The Henkel reaction of alkali metal salts of naphthalene acids is reported. The salts of 1-naphthoic acid, 2naphthoic acid, 1,3-NL)CA (naphthalene-1,3-dicarboxylic acid), 2,3-NDCA1 l,BNDCA, l,&NDCA, and 2,7-NDCA and mixtures containing all other diacid salts afford the naphthalene-2,Bdicarboxylate as the principal product. Reaction variables, the detection of intermediate acids, and physical state of reactants and their mechanistic implications are discussed. An intermolecular mechanism is supported; contentions for an intramolecular mechanism are examined.During the past decade, the cognoscenti of industrial chemistry have observed with interest the development of the Henkel reaction, which is the thermal rearrangement or disproportionation of aromatic carboxylates of alkali metals to symmetrical aromatic dicarboxylates. This reaction, sometimes referred to as the Raecke process, is usually carried out in an inert atmosphere between 350 and 500' in the presence of catalytic quantities of cadmium salts. The first results of studies on this reaction were reported by Raecke and co-workers at Henkel et Cie. in 1952.2 Strong emphasis was placed on the process variables of the benzenecarboxylates, since the principal product is terephthalic acid, a valuable component for polyester derivatives. Several important reactions exemplifying this process are listed below. COOK 2 ,,,-,+Q h K COOK +
KsIn the naphthalene series, our prime concern in this paper, potassium 1and 2-naphthoate disproportionate to dipotassium naphthalene-2,6-dicarboxylate12 which is also the product of the rearrangement of dipotassium naphthalene-1 ,&dicarboxylate. Similar reactions apply to the carboxylates of furan, thiophene, pyrrole, and pyridine. Potassium is the cation used most often in the Henkel reaction. Rubidium and cesium are quite satisfactory, whereas sodium and lithium give much lower yields a t temperatures higher than the usual range of 400-450'. Henkel chemists found that 2-10 mole yo cadmium, and sometimes zinc, is the preferred catalyst concentration. The reaction is carried out under pressure, usually with carbon di-(1) Part I : (2) The basic review article is due t o B. Raecke, Angew. Chem., TO, 1 E. MoNelis, J . 070. Chem., 28, 3188 (1963). (1958).oxide, between 10 and 140 atm. With certain notable exceptions, lower pressures give lower yields, as does replacement of carbon' dioxide with nitrogen. Water, hydrogen, acids, and oxygen are poisons.The mechanism of the Henkel reaction has been the focus of no little c o n t r o~e r s y .~~~ It is the purpose of this paper to resolve some apparent contradictions and to present certain cogent data aimed a t a resolution of the reaction mode. Toward this end, the naphthalene series serves as a model for certain mechanistic generalities.
ExperimentalSalt Preparation.-The naphthoic acid or the naphthalenedicarboxylic acid waa added to an aqueous solution of potaeaium hydroxide with stirring. The base concentration waa equivalent to the added acid. After filtration, the salt solution waa s...
