Question

why phenol OH group is not replaced when reacted with halogens

Answer 1

Halogens cannot be prepared by the action of phosphorus halides with phenols because it is difficult to replace the hydroxy group of phenols by a halogen atom. Due to resonance in phenol, the C-O bond in phenols aquire partial double bond character. Therefore the C-O bond in phenols is stronger than the carbon oxygen single bond in alcohols. Consequently it is difficult to break the C-O bond in phenols.