Abstract
Trans-oxanes are important liquid crystals. The commonly used techniques for the synthesis were to react 2-substituted propylene glycols with substituted formaldehydes. Such process produces toxic cis-oxanes, which are harmful to the environment. The cis to trans isomerization of wasted cis-oxanes was studied in the presence of p-toluenesulfonic acid as catalyst. The yield of cis to trans conversion was over 70%, which was much higher than 42–69% when traditional methods were employed. The total yield of the new method was increased to 90%. Further investigation of effects of catalysts, reaction times, temperatures on the cis-trans conversion was carried out. Proposed mechanism of this process for the conversion was discussed.
Highlights
The corresponding trans isomers were obtained by recrystallization and the ratio of cis/trans conversion maintained at 25:75 as temperatures were increased
It was known that the conversion of the cis-trans isomer might be a chemical dynamic equilibrium process[13], which was generally divided into three categories: photoisomerzation, thermoisomerization and catalytic isomerization
As the final cis-trans equilibrium was decided by the thermodynamic stability of each isomer, thermoisomerization and catalytic isomerization were classified into one group during the synthesis[14]
Summary
The commonly used synthesis was to react substituted formaldehydes (a) with 2-substituted propylene glycols (b) for mixed cis-trans oxanes (e)[5,6], followed by separation and purification to give pure trans-oxane monomer (d) (Fig. 1). Pure trans-oxanes (d) monomers were obtained by either high vacuum distillation, chromatography or recrystallization The latter left a large amount of cis-trans mixtures (f) with cis configuration dominant. Due to the repeated conversion and purification of the mixture f, the mother liquor generated in the process was reused in site repeatedly to reduce environmental pollution. This improves the utilization rate of the starting materials and reduces the cost of products. The factors and mechanism of conversion were analyzed throughout the experiments in order to provide an example for the optimization of the process, and a reference for the synthesis of similar oxanes
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