A very popular azo-dye known under dozens of trade names and widely used in textile, leather, food, — Physical Chemistry — Thermodynamics Chemistry Question
Acid orange 7
A very popular azo-dye known under dozens of trade names and widely used in textile, leather, food, cosmetics, as well as other industries, Acid Orange 7 (Acid Orange II, Persian Orange, listed in the Color Index as No. 15510) can be readily obtained by azo-coupling of diazotized sulphanilic acid with 2-naphtholate
[VISUAL]
Materials and hardware
* Sulfanylic acid, solid,
* 2-Naphthol, solid,
* sodium carbonate, solid,
* sodium nitrite, solid,
* sodium hydroxide, 5% aqueous solution,
* hydrochloric acid, conc.
* ice.
* glass beakers (150, 200, 500 cm3),
* thermometer,
* spatulas,
* magnetic stirrer,
* heating plate,
* vacuum filtration apparatus,
* desiccator.
The diazotization
Sulfanylic acid (8.66 g, 0.05 mol) is dissolved in the solution of 3 g of sodium carbonate in 50 cm3 water in a 150 cm3 glass beaker placed on a magnetic stirrer. 15 cm3 of concentrated HCl are added to this solution at vigorous stirring. After cooling to room temperature, the beaker is immersed in an ice bath (a couple of ice chunks can be added to the mixture to ensure good cooling) and the mixture is further cooled to 0 °C. A solution of NaNO2 (3.45 g, 0.05 mol) in 20 cm3 of water is added dropwise (warning! this operation should be done in a hood because of evolution of nitrogen oxides). The rate of addition should be controlled to keep the temperature near 0 °C as accurately as possible (warning! even a 2° – 3° increase leads to side-reactions which may lead to the formation of phenols giving unwanted azo-dyes which dramatically worsen the purity of color of the target dye). During the addition white precipitate of diazonium salt (diazotized sulfanylate is a betaine, an inner salt with zero net charge, therefore it is not well soluble in water) may sometimes form. The results of diazocoupling do not depend on whether the diazonium salt is in solution or suspension.
After addition of all nitrite solution, stirring is continued for 10 – 15 min (warning! temperature should be carefully controlled!). The diazonium salt solution (or suspension) should be used immediately after preparation.
The azocoupling
2-Naphthol (7.21 g, 0.05 mol) is dissolved in 40 cm3 of 5% NaOH solution. This solution is mixed with solution of 12.5 g Na2CO3 in 100 cm3 water in a 500 cm3 beaker. The resulting solution should be transparent, if any precipitate or suspension persists, it should be filtered off. The solution of naphtholate is cooled to 0 °C by ice (an ice bath plus a few ice chunks inside). The diazonium salt solution is slowly poured to naphtholate solution under vigorous stirring by a spatula or a glass rod. Attention should be paid to keep the temperature below 8 °C throughout the addition. Afterwards, the mixture is left for an hour, preferably on a magnetic stirrer. The dye partially precipitates as golden plates.
After an hour, the solution is heated to completely dissolve the precipitate, filtered hot (note: this filtration can be omitted if a hot filtration funnel is not available), and saturated by 50 g of sodium chloride (50 g) while hot (it is necessary to keep temperature above 50 °C during saturation, so the beaker should be placed on a heating plate). Dye precipitate formed by salting-out is filtered off by vacuum filtration from hot solution (note: if the temperature of solution being filtered drops below 50 °C, sodium chloride partially co-precipitates with the dye). The dye is dried in a desiccator over CaCl2. Orange solid, yield 25 g.
The quality of dye can be controlled by the UV/Vis spectroscopy. In aqueous solution λmax 487 nm (logε = 4.87).
Under the name tropaeolin 000 the dye is used as an acid-base indicator in aqueous solutions. Guess in which region of pH this dye changes its color:
strongly acidic (pH<2);
acidic (pH 2 – 6.5);
neutral (pH 6.5 – 7.5);
mildly alkaline (pH 7.5 – 9);
strongly alkaline (pH 9 – 14).
Model Answer
mildly alkaline (pH 7.5 – 9). Explanation: The most apparent functional group in the dye molecule which can account for pH-dependent changes is a phenolic hydroxyl. Thus, the compound is a weak acid, which can be deprotonated under weakly alkaline conditions. Actually, pH-range in which the transformation takes place is within 7.5 – 8.5. As phenolate oxygen is a stronger donor than hydroxy-group, the color deepens upon the addition of a base (from yellow-orange to reddish).
Write the reaction equation which accounts for the color change.
Model Answer
The phenolic hydroxyl group of Acid Orange 7 undergoes deprotonation under basic conditions to yield the phenolate form, which shifts the color from yellow-orange to reddish:
[VISUAL]
Equation:
Acid Orange 7 (phenolic form) + OH- → Acid Orange 7 (phenolate form) + H2O
Write the reaction equation of an azocoupling required to obtain chrysoidine dye.
[VISUAL]
Model Answer
Synthesis of chrysoidine dye via azo-coupling of benzenediazonium chloride with m-phenylenediamine under weakly acidic conditions (pH 5-5.5):
[VISUAL]
Equation:
Benzenediazonium cation + m-phenylenediamine → Chrysoidine + H+
Which pH region should be chosen for this azocoupling:
strongly basic,
weakly basic,
weakly acidic,
strongly acidic?
Model Answer
weakly acidic. Explanation: The azo-coupling of diazonium salts with aromatic amines (such as m-phenylenediamine) is typically carried out under weakly/mildly acidic conditions (pH 5-5.5) to ensure sufficient concentration of the free amine nucleophile while preventing decomposition of the diazonium species.