Methylene glycol, formaldehyde

The materials we shall discuss are all polymers of formaldehyde which may be viewed as methylene glycol in the presence of water ... 

Formaldehyde is produced and sold as water solutions containing variable amounts of methanol. These solutions are complex equiUbrium mixtures of methylene glycol, CH2(OH)2, poly(oxymethylene glycols), and hemiformals of these glycols. Ultraviolet spectroscopic studies (13—15) iadicate that even ia highly concentrated solutions the content of unhydrated HCHO is <0.04 wt%. 

Aqueous Formaldehyde. Water solutions of formaldehyde consist mainly of telomers of methylene glycol having <100 ppm of the formaldehyde as CH2O (5). Alcohols form hemiformals with aqueous formaldehyde according to the following, where n = 1,2,3, etc. 

Alkaline catalysts are also effective in the polymeri2ation—depolymeri2ation of methylene glycol. The mechanism of the formaldehyde addition to the phenolate is still not completely understood. The most likely mechanism involves the contribution of phenol hemiformals (10) (5). 

Novolacs are prepared with an excess of phenol over formaldehyde under acidic conditions (Fig. 7.6). A methylene glycol is protonated by an acid from the reaction medium, which then releases water to form a hydroxymethylene cation (step 1 in Fig. 7.6). This ion hydroxyalkylates a phenol via electrophilic aromatic substitution. The rate-determining step of the sequence occurs in step 2 where a pair of electrons from the phenol ring attacks the electrophile forming a car-bocation intermediate. The methylol group of the hydroxymethylated phenol is unstable in the presence of acid and loses water readily to form a benzylic carbo-nium ion (step 3). This ion then reacts with another phenol to form a methylene bridge in another electrophilic aromatic substitution. This major process repeats until the formaldehyde is exhausted. 

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). 

Much has been made about methylene glycol being the cause for formaldehyde s slow rate of fixation, succinctly expressed by Fox et al.7 Equilibrium between formaldehyde as carbonyl formaldehyde and methylene glycol explains most of the mystery of why formaldehyde penetrates rapidly (as methylene glycol) and fixes slowly (as carbonyl formaldehyde). Flowever, the equilibrium equation indicates the proportional amounts of carbonyl formaldehyde and methylene glycol, not the rate of conversion between the two... 

On Cu, the overall anodic reaction involving formaldehyde, or more correctly, the methylene glycolate anion may be depicted as ... 

In the polarography of aqueous formaldehyde solutions the only reducible species is the unhydrated aldehyde, and under suitable conditions the observed current is dependent on the rate of dissociation of methylene glycol. This was first shown by Vesely and Brdidka (1947) and by Bieber and Triimpler (1947a) later work (BrdiiSka, 1955) demonstrated catalysis by borate and hydroxide ions, but no extensive study has been made by this method. An exact mathematical solution of the diffusion problem is difficult to obtain a recent review has been given by Brdi5ka (1960). 

It has been shown by H naff (1963) that the rate of reaction of several carbonyl reagents (bisulphite, hydrazine, phenylhydrazine, semi-carbazide and hydroxylamine) with aqueous formaldehyde solutions is independent of the nature and concentration of the reagent, and is therefore determined by the rate of dehydration of methylene glycol. He obtained catalytic constants for hydrogen and hydroxide ions, and a detailed study of acid-base catalysis has been made by the same method by Bell and Evans (1966). 

Synonyms AI3-16096 Anesthenyl Bis(methoxy)methane BRN 1697025 Dimethoxymethane Dimethyacetal formaldehyde EINECS 203-714-2 Formal Formaldehyde dimethylacetal Meth-oxymethyl methyl ether Methylene dimethyl ether Methylene glycol dimethyl ether Methyl formal UN 1234. 

Methylenebis(oxy) ]bis(2-chloroformaldehyde), see Bis (2-chloroethoxy) methane Methylene chlorobromide, see Bromochloromethane Methylene dichloride, see Methylene chloride Methylene dimethyl ether, see Methylal Methyl 2,2-divinyl ketone, see Mesityl oxide Methylene glycol, see Formaldehyde Methylene glycol dimethyl ether, see Methylal Methylene oxide, see Formaldehyde Methyl ethanoate, see Methyl acetate (1 -Methylethenyl)benzene, see a-Methylstyrene Methyl ethoxol, see Methyl cellosolve 1-Methylethylamine, see Isopropylamine (l-Methylethyl)benzene, see Isopropylbenzene Methylethyl carbinol, see sec-Bntyl alcohol Methyl ethylene oxide, see Propylene oxide ds-Methylethyl ethylene, see cis-2-Pentene frans-Methylethyl ethylene, see frans-2-Pentene Methyl ethyl ketone, see 2-Bntanone Methylethylmethane, see Butane... 

Interaction Between Partial Reactions. The original mixed-p)otential theory assumes that the two partial reactions are independent of each other (1). In some cases this is a valid assumption, as was shown earlier in this chapter. However, it was shown later that the partial reactions are not always independent of each other. For example, Schoenberg (13) has shown that the methylene glycol anion (the formaldehyde in an alkaline solution), the reducing agent in electroless copper deposition, enters the first coordination sphere of the copper tartrate complex and thus influences the rate of the cathodic partial reaction. Ohno and Haruyama (37) showed the presence of interference in partial reactions for electroless deposition of Cu, Co, and Ni in terms of current-potential curves. 

Ferrocenylacetonitrile, 40, 4S Ferrous sulfate, oxidation ferf-butyl alcohol to a,a,a, a -tetramethyl tetra-methylene glycol by hydrogen peroxide and, 40, 90 Fluoboric acid as catalyst for diazomethane etherifications, 41, 9,10 Formaldehyde, reaction with diethyl malonate to form diethyl bis-(hydroxymethyl)malonate, 40, 27... 

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 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). 

Methylene glycol is formed by the reaction between formaldehyde and water ... 

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