Azo coupling

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In organic chemistry, an azo coupling is an reaction between a diazonium compound (R−N≡N+) and another aromatic compound that produces an azo compound (R−N=N−R’). In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile, and the activated carbon (usually from an arene, which is called coupling agent), serves as a nucleophile. Classical coupling agents are phenols and naphthols. [1] 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.

Contents

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). [2] 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. [3]

Prontosil, the first sulfa drug, 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 also be used to form or to modify proteins such as tRNA synthetase. [4]

Examples of azo C-coupling reactions

Illustrative is the reaction of phenol with benzenediazonium chloride to give a Solvent Yellow 7, a yellow-orange azo compound. The reaction is faster at high pH. [2] Many other azo dyes have been prepared by similarly. Several procedures have been described in detail. [5] [6] [7]

Azo-coupling-A-mechanism-2D-skeletal.png

Naphthols are popular coupling agents. One example is the synthesis of the dye "organol brown" from aniline and 1-naphthol:

Synthesis of organol brown from aniline and 1-naphthol Synthesis of Organol Brown N.svg
Synthesis of organol brown from aniline and 1-naphthol

Similarly, β-naphthol couples with phenyldiazonium electrophile to produce an intense orange-red dye.

Besides activated aromatic coupling agents, other nucleophilic carbons could also be coupled with diazonium salt:

Synthesis of C.I. Pigment Yellow 12, a diarylide pigment. PigmentYellow12corrected.png
Synthesis of C.I. Pigment Yellow 12, a diarylide pigment.

Examples of azo N-coupling reactions

Azo N-coupling under basic condition Azo N coupling.jpg
Azo N-coupling under basic condition

In alkaline media, diazonium salt can react with most primary and secondary amines, which exist as a free base to produce triazene. [8] This chemical reaction is called azo N-coupling, [9] or the synthesis of azoamines. [10]

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. [2]


Related Research Articles

<span class="mw-page-title-main">Amine</span> Chemical compounds and groups containing nitrogen with a lone pair (:N)

In chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Formally, amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives of ammonia are also called amines, such as monochloramine.

<span class="mw-page-title-main">Aniline</span> Organic compound (C₆H₅NH₂); simplest aromatic amine

Aniline is an organic compound with the formula C6H5NH2. 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 starting material for fine chemical synthesis. Its main use is in the manufacture of precursors to polyurethane, dyes, and other industrial chemicals. Like most volatile amines, it has the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds. It is toxic to humans.

<span class="mw-page-title-main">Nitrous acid</span> Chemical compound

Nitrous acid is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.

The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring. Friedel–Crafts reactions are of two main types: alkylation reactions and acylation reactions. Both proceed by electrophilic aromatic substitution.

<span class="mw-page-title-main">Imine</span> Organic compound or functional group containing a C=N bond

In organic chemistry, an imine is a functional group or organic compound containing a carbon–nitrogen double bond. The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bonds. Imines are common in synthetic and naturally occurring compounds and they participate in many reactions.

In organic chemistry, an aryl halide is an aromatic compound in which one or more hydrogen atoms, directly bonded to an aromatic ring are replaced by a halide. Haloarenes are different from haloalkanes because they exhibit many differences in methods of preparation and properties. The most important members are the aryl chlorides, but the class of compounds is so broad that there are many derivatives and applications.

<span class="mw-page-title-main">Azo compound</span> Organic compounds with a diazenyl group (–N=N–)

Azo compounds are organic compounds bearing the functional group diazenyl.

<span class="mw-page-title-main">Azo dye</span> Class of organic compounds used as 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 linkage. Azo dyes are synthetic dyes and do not occur naturally. Most azo dyes contain only one azo group but there are some that contain two or three azo groups, called "diazo dyes" and "triazo dyes" respectively. Azo dyes comprise 60-70% of all dyes used in food and textile industries. Azo dyes are widely used to treat textiles, leather articles, and some foods. Chemically related derivatives of azo dyes include azo pigments, which are insoluble in water and other solvents.

<span class="mw-page-title-main">Diazonium compound</span> Group of organonitrogen compounds

Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group [R−N+≡N]X 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 compound where R is hydrogen, is diazenylium.

<span class="mw-page-title-main">Thiophenol</span> Chemical compound

Thiophenol is an organosulfur compound with the formula C6H5SH, sometimes abbreviated as PhSH. This foul-smelling colorless liquid is the simplest aromatic thiol. The chemical structures of thiophenol and its derivatives are analogous to phenols. An exception is the oxygen atom in the hydroxyl group (-OH) bonded to the aromatic ring is replaced by a sulfur atom. The prefix thio- implies a sulfur-containing compound and when used before a root word name for a compound which would normally contain an oxygen atom, in the case of 'thiol' that the alcohol oxygen atom is replaced by a sulfur atom.

<span class="mw-page-title-main">Palladium on carbon</span> Chemical compound

Palladium on carbon, often referred to as Pd/C, is a form of palladium used as a catalyst. The metal is supported on activated carbon to maximize its surface area and activity.

<span class="mw-page-title-main">Triazenes</span> Organic compounds with a diazoamino group

Triazenes are organic compounds that contain the functional group R1−N=N−NR2R3, where the R are each any of various types of substituent groups. Some anti-cancer medications and dyes are triazenes. Formally, the triazenes are related to the unstable chemical triazene, H2N−N=NH.

The reduction of nitro compounds are chemical reactions of wide interest in organic chemistry. The conversion can be effected by many reagents. The nitro group was one of the first functional groups to be reduced. Alkyl and aryl nitro compounds behave differently. Most useful is the reduction of aryl nitro compounds.

<span class="mw-page-title-main">2-Nitroaniline</span> Chemical compound

2-Nitroaniline is an organic compound with the formula H2NC6H4NO2. It is a derivative of aniline, carrying a nitro functional group in position 2. It is mainly used as a precursor to o-phenylenediamine.

<span class="mw-page-title-main">4-Nitroaniline</span> Chemical compound

4-Nitroaniline, p-nitroaniline or 1-amino-4-nitrobenzene is an organic compound with the formula C6H6N2O2. A yellow solid, it is one of three isomers of nitroaniline. It is an intermediate in the production of dyes, antioxidants, pharmaceuticals, gasoline, gum inhibitors, poultry medicines, and as a corrosion inhibitor.

<span class="mw-page-title-main">2-Naphthol</span> Chemical compound

2-Naphthol, or β-naphthol, is a fluorescent colorless (or occasionally yellow) crystalline solid with the formula C10H7OH. It is an isomer of 1-naphthol, differing by the location of the hydroxyl group on the naphthalene ring. The naphthols are naphthalene homologues of phenol, but more reactive. Both isomers are soluble in simple alcohols, ethers, and chloroform. 2-Naphthol is a widely used intermediate for the production of dyes and other compounds.

<span class="mw-page-title-main">Benzenediazonium tetrafluoroborate</span> Organic compound containing an –N≡N+ function

Benzenediazonium tetrafluoroborate is an organic compound with the formula [C6H5N2]BF4. It is a salt of a diazonium cation and tetrafluoroborate. It exists as a colourless solid that is soluble in polar solvents. It is the parent member of the aryldiazonium compounds, which are widely used in organic chemistry.

1-Naphthol, or α-naphthol, is a organic compound with the formula C10H7OH. 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 homologues of phenol. Both isomers are soluble in simple organic solvents. They are precursors to a variety of useful compounds.

<span class="mw-page-title-main">Acetoacetanilide</span> Chemical compound

Acetoacetanilide is an organic compound with the formula CH3C(O)CH2C(O)NHC6H5. It is the acetoacetamide derivative of aniline. It is a white solid that is poorly soluble in water. This chemical and many related compounds (prepared from various aniline derivatives) are used in the production of organic pigments called arylide yellows. Acetoacetanilides usually exist as the keto-amide tautomer according to X-ray crystallography.

<span class="mw-page-title-main">3-Hydroxy-2-naphthoic acid</span> Chemical compound

3-Hydroxy-2-naphthoic acid is an organic compound with the formula C10H6(OH)(CO2H). It is one of the several hydroxynaphthoic acids. It is a precursor to some azo dyes and pigments. It is prepared by carboxylation of 2-naphthol by the Kolbe–Schmitt reaction.

References

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