MONOFUNCTIONAL REACTIVE DYE

Monofunctional systems:

Some of the most important monofunctional reactive systems contain only one possible reactive centre, such as the halogeno substituent in the aminohalotriazine dyes, or the activated terminal carbon atom in the vinylsulphone system. In others there are two equivalent replaceable halogeno substituents, as in the dichlorotriazine, difluoropyrimidine or dichloroquinoxaline heterocyclic ring systems. When one of these halogen atoms is displaced by reaction or hydrolysis, as in Scheme 4.2 for example, the reactivity of the remaining halogeno substituent is greatly decreased by the presence of the new hydroxy or cellulosyl substituent.

Dichloro-s-triazine dyes:

In a typical dye of this type, such as CI Reactive Red 1, the partial polarisation of the ring C=N and C–Cl bonds, because of the greater electronegativity of the N and Cl atoms, makes the 2- and 4-chloro substituents susceptible to nucleophilic displacement, although these influences are muted by feedback of electrons from the NH bridging group linking the triazine ring to the phenylazonaphthyl chromophore. Such dyes are stable in neutral solution at ambient temperature but subject to hydrolytic attack by hydroxide ions at alkaline pH and to autocatalytic hydrolysis under acidic conditions. To guard against this, a buffer is added to the solution to ensure stability during isolation and further buffer is added to the dyestuff paste before drying.

The dichlorotriazine dyes are highly reactive and can be readily fixed to cellulosic materials by pad–batch dyeing at ambient temperature or by batchwise methods at 30–40°C. This means that relatively small chromogens are preferred to ensure adequate mobility of dye on the fibre during the exhaustion stage. This requirement makes these dyes eminently suitable for bright dyeings but less satisfactory for deep tertiary hues, since the larger-size chromogens used for this purpose often fail to give acceptable performance by low-temperature application. A weakness with certain dichlorotriazine dyes, particularly red dyes based on H acid as coupling component, such as CI Reactive Red 1, is that under conditions of low pH the dye–fibre bond is broken by acid-catalysed hydrolysis, leading to deficiencies in fastness to washing or acid perspiration.

When partial hydrolysis occurs as in Scheme 4.2 to form the2-chloro-4- hydroxy-s-triazinylamino species, the dye does not have a further chance to achieve fixation via the remaining chlorine atom. Under the alkaline conditions of the fixation stage, ionisation of the acidic 4-hydroxy substituent leads to amassive feedback of negativity into the triazine ring, causing total deactivation of

the remaining 2-chloro substituent.

Controlled reaction of dichlorotriazine dyes with either amines or alcohols leads to two further classes of monofunctional dyes, the 2-amino-4-chloro- and 2-alkoxy- 4-chloro-triazines respectively. The latter are more reactive than the former but less reactive than the parent dichlorotriazine types. They are now only of historical

interest, the 2-isopropoxy-4-chloro system forming the basis of the Cibacron Pront

(CGY) range for printing. The bulky isopropoxy group was chosen in order to

disrupt the planarity of the substituted triazine system and thus favour removal of

unfixed dye from the printed fabric during the washing-off stage.

In a recent investigation, the relative reactivities of model 2-alkoxy-4-

chlorotriazine dyes were compared. Surprisingly, the hydroxide ion preferentially

displaced the alkoxy group rather than the chloro substituent. Increasing the size

and electron-donating capacity of the alkoxy group resulted in a decreasing

propensity for substitution, so that displacement of methoxide ion was 12 times

faster than displacement of isopropoxide ion [11].