Methylene glycol, formaldehyde compounds

Formaldehyde is usually described as a gas, but it also exists dissolved in water or other solvents. Because of very strong tendencies to hydrogen-bond, both formaldehyde and water combine avidly to make a hydrated compound called methylene glycol (Fig. 12.2). 

This may result from the nitrolysis of the compound (VI) at the bond C. This ester, like esters (VIII) and (IX), is unstable and readily decomposes. Finally, formaldehyde, split off from hexamine, may yield unstable methylene glycol nitrate (XVII) in the presence of anhydrous nitric acid. 

The hydroxylation theory of Bone2 and his co-workers has had wide acceptance as far as the oxidation of aliphatic hydrocarbons is concerned. The mechanism postulated involves the successive formation of hydroxyl compounds, which may add oxygen to form additional hydroxyl groups or which may lose water and decompose. In this way methane would first form methanol, then methylene glycol which would be decomposed to formaldehyde and water formaldehyde would be oxidized to formic acid or decomposed to carbon monoxide and hydrogen. The theory, however, is open to a number of criticisms. 

The addition of water to the carbonyl group in aqueous solutions can lead to the formation of hydrates. The reactions and stability of hydrates depend primarily on the inductive effect of substituents. Hydration of formaldehyde readily yields methylene glycol (methanediol), which polymerises to linear oligomers and also to polymers known as paraformaldehyde. Hydrates of a-dicarbonyl and a-hydroxycarbonyl compounds spontaneously dimerise into various cyclic 1,4-dioxanes (see Figure 4.62). These hydrates are intermediates of other a-dicarbonyl and a-hydroxycarbonyl compounds in the oxidation-reduction reactions and precursors of carboxylic acids. Dialkyl ketones do not form hydrates. 

Reactions of formaldehyde with aromatic hydrocarbons are similar in cane respects to those involving olefins and may involve a somewhat similar mechanism. However, reactions apparently proceed further than in the ca of olefins, and the simple addition products of methylene ycol or sahstituted methylene glycol have not been isolated. With aromatic hydrocarbons, formaldehyde and hydrogen halides, the primary reaction products isolated are compounds in which one or two halomethyl groups are substituted for hydrogen on the aromatic nucleus. On further reaction, compounds are obtained in which two or more aromatic nuclei are linked together by methylene gi oups. When sulfuric acid is employed as a reaction catalyst, methylene derivatives of this latter type are apparently the pi incipal products obtained. 

The use of [,3CJformaldehyde resulted in the formation of 80% of the dilabeled ethylene glycol, indicating that the ethylene glycol formation proceeds preferentially via reductive carbon-carbon coupling over hydro-formylation of formaldehyde the catalytic turnover is not given (166). Ru3(CO),2 was also found to catalyze the reductive alkylation of active methylene compounds with formaldehyde under synthesis gas. For example, pentan-2,4-dione is converted into 3-methylpentan-2,4-dione... 

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