Structural discussion of reactive dyes those suitable for cotton fibre

There are different types of reactive dyes that are usually compatible with cotton fibre. But molecular structure of cellolusic cotton fibre is complex. So the dyes which have more reactivity is suitable for that complex structure of cotton fibre. The molecular structures of reactive dyes resemble those of acid and simple direct cotton fibre dyes, but with an added reactive group. Usually the structure of  reactive dye assemble with azo anthraquinone  triphenodioxazine or copper phthalocyanine chromophores. The key structural features of a reactive dye are the chromophoric system, the sulphonate groups for water solubility, the reactive group, and the bridging group that attaches the reactive group either directly to the chromophore or to some other part of the reactive dye molecule. Each of these structural features can influence the dyeing and fastness properties of dyed textile material. Most of the commercial reactive dyes have a complete gamut of colours, many of which are particularly bright in color. Reactive dyes often have quite simple structures that can be synthesised with a minimum of coloured isomers and biproducts that tend to dull the shade of the more complex polyazo direct dyes. Some colours are difficult to obtain with simple chromophores. Dark blue and navy reactive dyes are often rather dull copper complexes of azo dyes and the production of bright green reactive dyes usually make. A wide range of possible fibre-reactive groups has been examined and evaluated by the dyestuff manufacturers. The final choices for commercial reactive dyes are limited by a number of constraints. The reactive group must exhibit adequate reactivity towards cotton fibre, but be of lower reactivity towards water that can deactivate it by hydrolysis. The hydrolysis of the dye’s reactive group is similar to its reaction with cellulose fibre but involves a hydroxyl ion in water rather than a cellulosate ion in the fibre. In addition, the dye–fibre bond, once formed, should have adequate stability to withstand repeated washing. Other factors involved are the ease of manufacture, the dye stability during storage and the cost of the final reactive dye.

Reactive groups are of two main types:

(1) Reactive dyes which are reacting with cellulose by nucleophilic substitution of a labile chlorine, fluorine, methyl sulphone or nicotinyl leaving group activated by an adjacent nitrogen atom in a heterocyclic ring.

(2) Reactive dyes those reacting with cellulose by nucleophilic addition to a carbon–carbon double bond, usually activated by an adjacent electron-attracting sulphone group. This type of vinyl sulphone group is usually generated in the dyebath by elimination of sulphate ion from a 2-sulphatoethylsulphone precursor group with alkali.

Although many of the early reactive dyes had only one reactive group in the dyestuff molecule, many of the newer reactive dyes are bifunctional with two or more identical or different reactive groups shows some typical fibre-reactive groups and the commonly used abbreviations for these groups. Dyes with nicotinyltriazine reactive groups (NT) will react with cotton on heating under neutral conditions. 

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