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Fischer Esterification: Synthesis of Isopentyl Acetate Introduction Dyes are important in industry and have been used for centuries to improve the appearance of fabrics. Dyes made from plant extracts were used in the past, but synthetic dyes, a relatively new discovery, have been used recently. The discovery of synthetic dyes came about when an 18 year old research assistant by the name of William Perkins attempted to oxidize N-allytoluidine to synthesize quinine, a natural white crystalline substance. Instead of producing a white crystalline substance, he produced a red solid. He repeated the experiment with aniline isolated from tar and contained a mix of aniline with o-toluidine and p-toluidine and derived black crystals that turned purple when dissolved in ethanol. This purple is known as aniline purple and was the first recorded preparation of a synthetic dye. Following this discovery, Peter Griess found that aniline could be converted into a diazonium ion which could be coupled with another aromatic compound to form azo dyes of all kinds of colors. A diazonium ion is a weak electrophile that reacts with electron rich aromatic compounds to induce electrophilic aromatic substitution reactions where ortho and para substitution products dominate due to electrophilic aromatic substitution. A Diazonium Ion

Azo dyes contain an azo group (N=N) and an aromatic compound attached to each N in the azo group. One of the aromatic compounds will be attached to a “G” which can be an OH, NR2, or other electron donating group, while the other aromatic compound will be attached to “Z” which can be a wide range

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of different atoms or functional groups. Azo dyes with this N=N group are important dyes used for food, cloth, pigments in paint, and printing ink. An Azo Dye

The process in which a diazonium ion reacts with a coupling, aromatic component to form an azo dye is known as diazotization. Diazotization

To form an azo dye from sodium nitrate, there are three main steps. First, an electrophilic nitrosonium ion is made by protonation of a nitrous acid followed by a loss of water. In the second step, the nitrosonium ion reacts with an aromatic amine, aniline in this case, which forms a Nnitrosoanillinium ion. After a proton transfer and tautomerization occur, a diazohydroxide intermediate is formed. Once the intermediate is protonated and loses water, the diazonium ion is formed. The third step involves a coupling reaction to make the azo dye via electrophilic aromatic substitution. To ensure the success of diazotization, the reaction must be carried out at 0°C, minimizing the reaction with water and producing a phenol. In addition, the reaction needs to proceed as quickly as possible with continuous stirring and cooling. The pH of the reaction is also important.

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In the experiment, groups were assigned an aromatic amine to diazotize and an electron rich coupling reagent to prepare to form an azo dye. The goal of the experiment was to find the azo dye absorbance in relation to wavelength using a U-V spectrum, to calculate the molarity of the dye, to observe color change in relation to pH, and to dye cloth strips directl in an acidic and basic solution. Shown below are the possible diazo components and coupling agents that could have been assigned. Diazo Components

138.124g/mol-1

137.14g/mol-1

172.20g/mol-1

Coupling Agents

110.111g/mol-1

110.111g/mol-1 144.170g/mol-1

144.170g/mol-1

Overall Reaction of 4-Nitroaniline and 1-Naphthol

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NH2

N

N +

Na

O

–

+

N

+

Na HO

+

N O

–

HO

N

0°C H2O

H2O 0°C HO

–

O

HCl

N

Cl

O N

N

+

+

–

O O

HO

+ OH

4-nitroaniline

diazonium salt and 1-naphthol coupling

4-(4-Nitrophenylazo)-1-naphthol

Mechanism for Synthesis of 4-Nitroaniline and 1-Naphthol

Procedure Table 1: Diazo Component (Aryl Amines) and Coupling Agents (Phenols) 4

Diazo Component

MW (g mol-1)

Coupling Agent

MW (g mol-1)

p-Nitroaniline

138.124

2-Naphthol

144.170

Sulfanilamide

172.20

1-Naphthol

144.170

4-Aminobenzoic acid

137.14

Resorcinol 110.111 (1,3-dihydroxybenzene) Catechol 110.111 (1,2-dihydroxybenzene)

Table 2: Mass of 10 mmol of each Diazo Component and Coupling Agent Mass of 10 Diazo Component

Mass of 10 Coupling Agent

mmol (g)

mmol (g)

p-Nitroaniline

1.381g

2-Naphthol

1.442g

Sulfanilamide

1.722g

1-Naphthol

1.442g

4-aminobenzoic acid

1.371g

Resorcinol 1.101g (1,3-dihydroxybenzene) Catechol 1.101g (1,2-dihydroxybenzene)

A. Preparation of Coupling Agent(1-Naphthol) Ten mmol of 1-naphthol (1.442g) was dissolved in 20 mL of 1M NaOH in a 125mL Erlenmeyer flask. The solution was then cooled to 0°C in an ice/salt bath. B. Diazotization of an Aromatic Amine (4-Nitroaniline) Under the hood, 10 mmol of 4-nitroaniline was dissolved in about 11mL of 3M HCl in a 50mL beaker. The beaker was placed on a stir plate while on a ring stand. A magnetic stir bar was aadded to the beaker and the solution was gently heated and stirred until 4-nitroaniline was mostly dissolved. After adding 7.2mL of deionized water was added to induce the dissolving of the aromatic amine, the solution was stirred and heated for a maximum of 20 minutes. After it dissolved, the solution was 5

cooled to room temperature and then further cooled to 0°C in an ice/salt bath. Five mL of 1.0M NaNO 2 was added dropwise to the beaker while stirring. The solution was tested with starch iodide paper and since the paper did not immediately turn blue-violet, 9.2mL of 1.0M NaNO2 was added until the starch iodide paper immediately turned blue-violet. Immediately after, the diazonium salt solution was added to the coupling agent made in part A. The solution was placed in an ice bath for 15 minutes while stirring until crystallization was complete. The solution was allowed to settle, and the azo dye was collected via vacuum filtration. After removing the stir bar, the crystals were allowed to dry thoroughly. The crystals were placed in a pre-weighed beaker (116.020g) and dried in a vacuum oven at 80°C for a week. After the beaker was re-weighed to get the mass of the dye(5.404g). The product was placed between two plastic weight boats and was ground to a find powder using a pestle. C. Recording the UV-Visible Spectrum of the Dye About 11.5 mg of dye was dissolved in 100mL of absolute ethanol using a 100mL volumetric flask. A small stir bar was added and was stirred for about 10 minutes. After 10 minutes, the solution was warmed slightly to dissolve the remaining undissolved product. The solution was cooled to room temperature. Next,5mL of the dye solution was added to a 25mL volumetric flask and about 20mL of deionized water was added to the flask. The molarity of the dye solution was calculated. Next a cuvette was filled with some of the aqueous dye solution. The spectrum was measured from 800 to 350nm in 25 nm intervals. To zero the spectrometer, 5 drops of absolute ethanol in 2mL of deionized water was used as the background solution.The final concentration in mg/mL and moles/L were calculated and recorded. D. pH Indicator Ability The aqueous solution from part C was used for this part of the lab. The color and the pH of the aqueous solution was recorded. Five mL of the dye solution used in part C was added to 20mL of 0.1M HCl in a small beaker. The solution was stirred with a glass stir rod and the color and the pH was recorded after mixing. Next, 5mL of the dye solution used in part C was added to 20mL of 0.1M 6

NaOH in a small beaker. The solution was mixed with a glass stir rod and the color and pH was recorded. E. Direct Dyeing with the Azo Dye Acidic Solution for Direct Dyeing In a 1L beaker, 0.15g of dye was suspended in 100mL of 0.1M HCl with a stir bar. The solution was stirred for 5 minutes while heating. After allowing the particles to settle to the bottom of the beaker, the beaker was removed from the heat and the pH was checked with pHydrion paper. The approximate molarity of dye used was recorded. Basic Solution for Direct Dyeing In a 1L beaker, 0.15g of dye was suspended in 100mL of 0.1M NaOH with a stir bar. The solution was stirred for 5 minutes while heating. After allowing the particles to settle to the bottom of the beaker, the beaker was removed from the heat and the pH was checked with pHydrion paper. The approximate molarity of dye used was recorded. How to Dye Cloth Strips Four pieces of special cloth with strips of different fabrics were collected. The fabric types are found in Table 5. The corner of the wool fabric was clipped. Two pieces of cloth were submerged in the acidic dye solution for 10 minutes and the other 2 pieces of cloth were submerged in the basic dye solution for 10 minutes. The cloth was removed from the solutions with tongs and then were rinsed with cold water. After rinsing, the cloth was allowed to air dry in the hood. The color of the unwashed fabric was recorded. To test for fade resistance, one of the acid-dyed pieces of cloth was washed with detergent five times and one piece of base-dyed cloth was washed with detergent five times. The colors of the cloths were compared and recorded after drying. The molarity of the dye used was calculated and recorded. Results Table 3: Wavelength vs. Absorbance 7

Wavelength Absorbance (nm) 350

0.220

375

0.225

400

0.226

425

0.302

450

0.344

475

0.299

500

0.214

525

0.146

550

0.095

575

0.064

600

0.042

625

0.028

650

0.021

675

0.015

700

0.008

725

0.008

750

0.009

775

0.009

800

0.005

Table 4: Color Change Versus pH Results Water

Water

0.01M HCl

0.01M HCl

0.01M NaOH

0.01M NaOH

Color

pH

Color

pH

Color

pH

Light orange

~5

Light

~1

Dark

~12

8

orange/slightl

blue/purple

y red

Table 5: Color of Fabric Dyed in Acidic vs. Basic Solutions Fabric

Color of

Color of washed

Color of

Color of washed

unwashed acid

acid dyed cloth

unwashed base

base dyed cloth

dyed cloth Spun Diacetate

dyed cloth

Olive

Bright

green/yellow

yellow/orange

Off-white

Off-white

spots Bleached Cotton

Tan

Peach/light brown White-ish

Off-white

Spun Polyamide

Green/yellow

Light olive green

Off-white

Light light yellow

Pinkish/white

Light pink/white

White

White

Very light tan

Lighter pink/tan

White

White

Dark blue

Mid/denim blue

Very light

Very light

yellow/tan

yellow/tan

(Nylon 6,6) Spun Polyester

spots

(Dacron 54) Spun Polyacrylic (Orlon 75) Worsted wool

Calculations Graph 1: UV-Visible Absorption Spectrum

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Absorbance Spectrum of Dye 0.4 0.35

Absorbance

0.3 0.25 0.2 0.15 0.1 0.05 0 350.00 400.00 450.00 500.00 550.00 600.00 650.00 700.00 750.00 800.00

Wavelength(nm)

The λmax for the dye was at 450nm. Therefore, the color absorbed is purple, while the color observed is yellow/green. Discussion To be a useful dye, the azo component has to be soluble in water. To be soluble in water the diazo component must have a polar or ionic substituent on the aromatic ring. Azo groups that are not soluble in water do not have an ionic group attached to the ring. In the introduction, you can see that all three diazo components have a polar or ionic substituent on the aromatic ring. However, the coupling components are insoluble in water 1. Discuss any observations about solubility of the azo dye, reference the dye structures in the introduction. Why would the dye not be fully soluble under the various conditions? 2. Explain the λmax value, graph 1. Is this what would be expected based on the observed color of your dye solution? (See the complementary color table on D2L) Would the λ max change in acidic or basic solutions, explain why or why not. 4. Compare and contrast the basic and acidic dye solutions. What colors were observed? Were the colors in acidic versus basic solutions the same or different? Why? Draw the structures of the dye under acidic and basic conditions. Indicate whether the dye produced could function as a pH indicator 5. Evaluate the effectiveness of the synthesized dye in the direct dyeing of the fabrics (information on how dyes bond to fabric on D2L). Give at least two reasons of why the various fabrics may have different dying capabilities with YOUR azo dye. 6. Give an overall conclusion/results for this experiment. Questions:

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1. What would be the major difference between the IR spectra of the diazo component vs the azo-dye final product? Give specific wavelengths and absorption peak shapes. -The diazo component assigned contains an NH 2 group and an NO2 group, while the azo-dye yielded in the reaction contains an NO 2 group and an OH- group, as shown in the introduction. Since both components contain a NO2 group there would not be much distinction when it comes to this peak. However, the NH2 in the diazo compound will have a doublet peak at around a wavenumber of 2900 to 3000 cm-1, while the OH- from the azo dye will show a singlet peak at around a wavenumber of 3250 cm-1. 2. In a proton NMR of the final product how many peak sets would be observed? Draw your final azo-dye product and label the peaks using the alphabet convention (a, b, c, etc.). 3. Draw the diazo component and the coupling agent needed to synthesize each of the following diazo dyes.

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