Why the yield of chlorobenzene is generally poor, When phenol is treated with PCl5 ???
21C6H5OH + PCl5→ C6H5Cl + POCl3 + HCl
3 C6H5OH + POCl3 → (C6H5)3PO4 + 3 HCl
The POCl3 formed in the first step consumes the phenol further by 3 moles to give mainly triphenyl phosphate.Thus if we use 4 moles of phenol we get only 1 mole of chlorobenzene just because of the POCl3 formed in the first step!
21The O-C bond in phenol acquires a double bond character due to resonance and so it is difficult to break that bond to substitute -Cl.
383If the O-C bond in phenol acquires a double bond, then O can release Hydrogen and O-H bond pair electrons can be contributed for making O=C bond. And Cl can form bond with H to produce HCl.
Is there any confusion!!............
21The l.p of electrons on the oxygen atom contributes for the partial double bond character of the O-C bond