Bondline / Reactions

Azo coupling

The diazonium ion is a weak electrophile, so it needs the very electron-rich phenoxide ion. Attack at the 4-position, then loss of a proton, gives a yellow-orange azo dye.

Aromatic substitution · 2 steps

Conditions

Benzenediazonium chloride added to phenol in cold NaOH(aq), below 10 °C.

Mechanism in 2 steps

Red curly arrows show where each pair of electrons moves.

O−N+NNNHOHO−++
Step 1Electrons flow from the O⁻ through the ring; the 4-position attacks the end nitrogen of the diazonium ion.
NNHOHO−NNO−H2OOH⁻++
Step 2Hydroxide removes the proton and the ring becomes aromatic again.

About aromatic substitution reactions

Benzene keeps its stable aromatic ring by substituting rather than adding. An electrophile attacks, then a proton is lost to restore aromaticity.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.

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