Synthesis of Methyl Orange and Its Application to Textiles

The practice of using dyes is perhaps the most ancient art of chemistry. Dyeing substances from plant, animal, or mineral sources has been known before written history. The accidental discovery of the purple dye, mauve, by W.H. Perkin in 1856 is generally considered to be the birth of the modern chemical industry. Several other synthetic dyes followed. One important group is known as the azo dyes, which are named after their unusual N=N, azo, functional group.

Methyl orange, 4-[4-(dimethylamino)phenylazo]benzenesulfonic acid, is an azo dye that forms orange crystals and is commonly used as an acid-base indicator, due to the fact that its anion form is yellow and its acid form is red:

O S N CH3 OH N N

CH3 Methyl orange In this experiment, you will synthesize methyl orange from sulfanilic acid and N,N˗dimethylaniline using a diazonium coupling reaction, a common reaction for treating an aliphatic amine to yield a carbocation. The reaction between a primary aliphatic amine and nitrous acid gives an unstable diazonium salt that loses N2 to give a carbocation. The carbocation may then either (1) lose a proton to give an alkene, (2) react with a nucleophile, or (3) rearrange, followed by (1) or (2). The nucleophile we are using here is dimethylaniline. Attack is in the para position due to steric hindrance at the ortho position by the bulky dimethylamine substituent. Because you are synthesizing an azo dye, your product purity will easily be determined using a Spectrophotometer.

O diazonium CH3 O + coupling O S N N + O S N CH3 - N CH - O 3 O + N N Na CH3 Diazontized N,N-dimethylaniline Methyl orange sulfanilic acid

OBJECTIVES In this experiment, you will  Synthesize methyl orange from the reaction of a primary amine with nitrous acid.  Calculate your product purity using a Spectrophotometer.  Observe the spectroscopic changes of methyl orange under acidic and basic conditions.  Explore how organic chemistry is an essential component of the textile industry.

Organic Chemistry with Vernier © Vernier Software & Technology 1 MATERIALS

Materials Part I: Synthesis of Methyl Orange sulfanilic acid 10% NaOH solution 2% sodium bicarbonate solution ice water bath sodium nitrite hot plate concentrated HCl glass stirring rod and spatula N,N-dimethylaniline Büchner funnel, filter, and filter paper glacial acetic acid pH paper 125 mL Erlenmeyer flask graduated cylinders, 10 mL and 100 mL two graduated Beral pipets watch glass

Materials Part II: Test the Colorimetric Absorbance of Methyl Orange synthesized methyl orange (from Part I) methyl orange solid LabQuest or computer 5% NaOH solution LabQuest App or Logger Pro 5% HCl solution SpectroVis Plus spectrophotometer

Materials Part III: Dying Textiles

synthesized methyl orange (from Part I) concentrated H2SO4 15% sodium sulfate solution small fabric strips of varying materials

PROCEDURE

Procedure Part I Synthesis of Methyl Orange 1. Obtain and wear goggles. Protect your arms and hands by wearing a long-sleeve lab coat and gloves. Conduct this reaction in a fume hood. 2. Dissolve 0.5 g of sulfanilic acid in 10 mL of 2% aqueous sodium bicarbonate solution in an Erlenmeyer flask. Record the mass of sulfanilic acid to the nearest 0.01 g in the table below. You can gently heat the flask for several minutes to speed up this process. Test one drop of the solution to make sure it is alkaline; if not, add a small amount (2–3 mL) of 2% sodium bicarbonate solution and check the pH again. 3. Dissolve 0.2 g of sodium nitrite in the solution of sulfanilic acid. Allow the flask to cool to room temperature. Record the mass of sodium nitrite to the nearest 0.01 g in the table below. 4. Cool the resulting mixture in an ice water bath, and add 0.5 mL of concentrated HCl one drop at a time with swirling. The solution should turn red-orange at this point. Allow the resulting solution to stand in the ice bath for 5–10 minutes. Keep this solution cold in the ice bath at all times. It now contains your diazonium salt, which will decompose if it becomes warm. It is only partially soluble in the aqueous solution and will precipitate as a blue-green solid. 5. Freshly prepare the N,N-dimethylaniline solution by using the Beral pipets to combine approximately 0.5 mL of N,N˗dimethylaniline and 0.5 mL glacial acetic acid. Then slowly add this solution to the diazonium salt suspension in the 125 mL Erlenmeyer flask. A dull, red-purple solid should appear.

2 Organic Chemistry with Vernier Synthesis of Methyl Orange and Its Application to Textiles

6. While the solution flask is still on ice, very slowly, two to three drops at a time, add 5 mL of 10% aqueous NaOH, with constant stirring with your glass stirring rod. This process should take 10–15 minutes. If the NaOH is added too quickly, then free dimethylaniline will separate out as an oily phase. This then leaves an equivalent amount of the diazonium salt unreacted. 7. Make sure the solution is basic using the pH paper, adding more NaOH if necessary. At the end of the coupling reaction a yellow-orange or golden color should be observed. 8. To recrystallize the product, heat the reaction mixture at a low setting on the hotplate for about 30 minutes. Everything should dissolve and the solution should be translucent (though colored). Gently stir the mixture periodically to prevent bumping. 9. Allow the mixture to cool slowly to room temperature to crystallize and then place the flask in an ice bath to get it as cold as possible. Important: Do not stir or shake the solution when it is cooling. Allow the crystals to form in an undisturbed flask. The crystals will have a higher purity if they form slowly in a motionless flask. 10. Filter the resulting gold-orange plates of methyl orange by suction. a. Set up a vacuum filtration using a Büchner funnel. Be sure to weigh and record the mass of the filter paper to the nearest 0.01 g before filtering the solid. b. Transfer the contents of the flask to a Büchner funnel assembly. Filter the mixture with vacuum suction. c. When most of the liquid has been drawn through the funnel, turn off the suction and wash the crystals with 5 mL of cold, distilled water. d. Gently transfer the filter paper with your product onto the watch glass to air dry. As directed by your instructor, either direct a gentle stream of air (low flow) to help dry the solid, or let them air dry until the next lab period. e. Weigh the dried recrystallized product on the filter paper and record the mass to 0.01 g 11. Weigh out and quantitatively dilute a small crystal of your synthesized methyl orange in water. Depending on purity, 5–10 mg of synthesized methyl orange in 500 mL of water should be sufficient. Record your mass and volume in the table below. You will need to know exactly how much of each you are measuring with the Spectrophotometer to determine the purity. Set this solution aside.

Procedure Part II Test the Colorimetric Absorbance of Methyl Orange You will analyze several samples to determine the amount of methyl orange in your synthesized sample. You can use this information to calculate its purity. Follow Steps 12–17 to prepare a set of methyl orange standard solutions and conduct testing to develop your own Beer’s law plot of the standards. Steps 18 and 19 will guide you through the testing of your synthesized methyl orange sample.

12. Quantitatively prepare a 4.0 × 10-5 mol/L stock methyl orange solution in water.

13. Prepare four dilutions of the methyl orange stock solution with the following concentrations: 3.0 × 10-5, 1.5 × 10-5, 8.0 × 10-6, 5.0 × 10-6 mol/L. Summarize the volumes and concentrations for the trials in the data table below.

14. Connect the SpectroVis Plus spectrophotometer to the USB port of LabQuest or a computer. Start the data-collection program, and then choose New from the File menu.

Organic Chemistry with Vernier 3 15. Calibrate the Spectrophotometer. a. Place the blank cuvette of water in the Spectrophotometer. To prepare a blank cuvette, rinse the cuvette twice with ~1 mL amounts and then fill it 3/4 full. Wipe the outside with a tissue and place it in the Spectrophotometer. b. Choose Calibrate from the Sensors menu of LabQuest or the Experiment menu of Logger Pro. c. When the warmup period is complete, select Finish Calibration. Select OK. 16. Determine the optimal wavelength for creating the standard curve and set up the mode. a. Empty the blank cuvette. Using the first methyl orange solution, rinse the cuvette twice with ~1 mL amounts and then fill it 3/4 full. Wipe the outside with a tissue and place it in the Spectrophotometer. b. Start data collection. A full spectrum graph of the solution will be displayed. Stop data collection. c. Change the mode to Events with Entry (absorbance vs. concentration) and select a wavelength for analysis:

 In LabQuest App, the displayed wavelength of maximum absorbance (max) is automatically identified on your graph with a point protector. Tap the Meter tab, then tap Mode. Change the mode to Events with Entry. Enter the Name (Concentration) and Units (mol/L). Tap OK.  In Logger Pro, click the Configure Spectrophotometer button, . Click Abs vs. Concentration as the Collection Mode. The wavelength of maximum absorbance (max) will be selected. Click OK. 17. You are now ready to collect data for the five standard solutions. a. Retain the cuvette in your Spectrophotometer. Start data collection. b. When the value displayed on the screen has stabilized, select Keep and enter the molar concentration. Select OK. The absorbance and concentration values have now been saved for the first solution. c. Discard the cuvette contents as directed by your instructor. Using the next methyl orange solution, rinse the cuvette twice with ~1 mL amounts, and then fill it 3/4 full. Wipe the outside, place it in the device, wait for the absorbance to stabilize and select Keep. Enter the molar concentration, and select OK. d. Repeat the procedure for the remaining methyl orange solutions that you prepared. e. Stop data collection to view a graph of absorbance vs. concentration. f. Record the absorbance and concentration values in your data table. You can determine these values by examining your data points along the curve, or by viewing the data table. g. Choose Curve Fit from the Analyze menu. h. Select Linear as the Fit Equation and select OK. 18. Retrieve the synthesized methyl orange sample dissolved in water you prepared for testing. 19. Measure and record the absorbance of the sample: a. Rinse and fill the cuvette 3/4 full with the sample. Cap the cuvette and place it in the Spectrophotometer.

4 Organic Chemistry with Vernier Synthesis of Methyl Orange and Its Application to Textiles

b. Read the absorbance value. If the absorbance value falls within the range of the methyl orange standard solutions, record it in your data table. If it does not, prepare a more dilute or more concentrated sample, depending on the absorbance value from your first test. 20. To determine the concentration of your methyl orange sample, interpolate along the regression line to convert the absorbance value of the unknown to concentration. c. Choose Interpolate from the Analyze menu. d. Click or tap any point along the regression curve (or use the ◄ or ► keys on LabQuest) to find the absorbance value that is closest to the absorbance reading you obtained in Step 19. The corresponding methyl orange concentration, in mol/L, will be displayed to the right of the LabQuest graph, or on your Logger Pro graph. e. Record the concentration of methyl orange in your data table.

21. Discard all solutions as directed. 22. Test the use of methyl orange as an acid-base indicator by dissolving a few crystals in several milliliters of water in a test tube. Add a few drops of 5% HCl solution to one part of the solution and a few drops of 5% NaOH solution to another part of the solution until the color changes are apparent in each case. 23. Obtain spectra for both the acidic and basic forms of methyl orange.

Procedure Part III Dying Textiles 24. To test the use of methyl orange as a fabric dye, take 0.1 g of methyl orange and place it into a 250 mL beaker. Add 50 mL of water to the beaker, 2 mL of 15% aqueous sodium sulfate, and 3 drops of concentrated H2SO4. 25. Add a stir bar and heat the mixture on a hot plate until it boils. 26. Remove from the heat and place the test fabric strip into the dye solution for 10 minutes. 27. Remove the sample with tongs and rinse well with water. 28. Repeat the procedure for as many different types of fabrics as provided.

DATA TABLE Part I Synthesis of Methyl Orange

Mass of sulfanilic acid used (g)

Mass of sodium nitrite used (g)

Mass of methyl orange and filter paper (g)

Mass of filter paper (g)

Mass of methyl orange synthesized (g)

Organic Chemistry with Vernier 5 Part II Standard Methyl Orange Absorbance

Trial Volume of 4.0 × 10-5 M Volume of water Concentration Absorbance number methyl orange stock (mL) (mol/L) (mL) 1

Part II Synthesized Methyl Orange Absorbance

Mass of synthesized Volume of water Concentration Absorbance methyl orange used used in dilution (mol/L) (g) (mL)

DATA ANALYSIS 1. Determine your product yield. 2. Determine your product purity based on your spectroscopy results. 3. How did the color of methyl orange change with pH? What is the structure of methyl orange at low pH? What is the structure of methyl orange at high pH? What property of methyl orange and related dyes gives them such intense color? 4. Comment on the results of the dying of the test fabric strip. Did all of the materials dye equally well? Explain in terms of the molecular structure of the test fabrics.

6 Organic Chemistry with Vernier Synthesis of Methyl Orange and Its Application to Textiles INSTRUCTOR INFORMATION 1. Do not attempt to take the melting point of your methyl orange as it decomposes on heating. 2. Be familiar with the MSDS information for each compound and follow safe handling and disposal practices. 3. Preparation of solutions:  2% sodium bicarbonate should be prepared 2% weight of sodium bicarbonate to volume water. This is also a 0.25 M NaHCO3 solution.  10% sodium hydroxide should be prepared 10% weight of sodium hydroxide to volume water. This is also a 2.5 M NaOH solution.  5% sodium hydroxide should be prepared 5% weight of sodium hydroxide to volume water. This is also a 1.3 M NaOH solution.  5% HCl should be prepared 5% volume HCl to volume of water. This is also a 0.6 M HCl solution. 5. Methyl orange has a low solubility in water and students will find it hard to dissolve. This is all right since it is also a strong chromophore; therefore, it is desirable for it to be very dilute. Depending on purity, 5–10 mg of synthesized methyl orange in 500 mL of water will give an absorbance of about 1.2. 6. We suggest cotton, wool, polyester and/or a polyester blend as testing fabric strips.

HAZARD ALERTS Acetone: Fire hazard (flash point 17.0°C). Store in dedicated flammables cabinet. Moderately toxic by inhalation or ingestion. Moderately toxic by ingestion. Vapor causes weakness, fatigue, nausea and headache. Skin and eye irritant. HMIS Classification: Health hazard2, Flammability3, Physical hazard0.

Sulfanilic Acid: Body tissue irritant. Avoid all body tissue contact. Combustible solid. HMIS Classification: Health hazard2, Flammability0, Physical hazard0.

Sodium Bicarbonate: Slightly toxic by ingestion. Dust may be irritating to respiratory system. Slowly decomposes in moist air. HMIS Classification: Health hazard1, Flammability0, Physical hazard0.

Hydrochloric acid: Highly toxic by ingestion or inhalation; severely corrosive to skin and eyes; wear gloves and eye protection when using this substance. HMIS Classification: Health hazard3, Flammability0, Physical hazard0.

N,N-dimethylaniline: Serious fire hazard. Irritating to body tissues. Severe eye irritant. Avoid all body tissue contact. Slightly toxic by ingestion. Vapor causes weakness, fatigue, nausea and headache. Skin contact causes dermatitis. HMIS Classification: Health hazard4, Flammability2, Physical hazard0.

Glacial Acetic Acid: Corrosive to skin and tissue; moderate fire risk (flash point 40°C); Moderately toxic by ingestion and inhalation. Causes eye burns. HMIS Classification: Health hazard3, Flammability2, Physical hazard0.

Organic Chemistry with Vernier 7 Sodium Nitrite: May intensify fire; oxidizer. Toxic if swallowed. Causes eye irritation. Fatal if inhaled. Very toxic to aquatic life. Toxic to aquatic life with long-lasting effects. HMIS Classification: Health hazard2, Flammability0, Physical hazard1.

Methyl Orange: Highly toxic by ingestion. Health hazard2, Flammability0, Physical hazard0.

The hazard information reference is: Sigma-Aldrich Co., 1-800-325-3010, www.sigmaaldrich.com/safety-center/msds-search.html

COMPOUND INFORMATION

Molar mass Density Boiling point Melting temperature Compound (g/moL) (g/mL) (°C) (C)

Sulfanilic acid 173.19 Solid 288

N,N-dimethylaniline 121.18 0.956 193–194 1.5–2.5

Sodium bicarbonate 84.01 Solid 270

Sodium nitrite 69.00 2.168 271

Acetic acid 60.05 1.049 116–117 15–16

Methyl orange 306 Solid Decomposes >300

8 Organic Chemistry with Vernier Synthesis of Methyl Orange and Its Application to Textiles SAMPLE RESULTS

Beer’s law analysis of pure methyl orange

Absorbance spectrum of lab-synthesized methyl orange

Organic Chemistry with Vernier 9