Prepare a pure sample of Methyl red from Anthranilic acid

Theory:

Methyl red is a dye that is used as an indicator in volumetric analysis. It is prepared from anthranilic acid by first creating a diazonium compound. This is done by treating anthranilic acid with sodium nitrite (NaNO2 ) and hydrochloric acid at a temperature between 0-5 degrees Celsius. The diazonium compound is then coupled with N, N-dimethyl aniline. The reaction mixture is then treated with a sodium acetate solution, stirred, and allowed to stand overnight. It is then decomposed with a 40% NaOH solution. The solid that forms are separated by filtration, washed with water, followed by 10% acetic acid, and then crystallized from alcohol.

Materials Required:

The materials required for preparing methyl red include:

• Anthranilic acid (6.5g)
• Sodium nitrite (3.5g)
• N-dimethyl aniline (8.5ml)
• Concentrated HCl 
• Sodium acetate (6.8g)
• 40% NaOH solution (2.5ml)
• Methyl alcohol (40g)

Chemical Equation:

The chemical equation for the production of methyl red from anthranilic acid by creating a diazonium compound and coupling it with N, N-dimethyl aniline is as follows:

• Diazonium compound formation:

C 7 H 7 NO 2 + NaNO 2 + HCl → C 7 H 7 N 2 Cl + NaNO 3 + H 2 O 

• Coupling with N, N-dimethyl aniline:

C 7 H 7 N 2 Cl + N,N-dimethyl aniline → C 14 H 14 N 2 O 2  + HCl

• Decomposition with sodium hydroxide:

C 14 H 14 N 2 O 2  + NaOH → C 14 H 14 N 2 O 2  + NaCl + H 2 O 

In this case, methyl red is not an acid or a base, it is a neutral compound, so it does not react with the base (NaOH) to produce any new products. Methyl Red acts as a spectator in this reaction and it remains unchanged.

• Filtration and crystallization:

C 14 H 14 N 2 O 2 + alcohol → Methyl red (crystals)

Procedure:

1. In a beaker, dissolve 6.5 g of crystallized anthranilic acid in 5 ml of concentrated HCl and 5 ml of water. Filter the solution to remove any suspended impurities.
2. Transfer the solution to a 250 ml beaker immersed in an ice bath, and add 20 g of crushed ice and 8.0 ml of concentrated HCl to it.
3. In a separate beaker, dissolve 3.5 g of NaNO2 in 10 ml of ice-cold water, then add it to the anthranilic acid solution.
4. Slowly add 8.5 g (8.9 ml) of dimethyl aniline to the mixture with vigorous stirring, keeping the temperature below 5°C.
5. In a separate container, dissolve 6.8 g of sodium acetate in 12 ml of water, then add 6.0 ml of this solution to the reaction mixture.
6. Continue stirring and allow the temperature to rise above 25°C. Then add 2.5 ml of 40% NaOH solution to precipitate the product.
7. Separate the solid formed by filtration through a Buchner funnel, and wash with water followed by 10% acetic acid.
8. Crystallize from methyl alcohol. Separate the crystals by filtration and dry between filter paper and finally in an air oven at 100-110 degrees Celsius.

Result:

• Yield: 7.5 g
• Melting point: 182 degrees Celsius

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