639. Mechanism of the Kolbe–Schmitt reaction. Part II. Influence of the alkali metal

Abstract

A study has been made of the behaviour on heating of the alkali-metal mono-and di-salicylates and *-hydroxybenzoates, and of some related substances. Some rate studies are reported for these reactions.When a mixture of 14C-labelled potassium carbonate and dipotassium salicylate was heated at 200" appreciable exchange accompanied the rearrangement with formation of labelled p-hydroxybenzoate. The mechanism of these reactions is discussed DIFFERENCES in behaviour of the alkali metals in organic reactions have often been noted.2739 4 The Kolbe-Schmitt reaction provides a classical examp1e.l' 2s 59 and Ost * drew attention to the remarkable difference in behaviour between sodium and potassium salicylate at 220". The former yielded disodium salicylate, the latter dipotassium $-hydroxybenzoate. There occur in the literature many conflicting reports concerning the thermal rearrangement of the alkali-metal salts of aromatic hydroxy-carboxylic acids. In large measure, these are due to the use of carbon dioxide as a supposedly inert atmosphere in the early studies of these rearrangement^,^^ lo for under these conditions it is possible for carboxylation to intervene.More recent studies have revealed differences not only in the ratio of orthoto parasubstitution, but also in the extent of carboxylation and in the tendency to form hydroxydicarboxylic acids, particularly at higher temperatures.6311 In the Marasse procedure also differences have been observed 11, I29l3 and these distinctions hold in the phenol, naphthol, and heterocyclic series.6In order to establish how far purely thermal rearrangements are involved in the Kolbe-Schmitt reaction, a study of the behaviour on heating of various alkali-metal derivatives of salicylic acid and other aromatic hydroxy-carboxylic acids was undertaken.Most of our experiments were conducted in vacuo; this served to minimise oxidation, and in those cases where carbon dioxide was evolved eliminated the possibility of recarboxylation. From our studies on the monoalkali-metal salts the following facts have emerged. Monosodium salicylate at 300" yields equimolecular amounts of phenol, carbon dioxide, and disodium salicylate. A mixture of sodium phenoxide and monosodium salicylate at 140" gives equimolecular amounts of free phenol and disodium salicylate.In 1875 Kolbe

Chemical Technologies for Exploiting and Recycling Carbon Dioxideinto the Value Chain

“…Salicylic acid is synthesized via the Kolbe-Schmitt method, [85][86][87][88] in which sodium phenolate is reacted with CO 2 at high temperatures and pressures. The high ortho-selectivity of salicylic acid is caused by the chelate effect of sodium ions.…”

“…[87] In contrast, potassium phenolate almost exclusively yields p-hydroxybenzoic acid (PHB) (Scheme 10). [88,89] Enhanced yields in p-hydroxybenzoic acid are obtained with the larger alkali and earth alkali cations. [90]…”

Flow Chemistry of the Kolbe‐Schmitt Synthesis from Resorcinol: Process Intensification by Alternative Solvents, New Reagents and Advanced Reactor Engineering

“…For activated, electron-rich phenols, with two or more electron-donating OH groups, the carboxylation can be carried out in a smooth variant of the Kolbe-Schmitt processing (aqueous solution, reflux at 100°C, ambient pressure, soft hydrogen carbonate as carboxylating agent), different from the autoclave processing with strong alkaline media typical of the less reactive phenols [42,43]. In this manner, 2,4-dihydroxybenzoic acid (2,4-DHBA; also known as b-resorcylic acid) is synthesized from resorcinol (Scheme 1).…”

Carbon Dioxide as a Carbon Resource – Recent Trends and Perspectives

“…Today the process is known as the Kolbe-Schmitt reaction, and either salicylic acid or p-hydroxy benzoic acid can be obtained selectively by choosing either sodium or potassium ions, respectively, to balance the anionic charge of the phenolate (Scheme 13). The ortho selectivity in the presence of sodium (and also lithium) ions is a result of the chelate effect exerted by these cations [121], while the larger potassium ions drive the reaction selectively towards the para-substituted product [122,123]. The synthesis of pharmaceuticals like acetyl salicylic acid (ASS, Aspirin), and of dyes and pesticides are the main applications for salicylic acid.…”