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Wittig olefination: methyltriphenylphosphonium bromide + benzaldehyde → styrene

[Ph3P=CH2] + Ar–CHO → Ar–CH=CH2 + Ph3P=O

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: Step 1 — 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). Step 2 — 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.

Expected yield

70–88 %

Expected duration

4–12 h at RT after −78 °C ylide formation

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