4,4'-Biphenol
4,4′-Biphenol is an organic compound with the formula / It is one of three symmetrical isomers of biphenol. It is a colourless crystalline solid with a high melting point. It is primarily used in the production of polymers, particularly liquid crystals where it imparts high thermal stability, and PPSU-type polysulfone.
Synthesis
The industrial synthesis of 4,4′-biphenol was developed by Allan Hay in the 1960s. As the direct oxidative coupling of phenol gives a mixture of isomers, 4,4′-biphenol is instead prepared from 2,6-di-tert-butylphenol, where para-coupling is the only possibility. A reaction with oxygen produces phenol-radicals which undergo rapid dimerisation, ultimately forming a diphenoquinone.This intermediate is reduced to the tetra-butyl-biphenyl derivative by a reaction with two equivalents of 2,6-di-tert-butylphenol, in an oxygen-free environment. In the final step, high temperature dealkylation is performed to remove the butyl groups, producing the desired 4,4′-biphenol product. If groups less bulky that t-butyl are used then polyphenylene ethers such as poly(p-phenylene oxide) can be produced.