Carbolic Acid

When phenol is passed through a red-hot tube a complex decomposition takes place, resulting in the formation of benzene, toluene, naphthalene, &c. (J.G. Kramers, Ann., 1877, 189, p. 129). Chromium oxychloride reacts violently on phenol, producing hydroquinone ether, O(C6H4OH)2; chromic acid gives phenoquinone, and potassium permanganate gives paradiphenol, oxalic acid, and some salicylic acid (R. Henriques, Ber., 1888, 21, p. 1620). In alkaline solution, potassium permanganate oxidizes it to inactive tartaric acid and carbon dioxide (O. Doebner, Ber., 1891, 24, p. 1755). When distilled over lead oxide, it forms diphenylene oxide, (C6H4)2O; and when heated with oxalic acid and concentrated sulphuric acid, it forms aurin, C19H14O3. It condenses with aceto-acetic ester, in the presence of sulphuric acid, to β-methyl coumarin (H. v. Pechmann and J.B. Cohen, Ber., 1884, 17, p. 2188).

The hydrogen of the hydroxyl group in phenol can be replaced by metals, by alkyl groups and by acid radicals. The metallic derivatives (phenolates, phenates or carbolates) of the alkali metals are obtained by dissolving phenol in a solution of a caustic alkali, in the absence of air. Potassium phenolate, C6H5OK, crystallizes in fine needles, is very hygroscopic and oxidizes rapidly on exposure. Other phenolates may be obtained from potassium phenolate by precipitation. The alkyl derivatives may be obtained by heating phenol with one molecular proportion of a caustic alkali and of an alkyl iodide. They are compounds which greatly resemble the mixed ethers of the aliphatic series. They are not decomposed by boiling alkalis, but on heating with hydriodic acid they split into their components. Anisol, phenyl methyl ether, C6H5·O·CH3, is prepared either by the above method or by the action of diazo-methane on phenol, C6H5OH + CH2N2 = N2 + C6H5·O·CH3 (H. v. Pechmann, Ber., 1895, 28, p. 857); by distilling anisic acid (para-methoxy benzoic acid) with baryta or by boiling phenyl diazonium chloride with methyl alcohol. It is a colourless pleasant-smelling liquid which boils at 154.3° C. Phenetol, phenyl ethyl ether, C6H5·O·C2H5, a liquid boiling at 172° C., may be obtained by similar methods. A. Hantzsch (Ber., 1901, 34, p. 3337) has shown that in the action of alcohols on diazonium salts an increase in the molecular weight of the alcohol and an accumulation of negative groups in the aromatic nucleus lead to a diminution in the yield of the ether produced and to the production of a secondary reaction, resulting in the formation of a certain amount of an aromatic hydrocarbon.

The acid esters of phenol are best obtained by the action of acid chlorides or anhydrides on phenol or its sodium or potassium salt, or by digesting phenol with an acid in the presence of phosphorus oxychloride (F. Rasinski, Jour. f. prak. Chem., 1882 [2], 26, p. 62). Phenyl acetate, C6H5·O·COCH3, a colourless liquid of boiling point 193° C., may be prepared by heating phenol with acetamide. When heated with aniline it yields phenol and acetanilide. Phenyl benzoate, C6H5·O·COC6H5, prepared from phenol and benzoyl chloride, crystallizes in monoclinic prisms, which melt at 68-69° C. and boil at 314° C.

Phenol is characterized by the readiness with which it forms substitution products; chlorine and bromine, for example, react readily with phenol, forming ortho- and para- chlor- and -bromphenol, and, by further action, trichlor- and tribrom-phenol. Iodphenol is obtained by the action of iodine and iodic acid on phenol dissolved in a dilute solution of caustic potash. Nitro-phenols are readily obtained by the action of nitric acid on phenol. By the action of dilute nitric acid, ortho- and para-nitrophenols are obtained, the ortho-compound being separated from the para-compound by distillation in a current of steam. Ortho-nitrophenol, C6H4·OH·NO2(1·2), crystallizes in yellow needles which melt at 45° C. and boil at 214°C. Para-nitrophenol, C6H4·OH·NO2(1·4), crystallizes in long colourless needles which melt at 114°C. Meta-nitrophenol, C6H4·OH·NO2·(1·3), is prepared from meta-nitraniline by diazotizing the base and boiling the resulting diazonium salt with water. By nitrating phenol with concentrated nitric acid, no care being taken to keep the temperature of reaction down, trinitrophenol (picric acid) is obtained (see Picric Acid). By the reduction of nitro-phenols, the corresponding aminophenols are obtained, and of these, the meta- and para-derivatives are the most important. Para-aminophenol, C6H4·OH·NH2(1·4) melts at 148° C., with decomposition. Its most important derivative is phenacetin. Meta-aminophenol, C6H4·OH·NH2(1·3), and dimethyl meta-aminophenol, C6H4·OH·N(CH3)2(1·3), are extensively employed in the manufacture of the important dyestuffs known as the rhodamines. The aminophenols also find application as developers in photography, the more important of these developers being amidol, the hydrochloride of diaminophenol, ortol, the hydrochloride of para-methylaminophenol, C6H4·OH·NHCH3·HCl(1·4), rodinal, para-aminophenol, and metol, the sulphate of a methylaminophenol sulphonic acid. Meta-aminophenol is prepared by reducing meta-nitrophenol, or by heating resorcin with ammonium chloride and ammonia to 200° C. Dimethyl-meta-aminophenol is prepared by heating meta-aminophenol with methyl alcohol and hydrochloric acid in an autoclave; by sulphonation of dimethylaniline, the sulphonic acid formed being finally fused with potash; or by nitrating dimethylaniline, in the presence of sulphuric, acid at 0° C. In the latter case a mixture of nitro-compounds is obtained which can be separated by the addition of sodium carbonate. The meta-nitro-compound, which is precipitated last, is then reduced, and the amino group so formed is replaced by the hydroxyl group by means of the Sandmeyer reaction. Dimethyl-meta-aminophenol crystallizes in small prisms which melt at 87° C. It condenses with phthalic anhydride to form rhodamine, and with succinic anhydride to rhodamine S.

Phenol dissolves readily in concentrated sulphuric acid, a mixture of phenol-ortho- and -para-sulphonic acids being formed. These acids may be separated by conversion into their potassium salts, which are then fractionally crystallized, the potassium salt of the para-acid separating first. The ortho-acid, in the form of its aqueous solution, is sometimes used as an antiseptic, under the name of aseptol. A thiophenol, C6H5SH, is known, and is prepared by the action of phosphorus pentasulphide on phenol, or by distilling a mixture of sodium benzene sulphonate and potassium sulphydrate. It is a colourless liquid, which possesses a very disagreeable smell, and boils at 168° C.

Various methods have been devised for the quantitative determination of phenol. J. Messinger and G. Vortmann (Ber., 1890, 23, p. 2753) dissolve phenol in caustic alkali, make the solution up to known volume, take an aliquot part, warm it to 60° C., and add decinormal iodine solution until the liquid is of a deep yellow colour. The mixture is then cooled, acidified by means of sulphuric acid, and titrated with decinormal sodium thiosulphate solution. S.B. Schryver (Jour, of Soc. Chem. Industry, 1899, 18, p. 553) adds excess of sodamide to a solution of the phenol in a suitable solvent, absorbs the liberated ammonia in an excess of acid, and titrates the excess of acid. See also C.E. Smith, Amer. Jour. Pharm., 1898, 369.