Ind. Eng. Chem. Res. 1992,31, 787-791
787
Sulfonation of 2-Naphthol by Sulfuric Acid: Rate Measurements and Kinetic Model Paolo Beltrame,t Giorgio Bottaccio,* Paolo Carniti,*i+and Giuseppina FelicioliI Dipartimento di Chimica Fisica ed Elettrochimica, Universitd di Milano, via Golgi 19, 1-20133 Milano, Italy, and Centro Ricerche, Acna Chimica Organica, via Fauser 4,I-28100 Novara, Italy
A kinetic study of the sulfonation of 2-naphthol was performed in homogeneous sulfuric acid solutions of concentrations ranging from 91 to 97%. Temperature was in the range 15-25 "C. Mono- and disulfonic acids typically represented >90% of the products. A kinetic model for the overall reaction has been proposed. The model parameters, as well as their dependence on temperature and acid concentration, were determined by optimization computer procedures. The activation energy of first and second sulfonation was found to be in the range 19-21 kcal/mol. The presence of an S03H group on the naphthalene rings played an activating or a deactivating role on further sulfonation, depending on the case. Introduction Sulfonation of 2-naphthol to 2-naphtholsulfonic acids (having one, two, or three SOsH groups) is an industrial process employed for production of various 2-aminonaphthalenesulfonic acids, useful for dyes manufacture. In particular, 2-naphthol-6-and -&sulfonic, 2-naphthol-3,6and -6,&disulfonic, and 2-naphthol-3,6,8-trisulfonic acids are obtained by 2-naphthol reaction with sulfuric acid. Acid concentrations range from 80 to 102%, and temperature from 60 to 115 "C, according to the product of interest. Since 2-naphthol does not quickly dissolve in sulfuric acid, the reaction conditions are initially heterogeneous; later, the reaction proceeds in a homogeneous phase. Batch reactors, where the sulfuric acid concentration is adjusted during the reaction, are usually employed. Information on the sulfonation products as functions of the process conditions has been available for a long time (Donaldson, 1958; Cerfontain, 1968). However, due to the complex nature of the system, a detailed kinetic model of the process seems to be required in order to enable one to better direct the industrial reaction. In the present study, kinetic runs on 2-naphthol in homogeneous solution have been carried out in mild conditions, as to both acid concentration and temperature, to limit the sulfonation products to mono- and disulfonic acids. Preliminary results of this study were in part presented previously (Beltrame et al., 1990). Experimental Section Materials. 2-Naphthol and all the 2-naphtholsulfonic acids employed as reagents and/or standards for HPLC analysis were commercial ACNA products or were prepared by standard procedures in the ACNA laboratories. All other chemicals were Carlo Erba or Fluka analytical grade products. Reaction Runs. The kinetic runs were carried out in a thermostated glass reactor equipped with a mechanical stirrer. For each run a known amount of sulfuric acid at the proper concentration was thermostated in the reactor; a weighed quantity of reactant (2-naphthol or an intermediate sulfonic acid) was then quickly added. At intervals, samples were withdrawn by a syringe preconditioned at the reaction temperature, quenched by a standard quantity UniversitA di Milano.
* Acna Chimica Organica.
of an aqueous solution of n-tetrabutylammonium hydroxide, weighed, diluted by a n-tetrabutylammonium hydrogen sulfate-sodium dihydrogen phosphate buffer solution (pH 5.2), and quantitatively analyzed. Temperature was kept constant (fO.l "C) at 15,20, or 25 "C. The sulfuric acid concentration was kept in the range 91-97%, the acid being in such an excess with respect to the other reactant as to maintain its concentration constant within 1% (at least 200 times more H,S04 than that required for monosulfonation). The initial concentration of 2-naphthol was around 0.08 M, while in the runs carried out on sulfonated derivatives (charged either as acids or as K or NH4 salts) the initial reactant concentration was in the range 0.007-0.017 M. In order to quickly attain homogeneous conditions, 2naphthol was introduced as a solution in a small amount of dimethyl sulfoxide (DMSO) in the main set of kinetic runs. When added, DMSO was in molar ratio
