Coupling Reactions & Azo Dyes

CHEDiazonium Salts & Reactions

Coupling joins a diazonium salt to an electron-rich aromatic (phenol or an aromatic amine) through a $\text{-}N=N\text{-}$ azo bridge, producing intensely coloured azo dyes. ISC expects the products, the colours, and the pH conditions that favour coupling at the para position.

Coupling with phenol (mild alkali)

C_6H_5N_2^+Cl^- + C_6H_5OH \xrightarrow{NaOH} p\text{-}HO\text{-}C_6H_4\text{-}N{=}N\text{-}C_6H_5

product is

p

-hydroxyazobenzene, an orange dye

Coupling with aniline (mild acid)

C_6H_5N_2^+Cl^- + C_6H_5NH_2 \rightarrow p\text{-}H_2N\text{-}C_6H_4\text{-}N{=}N\text{-}C_6H_5

product is

p

-aminoazobenzene, a yellow dye

Coupling with phenol (in mild alkali, $NaOH$ ) gives $p$ -hydroxyazobenzene, an orange dye, with the new $\text{-}N=N\text{-}$ bond at the $para$ position.
Coupling with aniline (in mild acid) gives $p$ -aminoazobenzene, a yellow dye.
The colour comes from the extended conjugation of the azo $\text{-}N=N\text{-}$ chromophore linking the two aromatic rings.
Coupling is electrophilic aromatic substitution: the diazonium ion is a weak electrophile, so it attacks only highly activated rings (phenols, amines).
pH matters: phenols couple best in mildly alkaline medium (forms the more reactive, more nucleophilic phenoxide ion); amines couple best in mildly acidic medium; strong acid/alkali suppresses coupling.
In strongly acidic medium phenol is not converted to phenoxide and the amine is protonated to $\text{-}NH_3^+$ (deactivated); in strongly alkaline medium the diazonium ion is converted to an unreactive diazohydroxide/diazotate.
Substitution prefers the $para$ position because it is least hindered; coupling goes $ortho$ only when $para$ is blocked.
Coupling with sulphanilic acid and N,N-dimethylaniline gives methyl orange, an azo indicator.
Coupling is a key colour test and the industrial basis of azo dyes used for textiles and indicators.

Where the marks go

Swapping the colours: phenol coupling gives orange $p$ -hydroxyazobenzene; aniline coupling gives yellow $p$ -aminoazobenzene.
Reversing the pH conditions — phenol couples in mild alkali (phenoxide), amine couples in mild acid; using strong acid or strong alkali stops the coupling.
Drawing the link as $\text{-}N\text{-}N\text{-}$ (single bond) rather than the conjugated azo $\text{-}N=N\text{-}$ that gives the colour.
Saying coupling works on benzene or simple arenes — the diazonium ion is a weak electrophile and needs a strongly activated ring (phenol or amine).
Placing the new azo bond ortho by default — it goes para unless the para position is already occupied.

How the board asks it

Predict the productcoupling with phenol in mild alkali giving p-hydroxyazobenzene
When benzenediazonium chloride is treated with phenol in the presence of dilute $NaOH$ , name and draw the structure of the coloured product formed, and state its colour.
Give reasonsphenoxide formation versus amine protonation under different pH
Account for the fact that the coupling of a diazonium salt with phenol is carried out in mildly alkaline medium, whereas its coupling with aniline is carried out in mildly acidic medium.
Conversiondiazotisation followed by coupling as a route to azo dyes
How will you convert aniline into $p$ -hydroxyazobenzene? Give the equations involved.
Identify / classifysulphanilic acid with N,N-dimethylaniline giving methyl orange
An azo dye used as an acid-base indicator is obtained by coupling diazotised sulphanilic acid with N,N-dimethylaniline. Identify the dye and give one use of azo dyes.
Give reasonsextended conjugation of the azo chromophore as the source of colour
Give reasons why azo compounds such as $p$ -aminoazobenzene are intensely coloured, whereas aniline itself is colourless.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.