DIAZO COMPOUNDS
Diazo compounds contain the N=N group. Their general structure is R-N=N-R’. Usually both R and R’ are aromatic groups, which help stabilize the azo group by delocalization. These compounds are generally formed by a coupling reaction between a diazonium salt and a coupling reagent.
DIAZONIUM SALTS
Diazonium salts are made by adding cold sodium nitrite solution to an aromatic amine dissolved in dilute acid at temperatures below 5°C. This process is called diazotisation. The diazonium salt formed is unstable and must be used immediately.
The acid reacts with sodium nitrite to form nitrous acid, which further reacts with the aromatic amine to generate the diazonium ion.
Mechanism of Diazotisation
- Formation of nitrosonium ion (NO+) Nitrous acid loses water to form the reactive nitrosonium ion.
- Attack of NO+ on aromatic amine
- Loss of proton
- Tautomerisation
- Loss of water to form diazonium ion
STABILITY OF DIAZONIUM IONS
- Aromatic diazonium salts are much more stable than aliphatic ones because the benzene ring stabilizes the N≡N+ group through electron donation.
- If the temperature rises above 5°C, benzenediazonium chloride decomposes to form phenol and nitrogen gas.
- Aromatic rings stabilize the diazonium ion by resonance.
COUPLING REACTION
A coupling agent is usually a benzene ring compound such as phenol or an aromatic amine. When a diazonium salt reacts with a coupling reagent, it forms a coloured azo compound. These reactions are widely used in dye manufacture.
Examples
- Benzenediazonium salt + alkaline phenol → Yellow-orange azo dye
- Benzenediazonium salt + alkaline naphthol → Red azo dye
- Benzenediazonium salt + aniline (phenylamine) → Yellow azo dye
Mechanism of Coupling Reaction
The coupling reagent attacks the diazonium ion (acting as an electrophile), followed by loss of a proton. A trans-azo compound is usually formed.
USES OF DIAZO COMPOUNDS
- Used in dye and pigment industry.
- Used in photocopying and document reproduction.
- Serve as intermediates for synthesizing a variety of aromatic compounds.
- Provide a method to prepare aryl halides (I, Br, Cl, F) and nitriles.
- Useful for introducing groups like –F, –Br, –Cl, –I, –NO₂, –OH and –CN into aromatic rings.
AZO DYES
Azo dyes contain two aromatic rings connected by an N=N double bond. These dyes are stable and show bright colours such as yellow, orange, red, brown and blue.
Why azo dyes are coloured?
- They absorb visible light leading to electronic excitation.
- The colour comes from chromophores (N=N, nitro, carbonyl groups).
- Auxochromes like –OH, –NH₂, –SO₃⁻ intensify colour.
Examples
Aniline Yellow, Orange G, Xylidine, Alizarin Yellow R, Bismark Brown Y, Congo Red.
Detailed Notes:
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PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/ DIAZOTISATION AND COUPLING.