1. Field of the Invention
Preferential complexation of one organic compound in a mixture of related compounds is a known technique for resolving mixtures of closely related compounds. Of particular interest herein are methods for isolating 3,5-xylenol and/or 3,4 xylenol from close boiling mixtures, by preferential complexation of one of the phenolics.
2. Description of the Prior Art
Mixtures of polymethyl phenolics are available as by-products from the coking of coal and the base extraction of petroleum fractions. The isolation and purification of individual phenolics is very difficult by the use of conventional separation methods such as fractional distillation and fractional crystallization. This fact may be seen from the table below which lists twelve phenols boiling within a 25.degree. range.
______________________________________ Boiling Point Melting Point Phenol (.degree.C.) (.degree.C.) ______________________________________ 2,3-xylenol 218 73 3,5-xylenol 221 63 3,4-xylenol 226 65 2,5-xylenol 211 74 2,4-xylenol 211 25 2,3,6-trimethylphenol 220 62 2,4,6-trimethylphenol 220 71 2,3,4-trimethylphenol 237 81 2,3,5-trimethylphenol 235 95 2,4,5-trimethylphenol 236 72 m-ethylphenol 218 -4 p-ethylphenol 219 46 ______________________________________
There are chemical processes known for separating closely-related organic compounds by methods other than, or in addition to, energy-intensive physical separation techniques such as fractional distillation or fractional crystallization. These chemical processes involve a step of preferential complexation of one component of a mixture of closely-boiling compounds over other components of the mixture. For example, U.S. Pat. No. 4,267,389 to Leston, describes treating a phenolic mixture comprising para-cresol, methylated phenols and ethylated phenols with an inorganic halide salt, such as calcium bromide, to remove para-cresol from the mixture. Removal of para-cresol from the mixture involves formation of a complex between para-cresol and calcium bromide, which complex forms preferentially over complexes between calcium bromide and other components of the phenolic mixture.
Mixtures of various alcohols may be resolved by treatment with a halide salt. For example, in Sharpless et al., J. Org. Chem., Vol. 40, No. 9, p.p. 1252-1257 (1975), there is reported a study of competition between pairs of mono-hydroxy alcohols and mono-hydroxy phenols for complex formation with a halide salt. This study finds that phenols as a class form poorer complexes than alcohols of comparable melting point, probably because the phenols are weaker bases than the comparable alcohols.
There remains need, therefore, for methods for resolution of mixtures of closely-related phenolics by chemical complexation methods, rather than by fractional crystallization or distillation.