Preparation of Methyl Orange PDF

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Experiment 2. Preparation of Methyl Orange Aim of this experiment: to understand diazotization and diazo-coupling reaction (electrophilic aromatic subsitution reaction), and to use them for synthesizing azo dyes. Goal of this experiment: Synthesis of methyl orange, a pH indicator, using diazotization and diazo-coupling reactions with sulfanilic acid and dimethylaniline.

Introduction Methyl orange is a pH indicator and due to its clear color change it is very often used in titration. Methyl orange changes color at the pH of a mid-strength acid and is usually used in titrations for acids. Unlike a so called universal indicator, methyl orange does not have a full spectrum of color change, but has a sharper end point.

p-Dimethylamino-Azobenzenesulfonic Acid (Methyl Orange)

In a solution becoming less acidic, methyl orange moves from red to orange and finally to yellow with the reverse occurring for a solution increasing in acidity. Methyl orange has a pKa of 3.47 in water at 25 °C. Methyl orange is prepared from sulfanilic acid and N,N-dimethylaniline. The first product obtained from the coupling is the bright red acid form of methyl orange, called helianthin. In base, helianthin is converted to the orange sodium salt, called methyl orange.

Acid-Base Indicator Properties of Methyl Orange

Color Theory The first real attempt to interpret the color of dyes in terms of chemical structure was made by Otto Witt in 1876. He proposed that dyes consist of conjugated Π-bonded systems, called chromophores, and salt-forming groups, called auxochromes, which are polar substituents that modify their colors. A chromophore is a simple, unsaturated, e- withdrawing group attached to an aromatic ring system. The extended conjugation due to the chromophore decreases ΔE (and increases λmax) of the Π→Π* transition, so that the λmax is in the visible range. Examples include nitro (-NO2), azo (-N=N-), carbonyl (C=O), and nitrile (-CN). An auxochrome is an e- donating group (by Π resonance) attached to an aromatic ring system. These also change ΔE and λmax. Examples include hydroxyl (-OH) and amino groups (-NH2, -NHR, and –NR2). The function of a dye is primarily to create a particular color or hue in association with the fabric or substrate. Both natural fibers (like wool, silk, and cotton) and synthetic substrates (like nylon) can be dyed. Dyeing is generally carried out in aqueous solution, and the process of attaching a dye molecule to the fiber is called adsorption. Dyes that are attracted to the textile by physical forces (physical adsorption, or physisorption), and have a direct attachment to the fiber (chemical adsorption, or chemisorption), are called direct dyes. Acid dyes, like methyl orange and tartrazine, are water-soluble anions (anionic dyes) that form ionic bonds between the dye and the fiber. Both wool and nylon are amide macromolecules that have terminal amino (-NH2) and acid (-COOH) groups. These can form ionic bonds with the dye molecules. As with amino acids, these groups often exist as zwitterions, that is the molecule has both a positively-charged group (-NH3+) and negatively-charged group (-COO-). Anionic (or acid) dyes have –SO3- groups that form ionic bonds, or salt linkages, with the – NH3+ groups on the fibers.

Reaction Scheme

Mechanism Diazotization:

Diazo Coupling:

Experimental Procedure Pretreatment before Diazotization 1. 2. (It may still appear cloudy.) 3. Gravity filter if solids are present, then rinse paper with 3-5 ml of hot water. Discard filtered solids, and cool filtrate to room temperature. Diazotization Reaction 4. 5.

6.

Coupling Reaction 7.

Caution: $N,N-dimethylaniline is toxic and is readily absorbed through the skin! Handle only with gloves! It is also volatile, and needs to remain completely in the hood until transfer is complete! 8.

Isolation of Crude Dye 9. 10. Discard filtrate in sink (~110 ml aq salt waste). 11. Remove 0.1 g of dye, and dry remainder in oven overnight to obtain yield. UV-Vis of MO solution in acidic and basic solution 12. Prepare a stock solution of MO in methanol (0.01 g in 5 mL MeOH). 13. Add a drop of stock solution in 2 mL of distilled water. Measure UV-Vis spectrometer. 14. To the aqueous solution of MO, add a drop of conc. HCl. Wait for 5 min. Measure UVVis spectrometer.

Reading Assignment Organic Chemistry, Wade, 7th Ed., 904-909. (Electrophilic aromatic substitution) 751-752. 890-893. 694-700. (UV-Vis spectroscopy)...