Arenediazonium Ions in Organic Synthesis: Para Red and para-Iodonitrobenzene An arenediazonium salt will be prepared from para-nitroaniline and used in two different reactions—with iodide ion to produce para-iodonitrobenzene and also with β-naphthol to produce an azo dye, Para Red. The Para Red will be used to dye fabric, its uV-visible spectrum will be determined, and its acid-base properties will be investigated.
Synthesis of para-nitrobenzenediazonium chloride. This arenediazonium salt will be prepared by the reaction of paranitroaniline with an aqueous solution of sodium nitrite and hydrochloric acid at low temperature (Reaction 1). The mechanism of this reaction is discussed in the Bruice text used for Chem 21 and Chem 22.
NH2
N
NaNO2, HCl (aq)
Cl-
0-5°C O2N
N
O2N
para-Nitroaniline
para-Nitrobenzenediazonium chloride
(Reaction 1)
Synthesis of para-iodonitrobenzene. Nitrogen gas can be displaced from arenediazonium salts by various nucleophilic reagents to produce a variety of different substituted aromatic rings. Here, addition of iodide ion (as KI) to para-nitrobenzenediazonium chloride will be used to prepare paraiodonitrobenzene (Reaction 2). Nitrogen gas is released during this reaction. Note that in the azocoupling reaction used to produce Para Red, no nitrogen gas is released. We will assess the purity of the product by measurement of the melting point (Technique 10).
N
I
N
KI
-
Cl
+ KCl +
N
N
O2N
O2N
para-Iodonitrobenzene
(Reaction 2)
Synthesis of Para Red. Arenediazonium salts can also react as electrophiles with strongly activated aromatic rings in electrophilic aromatic substitution reactions. The aromatic ring must be highly activated in order for the reaction to occur at the low temperatures (< 5°C) necessary to prevent decomposition of the arenediazonium salt. Here, para-nitrobenzenediazonium chloride will be reacted with β-naphthol to produce the azo dye Para Red (Reaction 3). The electrophile reacts with the ring carbon that is ortho to both the activating group (OH in this case) and the other ring.
NO2
N
N
N
OH
N OH
-
Cl
+
O2N
β -Napthol
Para Red
(Reaction 3)
Para Red (7/6/2016) - Dr. Kline - SMC - page !1 of !4
Fabric Dyeing Fabric strips containing six different types of cloth will be used for the dyeing. We note which type of cloth is dyed by the Para Red most effectively. The effectiveness of the dye is determined, at least in part, by the efficiency of the interactions between the dye and the cloth. A diagram of this type of fabric strip is shown below, after dyeing with a yellow dye. Our strips may or may not contain the same fabrics in the same order as in this example.
Acetate is cotton-derived fabric that has been modified to remove many OH-groups found on the surface of the threads.
Cotton is natural cellulose found in plant material.
Nylon is a synthetic material made of many linked amide groups, somewhat similar in structure to wool or silk.
Polyester (Dacron) is a synthetic material made from many linked ester groups.
Acrylic (Orlon) is another synthetic fabric made from many acryonitrile (CH2=CHCN) units linked together.
Wool and silk are natural polyamide or protein materials made from animal sources. This end of the strip is easily recognized by its thick "furry" appearance.
Para Red is an example of an azo dye and was the first azo dye used. The vivid colors of azo dyes are due to the delocalization of electrons in the aromatic rings and through the nitrogen-nitrogen double bond (the azo bond). Para Red will be synthesized in the fiber of the fabric by first dipping it in a basic solution of β-naphthol and then in the solution of the arenediazonium salt. This process is called “ingrain” dyeing and is typically used for azo dyes.
Once the fabric has been dyed the remaining portions of the β-naphthol and arenediazonium solutions will be combined to isolate the solid Para Red product.
Visible spectrum of Para Red. The visible spectrum of Para Red will be taken to determine whether or not the spectrum is consistent with the observed color of the dye. Recall that when a compound absorbs a portion of the visible spectrum, it appears colored. If no visible light is absorbed, the compound appears colorless or white. The apparent color of the compound is due to the portion of the visible spectrum that is not absorbed. A color wheel, or color circle, can be used to predict the observed color: the observed color is on the opposite side of the wheel from the absorbed color (wavelength). The wavelengths on the wheel at the right are of the absorbed light. Different azo dyes absorb light of different wavelengths due to their different structures. For example, more highly conjugated systems will absorb light with longer wavelengths (lower frequency; lower energy). Also, having electron-donating and electron-withdrawing groups para to each other on an aromatic ring increases both the wavelength of light absorbed and the intensity of the absorption (absorptivity in Beer’s law). Acid-Base Properties of Para Red. This compound can act as an acidbase indicator because the structures of its acid and conjugate base forms absorb light at different wavelengths. We will explore this property by adding acid and base, in turn, to a solution of the Para Red.
Color Wheel 400 nm
700 nm
Violet
425 nm
Red Indigo Blue
450 nm
Orange 600 nm
Green 520 nm
Yellow 550 nm
Para Red (7/6/2016) - Dr. Kline - SMC - page !2 of !4
Pre-lab Reading
• Chemistry of Diazonium salts - sections 23.10 and 23.11 in Klein, 2nd edition.
• Dyes and Dyeing Essay (link)
• Techniques
• Vacuum Filtration - Chapter 9 of Mohrig Techniques book, 4th edition
• uV-vis Spectroscopy - Chapter 25 of Mohrig Techniques book, 4th edition; Section 17.11 in Klein, end edition
Color - Section 17.10 in Klein; also information in Mohrig’s uV-vis Chapter
•
Procedure Safety Notes Wear gloves throughout this experiment. Sodium nitrite is an oxidizing agent and highly toxic. para-Nitroaniline is highly toxic and is readily absorbed through the skin. βNaphthol is an irritant and a suspected carcinogen. Solutions of HCl and NaOH are caustic. Para Red may be a carcinogen. Avoid touching the dyed fabric strip with your hands. List of Chemicals and Materials Needed
• para-nitroaniline (0.35 g/pair)
• β-napthol (2-naphthol, 0.13 g/pair)
• sodium acetate (1.5 mL of a solution containing 0.2 g) - a batch for class use may be prepared by the instructor
• potassium iodide (0.05 g/pair)
• 3 M HCl (8 mL/student)
• 10% NaOH (5 mL/student)
• 7% aqueous sodium nitrite solution (5 mL/pair) - a batch for class use may be prepared by the instructor
• acetone, higher quality than for washing if possible (15 mL/student)
• fabric strips with six types of material
Production of para-Nitrobenzenediazonium Chloride Prepare or obtain the following in advance and allow them to cool (separately) in an ice-water bath: 5 mL of 7% NaNO2 solution and 1.5 mL of a solution containing 0.2 g sodium acetate. The instructor may prepare batches of these for the entire class.
Place 0.35 g of p-nitroaniline and 10 mL of water in a 50-mL Erlenmeyer flask. The solid won’t significantly dissolve at this point. Add 4 mL of 3 M HCl and warm on a hot plate to dissolve the solid. You may use a magnetic stirring bar to stir the solution. Add more HCl dropwise, if necessary, to complete the dissolution of the solid. Then, cool the solution in a cold water bath (0–5 °C) to prepare for the production of the diazonium salt solution. Some solid may come out of solution at this point.
Add the chilled 7% NaNO2 solution all at once to the reaction mixture in the flask. Stir for 10 minutes, during which time the solution should become clearer. If the solution doesn’t become clearer, you may filter it, but this isn’t necessary. Keep the mixture cold (≤ 5°C) by periodically placing the solution in a ice-water bath. Just before you are ready to use the mixture in the following steps, add the sodium acetate solution you already prepared and continue to keep your diazonium salt in the ice-water bath.
Preparation of para-Iodonitrobenzene
Weigh 0.025 g of KI into a 25-mL beaker. Quickly add 0.5 mL of the cold diazonium salt solution you just prepared, using a Pasteur pipet with a 1-mL syringe. Swirl the mixture until gas evolution ceases. Isolate the solid by vacuum filtration with a Hirsch funnel. Wash the solid with two 1-mL portions of cold water.
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Dry the solid by pulling the vacuum for a few minutes, scrape it off the filter paper and into a labeled and weighed small beaker, and leave it to dry in your locker until the following lab period. The mass and melting point will be determined the following week.
Fabric Dyeing Obtain a fabric strip and, to keep track of which end of the strip is acetate and which end is wool, cut a notch in the wool end. Place 0.13 g β-naphthol in about 10 mL hot water (60-80 °C) in a 50-mL beaker. Then add 10% NaOH in 0.2-mL portions until the solid just dissolves. Do not add too much base, as the fabric decomposes in base. Soak the fabric strip in this solution for 2-3 minutes.
Remove the fabric strip from the β-naphthol solution with forceps and pat it dry using a paper towel. Don’t get any of the solution on your fingers because β-naphthol is an irritant. Place the strip in the chilled diazonium salt solution. After a couple of minutes, remove the strip (with forceps!), rinse it with water by dunking it in a beaker of water, and spread it out on a paper towel. Note the different colors observed on the different fabrics and how effectively the dye was accepted onto each fabric. Draw a diagram or take a photo. Dispose of the fabric strip in the waste: there will probably be a container just for those.
Isolation of Para Red
Chill the β-naphthol solution until it reaches about 5°C (you may start chilling it once you remove the fabric strip from it in the above paragraph). Once the fabric is removed from the diazonium solution, combine the remainder of the two solutions (basicified chilled β-naphthol and diazonium ion) together to yield the solid Para Red product. Isolate the solid by filtration on a Buchner funnel. Wash thoroughly with water. Dry on the funnel, scrape it off the paper on to a watch glass or beaker. Either dry it under a heat lamp, being careful not to decompose the dye under the lamp, or leave it to dry in your locker. Weigh it and, if you instructor indicates, determine the melting point the following week.
Acid-Base Properties
While you are waiting for the Para Red to dry, place a tiny (~1-3 mg) sample of the un-dry product in a small test tube. Add about half a milliliter each of acetone and deionized water. Note the color. Experiment with adding small amounts of NaOH and then HCl, using the solutions that are available in the lab. Note what happens to the color as you add base and then acid. This can also be done with dry Para Red.
Visible Spectroscopy
You may do this using either dry or not-so-dry Para Red. Make a solution consisting of a tiny amount of Para Red dissolved in about 25 mL acetone in a clean dry test tube (the solution should be orange as opposed to red; if it’s red then it’s too concentrated and it will be hard to get a good spectrum). Transfer an appropriate amount of the solution to a cuvette and, with your instructor’s help, take the visible spectrum from 400-800 nm. Sketch the spectrum in your notebook, being sure to write down the absorbance maximum or maxima (or take a photo of it now and sketch it before you leave). Transfer the solution back into the test tube and add just enough 10% NaOH solution that it changes color and the new color remains when the solution is mixed. Transfer the the solution back to the cuvette and take its spectrum again sketching the spectrum and absorbance max in your notebook.
Cleanup and Waste Disposal
Put all filtrates, excess reagents, and your dyed fabric strip in a waste container. Do not take your fabric strip home. Wash your glassware and filtration apparatus with soap and water. Any dye that remains may be rinsed with acetone.
Credits: This experiment is based on a lab done at the University of Colorado at Boulder and adapted by Dr. Jamey Anderson of Santa Monica College.
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