Formaldehyde disproportionation

The formaldehyde disproportionation has been examined by semi-empirical MO methods (Rzepa and Miller, 1985). With the MNDO procedure, transfer of hydride from hydrate mono-anion to formaldehyde is exothermic by 109 kJ mol-1, and the transition structure [29], corresponding to near symmetrical transfer of hydride, lies 72 kJ mol -1 above the separated reactants. Inclusion of two water molecules, to model solvation effects, stabilizes reactants and transition structures equally. Hydride transfer from the hydrate dianion was found to have a less symmetrical transition structure [30] not unexpected for a more exothermic reaction, but the calculated activation energy, 213 kJ mol-1, is unexpectedly high. Semi-classical primary kinetic isotope effects, kH/kD = 2.864 and 3.941 respectively, have been calculated. Pathways involving electron or atom transfers have also been examined, and these are predicted to be competitive with concerted hydride transfers in reactions of aromatic aldehydes. Experimental evidence for these alternatives is discussed later. 

The most commonly used emulsifiers are sodium, potassium, or ammonium salts of oleic acid, stearic acid, or rosin acids, or disproportionate rosin acids, either singly or in mixture. An aLkylsulfate or aLkylarenesulfonate can also be used or be present as a stabilizer. A useful stabilizer of this class is the condensation product of formaldehyde with the sodium salt of P-naphthalenesulfonic acid. AH these primary emulsifiers and stabilizers are anionic and on adsorption they confer a negative charge to the polymer particles. Latices stabilized with cationic or nonionic surfactants have been developed for special apphcations. Despite the high concentration of emulsifiers in most synthetic latices, only a small proportion is present in the aqueous phase nearly all of it is adsorbed on the polymer particles. 

Thermal Stability. Dimethyl sulfoxide decomposes slowly at 189°C to a mixture of products that includes methanethiol, formaldehyde, water, bis(methylthio)methane, dimethyl disulfide, dimethyl sulfone, and dimethyl sulfide. The decomposition is accelerated by acids, glycols, or amides (30). This product mixture suggests a sequence in which DMSO initially undergoes a Pummerer reaction to give (methylthio)methano1, which is labile and reacts according to equations 1—3. Disproportionation (eq. 4) also occurs to a small extent ... 

The synthetic importance of the reaction is limited, because as a consequence of the disproportionation, the yield of the alcohol as well as the carboxylic acid is restricted to 50%. However good yields of alcohols can often be obtained when the reaction is carried out in the presence of equimolar amounts of formaldehyde. The formaldehyde is oxidized to formic acid and concomitantly reduces the other aldehyde to the desired alcohol. This variant is called the crossed Cannizzaro reaction. 

Formaldehyde, in aqueous acidic solution, undergoes cyclotrimerization to trioxane (1,3,5-trioxacyclohexane), and also disproportionation to methanol and formic acid, with some resultant formation of methyl formate. The kinetic behaviour observed suggests a significant autocatalysis by formic acid. 

No evidence has been found that two methoxy radicals ever combine to give dimethyl peroxide, but they do disproportionate to give methanol and formaldehyde... 

In the literature, the simultaneous formation of the major NMMO degradation products N-methylmorpholine, morpholine and formaldehyde [20] is always attributed to the disproportionation of the primary aminyl radical 3, as a - not further defined - redox process between two molecules of 3, in which one is reduced to N-methylmorpholine (2) and the other oxidized to N-(methylene)morpholinium cation (17), which upon addition of water, forms morpholine and formaldehyde. Also this disproportionation can be rationalized as a radical coupling reaction which proceeds through recombination of N-centered 3 and C-centered 4, via the ammonium aminal intermediate 16 as the actual recombination product (Scheme 6). The intermediacy of this species was indeed confirmed by isolation from the reaction mixture and full characterization [17]. Compound 16 is quite labile and immediately de-... 

Although known as an explosive since 1894, PETN was used very little until after World War I when the ingredients to make the starting material became commercially available. The symmetrical, solid alcohol starting material, pentaerythritol, is made from acetaldehyde and formaldehyde, which react by aldol condensation under basic catalysis followed by a crossed Cannizzaro disproportionation to produce the alcohol and formate salt. Although the reaction takes place in a single mixture, it is shown below in two steps for clarity. 

This scheme of interrelated primary photochemical and subsequent radical reactions is comphcated by the back reaction of hydrogen atoms and hydroxyl radicals with formation of water (Fig. 7-16, reaction 2) or the dimerization of the latter with formation of hydrogen peroxide (Fig. 7-16, reaction 3). Furthermore, hydroxyl radicals are scavenged by hydroperoxyl radicals with formation of oxygen and water (Fig. 7-16, reaction 5) or by hydrogen peroxide to yield hydroperoxyl radicals and water (Fig. 7-16, reaction 4). In addition, hydroxymethyl radicals (HOCH ) formed by reaction 1 (Fig. 7-16) are able to dimerize with formation of 1,2-ethane-diole (Fig. 7-16, reaction 7) or they disproportionate to yield methanol and formaldehyde (Fig. 7-16, reaction 8). 

Compound 79 is structurally related to TIQ 80, obtained on condensation of norepinephrine with formaldehyde (164), and to TIQ 81, detected in animal tissues after exposure to acetaldehyde (165). Acid-catalyzed dehydration of TIQ 82, the N-methyl analog of TIQ 79, should lead to the iminium species 83 (166), which on two-electron oxidation or by disproportionation should give the isoquinolinium salt 84. Such reactions, if occurring in vivo, would parallel similar reactions seen with the neurotoxin MPTP in its conversions to MPDP and MPP (167) and could possibly explain the neurotoxic effects seen with 117 (168). 

At about 500 °K disproportionation is favoured as the products are mainly ethanol and acetaldehyde. Explosion leads to higher temperatures, and more ethoxy radicals decompose yielding more ethane and formaldehyde. 

Methanol co-adsorbed with water displaced most of the water on the surface methanol co-adsorbed with oxygen formed surface methoxides stable to 625 K. Oxygen pretreatment of the surface did lead to the formation of a species assigned as formaldehyde, which Henderson proposed to be formed via a disproportionation reaction between two methoxides. Spectroscopic probes of the controlling intermediate were inconclusive [71,72]. 

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