Azo coupling
Encyclopedia
An azo coupling is an organic reaction
between a diazonium compound
and a dialkylaniline (C6H5NR2), phenol
or other aromatic compound which produces an azo compound
. In this reaction the diazonium salt is an electrophile
and the activated
arene
is a nucleophile
in an electrophilic aromatic substitution
. In most cases, including the example below, the diazonium compound is also aromatic. The product will absorb longer wavelengths of light than the reactants because of increased conjugation. Aromatic azo compounds tend to be brightly colored due to the extended conjugated systems; many are used as dyes.
Azo couplings are important in the production of dye
s and pH indicator
s such as methyl red
and pigment red 170
. Prontosil
and other sulfa drugs are also produced using this reaction.
An example is the synthesis of the dye organol brown from aniline
and 1-naphthol
:
Organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...
between a diazonium compound
Diazonium compound
Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group R-N2+ X- where R can be any organic residue such alkyl or aryl and X is an inorganic or organic anion such as a halogen...
and a dialkylaniline (C6H5NR2), phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
or other aromatic compound which produces an azo compound
Azo compound
Azo compounds are compounds bearing the functional group R-N=N-R', in which R and R' can be either aryl or alkyl. IUPAC defines azo compounds as: "Derivatives of diazene , HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. PhN=NPh azobenzene or diphenyldiazene." The more...
. In this reaction the diazonium salt is an electrophile
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
and the activated
Activating group
In organic chemistry, a functional group is called an activating group if a benzene molecule to which it is attached more readily participates in electrophilic substitution reactions...
arene
Arene
Arene or Arênê or Arène may refer to:*an aromatic hydrocarbon*Arene , a genus of marine snails in the family Areneidae*Arene , the wife of Aphareus and mother of Idas and Lynceus in Greek mythology...
is a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
in an electrophilic aromatic substitution
Electrophilic aromatic substitution
Electrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...
. In most cases, including the example below, the diazonium compound is also aromatic. The product will absorb longer wavelengths of light than the reactants because of increased conjugation. Aromatic azo compounds tend to be brightly colored due to the extended conjugated systems; many are used as dyes.
Azo couplings are important in the production of dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....
s and pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...
s such as methyl red
Methyl red
Methyl red, also called C.I. Acid Red 2, is an indicator dye that turns red in acidic solutions. It is an azo dye, and is a dark red crystalline powder....
and pigment red 170
Pigment Red 170
Naphthol Red is an organic pigment extensively used in automotive coatings and painting.It is produced synthetically by converting p-aminobenzamide into the corresponding diazonium compound followed by diazotation with 3-hydroxy-2-naphththoic acid anilide.In the solid state the hydrazo tautomer...
. Prontosil
Prontosil
Prontosil, the first commercially available antibacterial antibiotic , was developed by a research team at the Bayer Laboratories of the IG Farben conglomerate in Germany...
and other sulfa drugs are also produced using this reaction.
An example is the synthesis of the dye organol brown from aniline
Aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...
and 1-naphthol
1-Naphthol
1-Naphthol, or α-naphthol, is a colorless crystalline solid with the formula C10H7OH. It is an isomer of 2-naphthol differing by the location of the hydroxyl group on naphthalene. The naphthols are naphthalene homologues of phenol, with the hydroxyl group being more reactive than in the phenols....
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