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Organic Chemistry II LabAzo Dyes Lab

**Please complete pre-lab take home assignment (see experiment pages to assist, 1st attachment). 

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Combinatorial Chemistry Parallel Combinatorial Synthesis of Azo Dyes This experiment is adapted from Gung, B.W., Taylor, R.T., J. Chem. Ed. 2004, 81(11) , 1630-1632. Background Combinatorial chemistry has become an increasingly important tool in the search for compounds with desired properties. The goal of combinatorial chemistry is to prepare a large number of structurally diverse but related compounds efficiently. A new journal has emerged that is devoted entirely to combinatorial chemistry. The pharmaceutical industry has embraced this new development and has invested millions of dollars into the area. The products from a combinatorial synthesis are usually called a library. The contents of a library can be screened for a desired activity, ranging from anti-tumor or anti-HIV properties to effective catalytic properties. In this experiment, the principle of combinatorial chemistry is shown through preparing azo dyes using the combinatorial approach. The coupling reactions that will be performed involve an aromatic diazo compound and an electron-rich, water-soluble aromatic compound as the coupling partners. The so- called “point of diversification” involves the structure variation on each reactant. With the different starting materials that are available, each student can be assigned a unique coupling reaction. Thus, the entire class will perform the same reaction (an azo coupling reaction) but each student will synthesize a different product. A fabric dyeing experiment follows the coupling experiment. The color of the dyed multi-fiber strip serves as the final assay for the experiment. At the end of the experiment, the colors of the dyed multi-fiber strips from the entire class should be compared. Any conclusions from the correlation of the product structure to the colored strips should be discussed in the lab report. Azo Compounds and Azo Dyes Compounds containing one or more azo groups (-N=N- linked to two carbon atoms) have a variety of uses. Aliphatic azo compounds, like azo bis isobutyronitrile (AIBN), can be used as radical initiators in the polymerization of alkenes to make plastics. Aromatic azo compounds are used as acid-base indicators, biological stains, and as colorants (dyes) for clothing, plastics, cosmetics, food and beverages. In fact, azo dyes form 60-70% of all synthetic dyes used as commercial colorants. Azo dyes have several advantages over other commercial dyes including their wide color range, good color fastness and ability to absorb light. They can also be synthesized cheaply because the starting materials are readily available, inexpensive compounds; most of the chemistry is completed at or below room temperature; and the environmental impact is low due to the use of water as a solvent in all of the reactions. Cost advantages tend to compensate for the lower resistance to bleaching and lower brilliance of azo dyes compared to anthraquinones, the second most used dye class. Azo dyes are generally prepared by a two-step process. First, an aromatic amine is converted to a diazonium salt by reaction with nitrous acid (HNO 2 , prepared by mixing NaNO 2 with HCl or H 2 SO 4 ). Then the diazonium salt is reacted with an electron-rich aromatic compound, usually one that contains an OH group (a phenol) or an amino nitrogen. Benzene rings that are not activated by one of these groups do not react readily with diazonium ions. The reactions are as follows:
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STEP 1: Formation of diazonium salt from an aromatic amine. ––––––– > STEP 2 (below) : Azo coupling reaction The azo coupling reaction creates a molecule with an extended system of conjugated double bonds that can absorb visible light. The color of the light absorbed depends on the nature of the substitutents X and Y. If these groups contribute to the delocalization of electrons, then they will shift the wavelength of light absorbed to longer wavelength. The example below shows the azo coupling reaction that is used to produce the dye solochrome orange M. Note that the diazonium salt adds to the phenol (salicylic acid) at the position para to the OH group. salicylic acid p-nitroaniline solochrome orange M EXPERIMENTAL PROCEDURES Warnings! Avoid skin contact with arylamines Sodium hydroxide is caustic. Avoid skin contact. Hydrochloric acid is highly corrosive. Handle it with care. Sodium nitrite is a toxic oxidizer. Naphthol derivatives are irritants. 1-Naphthol is toxic. Diazonium salts are explosive in the solid state. They should be kept in solution and used immediately after preparation. Azo dyes are irritants. Wear gloves when dyeing the fabrics. Couplings using the Diazonium Salt from Aminobenzenesulfonic Acid First, obtain an assignment letter from the instructor. The letter will tell you which TWO compounds from the chart below you should use for your coupling reaction. an azo dye diazonium salt
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Organic Chemistry Laboratory II (X36.1B) PRE-LAB QUIZ for AZO DYES EXPERIMENT Name ___________________________ 1. You can perform some of the calculations before you start the experiment. a) The experiment calls for 2.8 mmol of an aminobenzenesulfonic acid. What is the mass of 2.8 mmol of aminobenzenesulfonic acid? b) The experiment calls for 2.8 mmol of sodium nitrite (NaNO 2 ). What is the mass of 2.8 mmol of sodium nitrite? 2. After reading about azo coupling, write the structures of the compounds that would be formed in the reactions shown below. amine diazonium salt azo dye 3. Draw the structures of the two organic compounds that would be the starting materials for the synthesis of the following azo dyes. azo dye starting amine starting coupling compound Methyl Red Para Red
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4. Azo coupling is only one of many reactions of diazonium salts. In the boxes provided, write the structures of the products of the reactions shown below. (Information about the reactions of diazonium salts can be found in most organic chemistry textbooks - for example, Brown & Iverson, 7 th edition, pgs. 992-994). diazonium salt
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Combinatorial Chemistry Parallel Combinatorial Synthesis of Azo Dyes This experiment is adapted from Gung, B., Taylor, R. Chem. 2004, 81(11),...
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