This route to phenol involves the intermediate monochlorobenzene, which can be manufactured from benzene in two ways.

The classical chlorination of benzene, as was still run, for example, by Dow and Bayer until 1975 and 1977, respectively, was done at 25-50°C in the liquid phase with FeCl₃ catalyst. About 5% higher chlorinated benzenes are obtained as byproducts.

In the more economical Raschig-Hooker process - first operated in 1932 by Kh6ne-Poulenc in a 3000 tonne-per-year plant - hlorobenzene is obtained by oxychlorination of benzene with HCl/air mixtures at about 240°C and atmospheric pressure:

CuCl₂ FeCl₃/Al₂O₃ is used as the catalyst. The benzene conversion is limitcd to 10-15% to control the evolution of heat at the fixed-bed catalyst and to suppress the formation of dichlorobenzenes. HCl is converted as completely as possible. In oxychlorination, the fraction of dichloroben7,enes is 6-10%, some-what higher than with the classical chlorination.

The classical processes also differ from the Kaschig- Hooker process in the conversion of chlorobenzcne to phenol. In the classical proccsses, hydrolysis of chlorobcnzene is done with 10-15% caustic soda or sodium carbonate solution at 360-390 °C and a pressure of 280-300 bar. Diphcnyl ether, and o- and p-hydroxydiphenyl, are formed as byproducts.

HCl from the benzene chlorinalion is used to release the phenol from its sodium salt.

In the Kaschig-Hooker process, hydrolysis takes place catalytically over Ca₃(PO₄)₂/SiO₂ with H₂O at 400-450°C. Due to carbon deposition, the catalyst must be regenerated frequently. The conversion is about 10- 15%, with an overall selectivity of 70-85% for both steps. In the USA, all plants using this technology have been shut down; however, this classical chlorobcnzene/phenol route is still used to some extent in several countries, including Argentina, India, Italy, and Poland.

Based on thc nct equation for Dow and Bayer process:



and for the Raschig-Hooker process, in which HCl from the hydrolysis of chlorobenzene can be used for the oxychlorination of benzene, one would expect a large economic advantage for the latter:

Only about 0.02 tomes of HCl are consumed per tonne of phenol. However, hydrochloric acid is highly corrosive under the hydrolysis and oxychlorination conditions, calling for large investments for corrosion-resistant plants. Furthermore, due to the rcquired vaporization of dilutc hydrochloric acid, the Raschig-Hooker process uses large amounts of energy.

Chlorobenzene is not only a suitable starting matcrial for phenol, but is also used in thc manufacture of aniline and as a raw material for numerous other aromatic intermediates. In addition, chlorobenzene is an important solvent.