Multi-step Synthesis: Preparation of Organic Dyes

Objective: The purpose of this lab is to explore the synthesis of organic azo dyes.

Grading: 1. Write-up in your lab notebook 2. Successful dying of cloth sample 3. Answers to post-lab questions 4. TA evaluation of lab procedure

Introduction: Synthetic dyes Pigments were originally isolated from natural sources – plants, animals, and minerals. The first documented synthetic dye was actually discovered by accident in 1854. William Perkin was attempting to synthesize quinine, a potent anti-malarial agent, when he found a purple substance in the midst of a black sludge. The substance, mauve, was the beginning of a booming industry that led to the discovery of a large family of synthetic dyes. 1 size 12{ {} rSup { size 8{1} } } {} What makes this even more interesting is that the correct structure of his dye was not fully revealed until 1995! 2 size 12{ {} rSup { size 8{2} } } {} The largest group of dyes is the azo dyes, which come in a wide range of colors – yellow to red to blue. The general structure of azo dyes includes an azo group (N=N) attached to two aromatic ring systems (see Figure 1).

Figure 1. General structure for azo dyes. The aromatic rings typically contain substituents such as alcohol or amino groups. The colors of the dyes come about through a structural characteristic known as conjugated bonds. Conjugated systems are made up of alternating carbon-carbon single bonds and carbon-carbon double bonds. Nonconjugated double bonds contain a saturated carbon atom between the two double bonds; there is no alternating between single and double bonds as seen with conjugated systems. Examples of these compounds are shown in Figure 2.

Figure 2. Examples of dienes. Structures 1, 2, and 3 are examples of conjugated systems, as they contain alternating carbon-carbon single bonds and carbon-carbon double bonds. Structures 4, 5, and 6 are nonconjugated molecules, as they have a saturated carbon atom between the two double bonds. Conjugation gives dyes their ability to absorb different wavelengths of light and makes the compound appear colored. Examples are found throughout nature, as seen in the examples in Figure 3. The absorption wavelength shifts up by about 30-40 nm for each additional conjugated double bond added to the molecule. Addition of an alkyl group increases the absorption wavelength by ~5 nm. 3 size 12{ {} rSup { size 8{3} } } {}

Figure 3. The structure of lycopene contains 11 conjugated double bonds and two nonconjugated double bonds. Lycopene is the terpene responsible for the red color of tomatoes and is also found in guava and watermelon. Beta-carotene gives carrots their orange color, as it contains 11 conjugated double bonds. Azulene, a monoterpene, has 5 conjugated double bonds and appears as a brilliant blue. Indigo is another conjugated system that gives the deep blue color to blue jeans.

Synthesis of azo dyes Azo dyes are synthesized in a two-step reaction: formation of a diazo compound and a coupling reaction. We will not go into the mechanism of this reaction, but we should know a few basics. The first step of this synthesis involves transforming the aniline derivative (aniline – benzene containing an amino group) into a diazonium salt with sodium nitrite (NaNO2) size 12{ \( ital "NaNO" rSub { size 8{2} } \) } {} under acidic conditions. (Figure 4) Typically sodium nitrite is added stoichiometrically or in very small excess. Using an excess can cause the diazonium salt to decompose. 1 size 12{ {} rSup { size 8{1} } } {}

Figure 4. The first step in azo dyes synthesis involves transforming the amino group into a diazonium ion with sodium nitrite and acid. The gas nitrous acid is a side-product released as the reaction takes place. Here the 1, 4-disubstituted benzene is being used. The benzene being used may have different positions of substitutions. In this experiment we will look at 1, 3-disubstituted benzenes (notated with “m” for metasubstituted) and 1, 4-disubstituted benzenes (notated with a “p” for para-substituted).

Figure 5. The 1, 3-disubstituted benzene can also undergo transformation to a diazonium ion.

Figure 6. Structures of amino compounds for preparation of diazo compounds. Note that all of these compounds contain aromatic rings and at least an amino functional group. The next step of the reaction involves a “coupling” reaction with either an aminocontaining aromatic ring or an alcohol-containing aromatic ring (phenol – aromatic ring with a hydroxyl group). For example, we can take the diazonium salt from p-nitroaniline and base-treated 2-naphthol and form para red, the dye used in making American flags.(Figure 7)

Figure 7. Synthesis of para red from p-nitroaniline and 2-naphthol. With the reactions involving phenol derivatives (see Figure 8), the compound is first reacted with NaOH to deprotonate the hydroxyl group, forming a strong nucleophile. The negatively-charged nucleophile then attacks the electrophilic diazonium ion.

Figure 8. Structures of phenols for coupling reactions. Note that all four compounds contain an alcohol functional group on the aromatic ring. Alternatively, the coupling reaction can involve aniline derivatives (see Figure 9). These do not need treatment with base prior to addition to the diazonium salt. Instead, these are prepared in a weakly acidic solutions.

Figure 9. Structures of amines for coupling reactions. Aniline will be used for either preparation of the diazo compound or as a coupling agent. Note that all three compounds contain an amino functional group, with the middle compound having a disubstituted amino group. Azo dyes often have the same number of conjugated double bonds for each product. What, then, causes the differences in colors seen between each dye? Many theories have been developed to explain the changes in color, including resonance effects, molecular orbital explanations, electronic effects, and many more. It is difficult, then, to give one answer to what causes the shifts in the absorption wavelength. Generally we can say that addition of electron-withdrawing groups (such as −NO2 size 12{ - ital "NO" rSub { size 8{2} } } {}) shift the absorption wavelength UP, causing a darker color to appear. Addition of hydroxyl or amino groups tend to increase the color’s intensity. 1,4 size 12{ {} rSup { size 8{1,4} } } {}

Starch-iodine indicator paper In this experiment, one way we determine whether the reaction is complete is by using starch-iodine indicator paper, which tests for the presence of nitrous acid (HNO2) size 12{ \( ital "HNO" rSub { size 8{2} } \) } {}. A positive test is shown by the change of the colorless indicator paper to a deep blueblack color. Once our reactions are complete, free NO2 size 12{ ital "NO" rSub { size 8{2} } } {} in the reaction mixture can be protonated to form nitrous acid. Thus a positive test from starch-iodide paper signals the reaction is complete!

Ingrain Process Rather than mixing the two components together in a flask for the coupling reaction, we will instead use the “ingrain process”. This is a patented technique for dyeing cloth in which the reaction takes place within the grain of the cloth. The cloth is first soaked in the prepared coupling reagent and then dried. The dried cloth is then soaked in the diazonium salt. The presence of color indicates a successful experiment!

References: 1. Christie, R. M. Colour Chemistry. Cambridge: Royal Society of Chemistry: Cambridge, 2001. 2. Meth-Cohn, O.; Smith, M. J. Chem. Soc. Perkin Trans. 1994, 1, 5–7. 3. Wade, L. G., Jr. Organic Chemistry, 3rd ed. Prentice Hall: New Jersey, 1995. 4. Gordon, P. F.; Gregory, P. Organic Chemistry in Color. Springer-Verlag: New York, 1983.

Materials Required **Specific diazo/coupling reagents will be assigned in class. ** Equipment Chemicals • 10-mL beaker 3 M HCl • Forceps 1 M NaNO2 size 12{ ital "NaNO" rSub { size 8{2} } } {} • Stir bar 1 M NaOH • Ice bath 3 M Na2CO3 size 12{ ital "Na" rSub { size 8{2} } ital "CO" rSub { size 8{3} } } {} • Starch-iodine indicator paper Diazo/coupling reagents. Safety Wear gloves and safety glasses at all times. Do not touch the reagents without wearing gloves, as they are highly toxic and some are corrosive. Please follow the safety precautions by opening the containers in the fume hoods and not carrying open containers around the lab. Dispose of all diazonium waste in the organic waste container and not the solid waste container. Diazonium compounds can be extremely dangerous (explosive) in a solid state. KEEP DIAZONIUM COMPOUNDS WET (in solution) AT ALL TIMES.

Experimental: This lab will be done in PAIRS. Each person will prepare one diazonium salt and one coupling compound, resulting in 4 dyes per pair. Each pair will prepare one phenol and one amine coupling compound (see Figures 8 and 9). Diazo Component MW (g/mol) Coupling Component MW (g/mol) Aniline 93.1 Aniline 93.1 m-anisidine 123.2 N-methylaniline 107.2 m-nitroaniline 138.1 N,N-dirnethylaniline 121.2 m-toluidine 107.2 m-phenylenediamine 108.1 p-asidine 123.2 Phenol 94.1 p-nitroaniline 138.1 1-naphthol 144.2 p-toluidine 107.2 2-napthol 144.2 resorcinol 110.1

Table 1. Reagents to be prepared. A. Preparation of amine 1. Dissolve 2.0 mmol of the amine in 2 mL 1M HCl. **If using 1, 3-phenylenediamine use 4 mL 1M HCl. ** 2. Keep cool in an ice bath until Part D. B. Preparation of phenol 1. Dissolve 2.0 mmol of the phenol in 4 mL 1M NaOH. 2. Keep cool in an ice bath until Part D. C. Preparation of Diazonium salt 1. Mix 2.0 mmol of “diazo” component with 1.6 mL of 3M HCl in a 10-mL beaker. 2. If the compound doesn’t dissolve, gently heat (<50 °C) and add 1-2 mL 3M HCl. **Note – Some compounds may be difficult to dissolve in acid. Addition of 1-2 mL acetone may aid in dissolving your compound. 3. Cool this solution for at least 5 minutes in an ice bath with stirring. 4. Continue stirring as you slowly add 2.0 mL of 1M NaNO2 size 12{ ital "NaNO" rSub { size 8{2} } } {} drop wise over 3 minutes. 5. Test solution with starch-iodide paper; add NaNO2 size 12{ ital "NaNO" rSub { size 8{2} } } {} until a positive test (blueblack paper). 6. Divide the solution into roughly two equal parts for coupling—keep on icebath until you are ready to use. D. Dying Cloth via the Ingrain Process 1. Take one part of your coupling compound prepared in Part A or Part B and dilute with 8 mL of deionized water. 2. Soak piece of clean white cloth in this solution for 2-3 minutes. 3. Remove the cloth with forceps and blot between paper towels to remove most of the water. 4. Hang up the cloth to dry. 5. Mix 8-mL cold deionized water with diazonium salt from Part C. **Note – You may need to add 1-2 mL acetone if your salt is not fully dissolving. 6. Add the dry cloth and agitate with stir bar long enough to dye uniformly. 7. If your coupling component was an aromatic amine, briefly dip the cloth in a solution of 3M Na2CO3 size 12{ ital "Na" rSub { size 8{2} } ital "CO" rSub { size 8{3} } } {}. 8. Remove cloth and dry as before. 9. Prepare a table with compounds and respective dye colors.

Waste Disposal Organic waste should be disposed in the organic waste container