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 matter, organic materials, i.e., matter in its various forms that contain ...

, an azo coupling is an

reaction Reaction may refer to a process or to a response to an action, event, or exposure. Physics and chemistry *Chemical reaction *Nuclear reaction *Reaction (physics), as defined by Newton's third law * Chain reaction (disambiguation) Biology and ...

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. The parent, compou ...

() and another

aromatic compound Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...

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 (SEAr) is an organic reaction in which an atom that is attached to an aromatic ring, aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitut ...

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 Electric charge, charged, have an ...

, and the activated carbon (usually from an

arene Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...

, which is called coupling agent), serves as 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 are ...

. Classical coupling agents are phenols and naphthols. Usually the diazonium reagent attacks at the para position of the coupling agent. When the para position is occupied, coupling occurs at a ortho position, albeit at a slower rate.

Uses of the reaction

Aromatic azo compounds tend to be brightly colored due to their extended conjugated systems. Many are useful 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−N=N−C l ...

). Important azo dyes include methyl red and pigment red 170. Azo printing exploits this reaction as well. In this case, the diazonium ion is degraded by light, leaving a latent image in undegraded diazonium salt which is made to react with a phenol, producing a colored image: the

blueprint A blueprint is a reproduction of a technical drawing or engineering drawing using a contact print process on light-sensitive sheets introduced by Sir John Herschel in 1842. The process allowed rapid and accurate production of an unlimited number ...

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 ...

, the first

sulfa drug 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 antimicrobial agents th ...

, was once produced by azo coupling. The azo compound is a prodrug that is activated in-vivo to produce the sufanilamide. The reaction is also used in the Pauly reaction test to detect tyrosine or histidine residues in proteins. Additionally, through the azo coupling reaction between the aromatic diazonium ion and aromatic amino acid residues, this reaction can also be used to form or to modify proteins such as tRNA synthetase.

Examples of azo C-coupling reactions

Illustrative is the reaction of

phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...

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

. The reaction is faster at high pH. Many other azo dyes have been prepared by similarly. Several procedures have been described in detail. : Naphthols are used as coupling agents. One example is the synthesis of the dye organol brown (4) from

aniline Aniline (From , meaning ' indigo shrub', and ''-ine'' indicating a derived substance) is an organic compound with the formula . Consisting of a phenyl group () attached to an amino group (), aniline is the simplest aromatic amine. It is an in ...

(1) and

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

(3): :

Similarly,

β-naphthol 2-Naphthol, or β-naphthol, is a fluorescence, fluorescent colorless (or occasionally yellow) crystalline solid with the formula C10H7OH. It is an isomer of 1-Naphthol, 1-naphthol, differing by the location of the hydroxide, hydroxyl group on th ...

couples with

phenyldiazonium Benzenediazonium tetrafluoroborate is an organic compound with the chemical formula, formula 6H5N2F4. It is a salt of a diazonium compound, diazonium cation and tetrafluoroborate. It exists as a colourless solid that is soluble in polar solve ...

to produce an intense orange-red dye. Besides activated aromatic coupling agents, other nucleophilic carbons also couple with diazonium salts as illustrated by the synthesis of azo

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

s. :

Examples of azo N-coupling reactions

In alkaline media, diazonium salt can react with most primary and secondary amines, which exist as a free base to produce triazene. This chemical reaction is called azo N-coupling, or the synthesis of azoamines. : Azo N coupling

The dye called aniline yellow is produced by the reaction of aniline and a diazonium salt. In this case the C- and N-coupling compete.

References

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