Acylation of phenol with acetic acid over a HZSM5 zeolite, reaction scheme

The scheme of the gas phase phenol acylation with acetic acid on a HZSM5 zeolite was established from the effect of contact time (hence of conversion) on the product distribution. Phenyl acetate and o-hydroxyacetophenone are primary products, O-acylation being much faster than C-acylation. At high conversion, part of the o-hydroxyacetophenone results from the acylation of phenol with phenyl acetate. The formation of p-hydroxyacetophenone which does not occur through phenol acylation involves the hydrolysis of p-acetoxyacetophenone selectively formed through the autoacylation of phenyl acetate. The ortho-selectivity of phenol acylation can be related to a pronounced stabilization of the transition state while the para-selectivity of phenyl acetate autoacylation could be due to a steric hindrance to the approach of the acetyl group in the ortho-position of phenyl acetate.