Necessity of alkali for dyeing with reactive dye

Necessity of alkali for dyeing with reactive dye

 (Described by vinyl sulphone reactive dye)

Reactive dyes belonging to vinyl sulphone system has become popular with the dyers as those belonging to the chlorotriazine system.

The structure of a typical dye of this group Remazol Red B is shown below:

These dyes may be represented by the general formula, 

D-SO₂-CH₂-CH₂-OSO₃K

where D is the color forming part and contains one or more solubilising groups.

          In this form of the dye, the sulphonyl induces polarization of aliphatic chain attached to it, by which the alpha carbon atom in the sulphonyl group has an electron restraining effect and the carbon atom in the beta position gains a positive charge –

                              D-SO₂-CH₂-CH₂-OSO₃Na

          The electrons belonging to the hydrogen atoms linked to the alpha carbon atom are nearer the carbon atom than the hydrogen atoms. However, the electrons in the neighborhood of the beta carbon atom are not as strongly linked to this atom. At the alpha carbon atom there is a tendency to liberate a proton and the –SO₃Na group linked to the beta proton is likely to separate as an anion. Both these tendencies favor the following reaction in an alkaline medium.

          D-SO₂-CH₂-CH₂-OSO₃Na + NaOH→ D-SO₂-CH=CH₂ + Na₂SO₄ + H₂O

          This reaction takes place almost immediately on addition of an alkali to a solution of the original dye in water.

          The vinyl sulphonyl group (-SO₂-CH=CH₂) shows strong polarization caused by the sulphonyl (-SO₂) group:

         

                                                  D-SO₂-CH=CH₂

          This activated double bond can then react with water, alcohols and cellulose, forming ethers by the addition reaction:

                              D-SO₂-CH=CH₂ + R-OH → D-SO₂-CH₂-CH₂-OR

          In the dyeing of cellulose with vinyl sulphone dyes the reaction with cellulose is always accompanied by reaction with water with the formation of hydroxyl ethyl sulphone dye, D-SO₂-CH₂-CH₂-OH, which then cannot react with cellulose.

          Both the reactions (with fibre and with water) are catalyzed by alkali. Thus alkali is required in the dyeing of cellulose with these reactive dyes for converting the suphato ethyl sulphone dye to vinyl sulphone dye with the cellulosic fibre.

(in the first case, alkali is consumed while the other alkali acts only as a catalyst)

The hydroxy ethyl sulphone dye (the reaction product of the vinyl sulphone dye and water) cannot react with cellulose and has very low affinity for cellulose.

In neutral solutions, the sulphatoethyl sulphone dyes (as marketed) do not undergo any chemical reaction; the aqueous dye solution may ever by boil without the risk of losing the reactivity by the formation of the hydroxy ethyl sulphone form of the dye.