Carbolic Acid or Phenol

CARBOLIC ACID or PHENOL discovered in 1834 by F. Runge in coal tar. A. Laurent in 1841, first obtained it in a pure crystalline condition, determined its composition and named it phenyl hydrate or phenic acid. It is, in fact, to be regarded as derived from, benzene, the simplest hydrocarbon of the so-called aromatic series (see also CHEMISTRY : Organic), through the re placement of one hydrogen atom by the hydroxyl or alcoholic group, •OH. Thus it stands in the same relation to benzene as ordinary methyl alcohol (wood spirit), does to methane (marsh-gas), just as methyl alcohol is hydroxy-methane so is phenol hydroxy-benzene, and, having the formula is the prototype and simplest member of one of the most important groups of compounds in organic chemistry. Although, as ex plained above, it has certain analogies of composition with methyl and other alcohols of the aliphatic series, its properties differ greatly from those of an alcohol, properly so called, and this dif ference must be attributed to the great difference in structure and properties between the paraffin hydrocarbons, on the one hand (methane, ethane, etc.), and the coal-tar hydrocarbons (ben zene, toluene, naphthalene, etc.), on the other. It is found in nature as a product of animal metabolism, occurring in the urine of man, horse and other mammals; also in the degradation prod ucts of proteins. It is formed in many chemical processes, par ticularly by the decomposition of organic matter such as wood or coal at high temperatures (carbonization). It has been observed in small quantities in Galician and Russian petroleum. It is con tained in coal tar in proportions of from o• 1 to 1.o% and its principal commercial source is that fraction of coal tar which distils from C. For the manufacture of carbolic acid, this fraction is treated with an 8-1o% aqueous solution of caustic soda. The phenol and its homologues are thereby dissolved in this solution, forming carbolate (phenoxide or phenate), whilst the hydrocarbon oils together with small quantities of pyridine and other evil-smelling bases are left as an oily layer floating above the carbolate solution. The latter is drawn off, steamed for the removal of adhering or dissolved neutral hydrocarbons, and then treated with carbon dioxide, which decomposes the carbolate to give sodium carbonate and thus liberates the carbolic acid and the higher phenols or cresols, forming a black oily layer of crude carbolic acid of characteristic odour.

The separated crude carbolic acid contains about 15-2o% of water, traces of inorganic salts and other impurities from which it is freed by repeated fractional distillation, and, in this man ner, the crude product is separated into fractions rich in phenol which crystallize upon cooling and fractions containing the cresols (higher homologues of phenol) which fail to crystal lize. The raw crystals are passed through centrifuges in which they are freed from adhering "cresylic" liquid and they are then submitted to a final fractionation yielding a distillate of commer cial carbolic acid in form of crystals of melting point 39°-4o°C. The mixture of cresols or liquid carbolic acid forms an article of commerce, but is frequently further worked up and more or less efficiently separated into its constituents. Comparison of the boiling points of the pure compounds—phenol, 181.3°C; ortho cresol, para-cresol, 199.5°; meta-cresol, 200.0° ; symmetri cal xylenol, that by fractional distillation they may be separated into phenol, ortho-cresol and a not easily separable mixture of para- and meta-cresols. A product rich in meta-cresol is of importance for the manufacture of explosives.

At times when carbolic acid is in great demand or when slack ness in the demand for the products accompanying it in coal tar does not commercially justify its extraction, it is manufactured by synthesis from benzol. Only one process is carried out indus trially. Benzol is sulphonated with fuming sulphuric acid, the resultant benzenesulphonic acid after: neutralization with milk of lime is converted into its sodium salt and the sodium benzenesul phonate is fused at 340°C with an excess of sodium hydroxide in cast iron vessels provided with powerful stirrers. The melt, on treatment with a limited amount of water, yields a solution of sodium phenate (carbolate or phenoxide) and a solid residue of sodium sulphite. From the carbolate solution crude carbolic acid is liberated by means of carbon dioxide and distilled, as in tar works practice. As, however, the synthetic product is not asso ciated with cresols, fractionation is simpler, and a very pure carbolic acid is obtained.

Properties of Phenol or Carbolic Acid.

Phenol crystallizes in colourless needles of characteristic odour. If chemically pure, the crystals are not hygroscopic and remain colourless, but when containing even slight impurities they are highly deliquescent and slowly assume a pink colour. Phenol has melting point boiling point 181.3°C under 76omm. pressure; specific gravity (d2o°/2o°) 1.0722; calorific value 7,810 calories (14,06o B.T.U.). It forms a hydrate of melting point 17.2°C. By mixing phenol with water two immiscible solutions are formed, one of phenol in water and one of water in phenol, but on adding more water a clear solution results; zoo parts of water dissolve at z t ° C 4.8 and at 77°C I 1.8 parts of phenol, whilst at 84°C phenol and water are miscible in any proportion. It is readily soluble in alcohol, ether, carbon disulphide, chloroform, glacial acetic acid and somewhat in light petroleum. It is also readily soluble in caustic alkalis, slightly soluble in aqueous ammonia solution, but almost insoluble in sodium carbonate solution. Good solvents are also gallic acids and a solution of sodium benzene sulphonate. It is volatile in steam. It gives a violet coloration with ferric chloride and a white precipitate of tribromophenol with bromine water. Phenol is a weak acid and forms salts with alkalis which react alkaline to litmus. It is readily acted upon by chlorine, bromine, iodine, sulphuric and nitric acid, chloro form in alkaline solution, formaldehyde, oxidizing and reducing agents, and a large number of derivatives can be obtained from it by these and other reagents.

Uses of Phenol.

The ease with which phenol reacts with a great number of chemical compounds marks it as a substance of wide and varied applicability in the chemical and other industries. It is used to a moderate extent as an intermediate in dyestuff manufacture. It is a component of the azo-dyes, such as diamine green B, brilliant yellow, chrysophenin and aurin or rosolic acid. The last is used as an indicator (q. v.) in chemical analysis and for colouring varnishes or in form of its sodium salt, yellow corallin, for printing wall paper. By heating phenol with phthalic anhydride and sulphuric acid another important indicator, phenolphthalein, is obtained, which is colourless in neutral or acid solution but turns deep red with caustic alkalis. Phenolphthalein also finds applica tion as a purgative. A number of dye-stuffs are derived from the products of the nitration of phenol, ortho- and para-nitrophenol and their derivatives.

A very important product of the nitration, i.e., the interaction of nitric acid with phenol, is picric acid, or 2 :4 :6-trinitrophenol. It was formerly used as a yellow dye-stuff, but its importance lies in its application as an explosive (lyddite). At the outbreak of the World War it was most widely used for bursting charges in high explosive shells, although it was gradually replaced by the more reliable and efficient trinitrotoluol (T.N.T.). Another high explo sive, mainly used in the French army, is trinitro-meta-cresol, de rived from one of the three cresols, the higher homologues of phenol. In chemical warfare chloropicrin, a compound made by chlorination and oxidation of picric acid or its salts by means of bleaching powder, has been used as a lachrymator, mostly in com bination with stannic chloride. Picric acid has usefulness in an entirely different direction, namely as a substitute for "carron oil" for the dressing of burns, and in the treatment of skin diseases such as erysipelas and eczema.

To the non-chemist phenol, or carbolic acid as it is usually named in this connection, is mostly associated with its antiseptic and disinfectant properties. It is a strong germicide and parasiti cide. It now plays a much less important role in surgery than it did in the first days of antisepsis, and on account of its irritant action and the danger of absorption it has become unpopular even as a dressing or lotion after operation or injuries, when it may lead to gangrene, necessitating amputation. Although, therefore, in surgical practice it has been largely superseded by its deriva tives and other substances, it is still used as a standard of germi cidal efficacy for the comparison and evaluation of other disinfec tants. Amongst antiseptics directly obtained from phenol may be mentioned :—tribromophenol (bromol), phenolsulphonic acid (aseptol), di-iodophenol-para-sulphonic acid (sozoiodol). More over it is also widely used, either by itself in aqueous solutions or in soaps, tooth-powders and similar preparations.

One of the most important phenol derivatives is salicylic acid (q.v.), produced from dry sodium phenate (phenoxide) and car bonic acid at 13o° C, the sodium salicylate formed being acidified to produce the acid. It is used in therapeutics and as an in termediate for over ioo azo-dyes. It is a powerful food preser vative, though condemned by health authorities and not permitted in some countries for this purpose. Some derivatives of phenol are used in perfumery, the most prominent of which is coumarin (q.v.) synthesized by way of salicylaldehyde. Phenol finds a quan titatively important and steadily growing application in the pro duction of artificial resins and plastic materials made by its con densation with formaldehyde. (See RESINS, SYNTHETIC.) Several photographic developers, such as metol, ortol and rodinal, are obtained from phenol, as are also certain sensitizers of the photo graphic plate. By the hydrogenation of phenol with nickel as cata lyst at a temperature of 160°-180°C cyclohexanol or hexaline is obtained, a solvent used for intensifying the detergent action of soaps, particularly in the textile industry.

Physiological Properties.

Carbolic acid has a pungent and, in very dilute solutions, a somewhat sweet taste. It coagulates the proteins of the tissues, and forms, when concentrated, a white opaque scar on the skin which becomes red and shiny and falls off after a few days, leaving a light brown stain for several weeks. It acts as a caustic, and causes irritation and necrosis of the mucous membranes and even in dilute solution produces local anaesthesia which lasts for many hours. It is found in urine, mostly coupled with sulphuric or glucuronic acid, principally formed in the liver. It is oxidized in the body to hydroquinone (quinol) and pyrocate chol, these products causing a green coloration of the urine. Carbolic acid is formed by degradation of proteins.

Toxicology.

Carbolic acid acts upon the central nervous sys tem by absorption through the unbroken skin, the intestines, wounds or the respiratory organs. The larger portion of toxic doses is excreted, partly through the lungs, thus causing inflamma tion of the air passages and corrosion of the points of entry. It is a typical nerve poison acting first by exciting and then by para lyzing. Administered as a chronic poison it degenerates kidney and liver, but the effect varies for different individuals. The fatal dose may vary considerably, but is in the adult of the order of one gram when introduced into open wounds and eight grams when taken by the mouth. In the latter case the patient collapses, and the skin becomes cold and clammy. By paralysis of the respira tory organs, the breathing gets shallow, the patient dying in a state of coma.

As antidote in cases of carbolic acid poisoning soluble sulphates, such as sodium or magnesium sulphate, used to be administered. These have been found to be of little or no use, because the phenol does not combine with sulphates as such, but with organic sulphur while being oxidized to sulphuric acid in the body. The first treat ment is the removal of the poison by the stomach tube followed by the thorough rinsing of the stomach with water to which 1o% of alcohol has been added. Saccharated solution of lime is also recommended. When coma and collapse set in, the patient should be sustained by the external application of warmth and by such nerve stimulants as caffeine or strychnine; artificial respiration may eventually be used, although there is little prospect of resusci tation, where intoxication has advanced so far. (R. LE.) CARBOLOY: see TOOL STEEL.