The Synthesis of Methyl Red

Written by Teresa

Introduction

Methyl red is a type of dye that can be used as a pH indicator similar to the use of phenyphthalein. The purpose of this laboratory was to synthesize methyl red using a two-step synthesis. This is an important synthesis due to the use of two important reactions: changing a carboxylic acid by converting the amino group and then a diazocoupling reaction (two molecules are coupled by an azo bridge) through a pseudo electrophilic aromatic substitution. In the second reaction, the product of the first reaction is used as the electrophile. The end result of these two reactions in this laboratory is the azo dye methyl red. Methyl red is used in many ways in society including food chemistry as well as in the textile industry when making chemical dyes. These are important laboratory reactions to understand as recrystallization is used in many laboratories in organic chemistry, as seen this semester. Being able to perform recrystallizations and these two reactions are essential basic skills required in many reactions in many laboratories everywhere.

Observations

When the water was first added to the anthranilic acid, it looked like the color and consistency of plain oatmeal with bubbles and foam on the surface. When this mixture stopped swirling, a light tan solid formed at the bottom of the flask. As the HCl was added dropwise, the solution turned a medium and darker red-brown. The solution then turned back to a light peach solid with cooling in the ice bath. With stirring and the addition of the sodium nitrite solution, the solid turned into a clear, light amber liquid. As the N,N-dimethylanaline was added, the solution turned a deep, clear orange, and then turned clear red as the stirring apparatus was turned on. The red deepened after stirring for five minutes, and then with the addition of sodium acetate, it turned dark and opaque with a red-orange residue on the sides of the flask. It looked like thinner cough syrup. After the NaOH was added, it looked like grape juice. The final compound that was collected after the vacuum filtration was brick red and pasty.

Results

Table 1: Measurements/Calculations

Theoretical Yield: 2.1544 g

Conclusion

This laboratory taught the students that recrystallization is an essential basic skill for laboratory experiments when synthesizing different products. Recrystallization will need to be remembered for future laboratory classes as well as for when the students have graduated and perhaps work in a laboratory. This laboratory isn’t finished yet, and there will be steps next week in order to determine if methyl red was synthesized.

Editor’s note (October 27, 2026): The student’s measurements and text are left as written. Two things need correcting.

• The theoretical yield was calculated from the wrong reagent. The 2.1544 g figure is 0.008 mol × 269.30 g/mol, the molar mass of methyl red (Merck), so it matches the sodium acetate row of Table 1. Sodium acetate is added after the N,N-dimethylaniline to lower the acidity of the mixture so that coupling goes faster (Organic Syntheses), and it is not part of the methyl red molecule, so it does not set the theoretical yield. Anthranilic acid, sodium nitrite and N,N-dimethylaniline combine one to one to make methyl red, so the limiting reagent is the one with the fewest moles. In Table 1 that is anthranilic acid (0.0474 mol, against 0.0522 mol of sodium nitrite and 0.0711 mol of N,N-dimethylaniline), assuming enough HCl was used, which the post does not record. The theoretical yield is 0.0474 mol × 269.30 g/mol ≈ 12.8 g, not 2.15 g.
• No percent yield can be calculated. The post does not record the mass of the dried product, only that it was “brick red and pasty,” and the Conclusion says the lab was not finished. With a product mass, percent yield = (actual mass ÷ 12.8 g) × 100.

How the two steps work

First, anthranilic acid is dissolved in cold hydrochloric acid and sodium nitrite is added. The acid turns the nitrite into nitrous acid (HNO₂), which converts the amino group (–NH₂) into a diazonium salt; the carboxylic acid group is unchanged. This is diazotization, and it is done in an ice bath (0–5°C) because diazonium chlorides decompose rapidly at room temperature (LibreTexts). Second, the diazonium ion is the electrophile: it attacks the electron-rich ring of N,N-dimethylaniline, mainly para to the dimethylamino group (LibreTexts), and the two rings are joined by an azo bridge (–N=N–). The product is methyl red, 2-[4-(dimethylamino)phenylazo]benzoic acid.

As an indicator, methyl red is red below about pH 4.4, orange in between and yellow above about pH 6.2 (UT Austin); Merck’s specification gives 4.4 to 6.0 (Merck). For a related azo dye made the same way, see Synthesis of and Fabric Test for Methyl Orange, and for a worked recrystallization, see Purification of an Unknown Solid by Recrystallization.