1. Cyanides and isocyanides
Nitriles R–C≡N (alkanenitriles; CH₃CN ethanenitrile) and isocyanides R–N≡C (isocyanoalkanes): functional isomers.
Cyanides: RX + ethanolic KCN (ionic; C attacks; +1 C); amide + P₂O₅, heat. Isocyanides: RX + AgCN (covalent; N attacks); 1° amine + CHCl₃ + alcoholic KOH (carbylamine).
| Property | Nitrile | Isocyanide |
|---|---|---|
| Smell | Pleasant | Foul |
| Hydrolysis (H⁺) | RCOOH + NH₄⁺ | RNH₂ + HCOOH |
| Reduction | RCH₂NH₂ (1°) | RNHCH₃ (2°) |
| Heating | Stable | Isomerises to nitrile |
| Reagent with RX | Product |
|---|---|
| KCN | Nitrile |
| AgCN | Isocyanide |
Formulas
- Isocyanide from AgCN
Key terms
- Nitrile
A compound R–C≡N, named as an alkanenitrile.
- Isocyanide
A compound R–N≡C, isomeric with the nitrile.
2. Preparation and stability of diazonium salts
Diazotisation: at 273–278 K (ice); HNO₂ formed in situ; excess HNO₂ turns starch–iodide paper blue. Aryl diazonium ions are resonance-stabilised but decompose on warming (→ phenol + N₂); dry salts are explosive; alkyl diazonium ions decompose at once.
| Condition | Result |
|---|---|
| 273–278 K | Stable in solution |
| Warm | Phenol + N₂ |
| Dry | Explosive |
Formulas
- Decomposition on warming
Key terms
- Diazotisation
Conversion of a primary aromatic amine into a diazonium salt with nitrous acid in the cold.
3. Replacement reactions
| Reagent | Product |
|---|---|
| CuCl/HCl (Sandmeyer) | ArCl |
| CuBr/HBr (Sandmeyer) | ArBr |
| CuCN/KCN (Sandmeyer) | ArCN |
| Cu powder + HCl/HBr (Gattermann) | ArCl/ArBr |
| KI, warm | ArI |
| HBF₄; heat (Balz–Schiemann) | ArF |
| H₃PO₂ or CH₃CH₂OH | ArH |
| H₂O, warm | ArOH |
| ArN₂BF₄ + NaNO₂, Cu, heat | ArNO₂ |
In each case N₂ is evolved and the new group occupies the position of –N₂⁺.
| Group | Reagent |
|---|---|
| –F | HBF₄, heat |
| –I | KI |
| –H | H₃PO₂ |
| –CN | CuCN |
Formulas
- Balz–Schiemann
Key terms
- Sandmeyer reaction
Replacement of –N₂⁺ by Cl, Br or CN using copper(I) salts.
- Balz–Schiemann reaction
Preparation of aryl fluorides by heating arenediazonium fluoroborates.
4. Coupling and synthetic importance
Coupling (para): phenol in mildly alkaline solution → 4-hydroxyazobenzene (orange); 2-naphthol → orange-red dye (test for ArNH₂); aniline or N,N-dimethylaniline in mildly acidic solution → 4-aminoazobenzene (yellow) or butter yellow. Colour from the conjugated –N=N– system.
Importance: introduces F, I, CN, OH not possible directly; –NH₂ used as a removable director (aniline → 2,4,6-tribromoaniline → diazotise → H₃PO₂ → 1,3,5-tribromobenzene); dyes.
| Partner | pH | Product |
|---|---|---|
| Phenol | Alkaline | 4-Hydroxyazobenzene |
| Aniline | Mildly acidic | 4-Aminoazobenzene |
Formulas
- Coupling with aniline
Key terms
- Azo coupling
Electrophilic substitution of an activated arene by a diazonium ion to give Ar–N=N–Ar′.
Exam tips
Common mistakes
- AgCN giving nitriles (it gives isocyanides).
- Isocyanide hydrolysis giving a carboxylic acid with the same carbons.
- Diazotising at room temperature.
- Coupling phenol in acid or aniline in alkali.
How marks are usually awarded
- Replacement reactions (1 mark each): reagent, condition, product.
- Syntheses (1 mark per step): every intermediate named.
- Coupling (2 marks): equation with pH and product colour.