Cyanides & Isocyanides
Organic Chemistry – WBJEE Study Notes
Cyanides (R–C≡N) and isocyanides (R–NC) are isomeric functional groups that play pivotal roles in synthetic chemistry. While cyanides are widely known as toxic reagents, they are also versatile building blocks for nitriles, amides, and heterocycles. Isocyanides, on the other hand, are key players in multicomponent reactions such as the Ugi and Passerini reactions.
1. Simple Explanation
| Feature | Cyanide (R–C≡N) | Isocyanide (R–NC) |
|---|---|---|
| Structure | Terminal nitrile: carbon triple‑bonded to nitrogen. | Terminal isocyanide: nitrogen bound to carbon triple‑bonded to the R‑group. |
| Electronegativity | N is more electronegative → nitrile carbon is electrophilic. | C is more electronegative → isocyanide nitrogen is nucleophilic. |
| Typical Reactions | Nucleophilic addition to electrophiles (e.g. alkyl halides → nitriles). Conversion to amides, acids, alcohols. | Nucleophilic addition to electrophiles → multicomponent reactions (Ugi, Passerini). Metal‑catalyzed additions. |
| Uses | Synthesis of nitriles (precursors to amides, carboxylic acids, amines). Organic synthesis, organometallic chemistry. | Multicomponent “one‑pot” synthesis of peptidomimetics, natural products, pharmaceuticals. |
| Safety | Highly toxic (especially HCN). Handle under fume hood, use proper PPE. | Less toxic than HCN but still hazardous (irritant, strong odor). Handle in fume hood. |
2. Key Points
2.1 Nomenclature
| Generic Formula | Common Name | Systematic Name |
|---|---|---|
| R–C≡N | Cyanide | R‑cyanide (or R‑acetonitrile if R = CH₃) |
| R–NC | Isocyanide | R‑isocyanide |
- Use acetonitrile for CH₃CN, phenylisocyanide for PhNC, etc.
- Avoid confusing “cyanide” with the ion CN⁻.
2.2 Preparation of Cyanides
| Method | Typical Conditions | Example |
|---|---|---|
| Halide + NaCN (S_N2) | Alkyl halide, NaCN, dry solvent (DMF, DMSO) | Cl‑CH₂–CH₃ + NaCN → CH₃CH₂C≡N |
| Halide + KCN (S_N2) | Same as above, KCN is more soluble in DMF | Br‑CH₂‑CH₂‑Br + KCN → CH₂(CN)CH₂(CN) |
| Hydrolysis of nitriles | Acid or base hydrolysis → carboxylic acids / amides | CH₃C≡N + H₂O/H₂SO₄ → CH₃COOH |
| Reduction of nitriles | LiAlH₄ / NaBH₄ + NaCN | CH₃C≡N + LiAlH₄ → CH₃CH₂CN |
Safety Tip: Use solid NaCN or KCN with a hydrophobic phase to minimize HCN release. Perform reactions in a properly equipped fume hood.
2.3 Preparation of Isocyanides
| Method | Typical Conditions | Example |
|---|---|---|
| Dehydrohalogenation of α‑haloketones | Use strong base (KOH, NaH) / high temperature | CH₃COCH₂Cl + KOH → CH₃CH₂NC |
| Dehydrohalogenation of α‑halocarboxylic acids | Similar to above | CH₃COOCH₂Cl + KOH → CH₃CH₂NC |
| Carbene addition to nitriles | Rh(II) catalysis | CH₃CN + Rh(II) + carbene → CH₃CH₂NC |
| Pinner reaction / nitrile → isocyanide | Treat nitrile with acid → imidate → isocyanide | CH₃C≡N + H₂SO₄ → CH₃CH₂NC |
- Isocyanides are often prepared in one‑pot to avoid handling highly volatile compounds.
2.4 Reactivity Overview
| Cyanide | Isocyanide |
|---|---|
| Electrophilic nitrile carbon | Nucleophilic nitrogen |
| Adds to alkyl halides (S_N2) | Adds to aldehydes/ketones (Ugi) |
| Hydrolysis → carboxylic acids | Forms amidines / imidates |
| Can be protonated → HCN | Can be protonated → HNC (rare) |
| Participates in metal‑catalyzed cross‑coupling | Participates in metal‑catalyzed alkylation |
2.5 Common Synthetic Applications
| Reaction | Cyanide Role | Isocyanide Role |
|---|---|---|
| Formation of nitriles (alkylation) | S_N2 with alkyl halides | Not used |
| Ugi 4‑component reaction | Provides the amino‑acid component | Provides the isocyanide component |
| Passerini 3‑component reaction | Nitriles + aldehydes + isocyanides | Nitriles + aldehydes + isocyanides |
| Formation of amides | Hydrolysis of nitriles | Isocyanides → amidines → am |