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.
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