organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...

, an azo coupling is an

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 rea ...

between a

diazonium compound 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 prop ...

() and another

aromatic compound Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past grouping ...

that produces 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 ...

(). In this

electrophilic aromatic substitution Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic ni ...

reaction, the aryldiazonium cation is the

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that ca ...

and the activated

arene Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past grouping ...

is a

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they a ...

. In most cases, including the examples below, the diazonium compound is also aromatic.

Diazotization

The process of conversion of primary aromatic amines into its diazonium salt is called

diazotization 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 ...

. Diazonium salts are important synthetic intermediates that can undergo coupling reactions to form azo dyes and electrophilic substitution reactions to introduce

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the res ...

Uses of the reaction

Aromatic azo compounds tend to be brightly colored due to the extended

conjugated systems Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb from its basic form *Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics *Complex conjugation, the change o ...

. Many are used as dyes (see

azo dye Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl and substituted aryl groups. They are a commercially important family of azo compounds, i.e. compounds containing the C- ...

). Important azo dyes include

methyl red Methyl red (2-(''N'',''N''-dimethyl-4-aminophenyl) azobenzenecarboxylic acid), 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. Methyl red is a pH indi ...

and pigment red 170. Azo printing exploits this reaction as well. Azo coupling is also used to produce

prontosil Prontosil is an antibacterial drug of the sulfonamide group. It has a relatively broad effect against gram-positive cocci but not against enterobacteria. One of the earliest antimicrobial drugs, it was widely used in the mid-20th century but is ...

and other

sulfa drugs Sulfonamide is a functional group (a part of a molecule) that is the basis of several groups of drugs, which are called sulphonamides, sulfa drugs or sulpha drugs. The original antibacterial sulfonamides are synthetic (nonantibiotic) antim ...

Examples of azo C-coupling reactions

Many procedures have been described.

Phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...

reacts with benzenediazonium chloride to give a Solvent Yellow 7, a yellow-orange

. The reaction is base-catalysed. : The related dye called aniline yellow is produced from the reaction of aniline and the diazonium salt. Naphthols are popular acceptors. One example is the synthesis of the dye "organol brown" from

aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile start ...

and

1-naphthol 1-Naphthol, or α-naphthol, is a fluorescent organic compound with the formula . It is a white solid. It is an isomer of 2-naphthol differing by the location of the hydroxyl group on the naphthalene ring. The naphthols are naphthalene homologues ...

: :

Similarly β-naphthol couples with phenyldiazonium

to produce an intense orange-red dye.

Examples of azo N-coupling reactions

In alkaline media, diazonium salt can react with most primary and secondary amines that exist as a free base and produce

triazene Triazene is an unsaturated inorganic compound having the chemical formula N3 H3. It has one double bond and is the second-simplest member of the azene class of hydronitrogen compounds, after diimide. Triazenes are a class of organic compoun ...

. This chemical reaction is called azo N-coupling or the synthesis of azoamines. PigmentYellow12corrected

References

{{DEFAULTSORT:Azo Coupling Substitution reactions Carbon-heteroatom bond forming reactions