Synthesis of Methyl Orange
Jineui Jung

Abstract

The purpose of this lab was to synthesize a synthetic dye, methyl orange. Methyl orange is a dye produced from a synthesis reaction between diazonium ions and aromatic structures. Dyes have basic structure of Aromatic N+N-aromatic, azo group. methyl orange was synthesized by coupling reaction between diazonium ion and N,N-dimethyl aniline under basic condition. Diazonium ion created from sulfanillic acid monohydrate and N,N-dimethyl aniline under basic condition. Basicity was tested with pH paper after adding sodium hydroxide which resulted in light blue color, representing pH value of 10. Product yield was calculated to 71.53% with percent error of 28.46% so it was determined methyl orange was fairly produced.

Introduction

Methyl orange synthesis is diazo coupling reaction between the diazonium sale of sulfanilic acid and N,N-dimethylaniline. The initial product gained through coupling reaction is red acid form of methyl orange and this later turned to orange sodium salt, called methyl orange. Dyes are used to give colors to fabrics and Chromophores, functional groups that absorb light, give color to these dyes. The most common chromophores are azo, nitro, and carbon groups.
The method of diazotiation is dependent on how basic and soluble the amine being coupled is. As sulfanilic acid is insoluble in acid, sulfanilic acid is dissolved in a basic solution. Primary aromatic amines can be directly diazotized but amines with sulfuric or carboxylic acid group is diazotized by dissolving the amine in dilute acid or water. Weakly Amine must be diazotized by dissolving them in a concentrated strong acid solution. After it becomes diazotized, it yields white precipitate of the diazonium salt of sulfanilic acid. This diazonium salt is allowed to react with dimethylaniline in the solution in which it was precipitated.

Procedure

1. Preparation of the diazonium salt of sulfanilic acid

a. place 0.06 g of anhydrous sodium carbonate in 25mL Erlenmeyer flask and dissolve it in 5mL of water. b. Add 0.18g of anhydrous sulfanilic acid to the solution and heat it gently on hot plate c. Remove the flask from heat and cool to room temperature and add 0.8g of sodium nitrite. d. Place the flask in an ice-water bath until solution is well below 10 Celcius. e. Add 0.25mL hydrochloric acid. f. Keep the flask in iceboats and scratch the bottom of the flask until a fine, white precipitate has formed.

2. Synthesis of Methyl Orange

a. Place 0.14mL of N,N-dimethylaniline and add 0.10mL of glacial aceti acid in 3 mL colonial vial. b. Transfer solution to suspension of the diazonium salt of sulfanilic acid in 25mL Erlenmeyer flask. c. Keep this mixture chilled in the ice bath for 10 minutes with stirring. d. obtain 1.5mL of Sdium hydroxide in a small test tube. Add this slowly to the chilled reaction mixture, with thorough stirring and determine pH if solution is basic. e. Add boiling chip to the basic solution and hear solution to boil for 10 minutes. f. Remove the solution from heat and add 0.5g of sodium chloride and allow the mixture to cool. g. Place the flask in iceboats to complete crystallization. h. Obtain 5mL of saturated sodium chloride solution in a small test tube for rinse solution after filtration. i. Collect the product by filtration using buchner funnel and filter paper. j. Place the filtered solid plus filter paper in a 50mL beaker containing 15mL of boiling water. k. Continue boiling it for several minutes to allow the whole mass of solid to become dispersed. l. Cool it in ice bath for 10 minutes and collect the product by filtration using buchner funnel and filter paper. m. Let product dry before weighing it and determine percentage yield.

3. Use of Methyl Orange as a Dye, and the effect of a Bleach

a. Prepare a dye bath by adding 1 mL of sodium sulfate solution and 1 drop of concentrated sulfuric acid to 30mL of water and add 0.05g of methyl orange. b. Heat the solution to boiling. c. Obtain a piece of test fabric, cut it half, and in hot bath for 5-10 minutes. d. Make the dye bath basic by adding sodium carbonate, add sodium dithionite until the bath becomes colorless, add bleaching agent at the end. e. Compare this “ bleached” portion with the unbleached piece.

Data and Calculations

Mass of Na2CO3 | 0.0627 g | Mass of Sulfanalic Acid | 0.1899 g | Mass of Sodium Nitrate | 0.0898 g | Mass of NaCl | 0.5906 g | Mass of Methyl Orange | 0.2770 g |

Spectrometer
Helianthus red= 470nm with the absorbance value of 0.51
Methyl Orange= 510nm with the absorbance value of 1.50

Percent yield of methyl orange 0.001183 moles x 327.34g/moles MO = 0.3872 grams of Methyl Orange
Percent Yield (Actual / theoretical yield) x 100 = (0.2770/0.3872)* 100= 71.53 %

Conclusion

Methyl orange was synthesized from N,N-dimethylanilin and sulfanilic acid. With sodium nitrite, diazonium salt of sulfanilic acid was synthesized. Hydrocloric acid was added to the solution after first step to create precipitate of dizonium salt. Coupling reaction followed between a diazonium salt with N,N-dimethylaniline to form light orange colored methyl orange precipitates. This changed to red color with hydrochloric acid with pH value of 3 and turned to orange with that of 10 after being added with sodium hydroxide. Theoretical yield of methyl range was 0.3872g while actual yield was 0.2770g. Percent yield was calculated to 71.53%. Error might come from inaccurate measure from using different scale as well as possible loss between transfer. Also, it could have been drier before measuring mass of final product. Another reason might come from dissolving and crystallizing process completely as there was time limitation.

References 1 The Merck Index, 8th ed., Merck & Co., Rahway NJ, 1968. The index gives the following reference for details on the synthesis of methyl orange: 2 L. Gattermann, Die Praxis des organischen Chemikers, 40th ed., de Gruyter & Co., Berlin, 1961, pp 260-261.

Questions: 1. What kind of reaction is the reaction between dimethylaniline and the diazonium salt. a. Give a mechanism for the coupling reaction. b. Why does this reaction lead to coupling at the para position of the dimethyl-aniline ring? 2. In the formation of heliathin after the coupling reaction has occurred, a proton is transferred from the dimethyalimon group to one of the azo nitrogens, leading to a more stable product. Why is this product more stable than the first one, when the proton was on the dimethylamino group? 3. A colored compound absorbs light in the visible range. The light, which is not absorbed, gives the compound the color, which we see. What colors are absorbed by the acidic (red) and basic (orange-yellow) form of methyl orange.