Preparation, Purification and Some Properties of 1,3,5-Trioxane

5-Trioxane (TXN) is usually prepared by treating a 60-65 % aqueous solution of formaldehyde with acid (e.g. 2 % of sulfuric acid) and distilling off the azeotrope of TXN with water (70% of TXN, b.p. 91.3 °C). The trimer can be extracted from the aqueous distillate with e.g. methylene dichloride or trichlorobenzene. 

A recommended method of TXN purification involves recrystallization from water, diethyl ether or methylene dichloride, vacuum sublimation or distillation 44). TXN is inert towards alkalies thus, basic drying agents may be used to remove water. On the large scale TXN is conveniently dried by distillation of the crude trimer with solvent which forms ternary azeotropes with high water content (e.g. n-heptane)45). 

The high-purification in laboratory requires refluxing over sodium-potassium alloy followed by distillation under nitrogen 46). It is advisable to keep the distilled product in the molten liquid state, to prevent premature spontaneous polymerization due to crystallization-melting cycles. 

Some properties of TXN are listed in Table 7.4. Pure TXN is colorless, crystallizes in needles or rhombohedral form. It is soluble in alcohols, ketones, organic acids, esters, phenols, aromatic hydrocarbons and chlorinated hydrocarbons. Solubility in aliphatic and cycloaliphatic hydrocarbons is limited at room temperature but rises markedly with temperature. At 20 °C water dissolves about 20 % wt of TXN, and solubility in water increases with increasing temperature 47). 

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