Question

A problem in dyeing fabrics is the degree of fastness of the dye to the fabric. Many of the early dyes were surface dyes; that is, they did not bond to the fabric, with the result that they tended to wash off after repeated laundering. Indigo, for example, which gives the blue color to blue jeans, is a surface dye. Color fastness can be obtained by bonding a dye to the fabric. The first such dyes were the so-called reactive dyes, developed in the 1930 s for covalently bonding dyes containing - \(\mathrm{NH}_{2}\) groups to cotton, wool, and silk fabrics. In the first stage of the first-developed method for reactive dyeing, the dye is treated with cyanuric chloride, which links to the fabric through the amino group of the dye. The remaining chlorines are then displaced by the \(-\mathrm{OH}\) groups of cotton (cellulose) or the - \(\mathrm{NH}_{2}\) groups of wool or silk (both proteins). Propose a mechanism for the displacement of a chlorine from cyanuric chloride by (a) the \(\mathrm{NH}_{2}\) group of a dye and (b) by an - \(\mathrm{OH}\) group of cotton.

Short answer

Question: Propose a step-by-step mechanism for the displacement of a chlorine atom from cyanuric chloride by (a) the \(\mathrm{NH}_{2}\) group of a dye and (b) by an \(-\mathrm{OH}\) group of cotton. Answer: (a) The mechanism for the displacement of a chlorine atom from cyanuric chloride by the \(\mathrm{NH}_{2}\) group of a dye involves an \(\mathrm{S}_{\mathrm{N}}2\) reaction. The amine group acts as a nucleophile and attacks a nitrogen atom in cyanuric chloride, forming a new nitrogen-nitrogen bond and releasing a chlorine atom as a chloride ion. (b) The mechanism for the displacement of a chlorine atom from cyanuric chloride by an \(-\mathrm{OH}\) group of cotton also involves an \(\mathrm{S}_{\mathrm{N}}2\) reaction. The hydroxyl group acts as a nucleophile and attacks a nitrogen atom in cyanuric chloride, forming a new oxygen-nitrogen bond and releasing a chlorine atom as a chloride ion.

Step-by-step solution

Identify the structure of cyanuric chloride

Cyanuric chloride has the molecular formula \(\mathrm{C}_{3}\mathrm{N}_{3}\mathrm{Cl}_{3}\). It is a planar molecule with three chlorine atoms bonded to the three nitrogen atoms, each of which is double-bonded to a carbon atom, forming a six-membered ring. The structure can be represented as follows: \(\mathrm{Cl}_{2}\mathrm{C}\equiv\mathrm{N}-\mathrm{NCl}\equiv\mathrm{C}-\mathrm{NCl}\).

Understand nucleophilic substitution reactions

In a nucleophilic substitution reaction, a nucleophile (an electron-rich species) attacks an electrophilic center bearing a leaving group. The nucleophile donates an electron pair to form a new bond with the electrophilic center, and the leaving group departs with the electron pair it formerly shared with the electrophilic center. Nucleophilic substitution reactions are commonly designated as \(\mathrm{S}_{\mathrm{N}}1\) or \(\mathrm{S}_{\mathrm{N}}2\) reactions, depending on the mechanism they follow. In this case, we will assume that the reaction follows an \(\mathrm{S}_{\mathrm{N}}2\) mechanism.

Propose a mechanism for the displacement of chlorine by the \(\mathrm{NH}_{2}\) group of a dye

In an \(\mathrm{S}_{\mathrm{N}}2\) reaction, a nucleophile attacks an electrophilic center while the leaving group departs simultaneously. In the case of cyanuric chloride reacting with an amine group (\(-\mathrm{NH}_{2}\)) in a dye, the amine acts as a nucleophile and attacks one of the nitrogen atoms bonded to a chlorine atom in cyanuric chloride. The nitrogen in cyanuric chloride is electrophilic due to the presence of highly electronegative chlorine atoms that draw electron density away from nitrogen. The amine donates an electron pair to the nitrogen atom, leading to the formation of a new nitrogen-nitrogen bond. At the same time, the nitrogen-chlorine bond breaks, and the chlorine atom leaves as a chloride ion (\(\mathrm{Cl}^-\)). The overall reaction is: \(\mathrm{R-NH}_{2} + \mathrm{Cl}_{2}\mathrm{C}\equiv\mathrm{N-NCl}\leftrightarrow \mathrm{R-NH-NCl} + \mathrm{Cl}^-\), where \(\mathrm{R}\) represents the rest of the dye molecule.

Propose a mechanism for the displacement of chlorine by the \(-\mathrm{OH}\) group of cotton

When an \(\mathrm{OH}\) group in cotton reacts with cyanuric chloride, a similar \(\mathrm{S}_{\mathrm{N}}2\) reaction takes place. The hydroxyl group (-OH) acts as a nucleophile and attacks a nitrogen atom that is bonded to a chlorine atom in cyanuric chloride. The nitrogen in cyanuric chloride is electrophilic due to the presence of highly electronegative chlorine atoms that draw electron density away from nitrogen. The hydroxyl group donates an electron pair to the nitrogen atom, leading to the formation of a new oxygen-nitrogen bond. Simultaneously, the nitrogen-chlorine bond breaks, and the chlorine atom leaves as a chloride ion (\(\mathrm{Cl}^-\)). The overall reaction is: \(\mathrm{ROH} + \mathrm{Cl}_{2}\mathrm{C}\equiv\mathrm{N-NCl}\leftrightarrow \mathrm{RO-NCl} + \mathrm{Cl}^-\), where \(\mathrm{R}\) represents the rest of the cotton molecule. These mechanisms describe the process of nucleophilic substitution by both the amine group in a dye and the hydroxyl group in cotton, leading to the displacement of chlorine atoms from cyanuric chloride and bonding between the dye and cotton fabric.