Concept 04 / 04Amines
Diazonium Salts: Preparation, Stability and Synthetic Reactions
Preparation of arenediazonium salts by diazotisation, their stability and properties, reactions with loss of nitrogen (Sandmeyer and Gattermann reactions, iodide, fluoride by Balz-Schiemann, reduction to arenes, hydrolysis to phenols, nitro compounds) and coupling reactions retaining nitrogen to give azo dyes, and their importance in synthesis.
Diazonium salts have the general formula (benzenediazonium chloride, ). Aliphatic diazonium salts are too unstable to exist; arenediazonium salts are stable at low temperature because the positive charge is delocalised by resonance with the ring.
Preparation (diazotisation). Aniline is treated with sodium nitrite and hydrochloric acid at to :
.
The salt is used immediately in solution; it decomposes on warming, and the dry salt can be explosive (the fluoroborate is an exception: stable at room temperature).
Reactions with displacement of nitrogen (see figure). is an excellent leaving group, so it is replaced by many groups:
Sandmeyer reaction: ; ; .
Gattermann reaction: copper powder with HCl or HBr gives ArCl or ArBr.
Iodide: (no catalyst needed).
Fluoride (Balz-Schiemann): fluoroboric acid precipitates arenediazonium fluoroborate, which on heating gives ArF: .
Replacement by H: mild reducing agents (hypophosphorous acid , or ethanol) reduce the salt to the arene: . This removes an amino group after it has served as a directing group.
Replacement by OH: warming the aqueous solution gives phenol: .
Replacement by : heating the fluoroborate with aqueous and copper powder gives nitrobenzene.
Reactions retaining nitrogen: coupling. The diazonium ion is a weak electrophile that attacks strongly activated rings (phenols, aromatic amines) at the para position, forming azo compounds () with an extended conjugated system; they are brightly coloured and used as dyes.
With phenol in mildly alkaline solution (phenoxide ion): p-hydroxyazobenzene (orange dye).
With aniline in mildly acidic solution: p-aminoazobenzene (yellow dye).
Methyl orange is made by coupling diazotised sulfanilic acid with N,N-dimethylaniline.
Importance in synthesis. Diazonium salts introduce groups (F, I, CN, OH, H) that are difficult to introduce directly by electrophilic substitution, and give substitution patterns otherwise impossible (such as 1,3,5-tribromobenzene from aniline: tribrominate, diazotise, reduce with ).
Key Takeaways:
- Diazotisation: + HCl at to ; arenediazonium salts are resonance-stabilised.
- Sandmeyer (, CuCN), Gattermann (Cu/HX), KI (ArI), (ArF), (ArH), (ArOH).
- Coupling at para: phenol (alkaline) → orange; aniline (acidic) → yellow azo dye.
- Diazonium chemistry allows otherwise inaccessible substitution patterns.
- Pitfall: iodobenzene needs only KI; chloro and bromo derivatives need copper(I) salts or copper powder.
Brain-Teaser 1: How would you convert aniline into (a) benzonitrile, (b) fluorobenzene, (c) benzene?
Solution: Diazotise first (/HCl, to ). Then: (a) CuCN/KCN (Sandmeyer); (b) , isolate the fluoroborate and heat (Balz-Schiemann); (c) (or ethanol).
Brain-Teaser 2: Synthesise 1,3,5-tribromobenzene from aniline.
Solution: Bromine water converts aniline to 2,4,6-tribromoaniline (the directs all three Br to ortho and para positions). Diazotise (/HCl, cold), then reduce with to remove the diazonium group: 1,3,5-tribromobenzene, a meta pattern impossible to obtain by direct bromination of benzene.
Brain-Teaser 3: Why is coupling with phenol carried out in mildly alkaline and with aniline in mildly acidic solution?
Solution: In mildly alkaline solution, phenol is converted to the more strongly activated phenoxide ion, which couples readily; strongly alkaline solution would convert the diazonium ion into unreactive diazotate. With aniline, mild acid keeps the diazonium ion stable while leaving enough free aniline (not protonated) to couple; strong acid would protonate all the aniline.
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