Benzene, from reduction of chlorobenzene

Adams platinum oxide catalyst is readily prepared from chloroplatinio acid or from ammonium chloroplatinate, and is employed for catalytio hydrogenation at pressures of one atmosphere to several atmospheres and from room temperature to about 90°. Reduction is usually carried out with rectified spirit or absolute alcohol as solvents. In some cases (e.g., the reduction of benzene, toluene, xylene, mesitylene, cymene and diphenyl ), the addition to the absolute alcohol solution of 2-5 per cent, of the volume of rectified spirit which has been saturated with hydrogen chloride increases the effectiveness of the catalyst under these conditions chlorobenzene, bromobenzene, o-, m- and p-bromotoluenes, p-dichloro- and p-dibromo-benzene are reduced completely but the halogens are simultaneously eliminated. Other solvents which are occasionally employed include glacial acetic acid, ethyl acetate, ethyl acetate with 17 per cent, acetic acid or 8 per cent, of alcohol. In the actual hydrogenation the platinum oxide Pt02,H20 is first reduced to an active form of finely-divided platinum, which is the real catalyst allowance must be made for the consumption of hydrogen in the process. 

The electron affinities of the chlorobenzene isomers have been determined by scaling half-wave reduction potentials [22], With higher gas phase values higher values are obtained from reduction potentials. These are compared to the ECD and CURES-EC values in Table 11.9. The CURES-EC-calculated values for the above compounds support experimental quantities and suggest that the Ea of all halogenated benzenes can be calculated. The CURES-EC values are listed in Table 11.10. The Ea... 

Products and reaction intermediates observed in the present study are rationalized by a dual reactivity of the C02 radical anion with substituted benzenes, i.e. one-electron reduction of and radical addition to the aromatic ring. For substrates with electron-withdrawing substituents, i.e., with positive u values such as X = NO2, COOH and COH, the observed products can be explained by an electron transfer from CO2 to the substrates as the primary reaction channel. Benzene and chlorobenzene react by both electron transfer and radical addition. For toluene, with an electron-donating substituent, products of radical addition are found. The dual reactivity of COf radical anion has recently been reported for thymine," in agreement with our observations. 

The isolation of benzene and cyclohexane from chlorobenzene and thiophenol, and cyclohexane from fluorobenzene, suggests the preferential reductive cleavage of the substituent prior to hydrogenation of the ring. However, fluorocyclohexane decomposes slowly to cyclohexene, which could give rise to the cyclohexane higher yields of fluorocyclohexane are obtained at lower temperatures. 

---