The Chemistry of Dyestuff. Dyestuffs. XIV. Colour and Constitution.

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)
When a beam. of white light falls upon a coloured surface part of the light may be reflected aud part may pass through the material, a lowering in the intensity of light accompanying it. This will vary with different parts of the spectrum, consequently the reflected and transmitted light will have a different composition to the original source, it will therefore appear coloured^ A portion of the light being absorbed in the coloured material, the corresponding colour or hue of a surface, or substance is dependent on (1) the absorptive or selective power exhibited towards light, (2) the nature and composition of the light in which it is viewed, (3) the sensitiveness of the eye.

It thus happens that out of the indefinite number of light vibrations of different wave length, only the very limited o range from 8100 A. U. (red) to 3600 A. U. (violet) ( A. U.= Angstrom unit = 10~8cm.) is appreciated by the normal eye as colour sensations, and in some of the earlier civilisations the sense of colour perception was probably even still more limited. These facts cannot be ignored in seeking a relation between colour and chemical constitution.

The light absorbing properties of a substance are a principal factor in determining colour, and these absorption properties may well be related to the chemical constitution or arrangement of atoms in the substance. It is, however, only by investing the word "coloured" with a wider meaning than physiological limitations have caused to be attached to. it, that a true theory of colour and constitution can be constructed. The practical usefulness of the chromophore theory, and its extension, the quinonoid theory of colour, has gained for them wide acceptance while still very incomplete and imperfect.

The portion of characteristic bands in the spectrum of a dye solution is readily ascertainable by spectroscopic methods. On addition of certain reagents modifications are caused which render further data available for a system of analysis of dyes known as Forrnanek's method, which has been applied also to vat dyes by Grandmougin.

Some substances only allow one colour of the spectrum to be transmitted, others two colours, or the spectrum may contain one or more dark bands, or a line spectrum along with dark absorption bands. Spectroscopic analysis of dyes depends on the differences shown in this respect. In nearly all cases the spectrum depends not only upon the nature of the dissolved substance, but also upon the thickness of the liquid, the concentration of the solution, the nature of the solvent and the temperature.

With many aromatic compounds absorption also takes place in the ultra violet portion of the spectrum, viz., among the invisible rays of light beyond the violet end of the spectrum, and absorption also may occur among the invisible light rays of the huge infra-red field adjoining the red of the visible spectrum. Under powerful electro-magnetic influence spectrum lines can be resolved into doublets, triplets, etc., and this phenomenon is known as the Zeeman effect. In the earlier attempts to relate colour to constitution absorption outside the range of visible light vibrations was neglected.

The physiological conception of white is obtained by mixing the colours of the spectrum or by mixing two complementary colours of the spectrum. Since the combination of the colours of the spectrum gives white, then by absorption of one colour from the spectrum the complementary colour will be obtained.

The thickness of the coloured solution may not only cause a change in the intensity of the colour, but also a distinct change in the observed colour (dichroism). For example chrome alum solution in a thin layer is bluegrey and in a thicker layer violet-red. Brilliant Acid Green 6B is green in thin layers which gradually changes with increasing thickness through blue-green, blue, dark-blue, violet, purple, to red.
The nature of the solvent also plays an important part in the production of colour effects, since different solvents will have a different absorption value.

The temperature is also of importance, because increase in temperature tends to alter the absorption towards the red end, i.e., longer wave length, of the spectrum. A strong solution of copper chloride is blue, cold, but becomes green on heating.

Marked differences may also be observed between the colour of substances in the solid and liquid condition or between a solid and its solutions, e.g., crystals of p-nitrophenol or triphenylmethane are colourless, but when melted are yellow.

From a chemical standpoint the constitution of coloured substances is of great importance in connection with the theory of colour. By studying the constitution of a large number of organic compounds Graebe, Liebermann, Kostanecki, and Witt developed the Chromophore Theory, known as Witt's Theory. This theory states that the manner in which the atoms are grouped is the cause of compounds being coloured. The atom groupings are ternied Chromophores. They may be divided into the following classes:

1. Ethylene group [] (indefinite position and number).
2. Ketone group []
3. Double bonded carbon and nitrogen []
4. Azo group []
5. Nitroso group []
G. Nitro group []
7. Thio group [] or polysulphides.

Speaking generally, an increase in the number of chromophore groups has the effect of intensifying the colour.
The position of the chromophore group in the molecule is also of importance.

The following examples will illustrate the chromophore theory:
Fulvene [] is isomeric with benzene, the latter is colourless whilst fulvene is orange yellow.

In many cases it is necessary to have some ring system in the molecule in order to produce coloured bodies. Tetramethyl butadiene dicarboxylic acid is colourless, the tetraphenyl derivative [] is orange.

With hydrocarbons the following are examples of the ethylene chromophore structure:
[] Biphenylene-propylene (yellow)
[] Bibisphen virtue ethane (red)
[] Dinaphthylenediphenylene ethylene (violet red)

The ketone group as chromophore generally requires two or more keto groups or a certain ring system in order to produce colour.

CH3COCOCH3 yellow, [] colourless, [] orange.

The azo group as chromophore varies extremely in the intensity of colour which it produces in compounds.

Diazomethane [] is yellow.

Azobenzol [] red.

The nitroso group has very strong chromophore properties. Nitroso tertiary butane (CH3)3C.NO in solution or in liquid condition is deep blue; under similar conditions nitrosobenzol C6H5NO is green.

The nitro group has a feeble chromophore character, this may be observed in the colour of nitrobenzene or nitronaphthalene. The position of the nitro group is not without influence. In solution p-nitrotriphenylfulgide is deep red, while the ortho and meta compounds are only feebly coloured.

The thiocarbonyl group, = C = S, has stronger chromophore properties than the carbouyl group = C = O.

Benzophenone is colourless, thiobenzophenone is a blue oil.

A considerable intensification of colour, due to the carbouyl group, is observable when other double bonds are present in the same compound. Phorone
(CH3)2C = CH.CO.CH = C(CH3)2 is yellow, the grouping of the atoms
[] accounting for the colour.

The constitution of paraand ortho -ben zoquin one or of other corresponding orthoand para-quinones shows the effect of the proximity of double bonds in colour production.


The para compound is yellow, the ortho red. The nearness of the two keto and also the two HC = CH groups causes the more intense colour in the ortho compound.

With quinones it often happens that two forms exist. By careful oxidation of catechol a colourless form may be produced which transforms into the coloured form, due to the following changes in structure:
benzenoid (colourless) quinonoid (coloured)

There are many examples in which stereoisomerism causes a difference in the colour of compounds, corresponding to cis and trans forms.


Diethoxy-naphthostilbene exists in two forms, the unstable, higher melting-point form is colourless, the lower melting form is yellow.

β-Brom allocinnamic acid is colourless, β-Brom cinnamic acid is yellow.

A similar difference may be noticed with diazo compounds.

The origin of colour in a compound is due to the chemical nature of the chromophore groups. The coloured bodies that contain only chromophore groups, tiiat is, coloured unsaturated compounds, these being termed Chromogenes, pass into colourless bodies by reduction, forming leuco compounds.

The following are examples:
Chromogene (coloured)
Leuco compounds (colourless)

By the introduction of new groups into chromogenes the colour may be decreased or intensified. Most chromogenes are relatively reactive bodies which may be more or less easily changed by chemical action. In some cases the introduction of a new group is accompanied by intramolecular change, such as the formation of quinonelike chromophores. The most important groups that affect the colour of a chromogene are the hydroxy, ammo and substituted ammo groups. These are known as Auxochrome groups. The auxochrome groups are unequal in value, e.g., the amino group develops a far more powerful action than the hydroxy group in paranitraniline which is deep yellow, while p-nitrophenol is practically colourless. On the other hand Benzopurpurine 10B (dianisidine coupled with 2 mols. α-naphthylamine-4-sulphonate) is carmine red, while Benzoazurine G of similar constitution, but a naphthol derivative, is blue. The auxochrome character of the hydroxyl group becomes more apparent by salt formation with bases. The replacement of hydrogen by alkyl or aryl groups also influences auxochrome groups. Indophenol [] is violet, whereas the dimethyl derivative is blue. Salt formation with chromogenes plays a very important part; if sal formation takes place in the auxochrome the colour becomes lighter, by salt formation in thejabromophore the compound becomes darker. Ortho-aminobenzophenone is yellow, its hydrochloride is practically colourless.

Acridine is white, its salts are, however, yellow, also phenazine is yellow but it produces red salts. These are examples of salt formation in the chromophore.

Many coloured amines are tautomeric in such a way that the free base and its salts belong to different structural types. Generally the base contains the true benzol ring and the salts the quinonoid structure. In th^se cases salt formation is accompanied with intensification of colour. Para-amidoazobenzol is yellow, it gives bluish violet salts that dissolve in water with a red colour, para-para-diamidoazobenzol is similar; with the meta derivative salt formation reduces the colour intensity, as is also the case in the substances in which the auxochrome alone forms salts; m-m-diaminoazobenzol is dark orange, its salts are gold orange. In the case of the meta compound quinonoid structure cannot be set up. With the para compound transformation may take place, and since the para-quinonoid structure is a strong chromophore, such change intensifies the colour.

The triphenylmethane dyestuffs are particularly interesting. Hantzsch has been able to show that the free base and salt are of different types in this class of dyestuff. The free colourless bases, which he terms the leucohydrates, are to be considered as triphenylcarbinols, the coloured salts as quinonoid compounds.

The salts of Crystal Violet dissolve in excess of hydrochloric acid giving colourless solutions and are therefore derivatives of the normal carbinol.

Similar transformation has been observed with acridine derivatives. The colourless methyl-phenylacridol [] gives a dark brownish-black iodide having the following constitution: []

With most coloured phenols the auxochrome nature of the hydroxyl group is made obvious only by salt formation. The colour increasing on addition of alkali. It is possible that this change is due to transformation to the quinonoid structure []

This is however hardly probable since meta-nitrophenol gives the same colour change without the possibility of quinonoid structure.

By salt formation of amines with acids without change in the chromophore, the colour is decreased.

By salt formation of hydroxyl (phenolic) groups without change in the chromophore, the colour is intensified.

In addition to the colour change by salt formation there is also the change due to Halochrome groups (Baeyer). The halochrome groups of most importance are oxygen derivatives, in which the oxygen apparently possesses basic properties to a mild degree, passing from the divalent to the tetravalent basic form. Many ketones having the group [] with dry hydrochloric acid gas give a darker coloured compound. It has been shown that in these compounds the double bond is not attacked. The salts are easily decomposed, either by addition of water or placing in a vacuum.

Thianthrene [] has strong halochrome properties, dissolving in concentrated sulphuric acid and thereby giving a fine blue solution.

Groups or atoms that cause an intensification of colour when introduced into compounds have been called Bathochromes, and those that decrease the colour of a compound Hypsochromes.

Alkyl halides when combined with an auxochrome decrease colour, e.g., m-nitro-dimethyl aniline is yellow, but its methyl bromide addition product is white. If the alkyl halides are attached to the chromophore, then as a rule the colour is intensified, e.g., acridine is colourless and its methyl iodide addition product is red.

Acylation (e.g., Acetylation and Benzoylation) always gives hypsochrome properties, whether acting upon amines or hydroxy groups. In homologous series an increase in molecular weight is accompanied by increased intensity of colour, and the introduction of alkyl groups into ring compounds shows that these groups possess a bathochrome character. The naphthalene derivatives are more intensely coloured than the corresponding benzene derivatives. Halogenation is accompanied as a rule with bathochrome functions. The intensification is not proportional to the number of halogen groups introduced, but the position of the halogen groups is important. The sul phonic acid group to a mild degree acts both as bathoand as hypso-chrome.

From what has been stated already, it will be understood that what is called the quinonoid theory of colour, while it does not deny much that is assumed by the chromophore theory, definitely, attempts to relate the structure of dyes to either orthoor para-quinones. It is assumed that colour change, e.g., as the effect of a solvent or by salt formation, is accompanied by a change in the primary structure of its molecules, the setting up of a quinonoid type in place of benzenoid structure being accountable for a change from colourless to coloured, and vice versa. A large amount of experimental evidence collected by Hantzsch and others may be claimed in support of this view. It is however a theory for aromatic compounds only; it ignores the immense range of light vibrations in the infra red, thus being adopted on a limited and arbitrary definition of colour, but much may be said for its usefulness as a working hypothesis. The identity of mononitrosophenol and monoquinone oxime, and also a similar identity in corresponding naphthalene derivatives, has led to the assumption of isomeric changes fitting well with the quinonoid theory.


Investigations into the isomeric coloured and colourless ethers of Fluorescein and Phenolphthalein have also resulted in strong evidence of such isomeric change from benzenoid to quinonoid type, according to whether coloured or colourless. The quinonoid theory has acted as a check on the over-rapid application to dyestuffs of simple ionic explanations of colour phenomena, which often ignore the actual chemical processes involved, e.g., in the case of Fluorescein and Phenolplithalein. The student however must not build too much upon the rigidity of chemical conceptions, and hypotheses framed to overcome some difficulty or inconsistency in formulae. Thus the usual formulae of structural chemistry having proved unequal to the strain of explaining the identity of p-nitrosophenol and p-quinone oxime, use was made of the conception
of isomeric change; this affords an excellent working hypothesis, but the reality of any such change must still be doubtful in the extreme. The quinonoid theory as at present applied is open to many grave fundamental objections, more so than such a theory of colour and constitution as is put forward by Baly, who claims that the molecular force field theory gives a rational explanation of the phenomena of light absorption and attendant colour. As a working hypothesis for use in synthetic dyestuff chemistry however, the latter theory cannot claim to be anything like so fruitful as the quinonoid theory. To a student of applied chemistry such distinctions are valuable, and must be borne in mind. The force field theory has been developed from the notion of chemical affinity being the external evidence of the force fields surrounding atoms or molecules. It follows from what is known as the Zeemann effect, viz., the resolution of spectrum lines into doublets, triplets, etc., in a powerful magnetic field, that each atom must form the centre of an electro-magnetic field due to the rotation of its component electrons. If two molecules, the force fields of which are of opposite type, are brought together, the two force fields will condense with escape of energy. When a force field is closed the body cannot react. If the free force lines of another body are caused to interpenetrate the closed fields of the former, they will become opened up. In the case of the closed force field of a base, an acid is the most suitable substance to open it up. The fact that a salt is produced is regarded as of no consequence, e.g., in the case of phenol and alkali. Phenol in alcoholic solution shows two absorption bands in the ultra violet, and in the alkaline solution it also exhibits two absorption bands. The frequency difference between these four bands is about 160. A strong absorption band in the infra red also has a frequency of 160. Similarly with the nitrophenols, in neutral and alkaline solutions they exhibit different bands, but in each case the different absorption bands in the two solvents are directly connected by the frequency relation, and the nitrophenols must therefore possess exactly analogous constitutions in neutral and alkaline solutions. Such results are quite at variance with the assumption of isomeric quinonoid change occurring in these cases.


Additional information may be obtained from the following sources:
Baly, "Colour and Constitution," Jour. Soc. Dyers, p. 39, 1915.
Green, "Quinonoid Addition as the Mechanism of Dyestuff Formation," Jour. Chem. Soc., p. 925, 1913.
Bearder, "Fluorescence," Jour. Soc. Dyers, p. 270, 1911.
May, " Origin of Colour in Organic Compounds," Chem. News, pp. 283 and 295, 1907.
von Baeyer, "On Aniline Colours," Zeit.f. ang. Chem. 1906, abstract in Jour. Soc. Dyers, p. 335, 1906.
Kauffmann, "On the Relation between Constitution and Colour," Ahren's Sammlung 1907,°F. Enke, Stuttgart.
Formanek, "Spektralanalytischer Nachweis kunstl. organ. Farbstofte," Springer, Berlin.

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