CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
MODULE 1: DYE SYNTHESIS & DYEING
(Jan 24 – Feb 11)
For an interesting account on the history on the dye industry, please read the accompanying section on “Dyes” from Napoleon’s Buttons: How 17 Molecules Changed
History, by Penny LeCouteur & Jay Burreson, (Penguin, New York) 2003, p. 162-180.
Introduction
Since prehistoric times, man has been dyeing cloth. In the next three weeks, you will work in pairs to synthesize a total of five dyes (3 each), and test their adhesion to a variety of natural and synthetic fibers. The structures of the dyes appear in Figure 1. You will each perform your own synthesis and generate your own samples, but can share the dyed fabric samples with your partner.
The Dyes
During the first week, each pair will synthesize the cationic dyes Malachite Green and
Crystal Violet via similar Grignard reactions. Malachite Green will be formed by the reaction between p-dimethylaminophenylmagnesium bromide and methyl benzoate. Crystal Violet is formed by the same reaction, substituting diethyl carbonate for the methyl benzoate. These are examples of mordant dyes. One of the oldest known methods of producing wash-fast dyes involves the use of metallic hydroxides, which form a link or mordant (L. mordere, to bite) between the fabric and the dye. The compounds will be used to dye both regular and mordanted fabric samples. These dyes also have select uses based on their known antifungal properties.
The former is sometimes applied to the mouths of babies in neonatal units, while the latter is the active ingredient in anti-ick medications used in home aquariums.
During the second week, each pair will synthesize Methyl Orange and Orange II dyes by an azo coupling reaction. Azo dyes are among the most common types of dyes. These are both examples of direct dyes that can either be made in bulk and later applied to a fiber, or can be developed on and in a fiber by combining the reactants in the presence of the fiber.
Interestingly, Methyl Orange can be used as an acid/base indicator - while its acid-stable form
(pH < 3.2) is red, its alkali stable form (pH >4.4) is yellow due to a change in its protonation state. Orange II is made by coupling diazotized sulfanilic acid with 2-naphthol in alkaline solution; Methyl Orange is prepared by coupling the same salt with N,N -dimethylaniline in a weakly acidic solution.
In the third week, each person will synthesize Indigo according to the 1882 method of
Baeyer (of asprin fame) by heating o-nitrobenzaldehyde and acetone in the presence of base.
Indigo is an example of a vat dye, a term which applies to dyes that are reduced to a colorless
(leuco) form, applied to a fiber, and then later oxidized on the surface of the fiber. Indigo is unique in that it can be abraded from the surface of a fabric (think of the knees of old blue jeans), whereas other dyes penetrate the fiber.
1

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
(H
3
C)
2
N N(CH
3
)
2
N(CH
3
)
2
Cl
N(CH
3
)
2
Crystal Violet
Cl
N(CH
3
)
2
Malachite Green
NaO
3
S
HO
N
N
NaO
3
S N
N
CH
3
N
CH
3
O
H
N
Orange II Methyl Orange
O
Indigo
Figure 1.
Molecular structures of the dyes to be synthesized.
The Fibers
After each of these dyes has been synthesized, you will test their ability to directly stain a variety of fibers that have been woven into a piece of Multifiber fabric. A natural fiber such as cotton has a surface area of about 5 acres per pound. When dyes are absorbed from solution
(usually aqueous) onto the surface of the fiber, the dye penetrates the fiber’s pores. These dyes and most other dyes are non-covalently bound to the fiber by a combination of intermolecular interactions including electrostatic forces, van der Waals forces, and/or hydrogen bonding. In the case of fiber-reactive dyes, the dyes may be covalent bonded to the fiber. The stronger the intermolecular interactions are between the dye and fiber, the better the dye will adhere. In these experiments, you should be able to see quite clearly the marked differences in shade produced by the same dye on different types fibers. While binding strength is important, for a dye to be commercially viable as a textile colorant, additional factors must be considered. Resistance to light, heat, and washing are important commercial considerations, though these are not concerns to be taken into account in our experiments.
The pieces of Multifiber fabric (Testfabrics, Inc.) you will receive each contain 13 strips of different fibers woven into it. Below the black thread starting at the top, the fibers are: ( 1 ) acetate rayon (cellulose di-or triacetate), ( 2 ) SEF (Monsanto’s modacrylic Self-Extinguishing
Fiber), ( 3 ) Arnel (cellulose triacetate), ( 4 ) cotton, ( 5 ) Creslan (poly-acrylonitrile), ( 6, 7 ) Dacron
54 and 64 (polyester without and with a brightener), ( 8 ) nylon 6.6 (polyamide), ( 9 ) Orlon 75
(poly-acrylonitrile), ( 10 ) silk (polyamide), ( 11 ) polypropylene, ( 12 ) viscose rayon (regenerated cellulose), and ( 13 ) wool (polyimide). The molecular structures of most of these fibers are provided in Figure 2.
Cotton is pure cellulose. Acetate rayon is cellulose (from any source) in which about two of the hydroxyl groups in each unit have been acetylated so that it can be solubilized in acetone and then spun into fiber. There are a smaller number of hydroxyl groups in acetate rayon compared to cotton. Nylon (which we’ll synthesize in the next module) is a polyamide and made
2

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift by polymerizing adipic acid and hexamethylenediamine. Except for the terminal acid and amine groups, there are no strongly polar centers in nylon. Similarly Dacron, a polyester made by polymerizing ethylene glycol and terephthalic acid, has few polar centers within the polymer.
Orlon is a polymer of acrylonitrile. Wool and silk are polypeptides cross-linked with disulfide bridges. The acidic and basic amino acids (e.g., glutamic acid and lysine) provide many polar groups in wool and silk.
HOH
2
C
O
HO
O
RO
O
OR
HOH
2
C
OH
O
Cellulose (Cotton, R=H;
Acetate rayon, R=OAc)
O n
O
(CH
2
)
4
O H H
N(CH
2
)
6
N
Nylon n
CH
2
CH
C
N n
Polyacrylonitrile
(Orlon)
O R O R
O O
N
H
H
N
N
H n
O OCH
2
CH
2 n
O R
Wool (R = amino acid residue)
Polyethylene glycol terephthalate
(Dacron)
Figure 2.
Molecular structures of the fibers to be dyed.
One way to make dyes adhere to fibers more effectively is to apply a mordant (L. mordere , to bite), usually a metal hydroxide, to the fibers. This agent helps form a stronger link between the fabric and the dye. This is one of the oldest known methods of producing wash-fast colors. Other substances, such as tannic acid, also function as mordants. The color of the final product depends on both the dye used and the mordant. For instance, the dye Turkey Red
(alizarin) is red with an aluminum mordant, violet with an iron mordant, and brownish-red with a chromium mordant.
Some mordanted pieces of test fabric have been prepared ahead of time and will be available for use during the first week of experiments only. They were prepared by allowing regular test fabric strips to stand in a nearly boiling solution of 0.1g of tannic acid in 50mL of water for 30min. The tannic acid was fixed to the cloth by transferring the cloth to a hot bath made from 20mg of potassium antimonyl tartrate (tartar emetic) in 20mL of water. After 5 min, the cloth was wrung out and dried as much as possible. Pieces of cloth were then immersed in
0.1M solutions of mordants (either ferrous sulfate or copper sulfate) and kept at the boiling point for about 15-20min.
3

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
O
Alizarin
O
O
O
OH
2
Cr
O
H
2
O
O
Ce llul ose
Figure 3.
Example of a dye binding to a mordanted fiber.
Part A (work in pairs): Grignard Synthesis of Crystal Violet or Malachite Green
1
In 1912 Victor Grignard received the Nobel prize in chemistry for his work on the reaction that bears his name, a carbon-carbon bond-forming reaction by which almost any alcohol may be formed from appropriate alkyl halides and carbonyl compounds. The Grignard reagent is easily formed by reaction of an alkyl halide (in particular a bromide) with magnesium metal in anhydrous diethyl ether or THF. The Grignard reagent is a strong base and a strong nucleophile. As a base it will react with all protons that are more acidic than those found on alkenes and alkanes. Thus, Grignard reagents react readily with water, alcohols, amines, thiols, etc., so the reactants and apparatus must all be completely dry and free of acidic protons, otherwise the reaction will not start. The magnesium metal, in the form of a coarse powder, initially has a coat of oxide on the outside. Crushing the turnings (on a piece of weighing paper) under absolutely dry solvent in the presence of an organic halide exposes a fresh unoxidized surface. The reaction will begin at exposed surfaces, as evidenced by a slight turbidity in the solution and the evolution of bubbles. Another useful trick to start the reaction is to add a few drops of CH
3
I or a small crystal of I
2
, which is believed to convert a small amount of halide to
(the more reactive) iodide. Once the exothermic reaction starts, it proceeds easily – the magnesium dissolves, and a solution of the Grignard reagent is formed. The Grignard reagent is not isolated but most often reacts immediately with a carbonyl compound to give the magnesium alkoxide, a salt that is usually insoluble in ether. The great versatility of this reaction lies in the wide range of compounds that can undergo the reaction. Reactions with ketones and aldehydes, giving tertiary and secondary alcohols, respectively, are among the most common. Finally, in a simple acid/base reaction, the alkoxide formed is reacted with acidified ice water to give a covalent, ether-soluble alcohol and the ionic water-soluble magnesium salt.
The first step in the synthesis of both dyes is to form a Grignard reagent from pdimethylaminophenyl bromide. Before embarking on this second step, find a partner and decide which compound each of you will synthesize. Products will be exchanged at the end. In this second step of the reaction, an ester will be slowly (the reaction is endothermic!) added to the
Grignard reagent. Crystal Violet is formed when one mole of diethyl carbonate is added to three moles of the Grignard reagent; Malachite Green is synthesized in a similar fashion by reacting one mole of methyl benzoate and two moles of the Grignard reagent. When either of the resultant alkoxide salts is hydrolyzed with HCl, an intermediate tertiary alcohol is formed.
However, this compound immediately loses a molecule of water, leaving a highly stable colored
4

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift carbocation. Both dyes are highly conjugated, and this conjugation is responsible for the colors of these dyes. The positive charge is extensively delocalized due to the overlap of the orbitals on the cationic center with the conjugated π -system of the benzene rings and even the paradimethylamino groups. The visible λ max
can be obtained from the UV/Vis spectrum.
N(CH
3
)
2
CRYSTAL VIOLET
Br MgBr
H
3
CH
2
C
O
O
O
H
3
CH
2
C
(H
3
C)
2
N OMgBr
HCl
(H
3
C)
2
N
Cl
N(CH
3
)
2
Mg N(CH
3
)
2
N(CH
3
)
2
THF
N(CH
3
)
2
N(CH
3
)
2
MALACHITE GREEN
O
(H
3
C)
2
N
OCH
3
HCl
(H
3
C)
2
N OMgBr
Cl
N(CH
3
)
2
N(CH
3
)
2
Figure 4.
Reaction scheme for the Grignard synthesis of Crystal Violet and Malachite Green.
Procedure
Making the Grignard reagent (whole class)
1. Obtain the following reagents before setting up the reaction: 2.5g of 4-bromo-N,Ndimethylaniline, 0.40g Mg turnings, 30mL of dry tetrahydrofuran (THF), and 2-3 small crystals of iodine. Keep the THF covered with parafilm or a rubber stopper.
2. Prepare a drying tube filled with anhydrous calcium chloride ahead of time. All glassware used in this experiment must be dry because the Grignard reagents are very water sensitive. Rinse a 50-mL round-bottom flask with 2-3mL of THF, pour 2-3mL of THF through the condenser, and discard these rinses in the flammable waste container. Once the glassware is rinsed, the reagents need to be added quickly and the apparatus assembled to minimize exposure to atmospheric moisture.
3. Fit the condenser to the flask and adjust the water so that it flows gently through the condenser. Place the drying tube at the top.
5

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
4. Clamp a round-bottomed flask in place, temporarily remove the condenser, and add
2.5g of 4-bromo-N,N-dimethylaniline, 25mL of dry tetrahydrofuran (THF), and 2-3 small crystals of iodine.
5. Scrape the Mg turnings on a piece of weighing paper to expose a fresh surface and
6. add these to the RBF.
Heat the reaction with a 70-75 o C water bath. Maintain a gentle reflux for 30 minutes.
The initial dark color fades and is replaced by the greyish solution typical of Grignard reagents.
7. Cool the reaction flask in a beaker of tap water until it reaches room temperature. Put on gloves before starting the dye synthesis.
Malachite Green synthesis (½ class)
8. Weigh 0.21g of methyl benzoate into a small vial (set in a beaker so it will not tip
9. over) and add 1.0mL of THF to the vial.
Using a Pasteur pipette, add the ester solution drop by drop, swirling after each drop.
10. After the addition is complete, replace the condenser and heat the reaction mixture to reflux for 5 minutes.
11. Cool the flask to room temperature. Pour the reaction mixture into a 150mL beaker.
12. Slowly add 5mL of 5% HCl solution to the beaker with stirring; some bubbling will occur as the residual magnesium reacts with the acid.
Crystal Violet synthesis (other ½ class)
8. Weigh 0.30g of diethyl carbonate into a small vial (set in a beaker so that it willnot tip over) and add 1.0mL of THF to the vial.
9. Using a Pasteur pipette, add the ester solution drop by drop, swirling after each drop.
10. After the addition is complete, replace the condenser and heat the reaction mixture to reflux for 5 minutes.
11. Cool the flask to room temperature. Pour the reaction mixture into a 150mL beaker.
12. Slowly add 5mL of 5% HCl solution to the beaker with stirring; some bubbling will occur as the residual magnesium reacts with the acid.
Measurement of UV-Vis spectra (whole class)
13. Collect a UV/Vis spectrum on a small diluted portion of your solution and record the λ of the maximum absorbance.
Dyeing Test fabric strips (whole class)
14. You will each receive two different strips of Multifiber fabric. Each strip contains 13 different types of fabric. One of the strips is untreated, the other has been pretreated with a mordant (note the slightly different tinge of color).
15. Remove a small aliquot of either Crystal Violet or Malachite Green and add it to 20mL of boiling water. Dye the mordanted cloth in this bath for 5-10 min at a temperature just below the boiling point. Cover with a watch glass to minimize evaporation.
6

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
16. Dye another piece of cloth that has not been mordanted and compare the two. In each case, allow as much of the dye to drain back into the beaker as possible and then, using glass rods, wash the dyed cloth under running water with soap, blot and dry.
17. Cut each strip in half and give one to your partner. Leave dyed fabric strips in your drawer for comparison with other dyed cloths in subsequent weeks.
SAFETY INFO:
4-BromoN,N -dimethylaniline, diethyl carbonate and methyl benzoate are irritants. Avoid contact with skin, eyes and clothing. The dyes made in this experiment will stain skin, clothing and anything else they contact. Wearing old clothes to lab is highly recommended.
Wear gloves while dyeing the fabric samples and cleaning the glassware that contained the dye.
CLEANUP:
The dye solution should be poured into an aqueous inorganic waste container in the hood. Dye stains on glassware can be removed with a few milliliters of 6M HCl then washing with water.
Neutralize the acid washings with sodium carbonate before pouring them down the sink or pouring them into the container for aqueous inorganic waste. Remove dye stains on the beaker, if necessary.
PRE-LAB QUESTIONS:
1. The TA will instruct you to keep the bottle of tetrahydrofuran (THF) closed at all times except when you are dispensing reagents. Why is this good advice?
2. Why are the magnesium turnings scraped on a piece of weighing paper and not just in the round bottom flask?
3. Why is it important to not overheat your THF in the water bath?
4. Why is the ester added dropwise to the Grignard reagent?
Part B (work in pairs): Synthesis of Azo Dyes Orange II or Methyl Orange
2
Probably the most common type of dyes are the azo dyes. There are literally thousands of them. They are typically formed in a two-step reaction - by first oxidizing an aromatic amine to a diazonium salt, and then coupling the salt to an aromatic phenol (or a different methylated amine) via an electrophilic aromatic substitution reaction. The mechanisms of these reactions will be covered in greater detail during recitation, but additional information on the mechanisms can be found in Wade chapters 17 & 19. For the synthesis of Orange II and Methyl Orange, everyone will form the same diazonium salt of sulfanilic acid in the first step according to the reaction scheme in Figure 5. Sulfanilic acid exists as a zwitterion in aqueous solution.
Before embarking on the second step, consult with your partner to determine which compound each of you will synthesize. Like last week, products will be exchanged at the end.
The procedures for the electrophilic aromatic addition are similar on paper, but a bit different in
7

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift how they are executed experimentally. To make Orange II, the diazonium salt of sulfanilic acid is coupled with 2-naphthol in alkaline solution in the second step. To make Methyl Orange, the salt is coupled with N,N -dimethylaniline in a weakly acidic solution. These dyes will then be used to dye regular Multifiber fabric strips (the mordanted cloth gives pretty similar results).
Step 1:
N
NH NH
3
NH
2
Cl
2 HCl N
Na
2
CO
3
2
+ CO
2
+ H
2
O
NaNO
2
SO
3
H
Step 2:
SO
3
SO
3
Na
SO
3
OH O
3
S N N
NaOH
ORANGE II
OH
N
N SO
3
Na
METHYL ORANGE
N
CH
3
CH
3
O
3
S acetic acid
N N
NaO
3
S N
N
(Yellow Form pH >= 4.4)
N
CH
3
CH
3
OH H
NaO
3
S NH
CH
3
N N
(Red Form pH <= 3.2)
CH
3
Figure 5.
Synthesis of Orange II and Methyl Orange via azo coupling reactions.
Procedure
Formation of the diazonium salt (whole class)
1. In a 50-mL Erlenmeyer flask dissolve (with boiling if necessary) 1.2g of sulfanilic acid in 12.5 mL of 2.5% sodium carbonate solution.
2. Cool the solution under tap water, add 0.47g of sodium nitrite, and stir until it is dissolved.
8

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
3. Pour the solution into a flask containing about 7.5g of ice and 1.3mL of concentrated hydrochloric acid. In a minute or two a powdery white precipitate of the diazonium salt should separate and the material is then ready for use. The product is not collected but is used in the preparation of Orange II or Methyl Orange dye while in suspension. It is more stable than most diazonium salts and will keep for a few hours.
Orange II synthesis (½ class)
4. In a 250 mL beaker, dissolve 0.9g of 2-naphthol in 5mL of cold 3M sodium hydroxide solution.
5. Pour (with stirring) the suspension of diazotized sulfanilic acid into the 2-naphthol solution. Rinse the Erlenmeyer flask with a small amount of water and add it to the beaker. Coupling occurs very rapidly and the dye, being a sodium salt, separates easily from the solution because a considerable excess of sodium ion from the
6. carbonate, the nitrile, and the alkali is present.
Stir the crystalline paste thoroughly to effect good mixing and, after 5-10min, heat the mixture until the solid dissolves.
7. Add 2.5g of sodium chloride to further decrease the solubility of the product, bring this all into solution by heating and stirring, set the beaker in a pan of ice water, and let the solution cool undisturbed. When near room temperature, cool further by stirring and collect the product on a Buchner funnel.
8. Use saturated sodium chloride solution rather than water for rinsing the material out of the beaker and for washing the filter cake free of the dark-colored mother liquor.
The filtration can be somewhat slow. The product dries slowly and contains about
9.
20% of sodium chloride. The crude yield is thus not significant, and the material need not be dried before being purified.
This particular azo dye is too soluble to be crystallized from water; it can be obtained in a fairly satisfactory form by adding saturated sodium chloride solution to a hot, filtered solution in water and cooling, but the best crystals are obtained from aqueous ethanol.
10. Transfer the filter cake to a beaker, wash the material from the filter paper and funnel with water, and bring the cake into solution at the boiling point. Avoid a large excess of water, but use enough (about 12.5mL) to prevent separation of solid during filtration.
11. Filter by suction through a Buchner funnel that has been preheated on the steam bath.
Pour the filtrate into an Erlenmeyer flask, rinse the filter flask with a small quantity of water, add it to the flask, estimate the volume, and if greater than 15mL evaporate by boiling.
12. Cool to 80 o C, add 25-30mL of ethanol, and allow crystallization to proceed. Cool the solution well before collecting the product. Rinse the beaker with mother liquor and wash finally with a little ethanol. The yield of pure, crystalline material is about 1.7g.
Orange II crystallizes from aqueous alcohol as a dihydrate complex (i.e.
C
16
H
11
N
2
O
4
SNa . 2H
2
O) and allowance for this should be made in calculating the yield. If the water of hydration is eliminated by drying at 120 o C the material becomes fiery red.
9

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
Methyl Orange synthesis (other ½ class)
4. In a test tube, thoroughly mix 0.8mL of N,N -dimethylaniline and 0.63mL of glacial acetic acid.
5. To the suspension of diazotized sulfanilic acid contained in a 250mL beaker, add
(with stirring) the solution of dimethylaniline acetate. Rinse the test tube with a small
6. quantity of water and add it to the beaker.
Stir and mix thoroughly and within a few minutes the red, acid-stable form of the dye
7. should separate. A stiff paste should result in 5-10min and 9mL of 3 M sodium hydroxide solution is then added to produce the orange sodium salt.
Stir well and heat the mixture to the boiling point, when a large part of the dye should dissolve.
8. Place the beaker in a pan of ice and water and allow the solution to cool undisturbed.
When cooled thoroughly, collect the product on a Buchner funnel, using saturated sodium chloride solution rather than water to rinse the flask and to wash the dark mother liquor from the filter cake.
9. The crude product need not be dried but can be crystallized from water after making preliminary solubility tests to determine the proper conditions. The yield is about
1.25-1.5g.
Measurement of UV-Vis spectra (whole class)
13. Dissolve a small amount of dye in aqueous solution and record the λ max. Those working with Methyl Orange should measure the λ max in different solutions with pH
> 5 and < 3. (note: not every person’s sample must be measured if time runs short, but everyone in the section should have the λ max recorded for each sample type.)
Dyeing Test fabric strips (whole class)
14. For the direct dyes, Methyl Orange and Orange II, the dye bath is prepared from
50mg of Orange II or Methyl Orange, 0.5mL of 3M sodium sulfate solution, 15mL of water, and 5 drops of 3M sulfuric acid in a 30mL beaker. Place a piece of test fabric
(without mordant) in the bath for 5 min at a temperature near the boiling point.
Remove the fabric from the dye bath, allow it to cool, and then wash it thoroughly with soap under running water before drying it.
SAFETY INFO:
Avoid skin contact with diazonium salts. Some diazonium salts are explosive when dry, so always use in solution. Acids and bases should always be dispensed carefully in the hood.
CLEANUP:
The organic crystallization filtrate should be placed in the organic solvents container. The filtrate from the reaction should be poured into an aqueous inorganic waste container in the hood.
Dye stains on glassware can be removed with a few milliliters of 6M HCl then washing with water. Neutralize the acid washings with sodium carbonate before pouring them down the sink or
10

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift pouring them into the container for aqueous inorganic waste. Remove dye stains on the beaker, if necessary.
PRE-LAB QUESTIONS:
1. Why must the diazonium salt be added to a cold solution of 2-naphthol (for Orange II) when the same diazonium salt can be added to a room temperature solution of N,N -dimethylaniline (for
Methyl Orange)?
2. Why is the filter cake rinsed with saturated sodium chloride solution instead of water?
3. Why might it be better to measure the UV/Vis spectrum of a dilute sample rather than a concentrated one?
Part C (work in pairs): Synthesis of Indigo 3
The original source of indigo was the leaves of the tropical plants of the Indigofera species. When cut leaves are fermented in water, a colorless compound, indican, was extracted and subsequently cleaved to form glucose and another colorless compound, indoxyl. The fermented brew containing indoxyl was typically then transferred to a large open vat, the cloth added, and the mixture beaten with large sticks to aerate the solution. During this process the water-soluble indoxyl permeates the fibers and is air oxidized to form the intense blue, water insoluble dimer, indigo, that precipitates inside the fibers of the cloth.
OH
HO
HO
O H
OH
O ferment
OH O air O
2
O
H
N
N
H
N
H
N
H
N
H
Indigo
O
Indican Indoxyl
Figure 6.
Reactions associated with the isolation of indigo from plant sources.
Indigo has been produced commercially by chemical synthesis since 1897. In this experiment, you will prepare indigo by an almost forgotten method developed by J. F. W. Adolf von Baeyer (of asprin fame). This is not the method used commercially, but it is a good method for quickly generating the small quantities needed here. His 1882 synthesis of indigo starts with o-nitrobenzaldehyde, which is heated in acetone and base. The acetone enolate anion adds to the aldehyde to produce an intermediate hydroxy ketone that that been isolated and identified. The next steps are unknown (but in the lab report you will be asked to propose a viable mechanism).
Synthetic indigo is very insoluble in water. It must first be chemically reduced to a basesoluble yellow dihydro derivative (a.k.a. “indigo white” or the leuco form). This is typically
11

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift done with a chemical reducing agent such as sodium hydrosulfite. The fabric can then be dipped in the alkaline solution of the reduced dye so that the dye impregnates the cloth. Upon removal of the solution and exposure to air, the dye will be oxidized back to the original blue indigo.
CHO
OH O
O O
NO
2 base
H
3
C CH
3
H
2
C CH
3 base
NO
2
Indigo
Figure 7.
Laboratory synthesis of Indigo.
O
ONa ONa
H
N Na
2
S
2
O
4
+ NaOH
N
H
O
2
(air) N
H
N
H
O
Colored blue form
(Water insoluble)
Colorless leuco form
(Water soluble)
Figure 8.
Oxidation/reduction of blue and white Indigo.
Procedure
Indigo Synthesis and Dyeing (half class)
1. Place 6 mL acetone and 0.75 g (0.0017 mole) of o-nitrobenzaldehyde in a 25 mL round bottom flask.
2. Attach a reflux condenser and heat the flask in a water bath until the solution refluxes gently.
3. Add through the condenser over a period of 5 min 0.5 mL portions of a solution of 0.25 g
(3 pellets) sodium hydroxide in 5 mL of water (About 12 M NaOH).
4. Continue to reflux for another 20 min.
5. Remove from heat and allow the flask to cool.
6. Add 10 mL of water and swirl the contents to mix them.
7. Chill the flask in an ice bath.
8. Collect the solid on a Buchner funnel and wash the dark blue precipitate with a little water and several small portions of acetone.
9. Transfer the filter paper to a paper towel and allow it to air dry while you prepare for the next step.
(The following dyeing experiments can be run on a larger scale if people want to pool their samples together. Just scale the reagents accordingly.)
12

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
10. Place 50 mL of water and 2.5 mL of 10% sodium hydroxide, and 1 g of fresh sodium dithionate (Na
2
S
2
O
4
) in a 250 mL beaker.
11. Add all but a small sample of the blue indigo prepared, and stir the mixture until a yellow to green solution of the “indigo white” forms. (To disperse the dye in water, a bit of soap should be added to the dye bath. Although the liquid appears blue the dye is not dissolved, merely suspended.)
12. Heating the solution gently for a few minutes may help to improve the reduction. Note:
From the top, the liquid will always appear to be dark blue due to a film of the oxidized dye on the surface of the solution. Because Na
2
S
2
O
4
decomposes on storage, it is not easy to state exactly how much of this reducing agent should be used. If the mixture remains definitively blue, it may be necessary to add more reducing agent (in small portions) until the solution changes color.
13. Place the multifiber strips in the solution and heat gently for 5 minutes.
14. Remove the strips with a pair of tweezers, rinse them with water and allow them to dry.
The blue color should be fully formed in about 5 minutes. To increase the intensity of the dye, the process may be repeated several times with no drying. Note what happens during the drying process.
CLEANUP:
Add household bleach (5.25% hypochlorite solution) to the dye bath to oxidize it to the starting material. The mixture can be diluted with water and flushed down the drain. Check to make sure the pH is neutral, before diluting the dye bath with a large quantity of water and flushing it down the drain. Add household bleach (5.25% hypochlorite solution) to the dye bath to oxidize it to the starting material. The mixture can be diluted with water and flushed down the drain. Check to make sure the pH is neutral, before diluting the dye bath with a large quantity of water and flushing it down the drain.
PRE-LAB QUESTIONS:
1. Why is it important to not overheat your acetone in the water bath?
Lab Report
Short writing assignments (should be typed)
1. Though the syntheses you performed these past three weeks have already been reported in the chemical literature previously, lets pretend they haven’t. Write up succinct experimental procedures for the syntheses you performed these past two weeks, (i.e. I’m looking for 3 syntheses each, not 5) as though it were an experimental section of a journal article you plan to submit. (400 words max)
2. What can you conclude about the associations between dye and fabric on the basis of molecular structure (i.e. what is it about the chemical structures of the dye and the fabric that make dyeing effective)? Briefly compare and contrast the effectiveness of dyeing for each dyefabric combination. Note: You may want to create and refer to a data table to discuss this topic succinctly. (600 words max)
13

CHEM 118 Organic Lab II (Spring 2005)
Prof. Swift
3. If there were extra time (a few hours or possible a third week) allotted to this dye module, what experimental question would you be most interested in asking? Briefly describe an experiment(s) that could be performed to address the question you asked. (250 words max)
Questions (handwritten or typed)
1. Draw out the mechanism for the synthesis of Crystal Violet using curved arrow notation.
2. Interestingly, Methyl Orange can be used as an acid/base indicator - while its acid-stable form
(pH < 3.2) is red, its alkali stable form (pH >4.4) is yellow. The color change (red – yellow) is due to transition from one chromophore (azo group) to another (quinoid system). Using arrow notation, draw a mechanism showing how the yellow form is converted to the red form in the presence of acid.
NaO
3
S N N N
CH
3
H+
NaO
3
S
H
N N N
CH
3
CH
3
OH-
CH
3
Yellow at at pH > 4.4
Red at pH < 3.2
3. What are the Beilstein registry numbers and melting points of Crystal Violet and Indigo?
(note: sometimes an organic compounds will have multiple melting points listed in Beilstein. If this is the case, you should list them all, and indicate the one that was determined most recently based on the references.)
4. Propose a chemically plausible mechanism (curved arrow notation) for the synthesis of indigo shown in Figure 7. i.e. what are logical steps that could convert the hydroxyketone to indigo in the presence of base?
1 J. R. Mohrig, C. N. Hammond, T. C. Morrill and D. C. Neckers, Experimental Organic Chemistry A Balanced
Approach: Macroscale and Microscale (W.H. Freeman, NY) 1998.
2
3
L. F. Fieser, Williamson, K.L., Organic Experiments 8 th
C.F. Wilcox, Jr. and M. F. Wilcox, Experimental Organic Chemistry: A Small-Scale Approach, 2
Hall, Englewood Cliffs, NJ) 1988.
Ed. (Houghton Mifflin, NY) 1998. nd Ed. (Prentice
14