–, PhSOMe) that undergoes nitration.

Formates for green catalytic reductions via CO2hydrogenation, mediated by magnetically recoverable catalysts

Abstract Precious metal catalyst has been prepared by conventional wet impregnation method followed by precipitation and reduction with hydrogen finally passivated with water in air. The magnetically recoverable catalyst has been prepared starting from a stoichiometric Fe3O4 and ZrO2–Fe3O4 as supports prepared following a sequential precipitation procedure. Precious metal catalysts supported on carbon, alumina, magnetite and zirconia-magnetite nanocomposite has been used in the reduction of nitrobenzenes and acetophenone by using sodium and potassium formate as reducing agent in the presence and in absence of an aqueous phase. In addition, the same catalysts has been tested in CO2 and NaHCO3 hydrogenation, for verifying their potentiality in the CO2 as hydrogen carrier for hydrogenation processes.

New magnetically recoverable palladium-based catalysts active in the alkoxycarbonylation of iodobenzene

Abstract New magnetically recoverable catalysts have been synthesized by deposition of 1% palladium (Pd)-metal on the polymer poly(1-oxo-trimethylene), containing 15% of magnetite. The magnetite allows the complete recovery of the catalyst with the simple application of an external magnetic field. The activity of such a catalyst has been studied under phosphine-free conditions in the alkoxycarbonylation of iodobenzene to the corresponding benzoic acid esters.