The preparation of Azo Violet

Introduction

In the present post, I will describe the preparation of 4-[(E)-(4-nitrophenyl)diazenyl]benzene-1,3-diol, an interesting azo dye known under the common name of Magneson I or Azo violet. The compound’s synthesis starts with the preparation of the diazonium salt of p-nitroaniline. In the reaction, sodium nitrate is used under acidic conditions as a source of nitrosonium ions.

The produced nitrosonium ions are an electrophylic species that can attack the amine groups of p-nitroaniline leading to a nitrosilated aniline function. This nitrosilated intermediate is unstable and, through a series of proton transfer reactions, here shown as intermolecular steps for simplicity but in reality probabily involving the solvent, rearranges to form the desired diazonium salt.

Once prepared the diazonium salt can act as the electrophile in an electrophilic aromatic substitution reaction onto the very activated ring of the resorcinol molecule leading to the desired product.

Please notice how the substitution happens almost entirely on the site in positions 4 and 6 which are located in ortho to one of the hydroxy groups and in para to the other. Position 2, despite being in ortho to both hydroxy groups, is less favorable due to steric hindrance.

Experimental part

In a beacher, 1.66g of p-nitroaniline (12mmol, 1eq.) were dissolved in 20mL of 2M hydrochloric acid. The reaction mixture was cooled in an ice bath until its temperature dropped below 10°C. After that, 1g of potassium nitrite (11.7mmol, 0.98eq.) dissolved in a minimal amount of water was added dropwise to the reaction mixture monitoring that the temperature of the reaction did not exceed 5-10°C during the whole addition. The reaction mixture was left stirring for 15min and then 0.25g of urea (4.16mmol, 0.35eq.) was added to quench eventually remaining unreacted nitrosonium ions. 

To the obtained diazonium salt solution, 1.32g of resorcinol (11.99mmol, 1eq.) dissolved in 15mL of 2M sodium hydroxide were added dropwise under constant stirring. A red precipitate is immediately observed. The solution was removed from the ice bath and let stirring at room temperature for 1h. To the reaction mixture, 10mL of 2M hydrochloric acid were added and the solution was brought to a boil. The solution was then left cooling to allow the fine precipitate to partially recrystallize forming a coarser solid more easily recoverable by filtration. When completly cooled, the solid was collected using vacumm filtration, washed with water, and dried over calcium chloride. The obtained solid appears as a bright red fine powder.

The obtained reaction product
Solutions of azo violet in various conditions: neutral environment (left), alkaline solution (center), alkaline solution with magnesium ions (right)

The Magneson I molecule is a powerful azoic coloring compound and a few milligrams of dye are sufficient to strongly stain a liter of solution. Thanks to the weakly acidic phenolic protons, originally coming from resorcinol, azo violet can act as a pH indicator in strong alkaline environments changing its color from a pale yellow to a strong violet in the pH range going from 11 to 13. The compound can also be used as a qualitative test for magnesium ions since it can be adsorbed onto magnesium hydroxide forming a characteristic blue-stained fluffy/gelatinous precipitate. To run the test, the solution containing magnesium is treated with a few drops of a Magneson I solution and then brought to a stongly alkaline pH. The blue precipitate is immediately formed as a fine suspension that, in strong concentrations of magnesium ions, rapidly coalesces.

Lascia un commento

Il tuo indirizzo email non sarà pubblicato. I campi obbligatori sono contrassegnati *

Questo sito usa Akismet per ridurre lo spam. Scopri come i tuoi dati vengono elaborati.