Methylene glycol, hydrated formaldehyde

When aqueous solutions of formaldehyde are evaporated to dryness, a white crystalline solid (m.p. 121-123 C) known as paraformaldehyde is obtained. This comprises polyoxymethylene glycols formed by condensation of methylene glycol (hydrated formaldehyde see Section 8.3.2.) ... 

Similar ideas can be applied to formaldehyde oxidation. For bulk formaldehyde oxidation, we found predominant formic acid formation under current reaction conditions rather than CO2 formation. Hence, it cannot be ruled out, and may even be realistic, that formaldehyde is first oxidized to formic acid, which can subsequently be oxidized to CO2. The steady-state product distribution at 0.6 V is much more favorable for such a mechanism as in the case of methanol oxidation. On the other hand, because of the high efficiency of COad formation from formaldehyde, this process is likely to proceed directly from formaldehyde adsorption rather than via formation and re-adsorption of formic acid. Alternatively, the second oxygen can be introduced via formaldehyde hydration to methylene glycol, which could be further oxidized to formic acid and finally to CO2 (see the next paragraph). 

Figure 12.2 Structural formulas of formaldehyde and its hydrate, methylene glycol.

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). 

The efficient sink criteria cannot be overemphasized, however. When the experimental data for formaldehyde for either of the two different membrane scrubbers are considered, the theoretical predictions significantly exceed the experimental collection efficiencies. The uptake of formaldehyde at an aqueous interface is controlled by its rate of hydration to methylene glycol, a process that is acid- or base-catalyzed. The collection efficiency significantly increases in going from pure water to 0.1 M H2S04 as a scrubber liquid (55), but the uptake probability still remains a controlling factor in determining the collection efficiency. Obviously, in such cases theoretical predictions merely establish an upper limit. 

Aqueous formaldehyde as used for fixation contains mostly methylene glycol (-99%), its oligomers, and small amounts of formaldehyde. The proportion of the oligomers present depends inversely on the temperature. Formaldehyde solution cannot be obtained without the formation of methylene glycol. It is not the formaldehyde molecule that is primarily responsible for rapid penetration into the tissue but methylene glycol, which is the major component of formaldehyde solution. At concentrations of 2% or less, the formaldehyde in solution is present practically only as the hydrated monomer (HOCH2OH). 

Notably, formaldehyde in aqueous solutions is hydrated to methylene glycol [84] (D2C(OH)2 for the case of deuterated formaldehyde) with pK= 13, indicating that at pH= 13 methylene glycol is present at equal amounts with its anion [23,24]. Nevertheless, due to the absence of hydrogen... 

Since formaldehyde in its pure state is a highly reactive gas, it is commercially used either as a solution in water, known as formalin, or as a solid flake polymeric form known as paraforra. Formaldehyde in solution exists not as the pure aldehyde but rather in hydrated forms such as methylene glycol and low molecular weight gylcol ethers ... 

Several organic species (formaldehyde, formic acid, methanol, methyl-peroxide, etc.) are transferred from the gas to the aqueous-phase and contribute to the atmospheric aqueous-phase reaction system (Graedel and Weschler, 1981). The complex formation by S(IV) and aldehydes has already been discussed. Formaldehyde is very soluble in water because it hydrates to its diol form, methylene glycol ... 

In the United States, formaldehyde is obtained almost exclusively by the oxidation of methane, but in other countries it is mainly produced by dehydrogenation or oxydehydrogenation of methanol. Small amounts of formaldehyde are also obtained by the oxidation of dimethyl ether or higher hydrocarbons. The formaldehyde produced is absorbed in water, in which it exists mainly as the hydrate, methylene glycol. Solutions with up to 30% formaldehyde are clear amorphous paraformaldehyde, H(OCH2) OH, pre-... 

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. 

On the basis of these findings, Auerbach concluded that polymeric hy-di-ates other than the trimeric hydrate HO - (CH20)a-H ai C not present in appreciable proportions m solutions containing up to 30 per cent formaldehyde. Deviations encountered in the mass action constants for more methylene glycol form can be calculated from the apparent molecular weights. Values obtained in this way are shown in Table 3a. 

In the chapter dealing with the state of dissolved formaldehyde (Chapter 3), it has been pointed out that formaldehyde is hydrated and pai tially polymerized in aqueous solutions, being pre. t. as an equilibrium mixture of the monohydrate, methylene glycol, and polymeric hydrates, polyo> y-methylene glycols. The physical properti of formaldehyde solutions are such as would be expected in the light of this situation. They hehave like solutions of a comparatively non-volatile glycol they d<.> not Ijchave like solutions of a volatile gas. 

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