EXPERIMENT Synthesis and characterization of azo dye: methyl orange Purpose In this experiment students will explore dia

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Experiment Synthesis And Characterization Of Azo Dye Methyl Orange Purpose In This Experiment Students Will Explore Dia 1 (63.05 KiB) Viewed 44 times

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EXPERIMENT Synthesis and characterization of azo dye: methyl orange Purpose In this experiment students will explore diazotization reaction to obtain methyl orange. The newly synthesized azo dye - 4-[4-(Dimethylamino)phenylazo]benzenesulfonic acid (methyl orange) is formed upon coupling of diazotized sulfanilic acid with N,N-dimethylaniline in a weekly acidic solution. Methyl orange is commonly used as a dye and pH indicator and successful synthesis will confirm its activity at different pH and the ability to dye cotton and the nylon-6,6 polymers. Background information Dyes and pigments are important industrial chemicals. They can color fibers permanently, such that they will not lose this color when exposed to sweat, light, water, and many chemical substances. The world's first commercially successful synthetic dye - mauveine, was discovered by accident in 1856 by William H. Perkin (at the age of 18). By the end of the 19th century, over ten thousand synthetic dyes had been developed and used for manufacturing purposes. Azo dyes are diazotized amines coupled to an amine or phenol, with one or more azo bonds (- N-N-). They are synthetic compounds and account for more than 50% of all the dyes produced annually, showing the largest spectrum of colors. Nearly all the dyestuffs used by the textile industry are azo dyes, and they are also widely used in the printing, food, papermaking and cosmetic industries.² Azo compounds are compounds having azo group R-N-N-R' in which R and R' can be either aryl or alkyl group. The simplest example of an aryl azo compound is azobenzene (Figure 33). trans-azobenzene 320-380 nm 380-450 nm N=N cis-azobenzene Figure 32. Azobenzene photoisomerization
One of the most intriguing properties of azobenzene and other azo compounds is the photoisomerization of cis/trans isomers. The two isomers can be switched with particular wavelengths of light (Figure 33). Because of n-delocalization, aryl azo compounds have rich colors, especially reds, oranges, and yellows. Therefore, they are used as dyes, and are commonly known as azo dyes. Azo compounds are important to our society not only in colorization of fabrics. Some DVD-R/+R and some CD-R discs use blue azo dye as the recording layer. Azo pigments consist of colorless minerals as carriers colored using an azo compound. Azo pigments are important in a variety of paints including artist's paints. Azo compounds can be obtained in a "one-pot", two step reaction (Figure 34). You are to synthesize the one of the azo compounds-methyl orange. This compound dye wool, silk and skin, and you must work carefully to avoid getting them on your hands or clothes. The dye will eventually wear off your hands after some time. The reaction starts by diazotization of sulfanilic acid with sodium nitrite. The diazonium salt is then immediately used to react with N,N-dimethylaniline in an reaction of electrophilic aromatic substitution to form methyl orange. H₂N- -OH + 2 HCl + NaNO₂ sulfanilic acid -NEN to N,N-dimethylaniline -NEN 4-diazobenzenesulfonic acid + 2 NaCl + 2 H₂O Figure 33. Synthesis of methyl orange. methyl orange Na NOTE: The diazonium salts are unstable at temperatures above 5 °C even in solution. That's why they are generated in situ and used immediately in the next reaction. At higher temperatures and in the dry form, the diazo group decomposes violently decompose to produce N₂ (explosive!). The diazotization reaction is possible due to a nitrosonium ion (NO), which is generated in situ from hydrochloric acid and sodium nitrite. Next, amino group of sulfanilic acid reacts with nitrosonium ion to form 4-diazobenzenesulfonic acid. The latter one reacts rapidly with N,N- dimethylaniline to give methyl orange.
The essential reactants for methyl orange synthesis are provided in Table 1. Table 1. Compound Sulfanilic acid Sodium nitrite Na₂CO3 HCI (12M) Dimethylaniline Acetic acid NaOH (IM) MW amount used 173.19 (calculate) 68.99 Reactants used 105.98 36.46 121.18 60.05 40 (calculate) 5 mL (calculate vol.) (calculate vol.) (calculate vol.) 3 mL mmol 3 1 m.p. (°C) d (g/mL) 1.18 0.956 1.049
TOR 2. Write detailed, hand-written reaction mechanism of methyl orange synthesis starting from sulfanilic acid, sodium salt and nitrosonium ion. You must include arrows, electron flow and lone pairs.