The Chemistry of Dyestuff. Intermediate Compounds. IX. Acylation and Oxidation

A Manual for Students of Chemistry and Dyeing
By
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)Acylation.

By this term is meant the introduction of an acidyl group in place of hydrogen in hydroxy and amido compounds.

R.NH₂ + HOOC.R' = RNH.COR' + H₂O
R.OH + HOOC.R' = R.OOCR' + H₂O

The introduction of the acetyl group for example into aniline enables one on nitration to obtain almost pure para-nitraniline, whereas the nitration of aniline directly gives a large amount of tarry matter and a high proportion of meta-nitraniline.

In the oxidation of o-toluidine to anthranilic acid the acetylation of the amido group shields it against oxidation.

Acetylation of phenols is more difficult than that of amines; POC1₃ or PC1₃ may be used to remove water formed by the reaction.

The addition of a trace of pyridine assists acetylation.

In the production of some naphthylamine sulphonic acids the acetylation of the amido group shields certain positions in the naphthalene ring, e.g., 1.5-naphthylamine sulphonic acid. Without acetylation α-naphthylamine on direct sulphonation gives a mixture of 1.4 and other acids, with acetylation practically pure 1.5 acid is obtained.

The preparation of acyl derivatives of amido and oxy compounds plays an important part in the preparation of azo dyestuffs. Acylation modifies the properties of amines and phenols so that diazo bodies no longer condense with them. If a compound contains both amido and oxy groups, e.g. [-] , the oxy group may be shielded by acylation in alkaline solution, or in presence of dimethyl aniline by means of benzoylchloride; whereas the amido group may be acylated by means of concentrated acids or their anhydrides, generally at high temperature, or by the anhydride in neutral aqueous solution.

The most important acyl groups are formyl, acetyl, benzoyl and phthaloyl.


Alkylation and Arylation.

By these terms is understood the replacement of hydrogen by alkyl or aryl groups in amines, R.NH₂ + R'Cl = RNHR' + HC1, phenols, RONa + R'Cl = ROR' + NaCl, and carboxylic acids. These reactions are applied both to intermediate products and dyestuffs. The alkylation of amines as a rule increases their rate of condensation with diazo compounds to form azo compounds. The same reaction applied to phenolic substances completely prevents coupling or condensation to form azo derivatives. The alkylation of amido groups in dyestuffs greatly affects the colour, e.g., rosaniline is converted into a spirit soluble blue by phenylation, or into a violet by methylation. Alkylation or arylation causes a change in colour towards the violet end of the spectrum. The introduction of the phenyl or benzyl group greatly aids sulphonation in converting basic dyes into acid dyes.

The shielding influence of aryl groups is of great importance with oxy and thioxy derivatives, resulting generally in increased fastness, especially towards washing and alkalis.

The most important alkyl groups are methyl, ethyl, benzyl and phenyl, and they may be introduced by means of the alkyl halide, by amines, or by alcohols in presence of hydrochloric or sulphuric acid.

Naphthols are easier to alkylate than benzene derivatives. Instead of using methyl chloride or dimethyl sulphate it is sufficient to heat with alcohol and sulphuric or hydrochloric acid or zinc chloride as dehydrating agents.

Arylation may be promoted by the use of other agents; see Aniline Blue (p. 195) for the use of benzoic or acetic acids. (Figs. III and VIII, Appendix.)


Oxidation.

A large number of intermediate compounds are obtained by the use of oxidising agents the more important classes of compounds being aldehydes, ketones, acids and in some cases hydroxy compounds. Oxidation is also employed for the conversion of leuco compounds into dyestuffs.

For the preparation of these bodies the more important oxidising agents are air, chlorine, bichromates, permanganates, peroxides, e.g., lead, manganese, and hydrogen peroxides, persulphates, nitrites, nitrates, sulphuric acid particularly in presence of mercury salts (e.g.,phthalic acid from naphthalene), hypdchlorites, chlorates, ferricyanides, arsenates, and nitrobenzene and other nitro compounds.