Friedel–Crafts reaction of phenol
Like aniline, phenol too reacts to a very less extent during Friedel-Crafts reaction.  

The reason being that the oxygen atom of phenol has lone pair of electrons which coordinate with Lewis acid.  

In fact most substituents with lone pair would give poor yield.  

The two pathways involved in the reaction with phenol reduce the overall yeild: 


Phenols are examples of bidentate nucleophiles, meaning that they can react at two positions:         


on the aromatic ring giving an aryl ketone via C-acylation, a Friedel-Crafts reaction 


or, 


on the phenolic oxygen giving an ester via O-acylation, an esterification