The Chemistry of Dyestuff. Intermediate Compounds. X. Aldehydes and Carboxylic Acids

A Manual for Students of Chemistry and Dyeing
M. Fort, M.Sc. (Leeds) Late Lecturer in Dyeing in the Bradford Technical College and L. L. Lloyd, Ph.D. (Bern) Lecturer in Organic and Technical Chemistry in the Bradford Technical College
Cambridge: at the University Press 1919
(First edition 1917, reprinted 1919)

The aromatic aldehydes and also formaldehyde are largely employed in the manufacture of triphenylmethane, acridine, and xanthene dyestuffs.

Formaldehyde H.CHO (B.V. -21°C.) is obtained by passing a mixture of methyl alcohol vapour and air over a heated catalyst. Copper gauze in a copper tube is commonly employed. The tube is fitted with observation holes at each end, by means of which the approximate temperature may be gauged. The reaction mixture is passed through worms heated at about 40 U to 45°C., in which the unattacked methyl alcohol is condensed, and may be rectified and again passed over the contact material. The formaldehyde passes on and is collected in water. The commercial article is a solution in water and is termed "formalin." It contains from about 27 to 40 per cent, of formaldehyde. It is a pungent smelling gas. On keeping the aqueous solution it is slowly polymerised to "paraform" or trioxymethylene (CH2O)3, a crystalline solid of M.P. 152°C.

Formaldehyde condenses very readily with nearly all classes of aromatic compounds. Thus with aniline it forms anhydrofornmldehyde aniline C6H5=CH2, a white solid, or in cold acid solution equimolecules combine to form para amido-benzyl alcohol [-]

This body again readily condenses on heating with another molecule of aniline to form p.p-diamidodiphenyl methane.

In alkaline alcoholic solution two molecules of aniline condense with one molecule of formaldehyde to give methylene aniline (C6H5NH)2CH2.

Benzaldehyde C6H5CHO (B.P. 180°C., sp. gr. 1,05) is prepared by heating the mixture of benzal chloride and benzo-trichloride, obtained by the chlorination of toluene at the boil, with milk of lime under a pressure of from 4 to 5 atmospheres. Chalk is added along with the milk of lime in order to obtain a good emulsion. The berizaldehyde is distilled off with steam, whilst calcium benzoate, which is formed from the benzotrichloride, remains behind, and is subsequently converted into benzoic acid by treatment with mineral acid.
C6H5CHCl2 + Ca(OH)2 = C6H5CHO + H2 + CaCl2
2C6H5CCl3 + 4Ca(OH)2
= (C6H5COO)2Ca + 3CaCl2 + 4H2O

Benzaldehyde so obtained always contains some chlorine compounds. This renders it of little vajue for the scent industry, but it is quite suitable for the dyestuff industry. Benzaldehyde free from chlorine is obtained by the oxidation of toluene in sulphuric acid solution by means of manganese dioxide.

Benzaldehyde is a colourless liquid smelling of bitter almonds. It is sparingly soluble in water. Commercial benzaldehyde should dissolve almost completely in a ,varm aqueous solution of sodium bisulphite of sp. gr. 1,11; on extraction with ether and evaporation of the ether there should be no pungent odour of benzyl chloride. It is employed in the manufacture of triphenylmethane dyestuffs.


Ortho-nitrobenzaldehyde [] may be obtained as follows: -

(1) By the oxidation of o-nitrobenzylaniline or its sulphonic acid, by means of sulphuric acid and either manganese dioxide or sodium bichromate, to the corresponding o-nitrobenzylidene derivative, which is then hydrolysed by dilute acid, and distilled with steam.

(2) By the oxidation of o-nitrotoluene with sodium hypochlorite solution in presence of nickel oxide. It crystallises in long yellow needles, and is employed in the manufacture of indigo and triphenylmethane dyestuffs.

Meta-nitrobenzaldehyde [] (M.P. 58°C.) is obtained by nitrating one volume of benzaldehyde with a mixture of five volumes of fuming nitric acid and ten volumes of concentrated sulphuric acid; the temperature must not exceed 10°C. The reaction mixture is poured into ice water.

It is a pale yellow compound, and is employed in the manufacture of triphenylmethane dyestuffs.

Para-nitrobenzaldehyde [] (M.P. 106°C.) is obtained by similar methods to those employed in the preparation of the ortho compound, or by boiling p-nitrobenzyl chloride with a saturated solution of cupric nitrate. Oxidation takes place with an almost theoretical yield of p-nitrobenzaldehyde. It crystallises from water in long thin prisms.


These compounds are also employed in the manufacture of the triphenylmethane dyestuffs.

Otrho-chlorobenzaldehyde (B.P.214C., sp. gr. 1,29) is obtained by the oxidation of orthochlorotoluene, in sulphuric acid solution or suspension, by means of manganese dioxide; or by oxidation of benzyl sulphanilic acid, obtained by condensation of o-chlorobenzyl chloride with sulphanilic acid, to the benzylidene derivative, followed by hydrolysis (see o-nitrobenzaldehyde). It is a colourless liquid.

By heating with sodium sulphite under pressure it is converted into benzaldehyde-o-sulphonic acid, which is also obtained by oxidation of di-amido-stilbene-disulphonic acid (see p. 52).

Meta-chlorobenzaldehyde [] (B.P. 212°C., sp. gr. 1,246) is obtained by the chlorination of benzaldehyde.in presence of zinc chloride; or by diazotisation of m-amidobenzaldeliyde and treatment of the diazo-chloride with metallic copper. It is a colourless oil.

Para-chlorobenzaldehyde [] (M.P. 47,5°C., B.P. 213° to 214°C.) is often associated with the ortho compound, from which it may be separated by sulphonation with fuming sulphuric acid; or by the oxidation of p-chlorobenzyl aniline with sodium bichromate and sulphuric acid, followed by hydrolysis of the ben/ylidene compound with acid.

Dichlorobenzaldehydes are also employed in the manufacture of the triphenylmethaue dyestuffs.

The 2.5-compound is obtained by the chlorination of benzaldehyde in presence of iodine or antimony. The crude compound boils at 231° to 233°C.

The 2.6-dichlorobenzaldehyde is obtained from the corresponding dichlorobenzal chloride.

Similarly 2.3.4- and 2.4.5-trichlorobenzaI chlorides are converted into aldehydes when heated with fuming sulphuric acid at 100°C. The product is poured into ice water, the bisulphite compound is next obtained and decomposed by sodium carbonate.

Benzaldehyde-o-sulphonic acid is prepared by heating a strong solution of sodium sulphite with o-ehlorobenzaldehyde under pressure at 200°C.

Also by the oxidation of stilbene disulphonic acid with sodium permanganate in the cold.

Para-nitrobenzaldehyde-o-sulphonic acid [] is prepared similarly to benzaldehyde-o-sulphonic acid by oxidising a cold solution of the sodium salt of dinitrostilbenedisul phonic acid with sodium permanganate.

Meta-amidobenzaldehyde [] prepared from the sodium bisulphite compound of m-nitrobenzaldehyde by reduction with ferrous hydroxide. The compound readily polymerises and is therefore only obtained stable as salts.

Para-amidobenzaldehyde [] (M.P. 71°C.) is obtained by boiling p-nitro toluene with an alkaline solution of sodium polysulphide. In this reaction the nitro group acts as an oxidising agent within the molecule of the substance.

Also by the reduction of p-nitrobenzylidene aniline with sodium polysulphide solution followed by acid hydrolysis.

Para-amidobenzaldehyde and its derivatives may also be prepared by heating phenylhydroxylamine or its derivatives with formalin, followed by hydrolysis of the anhydro compound which is produced by boiling with water. For the production of alkyl derivatives, the hydroxylamine derivative is condensed with formalin and a dialkylaniline.

p-dimethylamidobenzaldehyde (M.P. 73°C.) crystallises from hot water.

p-diethylamidobenzaldehyde (M.P. 41°C.) crystallises from water in plates.

It easily polymerises to a yellow insoluble compound.


Of the three isomers only the meta and the para compounds are used for the preparation of dyestuffs. They are mainly employed for the production of triphenylmethane dyestuffs.

Meta-oxybenzaldehyde [] (M.P. 104°C., RP. 240°C.) is obtained from the corresponding amido compound by heating its diazo compound with water. It crystallises from hot water in colourless needles.

Para-oxybenzaldehyde [] (M.P. 115° to 116°C.) may be obtained by the Tiemann-Reiiner reaction of chloroform upon sodium phenate, in which case it is obtained along with salicylic aldehyde, the ortho and para compounds l>eing separated by steam distillation.

The para compound is manufactured by the Gattermann method, which consists in the treatment of phenol with hydrocyanic and hydrochloric acids; there is obtained the chlor-imide which on treatment with water gives ammonium chloride and p-oxybenzaldehyde.

Carboxylic Acids.

Benzole acid C6H5COOH (M.P. 121°C., B.P. 249C.). The major portion of this acid is obtained as a byproduct in the manufacture of benzaldehyde. During the preparation of benzal chloride some benzotrichloride is produced, which on hydrolysis with milk of lime gives calcium benzoate, from which benzoic acid is obtained by acidifying with hydrochloric acid.

Benzoic acid may be obtained by boiling one volume of benzyl chloride with three volumes of dilute nitric acid (16 per cent.) until the liquid no longer smells of benzyl chloride or benzaldehyde. Also by heating benzotrichloride with acetic acid and zinc chloride, the acetyl chloride obtained as a by-product being distilled off.

→ C6H5COOH + 2CH3COC1 + HCl.

The commercial benzoic acid usually contains some chlorine derivatives; the quantity should only be small, in which case it is not detrimental for general use.

It is employed for the production of benzoyl chloride and as an assistant in the arylation of amido compounds.

Benzoyl chloride C6H5COC1 (B.P. 198,5°C., sp. gr. 1,23) may be prepared by heating benzoic acid with phosphorus pentachloride, the phosphorus oxychloride is distilled off and the benzoyl chloride fractionated.

Benzoyl chloride is manufactured from calcium benzoate by treating with chlorine and sulphur dioxide.

It is a liquid of unpleasant odour, and is employed for the benzoylation of hydroxy and amido groups.

Salicylic acid [] (M.P. 156,7°C.). Phenol is dissolved in the equivalent quantity of concentrated caustic soda and the solution evaporated to dryness. The residue is powdered and dried in a current of an inert gas, the dried sodium phenate is saturated under pressure with dry carbon dioxide, sodium plienyl carbonate is produced, which on heating at 120 to 140°C. at ten atmospheres pressure is converted quantitatively into sodium salicylate, which crystallises in plates from water.


It is employed as the azo component in many mordant azo dyes, and in the production of some cotton dyestuffs. It is also used in the preparation of "salol" (phenyl salicylate), and of methyl salicylate (oil of wintergreen), which is used as a disinfectant.

Cresotinic acid [] (M.P. 163° to 164°C.) is prepared similarly to salicylic acid from ortho-cresol. It is volatile in steam.

Nitrosalicylic acid [] (M.P. 230°C.) is obtained by nitration of salicylic acid with a mixture of nitric and sulphuric acids. The nitro acid is precipitated by pouring into water.

Amidosalicylic acid is obtained from the above compound by reduction with cold sodium sulphide solution or by zinc dust and acid.

Meta-oxybenzoic acid [] (M.P. 188°C.) may be prepared by the action of nitrous acid upon m-amidobenzoic acid, or by heating m-chlorobenzoic acid, or m-cresol with sodium hydroxide under pressure. By heating with sulphuric acid (90 per cent.) at 210°C., anthraflavin and anthrarufin are produced.

Para-oxybenzoic acid [] (M.P. 213 to 214°C.) is manufactured by the same method as the ortho acid except that during the transformation of the sodium plienyl carbonate the temperature of the reaction is 220°C. The acid may also be obtained by heating sodium salicylate. It crystallises from water with one molecule of water of crystallisation.

Oxynaphthoic acids. These acids are very interesting on account of the ease with which some of them lose carbon dioxide.

The l-oxy-2-naphthoic acid (M.P. 187°C.) is obtained from dry sodium α-naphtholate and carbon dioxide under pressure at 120° to 145°C. The acid crystallises in needles. Nitrous acid reacts with it, with formation of 2-nitroso-α-naphthol and elimination of carbon dioxide. It couples with diazotised amines forming para-azo dyes.

2-oxy-1-naphthoic acid is obtained by heating dry sodium β-naphtholate at 120° to 140°C., after saturating with carbon dioxide under pressure. The acid when quickly heated melts at 156° to 157°C., but if slowly heated begins to decompose between 124° and 128°C. It is converted by prolonged boiling with water into β-naphthol and carbon dioxide. When treated with diazo salts carbon dioxide is split off giving azo dyes of β-naphthol.

2-oxy-3-naphthoic acid (M.P. 216°C.) is obtained by heating dry sodium β-naphtholate with the required quantity of carbon dioxide at 200° to 250°C. under pressure. It crystallises in yellow scales, and is characterised by its greater stability, being decomposed, with loss of carbon dioxide, only when boiled with sodium bisulphite solution. Heated with ammonia under pressure at 260° to 280°C., it gives 2-amino-3-naphthoic acid. Nitrous acid converts it into l-nitroso-2-oxy3-naphthoic acid. It couples with diazo salts to give azo dyes without elimination of the carboxylic acid group.

The anilides of oxy-naphthoic acid are employed, in place of β-naphthol, for coupling on the fibre in the production of insoluble azo dyes. (Compare Para-Red.) They are put on the market as Naphthol AS and NA (G.E.).

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