Materials Included In Kit

Additional Materials Required

Lab Hints

• Use extreme caution when handling sodium hydroxide, it is extremely dangerous to skin and eyes. Hot sodium hydroxide solutions should only be handled with chemical-resistant gloves. Chemical splash goggles must be worn.
• A 60-mm Büchner funnel setup works best for filtering the crude indigo. Larger Büchner funnels will also work but the product will be spread very thin over the larger filter paper. If no Büchner funnel is available, the product can be isolated by gravity filtration using a simple funnel and filter paper. If using gravity filtration, allow an extra 15 minutes for the lab and air dry the product overnight. Make sure the filter paper is opened up overnight to facilitate the drying process.
• After 15–30 minutes in an operating fume hood, the crude indigo product is dry and will begin to lift off the filter paper. It is easy to remove from the filter paper when dried and can be weighed for percent yield calculation.
• Depending on the number of students or groups in the lab and their propensity for mistakes, there is enough o-nitrobenzaldehyde (5 g) for each student to use 0.35 to 0.40 g. If starting with a larger amount of o-nitrobenzaldehyde, slightly increase the amount of acetone to around 3 mL. The yield of indigo will increase to 0.25 g or more.
• The amount of indigo dye used to dye the fabric can vary from 0.05 g to 0.1 g. More than 0.1 g of indigo is difficult to reduce to the leuco base using only a large reaction vial. If using closer to 0.1 g of indigo, increase the amount of sodium thionite to 0.25–0.3 g.
• Vary the amount of water in dyeing step 11, depending on the amount of indigo dye used. If the solution is too dilute, it will be a pale yellow and will not produce a deep blue color in the fabric. Your students should have enough indigo dye to perform at least two dyeing processes.
• To reduce the amount of oxidation of the leucoindigo solution during the fabric dyeing step (step 12), reduce the exposure to air by placing Saran Wrap™ or Parafilm^® over the top of the beaker.
• The yellow leucoindigo solution is a very good stain and will stain clothing, books, fingers, wood and unknown other items. Treat this solution with care! Wear chemical-resistant gloves, aprons, and hang the wet fabric over paper towels or a sink. Once the solution has oxidized back to indigo, it is a less permanent stain. Indigo is partially soluble in acetone which can be used to clean any spills on counters.
• Soak the fabric until it is a bright yellow, which only takes 3–5 minutes. When drying, make sure all the fabric is exposed to air, unfold any creases in the fabric.

Teacher Tips

• Glass Reaction Vial: Glass reaction vials used in this kit are the large, or 2-dram size (see Figure 6). The vials are made from borosilicate glass and are extremely durable. When dropped, they will usually resist breaking, and can be reused if cleaned.
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• The most important contributor to the success of Vial Organic labs is the vial cap which contains a PTFE seal. The vials and PTFE seals are designed to resist the organic solvents and contain the pressure generated as a reaction progresses.
• It is important that you tighten the cap of a reaction vial very well. If the seal fails, the reactants will leak out into the water bath. To obtain a tight seal, hold the cap and vial in opposite hands and tighten as hard as possible. Never use pliers to tighten the cap. Rubber or latex gloves or a wet towel are sometimes useful to provide extra grip for tightening the cap. After a little practice you will be able to “feel” the seal set without using too much force.
• When the vial is heated, the glass vial and plastic cap expand differently and the cap may no longer be sealed tightly. This will lead to a leak and may contaminate the reaction. A leak is evidenced by a small stream of tiny bubbles coming from the vial cap. If this happens, simply remove the vial from the water bath with tongs, allow it to cool for a few seconds and carefully tighten the vial cap. The vials cool down rapidly and the plastic caps do not retain heat so they will not be excessively hot. However, care must be taken not to burn oneself; use rubber gloves and carefully test the temperature of the vial before attempting to handle.
• Always use tongs to place the reaction vial into the boiling water bath or remove it from the bath. Test tube, utility, or crucible tongs work best. Do not drop the vial in the beaker; the beaker or vial may crack. The reaction vial does not have to stand upright in the hot water bath.
• Hot Water Bath and Immersion Heating Coil: This Vial Organic laboratory procedure requires heating which is safely performed in a hot water bath utilizing an immersion heating coil. The hot water bath is simply made by adding 300 mL of deionized water to a 400-mL or 600-mL Pyrex^® beaker (using deionized water greatly extends the life of your heater) and placing the immersion heater into the water (see Figure 7). The size of the water bath is not critical to Vial Organic. If 400-mL beakers are not available, 250-mL or a beaker larger than 400-mL will also work. Make sure the immersion heater coil is always submerged. If the beaker is too small, there will not be much room for the reaction vial. If the beaker is too large, it will take too long to heat up the water. Always make sure the beaker is Pyrex or borosilicate glass. The immersion heater does not have to be attached to the side of the beaker; it can rest on the bottom. There are five simple rules that must be strictly observed whenever the immersion heater is used:
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  1. Place the coil into the water before plugging it in.
  2. Unplug the coil before removing it from the water.
  3. Always unplug the immersion heater and remove it from the water bath as soon as the reaction is complete.
  4. Only heat water with the coil. Do not heat oils or organic solvents.
  5. Never drink water heated by the immersion heater.
  A hot plate can also be used to heat the hot water bath but it will take longer and is considerably more expensive. It is a good practice to start the hot water bath as soon as the laboratory begins so the bath is boiling by the time the reaction vial is ready. If the above rules are followed and deionized water is used, the immersion heater will last a long time.

Discussion

The production of indigo is a simple organic synthesis lab that does not require any special purification steps other than filtration. The real excitement of this lab comes when your students use their product to dye fabric. Watching their yellow fabric turn green and then a rich blue before their very eyes will bring to life the excitement and usefulness of organic chemistry. It is sure to be one of your student’s favorite labs.

It is best if this lab is done in two 50-minute lab periods. If you need to complete the lab in one 50-minute lab period; shorten the initial reaction time to 10 minutes, do not completely dry the crude indigo dye, and only dye the fabric for 2–3 minutes. The use of a Büchner funnel setup greatly facilitates and speeds up the filtration and drying of the product. Typical yields of crude indigo should be around 0.16 g or about 50% yield. This is enough material for at least two separate dyeing processes.

The fabric test strips are included in this kit to introduce textile science to your students. Clothes are very important to many students and they may not be aware that chemistry is very important in the preparation of the fabric and in the developing of dyes that work well with the different families of fabrics. A fabric test strip will be used to determine how well indigo dyes various types of fabrics. The mechanism by which the dye molecules absorb onto the fibers depends on the chemical composition and physical structure of the polymeric chains that comprise the fibers. The porosity, or the packing of the polymer chains, determines how easily the dye molecules penetrate the fibers. Chemical attractions, such as hydrogen bonding, salt formations, or Van der Waal’s forces help keep the dye molecules absorbed on the fibers.

The fabric test strips consist of six different types of fabrics and are identical to those used in the textile industry to study dyes and their color fastness (see Figure 4). These fabrics represent major families of polymers used in fibers.

The test strips will have to be cut into ½"- to 1"-wide strips, depending on how many students are dyeing. If one end of the test strip is marked with a permanent marker, it will make fabric identification easier. The wool fabric is beige and is a good marker.

The indigo will dye the six fabrics differently depending on how well the fabric accepts or holds on to the dye. The chemical structure of the fabric polymer and its porosity are key properties in a fabric’s ability to be dyed. Cotton is easily dyed because of the relatively open structure of the cellulose polymer. In polyester, the polymer chains can get closer, resulting in a dense, more closely packed structure. The dye molecules are simply too large to penetrate the polyester fiber.

When dying the test fabric, cotton will be the darkest, followed by wool. The acetate and nylon are also blue, but lighter. The polyester and acrylic will be the lightest blue.

In addition to dying the fabric test strips, we recommend dyeing 100% cotton fabric. Indigo is a great dye for cotton and the result looks very much like a new pair of blue jeans (the unwashed type). An economical way to obtain cotton fabric is to ask your students to bring an old white T-shirt, towel, sheet or pillow case. Cut the item into 2" x 10" strips which will fit nicely into the 100-mL flask. By combining several solutions into a larger flask, large items like socks can be dyed.

Introduction

To prepare a synthetic dye and dye fabric using vat dye techniques.

Background

The preparation of dyes and the process of dyeing fabric are two of the oldest chemical processes developed by humans. The use of dyes is an ancient art that was first practiced in Egypt, Persia, China and India more than 5,500 years ago. The earliest dyes include madder, a red dye, and indigo, a blue dye. Indigo was originally extracted from the Indigofere Sumatrana plant found primarily in India and later from the Isatis Tinctoria plant in Europe. The Indigofere plant produced a much richer dye and during the Roman Empire, a vigorous trade route was established between India and Europe to supply this rich blue dye to Europe’s growing textile market.

The extraction and purification of natural dyes continued as a growing industry until the late-1800s when synthetic dyes were developed.The discovery of synthetic dyes was due to scientists beginning to understand more about the structure of molecules, chemical reactions, and the synthesis of common materials, such as dyes. Much of the early history of organic chemistry involved the study and synthesis of dyes which led to the development of the modern chemical industry.

The first synthetic dye was prepared in 1856, when William Henry Perkin, a student at the Royal College of Chemistry (England) accidentally discovered a synthetic mauve dye. This led to the first commercial development of an organic compound. Not long afterwards, German chemists developed additional dyes and the synthetic dye industry grew rapidly in Germany. In the 1880’s, Adolph von Baeyer undertook the study of indigo and determined its molecular structure and synthetic routes. For his work on indigo, von Baeyer received the Nobel Prize in chemistry in 1905. However, because of the difficulties in scaling up the process, synthetic indigo was not commercially available until after 1900. By World War I, almost all the world’s manufactured dyes were produced in Germany and synthetic dyes had almost entirely replaced the extraction of dyes from natural sources.

Today, almost all dyes are still synthetically produced and are still an important part of the chemical industry. The thousands of available dyes come in all colors and are used to color paper, plastics, leather, foods, cosmetics and fabrics, such as clothes, linens and carpets.

Science of Dyes

Dyes are intensely colored molecules that strongly absorb light in the visible region (400–700 nanometers). Only organic molecules with several connected (also called conjugated) double bonds are capable of absorbing enough light in this region to be effective dyes. Conjugated systems connected to aromatic rings with electron withdrawing and attracting groups are a common feature of dyes. Many dyes belong to similar families and have the same general molecular structures, differing only by a different electron-withdrawing or attracting group. Some common dyes are shown in Figure 1.

Indigo belongs to a class of dyes called vat dyes. Vat dyes are the oldest known dyes and the term “vat” applies to the vessel used to extract and ferment the dye from its natural sources. A key feature of vat dyes is that they are water insoluble dyes that are reduced to a water-soluble species known as a leuco base. There are many different types of vat dyes but they share one common trait: they contain one or more carbonyl groups. The carbonyl group in a vat dye is reduced to the sodium salt by treatment with a reducing agent in the presence of a base. The sodium salt is then soluble in water.

Leuco bases are attracted to cellulose fibers such as cotton and paper and form strong hydrogen bonds to the cellulose structure. The dye molecule also penetrates the fiber structure and attaches to the cellulose molecule inside the fiber. Leuco bases are oxidized back to an insoluble dye when exposed to air. By this time the insoluble dye molecule is trapped within the molecular structure of the cellulose polymer and is not easily removed. This physical trapping and chemical insolubility of the dye molecule gives vat dyes their unusual resistance to fading. The most common vat dye is indigo which is used to give the traditional blue color to blue jeans.

Preparation of Indigo

There are many synthetic routes to prepare indigo. One of the simplest routes is the reaction of o-nitrobenzaldehyde and acetone in the presence of a base. While the route may seem straightforward, the reaction is quite complex and involves an aldol condensation, ring closure, dehydration and a disproportionation (see Figure 2). Dyeing with Indigo

Indigo is insoluble in water but is easily reduced by sodium dithionite (sodium hydrosulfite, Na₂S₂O₄) in a strong alkaline solution to produce a water-soluble leucoindigo (see Figure 3). This leuco base is strongly attracted to cellulose. After the reduced dye has been absorbed on the fiber, the original insoluble dye is reformed by oxidation with air or chemicals. The colors from this dyeing process are very resistant to washing because the dye is insoluble in water. A fabric test strip will be used to determine how well indigo dyes various types of fabrics. The mechanism by which the dye molecules absorb onto the fibers depends on the chemical composition and physical structure of the polymeric chains that comprise the fibers. The porosity, or the packing of the polymer chains, determines how easily the dye molecules penetrate the fibers. Chemical attractions, such as hydrogen bonding, salt formations, or Van der Waal’s forces help keep the dye molecules absorbed on the fibers.

The fabric test strip consists of six different types of fabrics (see Figure 4). These fabrics represent six major families of polymers used in fibers. Reaction and Physical Properties

Materials

Safety Precautions

o-Nitrobenzaldehyde is moderately toxic by ingestion, LD₅₀: 3310 mg/kg. Acetone is flammable and a dangerous fire risk; mildly toxic by ingestion and inhalation, LD₅₀ 10.7 mL/kg. Sodium hydroxide solution is corrosive to skin and eyes; skin burns are possible; very dangerous to eyes. Indigo dye and especially the leuco base are very permanent dyes; they will stain clothes, paper, wood products and hands; wear gloves when performing the dyeing process. Always place the immersion heater in the water before plugging it in. Always wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Setup

1. Add approximately 300 mL of deionized water to a 400-mL beaker. Place an immersion heater in the water, plug it in and allow the water to come to a boil. Note: Do not plug in the immersion heater until after it is placed in the water.
2. Place 0.3 g of o-nitrobenzaldehyde and 2.5 mL (2.0 g) of acetone in a large reaction vial. If performing percent yield calculations, weigh the o-nitrobenzaldehyde and acetone as it is being added to the vial. Gently mix the solution to dissolve the o-nitrobenzaldehyde.
3. Add 4 mL of 1 M sodium hydroxide solution to the vial. The yellow o-nitrobenzaldehyde in acetone solution will turn dark almost immediately and begin to get warm. Quickly seal the vial with the cap. Make sure the cap is on tight. Shake the reaction mixture again.
4. Using tongs, place the sealed vial in the beaker containing boiling water and immersion heater. If, at any time, small bubbles start coming out of the vial, the cap is not tight enough. Remove the vial, allow it to cool, and retighten the cap.
5. After 15 minutes, use tongs to remove the vial from the boiling water bath and place on the counter for a few minutes to cool.

Isolation of Product

6. Place reaction vial in a cold water bath for a few minutes.
7. Carefully open the reaction vial. Wear chemical-resistant gloves because the reaction vial may be pressurized and some solution may squirt out.
8. Filter the indigo using a Büchner funnel or gravity filtration setup. Rinse the vial out with water to remove all the product and then wash the product twice with 4–5 mL of cold water or until the filtrate is clear.
9. The blue-black solid on the filter paper is crude indigo. Remove the filter paper and indigo from the funnel and allow it to dry for at least an hour.
10. After the indigo is dry, weigh it if performing weight percent calculations.
11. The aqueous solution can be disposed of by flushing down the drain with excess water.

Vat Dyeing with Indigo

Setup

1. Add approximately 300 mL of deionized water to a 400-mL beaker. Place an immersion heater in the water, plug it in and bring the water to about 50 °C, do not bring it to a boil. The immersion heater may have to be unplugged during the procedure so the water bath does not get too hot. Do not plug in the immersion heater until after it is placed in the water.
2. Place about 0.07 g of indigo dye (about ½ of the product) and 5 mL of 1 M sodium hydroxide solution in a large reaction vial.
3. Seal the vial again with the cap. Make sure the cap is on tight. Shake the reaction mixture. The indigo will not completely dissolve in the sodium hydroxide solution.
4. Using tongs, place the sealed vial in the beaker containing hot water and immersion heater.
5. After 5 minutes, use tongs to remove the vial from the hot water bath and place the vial on the counter. As soon as the vial is cool enough to touch (a few seconds), carefully open the reaction vial. Wear chemical-resistant gloves because the reaction vial may be pressurized and some solution may squirt out.
6. Add 0.5 g of sodium dithionite to the reaction vial. Seal the vial again with the cap. Make sure the cap is on tight. Vigorously shake the reaction mixture.
7. Using tongs, place the sealed vial in the beaker containing hot water and immersion heater.
8. Continue heating the indigo solution until the indigo completely dissolves and a clear yellowish solution is produced. The yellow solution indicates the formation of the leucoindigo form of the dye. If the solution turns yellow but a blue solid remains on the bottom of the vial, remove the vial and shake the vial again. Heat until the solution is clear.
9. (Optional) If after 10 minutes the solution is still dark, remove the vial from the hot water bath and place it on the counter to cool. As soon as the vial is cool enough to touch (a few seconds), carefully open the reaction vial. Wear chemical-resistant gloves because the reaction vial may be pressurized and some solution may squirt out. Add another 0.1 g of sodium dithionite solution to the vial. Seal the vial again and shake vigorously before placing it in the hot water bath. Heat until the solution is a clear yellow.
10. Continue to heat the solution for 2–3 minutes after a clear yellow solution has been produced. Use tongs to remove the vial from the hot water bath and place on the counter for a minute to cool.
11. Place 40–50 mL of tap water and 0.2 g of the sodium dithionite solution in a 100 mL beaker. Stir briefly with a glass stirring rod or metal spatula.
12. Carefully open the reaction vial. Wear chemical-resistant gloves because the reaction vial may be pressurized and some solution may squirt out.
13. Quickly add the yellow leucoindigo solution to the beaker containing dilute sodium dithionite solution. The solution should remain clear and yellow but may turn blue at the surface.

Dyeing the Fabric

14. Carefully place the fabric test strip in the leucoindigo dye solution. Using a glass stirring rod or metal spatula, work the fabric into the solution until it is completely saturated with the leucoindigo solution. As the fabric soaks in the solution, it will turn a bright yellow. The top of the solution will be blue due to air oxidation of the leucoindigo.
15. After the fabric has soaked for about 5 minutes, it is ready to be removed. Wearing chemical-resistant gloves, carefully remove the fabric test strip using the glass stirring rod or metal spatula and hang it somewhere to dry. Indigo dye solution will drip from the fabric, be careful not to drip any indigo dye solution on any fabric or other cellulose type materials (e.g., books, papers), it will stain. Place several paper towels below the dripping fabric to catch any drips. Wear chemical-resistant gloves to avoid getting any of the dye on your hands.
16. As the fabric is exposed to air, the yellow leucoindigo is oxidized back to blue indigo. This reaction can be followed by watching the fabric turn from yellow to a green to blue green and finally to a deep blue. Make sure all the fabric is exposed to air and turns blue.
17. After 5–10 minutes, the fabric should be completely blue. Wearing chemical-resistant gloves, check the fabric to make sure it is completely blue before thoroughly washing the fabric with cold water until the washings are no longer blue.
18. The aqueous solution of leucoindigo can be disposed of by flushing down the drain with excess water.