group a group of the formula ##STR1## in which both R1 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 ??-thiosulfatoethyl, ??-phosphatoethyl, ??-chloroethyl or ??-sulfatoethyl group, this fiber-reactive grouping being bonded to the radica