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InExperiment 4 – Colorful Chemistry DYES AND PIGMENTS are colorful compounds used to change the appearance of objects. Nature produces them to make flowers attractive to insects and to people, to tell predators to back off, and to catch the sunlight for energy. Humans have learned to use such naturally colored substances from a very early time, as cave paintings and ceramic artifacts testify. It was not until the past century or so that we have discovered how to make our own dye molecules. The creation of new colors and their applications in the textile and printing industries was at least partially responsible for bringing synthetic organic chemistry to the foreground of scientific research! THE DIFFERENCE BETWEEN DYES AND PIGMENTS is that dyes are water-soluble and pigments are not. Dyes can be classified according to their structures and also based on their mode of application to fibers. According to structural differences, the most common dyes can be classified as azo, cationic, anthraquinone, and indigo (Figure 1). Depending on their mode of application, dyes can be grouped into the following types: direct, mordant, ingrain, vat, disperse, reactive, and solvent. Figure 1. Structural families of dyes LET’S BRIEFLY CONSIDER THE NATURE OF COLOR. As you have learned in your general chemistry classes, objects appear colored because they absorb certain wavelengths of the visible spectrum (400 – 750 nm) and reflect the complementary colors. Thus, a compound that absorbs blue light will appear orange and one that absorbs red will appear green. But what is it about the structures of these compounds that make them absorb certain wavelengths? A short explanation is the extent and nature of the conjugation present in the compound. A conjugated compound has a network of linked p-orbitals (forming pi-bonds), appearing structurally as alternating double and single bonds. This is apparent in all of the examples of dyes in Figure 1 above. In general, the more extended a conjugated system (the larger the number of pi electrons involved), the longer the wavelength absorbed (towards red), and the shorter the wavelength emitted (towards violet). There are many other factors involved, as you will observe, including contribution of ortho/para-activators and meta-deactivators. Azo Dyes encompass the largest family of dyes. They contain an azo group, -N=N-, linking two aromatic rings. Because of their extended conjugated pi-orbital systems, these aromatic compounds absorb in the visible region of the electromagnetic spectrum and are deeply colored, often vibrant orange. This implies that azo dyes absorb relatively short wavelengths of light. TO BE USEFUL AS DYES, AZO COMPOUNDS MUST BE SOLUBLE IN WATER. This can be achieved by having polar and ionic groups attached to the aromatic rings. Sodium salts of sulfonic (-SO 3 Na) and carboxylic (-CO 2 Na) acids work well for this purpose. Easter purple and American flag red are examples of ionic azo dyes (Figure 2). Recalls that nitro (NO 2 ) group contain a positively charged nitrogen and negatively charged oxygen. Simple changes in substituents and substitution patterns can make significant enough changes to be noticed by the naked eye, as evidenced by the comparison of Solochrome Orange M to AFR.
Figure 2. Examples of water-soluble azo dyes THE SYNTHESIS OF AZO DYES is accomplished in two steps (Figure 3). In the first step, an aromatic amine (aniline) is transformed into a diazonium salt by the reaction of nitrous acid (HNO 2 ) obtained in situ by mixing sodium nitrite and a mineral acid. Diazotization reactions are usually performed at low temperatures to avoid the decomposition of the diazonium salts. These compounds are unstable at higher temperatures due to their tendency to expel nitrogen gas. Some diazonium salts are explosive when dry and must be kept in solution. In the second step, the diazonium salt is coupled to an aromatic compound, usually an aniline or phenol derivative, to yield an aromatic azo compound. Figure 3. Synthesis of azo dyes. THE COUPLING REACTION BETWEEN DIAZONIUM SALTS and aromatic compounds takes place by an electrophilic aromatic substitution (EArS) mechanism (Figure 4). The diazo-nitrogen farthest from the ring is attacked by the aromatic ring. The coupling typically takes place in the para-position relative to the electron-donating group. When used as a direct dye, a solution of the dye is made in water and the fabric is soaked in the solution. Ingrain dyeing is also useful for applying these water-insoluble dyes. The coupling reaction between the diazonium salt and the activated aromatic ring takes place directly on the fabric. For instance, if cotton is immersed in a solution with 2-naphthol at pH 10, removed from the liquid, and then treated with a solution of the diazonium salt of 4-nitroaniline, American flag red is obtained. Figure 4. Mechanism for Diazo Coupling NH SO 3 - NH 4 + O 2 N N N Easter Purple N N OH NO 2 American Flag Red N HO^ N NO 2 Solochrome Orange M
indigo are insoluble in water but dissolve upon reduction with sodium dithionite (Na 2 S 2 O 4 ) under basic conditions. The reduced dye, called the leuco form, is soluble in water and is applied onto the fiber by immersion. Upon drying and exposure to atmospheric oxygen, the dye is re-oxidized and acquires its original color (Figure 7b). Notice that the carbonyl carbon of blue indigo is reduced (how many C-O bonds are in the reactant vs. product?). Dithionite is oxidized to sulfite in the process. While in the leuco-indigo solution, the fabric is yellow, but it quickly turns blue after it is removed. Figure 7. (a) Synthesis of Indigo and (b) reduction to leuco-indigo Dyeing TO UNDERSTAND THE PROCESS OF DYEING we must consider the chemical nature of fibers and fabrics. Different fibers subjected to the same dyeing process produce different color shades because each type of fiber reacts with the dye molecules in a unique way. Fibers with an abundance of polar groups, such as cotton and wool, are easier to dye than others with only a few polar residues, such as silk, polyesters, acetates, and acrylics. In general, synthetic fibers are less absorbent than natural ones and require special methods for color application. For example, polyesters are generally dyed using high pressure and temperature. Figure 8. Structures of the repeating units of fibers
EXPERIMENT OVERVIEW – General notes for clarification, please read carefully! In this experiment, students will synthesize dyes and use them to dye fabric swatches containing multiple types of fabric (Figure 8). STUDENTS ARE WELCOME TO BRING SMALL PIECES OF FABRIC FROM HOME TO DYE IN ADDITION TO THE SWATCH. SHOELACES ARE AN EXAMPLE OF AN APPROPRIATE-SIZED MATERIAL. The overall objective is to observe the different colors or qualities of fabrics that can be obtained with one dye, as well as comparison of different dyes and dyeing methods. Students will not perform every part of the experiment themselves, so it is important that the class pools their results together to make full comparisons. Thread a paperclip through the acetate (smoother) end of the fabric swatch before dyeing. The order is: acetate, cotton, nylon, polyester, acrylic, and wool. Each fabric strip should be labeled with your name, the dye, mordant, or other conditions if applicable on a securely fastened tag. Attach this tag to the paperclip immediately after dyeing and rinsing. Rough notebook guidelines are given for each part. Start a new notebook page for each reaction. Please write neatly on these pages. Include the products and % yield where appropriate. All reagent tables should include the following: mmol, mg or mL, MW, bp/mp, and density. Safety notes should follow the table and the comic strip / diagram procedure is directly after that. Refer to the Safety & Clean-up table that follows the procedure and write pertinent notes for each reaction. There are several parts of the experiment that are split up between students. Your notebook should include only the steps that you are assigned. Use the summary table below to ensure your preparation is complete. The experiment is separated by “Team Cat” and “Team Dog.” Look for the adorable icons to see which parts of the experiment to prepare for each week. Table 1. Team Assignment and Notebook Overview Team Cat Pair closer to the chalkboard Team Dog Pair farther from the chalkboard Day 1 *Part A *American Flag Red *B.1 - ingrain dyeing
- Part C – mordant dyeing w untreated or CuSO 4 treated swatch *Part A *Easter Purple *B.2 - ingrain dyeing
- Part C – mordant dyeing with AlKSO 4 or FeSO 4 treated swatch Print Table 3 (page 12) and bring with you to lab both days. Day 2 *Part D - indigo *Part E (Malachite green) *Part D - indigo *Part E (Eosin Y) Before leaving the lab both days, record your observations (Table 3, p. 12) for your dyes as well as for the other teams’ dyes. Make your own observations with descriptions and depth of color! Do not copy someone else’s descriptions. Down time during reactions or dyeing? If you are prepared for the next steps of the experiment and looking for a way to kill time, take this opportunity to write a draft of the experimental methods section. It is to your benefit to do this in lab while you can ask your TA questions!
2.5 M NaOH solution. Warm on a hotplate until the solid dissolves. Thread a paperclip through the acetate (smooth, bright white) end of each fabric swatch before dyeing. The order is: acetate, cotton, nylon, polyester, acrylic, and wool. Each fabric strip should be labeled with your name, the dye, mordant, or other conditions if applicable on a securely fastened tag. Attach this tag to the paperclip immediately after dyeing and rinsing. Immerse the fabric in the 2-naphthol solution for 2 minutes, stirring occasionally. Remove the fabric with tweezers and dry between paper towels. Immerse this pretreated fabric into the dilute diazonium salt solution in the 100-mL beaker. Let it stand in the solution at room temperature for 5 minutes. Remove the strip from the bath with tweezers, soak in tap water for a minute, then rinse with tap water into the waste until the rinse water runs clear. Dry on paper towels. Dispose of the solutions in the liquid waste. B.2. Coupling with ANS: Ingrain Dyeing with Easter Purple On a new page, copy the reaction scheme, make a table of reagents (amounts, properties, safety), and write the procedure below the table. Figure 13. Synthesis of Easter Purple. Dilute the suspension of the diazonium salt from Part A in a 100-mL beaker with 30 mL of water. In a separate 100-mL beaker, dissolve 100 mg of 8-anilino-1-naphthalenesulfonic acid ammonium salt (ANS) in 35 mL of water. Thread a paperclip through the acetate (smooth, bright white) end of each fabric swatch before dyeing. The order is: acetate, cotton, nylon, polyester, acrylic, and wool. Each fabric strip should be labeled with your name, the dye, mordant, or other conditions if applicable on a securely fastened tag. Attach this tag to the paperclip immediately after dyeing and rinsing. Immerse the fabric in the ANS solution for 2 minutes with occasional stirring. Remove the fabric using tweezers and pat dry on paper towels. Immerse the fabric in the dilute solution of diazonium salt and let it stand at room temperature for 10 seconds. Remove the strip from the bath with tweezers, soak in tap water for a minute, then rinse with tap water into the waste until the rinse water runs clear. Dry on paper towels. Dispose of the solutions in the liquid waste. Part C. Mordant Dyeing with Alizarin Start a new page, draw the structure of alizarin, make a table of reagents (amounts, properties, safety), and write the procedure below the table. In a 100-mL beaker place 20 mg of alizarin and 20 mL of 0.5 % NaHCO3 (aq). Warm on a hot plate to dissolve the dye, then bring to a gentle boil. Pre-treated mordant fabric swatches will be available in the lab. Add your assigned strip of fabric (untreated, CuSO 4 , AlKSO 4 , or FeSO 4 ) and gently boil for 5 minutes. Remove the strip from the bath with tweezers, soak in tap water for a minute, then rinse with tap water into the waste until the rinse water runs clear. Dry on paper towels. Dispose of the solutions in the liquid waste. Record your observations and those for the other three fabric swatches.
DAY 2 PROCEDURE
D. Vat Dyeing with Indigo - Start a new notebook page. Copy the reaction scheme (Figure 7), make a table of reagents (amounts, properties, safety), and outline the procedure. Include pertinent clean up and safety notes in a table after the procedure. D.1. Synthesis of Indigo Perform this reaction in the fume hood. In a 50-mL beaker add a stir bar, 100 mg of o- nitrobenzaldehyde, 1 mL of acetone, and 1 mL of water. Stir the suspension on a stir-plate and add 1 mL of a 2.5 M NaOH solution drop-wise. Dark blue indigo should start to form immediately as a black-blue sludge. Bring the beaker back to your bench-top and let the reaction mixture stand undisturbed at room temperature for 10 minutes. Transfer to an ice-water bath for an additional 10 minutes. Collect the solid by vacuum filtration onto pre-weighed filter paper, performed at your bench-top. Use a magnet to keep the stir bar from falling into the funnel. Wash the solid on the filter with 2 mL COLD water, allowing all the liquid to pass through before following with 2 mL of ethanol.^ Let the solid air dry with the vacuum on for 15 minutes, weigh the product, and calculate the % yield. If the yield is greater than 100%, place the filter paper with solid back on the funnel and dry for an additional 10 minutes. Rinse the stir bar over the liquid waste, then leave it in a shared dithionite bath for at least 10 minutes, which should remove any residual blue indigo and make for easier cleaning. D.2. Dyeing with Indigo To a 100-mL beaker equipped with a magnetic stir bar, add 25 mL of water and dunk the filter paper containing indigo directly into the beaker with the aid of tweezers. If possible, take out the filter paper after most of the indigo has dissolved, otherwise the paper can remain in the beaker. Add 5 mL of 2.5 M NaOH and cover with a watch glass.^ Boil on a hotplate with stirring with magnetic stir bar. Once the solution is boiling, add 7 mL of a freshly made solution of sodium dithionite. Boil and observe any color change. If the blue color persists add more sodium dithionite (1 mL at a time), allowing the solution to return to a boil between additions (up to 3 mL), until most of the solid has dissolved and solution turns yellow.^ The solution may not be clear yellow. Once 10 mL is added and the solution is boiling, proceed to the next step. Turn off the heat, add a strip of fabric (don’t forget to paperclip the acetate side!) and let it sit in the hot bath for 3 minutes. Use tweezers to remove from heat. Rinse well into a labeled waste beaker, dry with paper towels, then let it air dry. Wait a few minutes to record observations, as it takes time for the indigo to dry and completely undergo oxidation in the air. E. Direct Dyeing with Malachite Green and Eosin Y – Start a new notebook page. Include the structure of the assigned dye and the written procedure on one page of your notebook. Perform this step in the fume hood. Obtain 20 mL of the Malachite Green (Cat Team) or Eosin Y (Team Dog) solution in a beaker and cover with a watch glass. Add a strip of fabric and bring the system to a gentle boil on a hot plate, keeping it covered.^ After about 3 minutes of gentle boiling, use tweezers to remove from the heat and rinse the swatch into the waste beaker. Pat dry between paper towels and clean the tip of the squirt bottle.**
Pre-lab Questions Week 1
- What structural characteristics give dyes their color? List two examples that fit this trend.
- List the main functional group and associated intermolecular force (IMF) in the fibers of cotton, wool, nylon, polyester, acetate, and acrylic. Examples of IMF’s include hydrogen- bonding (H-bonding) and dipole-dipole interactions. “Polar” and “non-polar” are technically not IMF’s!
- Consider the dye American Flag Red and indicate its predominant IMF (Figure 2). Do you expect this dye to adhere better to cotton or to polyester? Base your response on the information in #2 above.
- Briefly explain the difference between ingrain and mordant dyeing. What is the role of the mordant and of the sodium bicarbonate solution? Why do mordant-dyed fabrics keep their colors so well? Week 2
- Explain the general process of vat dyeing. Redraw the structure of blue indigo and circle the atom that is being reduced in the vat dyeing process (Figure 7b).
- Calculate the mmoles of o-nitrobenzaldehyde and acetone used. Determine the limiting reagent and report the theoretical yield of blue indigo in mmol and mg. Show your work.
- Consult the 8M Technical Writing Guidelines (worksheet from the first lab meeting) and write a draft of the experimental methods section for Part D.1 (Indigo synthesis). Type double-spaced and print on a separate sheet from pre-lab #5 & 6.
In-lab Questions
- Print and bring Table 3 (p. 12) with you to lab and complete as you work. Record your own observations. DO NOT COPY YOUR PARTNERS’ DATA! You do not need to type up this table for the report but do make sure your observations are neat and easy to read. It may be necessary to print the table a second time to make your observations more organized.
- Draw the complete arrow-pushing mechanism for the synthesis of either easter purple (team dog) or American flag red (team cat) from 4-nitroaniline diazonium salt (Part B).
- Report the yield (in mg and %) for the synthesis of indigo in one complete sentence. Show your work.
- For any one dye under normal conditions (without mordant), briefly comment on the abilities of different fabrics to absorb the dye (color intensity). Include comments on the structural features of both the fiber and the dye.
- Discuss the results of mordant dyeing. What were the differences using the dye with and without a mordant (metal salt)? What were the differences using the same dye with different mordants?
- Comment on how the extent in conjugation (number of pi-electrons, electron withdrawing/donating groups) effected the color of the compounds. Choose 2-3 examples that best exemplified this trend.
Exp 4 - Colorful Chemistry Name _____________________________ Section Day _____ Time _____ TA Name __________________________ CHEM 8M CHECKLIST / RUBRIC – Do Not Turn In ** All assignment descriptions & due dates on Canvas ** SECTION INSTRUCTOR COMMENTS POINTS ASSIGNED IN-LAB QUIZZES Available on Canvas 24 hours before enrolled lab section, due by lab start time
INTRODUCTION Responses to pre-lab questions uploaded to GradeScope before enrolled section start time. Different assignments for week 1 & week 2.
LAB REPORT
RESULTS The main results are stated, as outlined in the in-lab questions, using complete sentences.
EXPERIMENTAL DETAILS (no characterization or dyeing) Guidelines online. General Methods plus separate paragraph for Part D.
NOTEBOOK PAGES Proper format: reaction scheme, chemical info table, ‘comic book’ procedure including all equipment, chemical names, & amounts, waste and clean-up note.
NEATNESS, ORGANIZATION, & ZOOM PARTICIPATION Proper order and format; proper spelling & grammar. Participate in polls / chat window.
LAB REPORT TOTAL
