The Chemistry of Dyeing

There are many different kind of dyes available for dyeing. Therefore, scientists try to classify them into groups. One method of classification is to look at certain structural features in the dye molecule. Methyl orange is an example for an azo dye (it contains the azo group –N=N-), malachite green is one of the triphenylmethane dyes (the triphenylmethane moiety consists of three benzene rings attached to a central carbon atom). The yellow dye alizarin is called an anthrachinone dye.

methyl orange N(CH ) (an azo dye) 3 2 N Cl- N +

NaO3S

(CH3)2N N(CH3)2 O OH malachite green (a triphenylmethane dye) OH

O alizarin (a anthrachinone dye)

Another way of classification is to look at the way of bonding of the dye to the fiber. Mordant dyes use a binding agent that assists in the binding. The mordant (often a metal hydroxide) bonds to the fiber and to the dye; the binding agent serves as a tether between the dye and the fiber. Vat dyes involve the application of the dye in a water-soluble form that then “precipitates” on the fiber. Reactive dyes literally react with certain functional groups of the fiber and bind covalently to the fiber.

fiber

mordant

dye

1 Indigo – A Vat Dye

Indigo has been used in many different cultures for thousand of years. It can be obtained from different plants through a fermentation process. It was valued very highly for its deep blue color. In some cultures it was considered sacred due to the resemblance to the color of the sky. Book at the FSC library about indigo are listed at the end of this document.

There are several ways to produce indigo synthetically. We are going to follow the Baeyer synthesis, which is easy and produces very pure indigo.

Procedure for indigo synthesis

Dissolve 0.1 g of 2-nitrobenzaldehyde in 2 mL acetone in a 50 mL beaker, and dilute the solution with 3.5 mL of water. Stir the solution vigorously using a magnetic stirrer and add 1 mL of 2 M NaOH. Continue to stir the mixture for 5 minutes. Write your observations into your notebook. Collect the product by vacuum filtration. Wash the product with water until the washings are not yellow any more (you will need about 10 mL). To dry the indigo, wash with 5 mL ethanol and continue suction for 5 to 10 minutes. Record the yield (in grams) of your product.

O O H N H NaOH 2 + 2 CH3COCH3 N NO2 H O 2-nitrobenzaldehyde acetone indigo

+ 2H2O + 2CH3CO2H

Vat dyeing with indigo

Indigo is virtually insoluble in water and therefore cannot be applied to the fiber in this form. A chemical process, called reduction, uses sodium dithionite (aka sodium hydrosulfite) to convert indigo into leukoindigo, which is water-soluble. Now the compound can be applied to the fiber. Oxygen in the air converts the leukoindigo back into indigo (the process is called oxidation), which due to its insolubility remains in the fiber. NaOH

Na S O 2 2 4 Na+ O H O H sodium hydrosulfite or N sodium dithionite N

N N H O H O oxygen (air) Na+ indigo leukoindigo 2 Procedure for fabric dyeing with indigo

Place 50 mg of indigo powder into a 50-mL Erlenmeyer flask, add a few drops of ethanol and mix it into a paste with a stirring rod. Add 200 mg of sodium dithionite, 2 mL of 3 M sodium hydroxide solution, and 20 mL of water. Stopper the flask to exclude air, shake it for several minutes, and with the stopper slightly loosened, heat the flask for a few minutes with swirling on a hot plate. The solution will acquire a deep yellow-green color. Dip a strip of each available fabric into the solution with tweezers for 10-15 seconds. Take the fabric out of the solution and observe. After about 15 minutes wash the strips with a little bit of soapy water. Note the results.

Reactive Dyes

The reactive dyes that we are going to use today all have a 2-chloro-1,3,5-triazine moiety as part of their structure. The carbon that is bonded to the chlorine atom is Lewis-acidic and reacts with Lewis bases in the fiber (the oxygen of alcohols, and the nitrogen of amines) in a substitution reaction.

NHR Cl R-OH OR alcohol substitution N N N N N N HCl or or + + reaction X N Y amine X N Y X N Y R-NH 2 dyes that are covalently bonded a 2-chloro-1,3,5-triazine in the fiber molecules to the fiber

The companies that produce these dyes usually don’t provide structures. However, we were able to obtain the structures of the two dyes that you are using today. Reactive Blue 2 is an anthrachinone dye; Reactive Red 2 is an azo dye.

3 O NH2 Reactive Blue 2 (Procion Blue HB), [12236-82-7] SO3Na MW = 840.12 g/mol, lambda max = 607 nm

Cl O N H N N SO3Na N N N H SO3Na H

NaO3S SO3Na

N N Cl N N OH H N N Reactive Red 2 (Procion Red MX-5B), [17804-49-8] Cl MW = 615.34 g/mol, lambda max = 538 nm

The reactive dyes work very well with cotton fabrics. Cotton is made of cellulose which is a polymer made of glucose (dextrose) subunits. The oxygen of the alcohol groups reacts with the dyes.

OH OH OH OH OH O O O O O O O O O O OH OH OH OH OH OH OH OH OH OH

cellulose

NaO S SO Na NaO3S SO3Na 3 3

N N N N Cl N N OH Cl N N OH H H N N N N

O OH OH O OH O O O O O O O O O O OH OH OH OH OH OH OH OH OH OH

two procion red molecules bonded to cellulose

4 Procedure for dyeing with reactive dyes

The fiber reactive dyes are already prepared in the three primary colors (red, blue, and yellow). Dip a strip of each available fabric into 10 mL of one of the fiber reactive dye solutions with tweezers for 10-15 seconds. Take the fabric out of the solution and observe. After about 15 minutes wash the strips with a little bit of soapy water. Note the results.

After tying, folding, knotting, etc. place your shirt in the bucket of soda ash to prepare the surface of the fabric to bond with the dye. After soaking for 10 minutes, ring out shirt to remove as much of the solda ash solution as possible. Using squirt bottles of dye provided add dye to your shirt, remembering which combinations make which colors. Place shirt in freezer bag and zip closed. Take home and allow dye to react for at least 24 hours. Rinse out, by hand, as much of the dye as possible with cold water. Wash separately dry and wear to next week’s lab.

Question

1. Calculate the % yield for your synthesis of indigo.