Preparation by reaction of 4-hydroxybenzoic acid with resorcinol,
? in the presence of zinc chloride at 160° (Nencki reaction);
? in the presence of zinc chloride and phosphorous oxychloride for 4 days at r.t. (78%);
in the presence of zinc chloride and a mixture of polyphosphoric acid/85% phosphoric acid (60:40) at 27°. Then, during 2 h, phosphorous trichloride was added between 27° and 37° and the mixture heated at 60° for 16 h (quantitative yield);
? in the presence of hydrofluoric acid at 75° in an autoclave.
