Formaldehyde, chain branching

High Temperature Reaction. Reaction in the high temperature regime produces carbon monoxide, water, methane, formaldehyde, and methanol (8) the two higher ketones also form ethylene (I). The intermediate responsible for chain branching appears to be formaldehyde. The concentration of formaldehyde and the rate of reaction run parallel over the whole of the reaction, as shown in Figure 4 for diethyl ketone. 

The methoxy radical may subsequently react to form formaldehyde (H atom abstraction) or methanol (H atom addition). The sequence of reactions (R15) through (R17) is strongly chain branching and serves to build up a radical pool. Once this radical pool is established, another chain-branching oxidation route becomes dominating. Methane consumption now occurs mainly by the reactions [254]... 

Formaldehyde is a product of the combustion of all hydrocarbons. Studies of the reactions of formaldehyde are important in leading to a better understanding of the mechanism of hydrocarbon oxidation. Its role in the low temperature region is variable but minor, and depends on the individual hydrocarbon and conditions. In sufficient quantities it appears able to suppress cool flames. In hydrocarbon oxidation above 400° C. formaldehyde is an important intermediate responsible for degenerate-chain branching. 

Kuzminskii (22) suggests that chain branching in the free radical reaction occurs through further oxidation of formaldehyde,... 

Reaction (86) is relatively slow for a chain branching step nevertheless, it is followed by the very rapid decay reaction for the methoxy [reaction (93)], and the products of this two-step process are formaldehyde and two very reactive radicals, O and H. Similarly, reaction (92) may be equally important and can contribute a reactive OH radical. These radicals provide more chain branching than the... 

There is good evidence that chain branching at 400 °C involves formaldehyde because (i) its concentration reaches a peak at the time of maximum rate (ii) the addition of formaldehyde reduces the induction period without affecting the maximum rate, and (Hi) the amount of formaldehyde needed to reduce the induction period to its minimum value is almost exactly that normally present at the maximum rate. 

Most of the current results on the selective oxidation of methane over metal oxide catalysts may be interpreted in terms of methyl radical chemistry. These radicals may either react with the oxides themselves to form methoxide ions or they may enter the gas phase. The methoxide ions on supported molybdena decompose to form formaldehyde or they react with water to yield methanol. On the basic oxides methoxide ions result in complete oxidation. Those radicals which enter the gas phase undergo typical free radical chemistry which includes coupling reactions to give ethane and chain branching reactions to give nonselective oxidation products. Secondary surface reactions, particularly with ethylene, also may result in complete oxidation. If further improvements in yields of partial oxidation products are to be achieved, ways must be found to more efficiently utilize the methyl radicals, both with respect to surface reactions and to gas phase reactions. In addition, if ethylene is the desired product, catalysts must be fine-tuned to the point where they will activate methane, but not ethylene. 

PdCb-CuCb catalyzes the condensation of branched-chain alkenes with formaldehyde to give the l,3-dioxanes 96a and 96b (Prins reaction)[73]. The yields are much higher than in the conventional acid-catalyzed Prins reaction. 

Henry reaction of nitro sugar 11 with formaldehyde allowed the introduction of two hydroxymethyl groups at the carbon bearing the nitro group, and hence opened a specific route for the preparation of branched-chain imino sugar 57 and analogues (Scheme 20).44... 

Because of the importance of carbonyl groups to the mechanism of condensation reactions, much of the assembly of either straight-chain or branched-carbon skeletons takes place between compounds in which the average oxidation state of the carbon atoms is similar to that in carbohydrates (or in formaldehyde, H2CO). The diversity of chemical reactions possible with compounds at this state of oxidation is a maximum, a fact that may explain why carbohydrates and closely related substances are major biosynthetic precursors and why the average state of oxidation of the carbon in... 

Formaldehyde, in sufficient quantities, can suppress cool-flame formation. Jost (27) presents evidence indicating that cool flames are a form of branched-chain explosions. It has been suggested that the cool-flame reaction is quenched by its own reaction product, formaldehyde, and arrested short of complete release of chemical enthalpy. This seems unlikely, however, because in systems exhibiting multiple cool flames the concentration of formaldehyde after the first cool flame does not drop in some cases it increases, and yet does not suppress subsequent cool flames. Bardwell (5), and Bard well and Hinshelwood (4) explain cool flame phenomena by a modified theory of Salnikov. This thermal theory is further supported by the results of Knox and Norrish (30) in the ethane-oxygen system. The key intermediate is presumed to be a peroxide by Bardwell and Hinshelwood (4). Formaldehyde is considered an inert, stable product with little effect on the reaction. 

If it is accepted that the activation energy of a chain reaction is largely that of the process generating chains, then the parallelism in the behavior of the energies of activation for the ethylene and formaldehyde oxidations may be interpreted on the basis of the degenerate-branching reaction the former is identical with the initiation reaction for the latter. Two possible reactions were suggested... 

The importance of olefins in chain transfer has been referred to. Ketones and aldehydes formed by Reactions 7 and 8 may also modify the combustion process. The greater reactivity of aldehydes compared with ketones was used by Pope and coworkers (168) to explain the relative oxidation resistance of straight-chain and branched paraffins and has been repeatedly cited (20, 24, 25, 224). Several workers have discussed the parts played by formaldehyde and other aldehydes in knock (25, 45, 111, 217, 231). 

The t2 period includes the cool flame reaction, which may be followed by a period of decreased reaction velocity, and leads up to autoignition. Lewis and von Elbe (108, 110) believe that in the r2 region unbranched chains are initiated by reaction of formaldehyde and perhaps other aldehydes with oxygen. Second-stage ignition is not believed to be of the branched-chain type but occurs as a result of unbalancing of thermal equilibrium. 

We thus investigated a range of such one-carbon electrophiles (Scheme 10). Formaldehyd, as the most reactive one, gave the desired product directly (28). Using benzaldehyde as the electrophile likewise lead to a C-2 branched aldonolactone in a high yield and stereospecificity. The two diastereoisomeric compounds obtained, due to the new chiral center formed in the side chain, were both isolated in a crystalline state (28). 

---