Phenols from halobenzenes

This phenol synthesis is different from the nucleophilic aromatic substitutions discussed in the previous section because it takes place by an elimination addition mechanism rather than an addition/elimination. Strong base first causes the elimination of HX from halobenzene in an E2 reaction, yielding a highly reactive benzyne intermediate, and a nucleophile then adds to benzyne in a second step to give the product. The two steps are similar to those in other nucleophilic aromatic substitutions, but their order is reversed elimination before addition for the benzyne reaction rather than addition before elimination for the usual reaction. 

It is difficult to replace aromatically bound halogen by a hydroxyl group. Hydrolysis of halobenzenes has been effected under pressure in the presence of alkali or after addition of catalytically active substances.526 There are numerous patents describing, e.g., the conversion of chlorobenzene into phenol 527 and phenols have been obtained from bromobenzene derivatives in an autoclave in the presence of copper catalysts.528 Hydrolysis of halo-benzene derivatives is easier if they contain electron-attracting substituents in the ortho- or /wra-position to the halogen. 

With aii this in mind, how would you expect fluorobenzene to react Most electron density is removed first from the ortho positions by induction, then from the meta positions, and then from the para position. Any conjugation of the lone pairs on fluorine with the it system would increase the eiectron density in the ortho and para positions. Both effects favour the para position and this is where most substitution occurs. But is the ring more or less reactive than benzene This is hard to say and the honest answer is that sometimes fluorobenzene is more reactive in the para position than benzene (for example, in proton exchange and in acetyiation—see later) and sometimes it is less reactive than benzene (for example, in nitration, as shown by the table above). In all cases, fluorobenzene is significantly more reactive than the other halobenzenes. We appreciate that this is a rather surprising conclusion, but the evidence supports it. For example, fluorobenzene reacts with bromine and an iron catalyst (it does need a cataiyst it is not as reactive as phenol) at only -20 °C to give the para-biomo derivative. 

The reaction of diphenyl ether with both Br" and Cl" has been investigated in the gas phase at 750 K. Splitting to the respective halobenzene and phenoxy radical occurred in competition with abstraction of o-, m-, or p-hydrogen atoms. This suggests that formation of dioxins from halogenated phenols in combustion may proceed by combination of phenoxy radicals instead of displacement of o-halogen atoms from the halophenol. ... 

In cases of insufficient data, tables in Dreisbach (1952) can be used from the knowledge of the boiling point at 760 torr and the chemical family to which the compound belongs. The tables developed the Cox Chart chemical families using Antoines equation, a modified version of equation D-2 (Appendix D). Charts are available for several organic compounds such as naphthalenes, halobenzenes with side chains, and phenols. The appropriate table is then referred to for an estimated vapor pressure. 

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