Appel reaction template

PPh₃ takes Br⁺ from CBr₄; CBr₃⁻ deprotonates the alcohol, the alkoxide attacks phosphorus and bromide displaces Ph₃P=O (SN2).

Mechanism step by step

PPh₃ and CBr₄ turn an alcohol into an alkyl bromide under neutral conditions

1. Bromophosphonium ion – Phosphorus attacks a bromine of CBr₄
2. Deprotonation – CBr₃⁻ removes the O–H proton
3. Alkoxyphosphonium ion – The alkoxide attacks P; Br⁻ leaves
4. SN2 – Bromide attacks carbon; Ph₃P=O leaves

Key points

• Inversion of configuration at a stereocentre (the last step is SN2).
• Neutral and mild: no HBr, so acid-sensitive groups survive.
• CCl₄ gives chlorides; I₂/PPh₃/imidazole gives iodides.
• Ph₃P=O formation is the driving force (as in Wittig and Mitsunobu).

Drawn with 80 atoms, 13 arrows, 6 shapes – every one editable in MolDraw.

Sources

• Appel, Angew. Chem. Int. Ed. Engl. 1975, 14, 801 Original paper
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook