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Salicylic Acid

SALICYLIC ACID (ortho-hydroxybenzoic acid), an aromatic acid, C 6 IL.(OH)(CO 2 H), found in the free state in the buds of Spiraea Ulmaria and, as its methyl ester, in gaultheria oil and in the essential oil of Andromeda Leschenaullii. It was discovered in 1838 by Piria as a decomposition product of salicin. It may be obtained by the oxidation of saligenin and of salicylic aldehyde; by the distillation of copper benzoate; by the decomposition of anthranilic acid with nitrous acid; by fusion of ortho-chlor or ortho-brom benzoic acid with potash; by heating orthocyanphenol with alcoholic potash; by heating a mixture of phenol, carbon tetrachloride and alcoholic potash to 100 C. (F. Tiemann and K Reimer, Ber., 1876, 9, p. 1285); and by the action of sodium on a mixture of phenol and chlorcarbonic ester (T. Wilm and G. Wischin, Zeit.f. Chemie, 1868, 6).

It is manufactured by Kolbe's process or by some modification of the same. Sodium phenolate is heated in a stream of carbon dioxide in an iron retort at a temperature of 180-220 C., when half the phenol distils over and a basic sodium salicylate is left. The sodium salt is dissolved in water and the free acid precipitated by hydrochloric acid (H. Kolbe, Ann., 1860, 115, p. 201). R. Schmitt (Jour. prak. Chem., 1885 (2), 31, p. 407) modified the process by saturating sodium phenolate at 130 C. with carbon dioxide, in an autoclave, sodium phenyl carbonate CjH&OCOzNa being thus formed; by continuing the heating under pressure this carbonate gradually changes into mono-sodium salicylate. S. Manasse (German patent 73,279) prepared an intimate mixture of phenol and potassium carbonate, which is then heated in a closed vessel with carbon dioxide, best at 130-160 C. The Chemische Fabrik vorm. Hofmann and Schotensack decompose a mixture of phenol (3 molecules) and sodium carbonate (4 mols.) with carbonyl chloride at 140-200 C. When 90 % of the phenol has distilled over, the residue is dissolved and hydrochloric acid added, any phenol remaining is blown over in a current of steam, and the salicylic acid finally precipitated by hydrochloric acid. The acid may also be obtained by passing carbon monoxide over a mixture of sodium phenolate and sodium carbonate at200C.:Na 2 CO 3 + C 6 H 2 ONa+CO = C 7 H 4 O 2 Na 2 + HCO 2 Na;and by heating sodium phenolate with ethyl phenyl carbonate to 200 C. : C6H 6 O-CO 2 C 2 H+C6H 6 ONa = HO-CeH4COjNa-|-CeH 6 -C 2 H5. It isto be noted in the Kolbe method of synthesis that potassium phenolate may be used in place of the sodium salt, provided that the temperature be kept low (about 1 50 C) , for at the higher temperature (220 C.) the isomeric para-oxybenzoic acid is produced.

Salicylic acid crystallizes in small colourless needles which melt at 155 C. It is sparingly soluble in cold water, but readily dissolves in hot. It sublimes, but on rapid heating decomposes into carbon dioxide and phenol. It is volatile in steam. Ferric chloride colours its aqueous solution violet. Potassium bichromate and sulphuric acid oxidize it to carbon dioxide and water; and potassium chlorate and hydrochloric acid to chloranil. On boiling with concentrated nitric acid it yields picric acid. When heated with nesorcin to 200 C. it gives trioxybenzophenone. Bromine water in dilute aqueous solution gives a white precipitate of tribromophenol-bromide CeH 2 Br 3 -OBr. Sodium reduces salicylic acid in boiling amyl alcohol solution to w-pimelic acid (A. Einhorn and R. Willstatter, Ber., 1893, 26, pp. 2, 913; 1894, 27 p. 331). Potassium persulphate oxidizes it in alkaline solution, the product on boiling with acids giving hydroquinone carboxylic acid (German Patent 81,297). When boiled with calcium chloride and ammonia, salicylic acid gives a precipitate of insoluble basic calcium salicylate, CeKU <^ Q 2 ^> Ca, a reaction which serves to distinguish it from the isomeric metaand para-hydroxybenzoic acids. It yields both esters and ethers since it is an acid and also a phenol.

Methyl Salicylate, CH 4 (OH)-COjCH,, found in oil of wintergreen, in the oil of Viola tricolor and in the root of varieties of Polygala, is a pleasant-smelling liquid which boils at 222 C. On passing dry ammonia into the boiling ester, it gives salicylamide and dimethylamine. When boiled with aniline it gives methylaniline and phenol. Ethyl salicylate, C,H4(OH)-CO 2 CtH s , is obtained by boiling salicylic acid with alcohol and a little sulphuric acid, or by dropping an alcoholic solution of salicylic acid into 0-naphthalene sulphonic acid at a temperature of 140-150 C. (German Patent 76,574). It is a pleasantsmelling liquid which boils at 233 C. It is practically unchanged when boiled with aniline. Phenyl salicylate, CH4(OH)-C-OjC 5 H 5> or salol, is obtained by heating salicylic acid, phenol and phosphorus oxychloride to 120-125 C. ; by heating salicylic acid to 220 C.; or by heating salicyl metaphosphoric acid and phenol to 140-150 C. (German Patent 85,565). It crystallizes in rhombic plates which melt at 42 C. and boil at 172 C. (12 mm.). Its sodium salt is transformed into the isomeric CH(OC 6 H S ) CO 2 Na when heated to 300. When heated in air for many hours it decomposes, yielding carbon dioxide, phenol and xanthone. Acetyl-salicylic acid (salacetic acid), CeH 4 (O-COCH,)-COsH, is obtained by the action of acetyl chloride on the acid or its sodium salt (K. Kraut, Ann., 1869, 150, p. 9). It crystallizes in needles and melts at 132 C. (with decomposition). Hydrolysis with baryta water gives acetic and salicylic acids. It is used in medicine under the names aspirin, acetysal, aletodin, saletin, xaxa, etc. It has the same action as salicylic acid and salicylates, but is said to be much freer from objectionable secondary effects. Salicylo-salicylic acid O- (C,H 4 COjH)j is obtained by continued heating of salicylic acid and acetyl chloride to 130- 140 C. It is an amorphous yellow mass which is easily soluble in alcohol.

Applications. The addition of a little of the acid to glue renders it more tenacious; skins to be used for making leather do not undergo decomposition if steeped in a dilute solution; butter containing a small quantity of it may be kept sweet for months even in the hottest weather. It also prevents the mouldiness of preserved fruits and has been found useful in the manufacture of vinegar. The use of salicylic acid as a food preservative, was, however, condemned in the findings of the commission appointed by the government of the United States of America, in 1904.

Medicine. The pharmacopeial dose of the acid is 5-20 grains, but it is so unrelated to experience and practice that it may be ignored. The British Pharmacopeia contains only one preparation, an ointment containing one part of acid to 49 of white paraffin ointment. Salicylic acid is now never given internally, being replaced by its sodium salt, which is much cheaper, more soluble and less irritating to mucous membranes. The salt has a sweet, mawkish taste.

Salicylic acid and salicin (j[.r.) share the properties common to the group of aromatic acids, which, as a group, are antiseptic without being toxic to man a property practically unique ; are unstable in the Body; are antipyetic and analgesic; and diminish the excretion of urea by the kidneys. As an antiseptic salicylic acid is somewhat less powerful than carbolic acid, but its insolubility renders it unsuitable for general use. It is much more powerful than carbolic acid in its inhibitory action upon unorganized ferments such as pepsin or ptyalin. Salicyclic acid is not absorbed by the skin, but it rapidly kills the cells of the epidermis, without affecting the immediately subjacent cells of the dermis (" true skin "). It has a very useful local anhidrotic action. Salicylic acid is a powerful irritant when inhaled or swallowed in a concentrated form, and even when much diluted it causes pain, nausea and vomiting. When salicin ia taken internally no irritant action occurs, nor is there any antisepsis. Whatever drug of this group be taken, the product absorbed by the blood is almost entirely sodium salicylate. When the salt is taken by the mouth, absorption is extremely rapid, the salt being present in the peripheral blood within ten minutes.

Sodium salicylate circulates in the blood unchanged, decomposition occurring in the kidney, and probably in tissues suffering from the Diplococcus rheumaticus of Poynton and Paine. It used to be stated that these drugs are marked cardiac depressants; and the heart being invariably implicated in rheumatic fever, it is supposed that these drugs must be given with great caution. It has now been established that, provided the kidneys be healthy, natural salicylic acid, sodium salicylate prepared from the natural acid, and salicin, are not cardiac depressants. Of the two latter, 300 grains may be given in a dose and ij oz. in twenty-four hours, without any toxic symptoms. The artificial acid and its salt contain ortho-, para- and meta-cresotic acids, which are cardiac depressants. The vegetable product which is extremely expensive must be prescribed or the synthetic product guaranteed ' physiologically pure," i.e. tested upon animals and found to have no toxic properties. Salicylates are the next safest to quinine of all antipyretics, whilst being much more powerful in all febrile states except malaria. Sodium salicylate escapes from the blood mainly by the kidneys, in the secretion of which sodium salicylate and salicyluric acid can be detected within fifteen minutes of its administration. After large doses haematuria has been observed in a few cases. The rapid excretion by the kidneys is one of the cardinal conditions of safety, and also necessitates the very frequent administration of the drug.

Therapeutics. Salicylic acid is used externally for the removal of corns and similar epidermic thickenings. It causes some pain, so that a sedative should be added. A common formula has II parts of the acid, 3 of extract of Indian hemp, and 86 of collodion. There is probably no better remedy for corns. Perspiration of the feet cannot be attacked locally with more success than by a powder consisting of salicylic acid, starch and chalk.

These drugs are specific for acute rheumatism (rheumatic fever). The drug is not a true specific, as quinine is for malaria , since it rarely, if ever, prevents the cardiac damage usually done by rheumatic fever; but it entirely removes the agonizing pain, shortly after its administration, and, an hour or two later, brings down the temperature to normal. In thirty-six hours no symptoms are left. If the drug be now discontinued, they will return in over ox>% of cases. In acute gonorrhoeal arthritis, simulating rheumatic fever, salicylates are useless. They may thus afford a means of diagnosis. In rheumatic hyperpyrexia, where the poison has attacked the central nervous system, salicylates almost always fail. The mode of their administration in rheumatic fever is of the utmost importance. At first 20 grains of sodium salicylate should be given every hour: the interval being doubled as soon as the pain disappears, and extended to three hours when the temperature becomes normal. The patient should continue to take about 100 grains a day for at least a fortnight after he is apparently convalescent, otherwise a recrudescence is very probable.

Salicylate of soda may occasionally be of use in cases of gallstone, owing to its action on the bile. It often relieves neuralgia, especially when combined with caffeine and quinine.

Salicylism, or salicylic poisoning, occurs in a good many cases of the use of these drugs. Provided the kidneys be healthy, the symptoms may be ignored. If nephritis be present, it may be seriously aggravated, and the drug must therefore be withheld. The headache, deafness, ringing in the ears and even delirium of salicylism, are practically identical with the symptoms of cinchonism. The drug must be at once withheld if haemorrhages (subcutaneous, retinal, etc.) are observed. As in the case of quinine, the administration of small doses of hydrobromic acid often relieve the milder symptoms.

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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