"group a group of the formula ##STR1## in which both R.sup.1 s, identical to or different from one another, denote a hydrogen atom or an optionally substituted alkyl group of 1 to 4 carbon atoms, R is a hydrogen atom or a sulfo group and Y represents a vinyl group or a .beta.-thiosulfatoethyl, .beta.-phosphatoethyl, .beta.-chloroethyl or .beta.-sulfatoethyl group, this fiber-reactive grouping being" . . . . . . .