Azo violet (Magneson I; p-nitrobenzeneazoresorcinol) is an

azo compound Azo compounds are organic compounds bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups). IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted ...

with the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...

C₁₂H₉N₃O₄. It is used commercially as a violet dye and experimentally as a

pH indicator A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, ...

, appearing yellow below pH 11, and violet above pH 13. It also turns deep blue in the presence of

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

salt in a slightly alkaline, or basic, environment. Azo violet may also be used to test for the presence of ammonium ions. The color of ammonium chloride or

ammonium hydroxide Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...

solution will vary depending upon the concentration of azo violet used. Magneson I is used to test Be also; it produces an orange-red lake with Be(II) in alkaline medium.

Properties

The intense color from which the compound gets its name results from irradiation and subsequent excitation and relaxation of the extended π electron system across the R-N=N-R' linked phenols. Absorption of these electrons falls in the visible region of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...

. Azo violet's intense

indigo Indigo is a deep color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine, based on the ancient dye of the same name. The word "indigo" comes from the Latin word ''indicum'', m ...

color (λ_max 432 nm) approximates

Pantone Pantone LLC (stylized as PANTONE) is a limited liability company headquartered in Carlstadt, New Jersey. The company is best known for its Pantone Matching System (PMS), a proprietary color space used in a variety of industries, notably graphi ...

R: 102 G: 15 B: 240.

Synthesis

Azo violet can be synthesised by reacting 4-nitroaniline with

nitrous acid Nitrous acid (molecular formula ) is a weak and monoprotic acid known only in solution, in the gas phase and in the form of nitrite () salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagent ...

(generated ''in situ'' with an acid and a nitrite

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

) to produce a

diazonium Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide. General properti ...

intermediate. This is then reacted with

resorcinol Resorcinol (or resorcin) is an organic compound with the formula C6H4(OH)2. It is one of three isomeric benzenediols, the 1,3-isomer (or '' meta''-isomer). Resorcinol crystallizes from benzene as colorless needles that are readily soluble i ...

, dissolved in a sodium hydroxide solution, via an

azo coupling In organic chemistry, an azo coupling is an organic reaction between a diazonium compound () and another aromatic compound that produces an azo compound (). In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the ...

reaction. :: Azo Violet Synthesis

This is consistent with the generalized strategy for preparing azo dyes. :: Generalized Azo Dye Synthesis

Reactivity

The chemical character of azo violet may be attributed to its azo group (-N=N-), six-membered rings, and hydroxyl side groups. Due to steric repulsions, azo violet is most stable in the ''trans''-configuration, but

isomerization In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomeriz ...

of azo dyes by irradiation is not uncommon. The ''para''-position

tautomerization Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hyd ...

of azo violet provides mechanical insight into the behavior of the compound in an acidic environment, and thus its use as a basic

. The predicted ¹H-NMR of pure azo violet shows the hydroxyl protons as the most deshielded and acidic protons. The participation of these hydroxyl groups' electron-donation to the conjugated π system likewise influences azo violet's λ_max and p''K''_a value. Pure Azo Violet NMR

References

{{reflist Azo dyes Nitrobenzenes Resorcinols PH indicators