Nunziata C. Marziano

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.

The Mc activity coefficient function for acid–base equilibria. Part 5. The Mc activity coefficient for a reliable estimate of thermodynamic values

The general validity of the relationship log (fZifAi/fDi)=nij(log fZjfAj/fDj)(Z = base, A = acid, D = conjugated acid) has been tested using different weak bases involved in protonation, ionization, and rate processes. Experimental data in aqueous sulphuric and perchloric acid have been treated simultaneously by a mathematical procedure which allows the transference of data from one acid to another. The analysis of the thermodynamic values of equilibria and rate processes shows the advantages both of the new treatment and of the latter estimated Mcf(x) functions.

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.

Raman and kinetic evidence of NO2+ ion in solid acid catalysts

Samples of H2SO4=SiO2 loaded with HNO3 and analysed by Raman spectroscopy exhibit the band of the NO a ion, which is formed by the protonation‐dehydration equilibrium of HNO3 in strong acidic media. The eAectiveness of NO a as a nitrating species is tested in the nitration of aromatic compounds towards substrates with high acid requirements for the conversion reagents‐products. The acidic properties of the acid systems used as catalysts are also described, and the thermodynamic parameters related to the acid‐base proton transfer process are discussed. ” 2000 Elsevier Science B.V. All rights reserved.

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.

The MC activity coefficient function for acid–base equilibria. Part 3. Improvement on the MC function by mathematical treatment

The empirical relationship log fBjfH+/fBjH+=njilog fBifH+/fBiH+ for protonation equilibria in aqueous sulphuric acid, between any two weak bases i and j has been tested. The weak bases examined are primary nitroanilines, amides, tertiary amines, indoles, benzophenones, sulphoxides, and hydroxy- and alkoxy-benzenes. A mathematical treatment which accounts for all the indicators, independently of their structure or acidity range, has been used. The results obtained show in all cases the validity of the relationship examined. Activity coefficient functions have been calculated by using both structurally similar indicators and all the compounds. The results show the identity between the particular functions and the general one. A single function [MCf(X)‘generatrix function’] able to give the dependence on acid concentration of {log ([BH+]/[B])– log [H+]} of any indicator is obtained. The analysis of relative nji values for different or structurally similar indicators show the limitation of classing indicators by series.

Thermodynamic nitration rates of aromatic compounds. Part 2. Linear description of rate profiles for the nitration of aromatic compounds in the range 40–98 wt% sulphuric acid

New studies of nitration in sulphuric acid have been made at high acidities (80–98 wt% H2SO4) where at 90 wt% a maximum occurs in the observed rate constant for aromatic compounds. A new treatment of experimental data is reported which takes into account the effective concentration of solutes and changes in the fArfNO2+/f ratio with acidity, using the Mc function. Compounds nitrating at low and high acidity are tested, and representative examples of the results obtained are given. Experimental evidence points to two significant findings in this study. The variation in the observed rate constants below and above 90 wt% H2SO4 is related to ionization of nitric acid to give NO2+ ions. The new procedure applied to the kinetic data allows us to obtain a linear description of rate profiles for substrates reacting as free bases in the range 40–98 wt% H2SO4.

Determination of the basicity of nitric acid in concentrated sulphuric acid by Raman and ultraviolet spectroscopy

The ionization ratio [NO2+]/[HNO3] of nitric acid, in 80–96% sulphuric acid, has been evaluated by Raman and u.v. methods; the thermodynamic pKa of the equilibrium involved has been calculated by using the Mc activity coefficient function.