Chemical Constitutions in the Colour Index
Third Edition (1971) — Volume 4
The organisation of colorants (dyes and pigments) into structural groups is set out in Volume 4 of the Third Edition of the Colour Index, published in 1971. At the time, this volume contained current as well as historical structural information. Hard copy supplements were published in 1975 (Volume 6), 1982 (Volume 7), 1987 (Volume 8) and 1992 (Volume 9). In between these updates, Additions & Amendments booklets were published quarterly. A separate Pigments and Solvent Dyes extract appeared in 1982, to be followed, in 1997, by an entirely new edition. More recently, a Fourth Edition of the Colour Index International has become available on-line.
Colour Index™ Constitution Numbers (CICNs)
In the Third Edition of the Colour Index, each essential colorant disclosed for publication was classified and allocated a separate five-figure C.I. Constitution Number. The original numbering of colorants was in multiples of five so as to leave space for future structurally similar dyes and pigments to be assigned numbers in the correct chemical sequence. Since 1997, however, new disclosures have been allocated six-figure C.I. Constitution Numbers to alleviate the congestion experienced in certain areas, notably monoazo colorants. In each category, the various colorants are arranged strictly on the basis of their chemical structures. Consequently, dyes and pigments that are chemically related, but belong to different application classes, can be found together.
It was pointed out in the Third Edition that where dyes or pigments differ only in the metal or acid used for salt formation, a subdivision has been made by addition of a sixth figure after a colon (currently, a seventh figure after a colon). Colon numbers have also been used in other ways. Details can be found in Introduction to the Colour Index: Classification System and Terminology.
The main categories used to classify colorants in Volume 4 (1971) are as follows:
Incorporated within many of these ranges are further sub-classes.
Within a particular class, simpler colorants precede more complex structures, with aliphatic derivatives appearing before aromatic compounds, which follow the order benzene, naphthalene, anthracene and heterocyclic types.
The colorants in each chemical class are usually arranged accordingly as they contain amino, hydroxyl or both amino and hydroxyl groups. The arrangement within the various sub-classes is, in general, in the substituent sequence halogen, nitro, acylamino, carboxamido, sulfonamido and sulfonyl. In the case of azo colorants, compounds devoid of carboxylic acid and sulfonic acid groups are separated from those containing these substituents.
More comprehensive details of chemical progression are provided at the start of each class or subclass, as appropriate.
Organic Structures — Historical
The chemical constitutions depicted in Volume 4 (1971) are primitive, uninformative and inaccurate. Thus, no account is taken of aromaticity, tautomerism or hydrogen bonding. More realistic structural representations did not appear until the publication of Volume 9 (1992).
All the chemical structures disclosed in the Pigments and Solvent Dyes edition (1997) were revised and updated. In particular, dominant ketohydrazone tautomers are given for derivatives of phenylazonaphthols and for acetoacetanilide and pyrazolone colorants. Wherever possible, structures are drawn in conformations dictated by the requirements of hydrogen bonding. The positive charge associated with the triarylmethane colorants is located at the central carbon atom for convenience. This publication marked the start of a complete revision of all the structures currently appearing in the Colour Index on-line.
Organic Structures — Current
Benzene rings are shown as Kekulé structures. Although the circle in a hexagon notation is a useful shorthand representation of the aromatic sextet in benzene, it is simplistic. Apart from benzene itself, bond type alternation occurs in all its derivatives except for fully symmetrical structures such as hexafluorobenzene. Fusion between a benzene ring and a heterocyclic ring system is always shown at a double bond.
Azo bonds are never linear. The intermediates used to form azo metal complex dyes are depicted as hydroxyazo structures.
In the case of naphthalene, the central bond is always shown as a double bond, in accordance with reality — alternating bond type occurs in naphthalene itself and in all its derivatives. In the case of ketohydrazone structures the uninvolved ring retains its three double bonds.
The central ring of anthraquinone and its derivatives possesses quinonoid character so that the Kekulé-type terminal benzene rings are shown as mirror images. As far as possible, a similar approach is used with larger polycyclic quinones.
GH, September 2013