Wittig olefination: methyltriphenylphosphonium bromide + benzaldehyde → styrene
Ready to scaleOlefination
Canonical Wittig olefination — the textbook 'make an alkene from an aldehyde' demo. The ylide (methylenetriphenylphosphorane) is generated in situ from methyltriphenylphosphonium bromide + base (typically n-BuLi at −78 °C, or NaH in DMSO), then reacts with benzaldehyde to give styrene. The triphenylphosphine oxide byproduct precipitates from cold ether and is the diagnostic end-of-reaction signal.
Conditions
- Suspend Ph3P+CH3 Br− (1.05 equiv) in dry THF under N2; cool to −78 °C; add n-BuLi (1.05 equiv, 2.5 M in hexanes) dropwise. Stir 30 min — solution turns yellow-orange (ylide formation).
- Add benzaldehyde (1.0 equiv) dropwise; warm to RT over 1–2 h; stir 4–12 h.
Workup
Quench with sat. NH4Cl; extract with Et2O; wash brine; dry MgSO4. Concentrate carefully (styrene is volatile, bp 145 °C). Triphenylphosphine oxide precipitates from cold pentane/ether — filter off. Distil residue under reduced pressure for clean styrene.
| Compound | Role | Equiv | MW (g/mol) | ρ (g/mL) | Safety |
|---|---|---|---|---|---|
| Benzaldehyde100-52-7 · C7H6OBenzaldehyde — the substrate aldehyde; freshly distilled to remove benzoic acid (white film on bottle indicates oxidation). | reactant | 1.0 | 106.12 | 1.050 | WarningSDS |
| Methyltriphenylphosphonium bromide1779-49-3 · C19H18BrPMethyltriphenylphosphonium bromide — ylide precursor; slight excess so the aldehyde is the limiting reagent. | reactant | 1.05 | 357.23 | — | Danger |
| n-Butyllithium109-72-8 · C4H9Li | reagent | 1.05 | 64.06 | — | DangerSDS |
| Tetrahydrofuran109-99-9 · C4H8OTetrahydrofuran — anhydrous; degas with N2 sparge before use. DMSO is a common alternative when NaH is the base instead of n-BuLi. | solvent | — | 72.11 | 0.890 | DangerSDS |
| Styrene100-42-5 · C8H8Styrene — colourless liquid, bp 145 °C; polymerises on standing (stabilise with 4-tert-butylcatechol if storing). Pungent sweet odour. | product | — | 104.15 | 0.910 | DangerSDS |