1,3-Dioxolane, hydration water

The use and reactions of glyoxal as a fixative have been reviewed extensively.3334 Glyoxal is the second smallest aldehyde, being like two formaldehyde molecules arranged back-to-back (Fig. 12.8, left). It too forms hydrates with water, the most common of which is 1,3-dioxolane (Fig. 12.8, right). 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

As with polystyrene sulfonic resins, Nafion-based acid catalysts are highly efficient for hydration and dehydration processes and, in general, for condensation reactions that occur with the formation of water or similar secondary products. Formation of ethers has been studied for various alcohols [109-111]. Dehydration of 1,4- and 1,5-diols at 135 °C affords the corresponding cyclic ethers such as 20 in excellent yields (Scheme 10.7), while 1,3-diols experience different transformations depending on their structure [112]. The dehydration of 1,2-diols mainly proceeds via the pinacol rearrangement. Further condensation of the initially formed carbonyl compound and unreacted diol affords 1,3-dioxolanes [113]. The catalyst could be efficiently reused following a reactivation protocol. Formation of aryl ethers is also possible, and the synthesis of dibenzofurans 21 (X = O) from 2,2 -dihydroxybiphenyls has been reported (Scheme 10.7) [114]. The related reaction... 

Far-IR transmission spectra were recorded of Structure I [ethylene oxide (EtO) hydrate] and Structure 11 [cyclopropane, trimethylene oxide (TMO), 1,3-dioxolane, tetrahydrofuran (THE), and cyclobutanone hydrates] hydrates. The slight water lattice stretching due to the large size of cyclobutanone was indicated by the most... 

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