Formaldehyde addition reactions

Pentaerythritol is produced by reaction of formaldehyde [50-00-0] and acetaldehyde [75-07-0] in the presence of a basic catalyst, generally an alkah or alkaline-earth hydroxide. Reaction proceeds by aldol addition to the carbon adjacent to the hydroxyl on the acetaldehyde. The pentaerythrose [3818-32-4] so produced is converted to pentaerythritol by a crossed Cannizzaro reaction using formaldehyde. All reaction steps are reversible except the last, which allows completion of the reaction and high yield industrial production. 

The most common catalysts are sodium hydroxide and calcium hydroxide, generally used at a modest excess over the nominal stoichiometric amount to avoid formaldehyde-only addition reactions. Calcium hydroxide is cheaper than NaOH, but the latter yields a more facile reaction and separation of the product does not require initial precipitation and filtration of the metal formate (57). 

Textile Flame Retardants. The first known commercial appHcation for phosphine derivatives was as a durable textile flame retardant for cotton and cotton—polyester blends. The compounds are tetrakis(hydroxymethyl)phosphonium salts (10) which are prepared by the acid-cataly2ed addition of phosphine to formaldehyde. The reaction proceeds ia two stages. Initially, the iatermediate tris(hydroxymethyl)phosphine [2767-80-8] is formed. 

Quinone Methides. The reaction between aldehydes and alkylphenols can also be base-cataly2ed. Under mild conditions, 2,6-DTBP reacts with formaldehyde in the presence of a base to produce the methylol derivative (22) which reacts further with base to eliminate a molecule of water and form a reactive intermediate, the quinone methide (23). Quinone methides undergo a broad array of transformations by way of addition reactions. These molecules ate conjugated homologues of vinyl ketones, but are more reactive because of the driving force associated with rearomatization after addition. An example of this type of addition is between the quinone methide and methanol to produce the substituted ben2yl methyl ether (24). 

Fig. 1. Influence of pH on A, the addition reaction of urea and formaldehyde (1 1) and B, the condensation of methylolurea with the amino hydrogen of a...

Condensation of vinyl chloride with formaldehyde and HCl (Prins reaction) yields 3,3-dichloro-l-propanol [83682-72-8] and 2,3-dichloro-l-propanol [616-23-9]. The 1,1-addition of chloroform [67-66-3] as well as the addition of other polyhalogen compounds to vinyl chloride are cataly2ed by transition-metal complexes (58). In the presence of iron pentacarbonyl [13463-40-6] both bromoform [75-25-2] CHBr, and iodoform [75-47-8] CHl, add to vinyl chloride (59,60). Other useful products of vinyl chloride addition reactions include 2,2-di luoro-4-chloro-l,3-dioxolane [162970-83-4] (61), 2-chloro-l-propanol [78-89-7] (62), 2-chloropropionaldehyde [683-50-1] (63), 4-nitrophenyl-p,p-dichloroethyl ketone [31689-13-1] (64), and p,p-dichloroethyl phenyl sulfone [3123-10-2] (65). 

By far the preponderance of the 3400 kt of current worldwide phenolic resin production is in the form of phenol-formaldehyde (PF) reaction products. Phenol and formaldehyde are currently two of the most available monomers on earth. About 6000 kt of phenol and 10,000 kt of formaldehyde (100% basis) were produced in 1998 [55,56]. The organic raw materials for synthesis of phenol and formaldehyde are cumene (derived from benzene and propylene) and methanol, respectively. These materials are, in turn, obtained from petroleum and natural gas at relatively low cost ([57], pp. 10-26 [58], pp. 1-30). Cost is one of the most important advantages of phenolics in most applications. It is critical to the acceptance of phenolics for wood panel manufacture. With the exception of urea-formaldehyde resins, PF resins are the lowest cost thermosetting resins available. In addition to its synthesis from low cost monomers, phenolic resin costs are often further reduced by extension with fillers such as clays, chalk, rags, wood flours, nutshell flours, grain flours, starches, lignins, tannins, and various other low eost materials. Often these fillers and extenders improve the performance of the phenolic for a particular use while reducing cost. 

Reasonable procedures for manufacturing resoles and novolacs are presented in subsequent sections. These procedures utilize the a concept known in the industry as programmed formaldehyde addition to avoid the problems mentioned above as well as aiding in control of the exothermic reactions resulting from the manufacture of the desired phenol-aldehyde products. These reactions are also extremely exothermic. 

It is important that the formaldehyde addition rate be balanced with the alkali content of the system and the engineering control capability. At high alkali contents, the exotherm will be more vigorous and create more load on the heat exchangers. At low alkali contents, the reaction rate may be quite slow. While this temporarily reduces the difficulty in instantaneous heat load, it may permit potentially hazardous levels of unreacted formaldehyde to accumulate. Such accumulations could become dangerous as batch temperature rises. In both cases. 

The nucleophilic addition reaction of urea to formaldehyde produces mainly monomethylol urea and some dimethylol urea. When the mixture is heated in presence of an acid, condensation occurs, and water is released. This is accompanied by the formation of a cross inked polymer ... 

These resins are prepared by an addition reaction of formaldehyde with either phenols, urea or melamine to prepare an intermediate such as the following ... 

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

Dynamic DSC scans of resole resins show two distinguishable reaction peaks, which correspond to formaldehyde addition and die formation of edier and metiiy-lene bridges characterized by different activation energies. Kinetic parameters calculated using a regression analysis show good agreement widi experimental values.75... 

Figure 3.1 Two essential steps of chemical reaction of formaldehyde (HCHO) with nucleic acid exemplified by adenine that are similar to formaldehyde-protein reactions, (a) Addition reaction as the first step, resulting in a methylol derivative, methylol adenylic acid (b) Second step is a condensation reaction, a stable product methylene-bis-adenylic acid is derived between the methylol derivative and another adenine. Reproduced with permission from Shi et al.,AIMM 2001 9 107-116. 

Some fixatives work by combining with tissue molecules, hence the term addition reactions. This may continue as cross-linking, whereby the original adducted (added-onto) molecule attaches to another portion of the same molecule or to an adjacent molecule. A small branched polymer is thus created. Formaldehyde is the prime example of an additive and cross-linking fixative. 

Gonzales, C., Sosa, C, and Schlegel, H. B., Ai initio study of addition reaction of the methyl radical to ethylene and formaldehyde, J. Phys. Chem. 93, 2435 (1989). 

In the research aiming at the application of the addition reaction, Pringle and co-workers found that bifunctional phosphine starting materials like H2PCH2CH2PH2 spontaneously react with formaldehyde in the absence of the catalyst, forming water-soluble chelating phosphines [9]. 

Trinitrohydroxyethylbenzene nitrate or trinitrophenyl-/J-ethyl nitrate, melting at 83°C, is obtained from trinitrotoluene, that undergoes an addition reaction with formaldehyde in an alkaline medium to produce the corresponding alcohol (m.p. 112°C), which is then nitrated (Vender [87]) ... 

Methane reacts only slowly with oxygen below 400° C. Ethane oxidation was observed by Bone and Hill (S) at 290° to 323° C. Formaldehyde, a reaction product, was found to increase, reach a maximum, and then decrease. Addition in amounts of 1% to a 3 to 1 ethane-oxygen mixture at 316° C. and 720 mm. eliminated the induction period, but other additives such as nitregen dioxide, acetaldehyde, ethyl alcohol, or water, were also more or less effective. 

The low-temperature oxidation represents a complex system and can be better interpreted when the elementary reactions are firmly established. We arc inclined to assign formaldehyde only a minor role in the low-temperature regime. Further experimental work is required to clarify the interactions between formaldehyde and peroxides, the radical-induced formaldehyde oxidation, and the effect of formaldehyde addition in the low-temperature hydrocarbon-oxygen systems. It has been established that mercury vapor is effective for the destruction of peroxides. Mercury vapor addition to systems in the cool-flame zone would perhaps be of value in assessing not only the role of peroxides, but also that of formaldehyde in this interesting region. 

The third sample, resin C, was synthesized from a different reaction procedure. The initial Na/P mole ratio was 0.5, with the formaldehyde added slowly over a 3-hour period at 95 °C. The intention of this procedure was to provide conditions to maximize condensation and to minimize addition, producing maximum polymer size. The mole ratios after completion of the formaldehyde addition were F/P/Na = 2.0/1.0/0.50. A second addition of sodium hydroxide equivalent to 0.20 mole was then made, and the polymerization was completed at 85-95 °C. Table I lists some of the general comparative properties for the three resins. 

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