Reactions

First, triphenylphosphine (PPh₃) attacks a halogen atom of the tetrahalomethane (CX₄, where X = Br or Cl) to form a phosphonium intermediate and a trihalomethane anion. The latter acts as a strong base and deprotonates the alcohol, generating a strong nucleophilic species. Then, the alkoxide attacks the phosphonium intermediate to form an alkoxyphosphonium ion with departure of the halide. Finally, the halide attacks the carbon atom in α position relative to the oxygen atom via an SN2 mechanism, resulting in the formation of the substituted product with inversion of stereochemistry (Walden inversion).