Condensation of Phenols with Formaldehyde I.
Formation of Phenol Alcohols
F. S. GRAKGER, Combustion Utilities Corporation, Linden, N. J.
T
HE simplest products obtained by reaction between phenols and aldehydes are the more or less watersoluble and crystallizable phenol alcohols, formed by simple addition of single molecules of the reactants: HOCeHs HCHO + HOCsHaCHzOH
+
Crystalline p r o d u c t s f r o m phenol, the cresols, thymol, and guaiacol were described, to the e x t e n t of m e l t i n g point and behavior with one or two solvents; but actual working details were given only in the case of phenol and one or two others. Yields and by-products were not m e n t i o n e d . There was, therefore, no indication as to whether or not the p r o d u c t s mentioned were the only ones formed in the various instances. The properties d e s c r i b e d by one investigator did not always tally with those of the other. The method of Manasse was adopted as the standard method and has remained so. I n the course of the following decade, the preparation of a number of alcohols was described in greater detail and with attention to establishing their identity, mainly by Auwers and his associates, working with xylenols (3). In 1907, Auwers (3) published a general review of the subject.
This is the first of u series of papers contributing further information, both fundamental and practical, of the general type not extensively covered in the literature, relating to the nature of phenol-aldehyde condensation and the products thereof. The present paper discusses the preliminary combination of the original materials, which precedes the condensation to resins. The later papers will deal with the chemistry and characteristics of the condensation process and of the products under alkaline and acid conditions and with different phenols.
This reaction, in all probability, constitutes the first step in all phenol-aldehyde condensations. Any new information as to what a c t u a l l y occurs in this initial stage, therefore, is of some importance in connection with the general chemistry of the subject.
PREVIOUS WORK Phenol alcohols and their conversion into resins (called “saliretins”) on heating, etc., have been known since 1843 (9); that is, long before even the reaction of phenols with aldehydes a t all was known; and, when the latter fact was discovered by von Baeyer in 1872 ( 5 ) , he attributed the formation of the resins and diphenyl methane derivatives which he obtained to the condensation of phenol alcohols primarily formed. The actual formation of phenol alcohols in this way, however, was not demonstrated until later, owing to the fact that the early investigators worked under acid conditions. The relatively rapid rate of condensation of phenol alcohols to the more complex condensation products under these acid conditions is such as to have prevented their detection. As a forerunner of the direct synthesis, Greene (G), in 1880, announced the preparation of saligenin (orthohydroxybenzyl alcohol) by reaction of phenol in alkaline solution with methylene chloride; and, when formaldehyde became a commercial product in the early nineties, the direct synthesis of phenol alcohols in general soon followed. This was accomplished by Manasse and Lederer ( 7 ) independently of each other, using alkaline conditions under which the further condensation of phenol alcohols is usually much slower than their formation. The broad general claims of these two investigators were practically identical, covering reaction in the presence of basic catalysts in general, that is, without reference to temperature, etc. The actual methods used in their examples differed, however. hlanasse favored the use of a strong alkali in aqueous solution, in proportions approximately equimolar to the phenol. The reaction was allowed to take place a t ordinary temperature, the mixtures usually standing several days until the odor of formaldehyde had disappeared. Lederer used a weakly basic substance and heated the reaction mixture, but only long enough to complete the combination. The heating had to be stopped a t this point to avoid excessive resinification. The claims of both investigators were combined in the Bayer and Company patent. I n either case the method amounted simply to limiting the reaction to the phenol alcohol stage by means of the temperature in the one case, and of the time of heating in the other.
POLYALCOHOLS Up to this point, except in one instance, only equimolar proportions of the phenol and formaldehyde had been used, and the products were assumed to be monoalcohols (that is, combinations of one molecule of phenol with only one of aldehyde), if not actually identified as such. RIanasse had observed in the case of guaiacol, however, that, if two molecular proportions of aldehyde were used instead of one, a second crystalline product was obtained. In this product two molecules of aldehyde were combined with one of guaiacol, and it gave up one of these two molecules on heating, yielding the original monoalcohol. The compound was not regarded as a dialcohol but as some looser form of combination. It is known, however, that aromatic nuclear carboxylic acids split off a second carboxyl group, as carbon dioxide, more readily than a single one. iluwers made the general observation that the aldehyde entered only the ortho and para positions with reference to the phenolic hydroxyl. In the case of paracresol this limited the number of possible alcohols to two-namely, the orthomonoalcohol and the diorthodialcohol. Using the dimolecular proportion of formaldehyde, he obtained as the sole product the dialcohol, OH HOCHZ- &H20H
I
CH,
which had been mistaken for a monoalcohol by Lederer. With equimolar proportions he obtained a mixture of the monoalcohol and the dialcohol, which he separated by means of solvents, leaving a corresponding quantity of cresol free.
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April, 1932
I N D US TR IA L A N D E N G I N E E R IN G C H E & I SIT R Y
Ullmann and Brittner (IO) improved the preparation of this dialcohol by using a concentrated reaction mixture. Under these conditions the. sodium salt crystallizes out in a pure state and is converted into the free alcohol by dissolving in water and neutralizing with acetic acid. The product then crystallizes out in high yield. On heating with dilute acids, it splits off formaldehyde and resinifies. Its constitution was established by analysis and derivatives. In 1927 Amann and Fonrobert (1) patented a general method for making polyalcohols, which they recommended as materials for resin manufacture. It consisted in adding the phenolate solution gradually to a large excess of the aldehyde, several moles to one of the phenol. Their examples included the dialcohol of paracresol and several indefinite liquid products. POSSIBILITIES O F ALCOHOL FORM.4TION
In the present investigation i t was found, as will be shown, that polyalcohol formation is of much more general occurrence than would appear from the literature, and that it does not require the presence of a large excess of aldehyde. Its occurrence, to a considerable extent, even in equimolar reaction mixtures is not an isolated phenomenon peculiar to paracresol. It was found also with phenol and with orthocresol and is to be expected with any phenol that forms a polyalcohol, a t all. For it is then evident that the monoalcohol, which is formed first, is capable of combining with a further quantity of aldehyde; and so, as soon as any of it is formed, it must compete to some extent with the remaining phenol for the remaining aldehyde, whatever the quantity of the latter may be. An idea of the actual extent to which this occurs is gained from the results of the present work. The possibility of polyalcohol formation and the number of alcohols possible are limited by the number of active positions available. The fact that only the ortho and para positions are active in this connection is consistent with the well-known rule of orientation and finds further confirmation in the present work. In a series of experiments in which fifteen different phenols were each heated with formaldehyde in alkaline solution, mesitol (the only one with no free active position, but with the two meta positions unoccupied) was distinguished from all of the others by failure to react. This then limits the number of molecules of aldehyde that may be taken up in any case to three, but the maximum number of possible alcohols, as in the case of metacresol, is seven.
POLYALCOHOL FORMATION BY PHEKOL Phenol is thus theoretically capable of forming two monoalcohols, two dialcohols, and one trialcohol. Of these only the monoalcohols are known. They were obtained by hlanasse by the following procedure: The approximately equimolar mixture of phenol, formaldehyde,
and a 10 per cent sodium hydroxide solution is allowed to stand 1 day or more, until the odor of formaldehyde has disappeared, and is then neutralized. The monoalcohols are extracted with ether and, after removal of the free phenol by steam distillation or a suitable solvent, are separated from each other by means of benzene, in which the ortho compound is more soluble. Kothing is said about yields or other products.
The writer’s experience n-ith this method has been as follom: With phenol the ether extract from an equimolar reaction mixture amounted to about two-thirds of the theoretical, and about half of it was free phenol. Although the monoalcohols were readily estracted with ether, repeated extractions continued indefinitely to remove small quantities
