Amino acids formaldehyde reaction with, methylol

Figure 15.1 Reactions of formaldehyde with peptides and amino acids. Shown are the four types of reaction products seen when peptides or amino acids are treated with formaldehyde in aqueous solution. These reaction products are methylol (hydroxymethyl) adduct (reaction 15.1), Schiff-base (reaction 15.2), 4-imidazolidinone adduct (reaction 15.3), and one type of methylene bridge [cross-link] (reaction 15.4).

The methylolation reaction may be catalyzed by either acids or bases. Under acid conditions, the formaldehyde probably picks up a proton forming a carbonium ion that then attacks the unshared pair of electrons on the amino nitrogen. In alkaline solution, the amino compound may react with 0H ion to form an amino anion that can then react with formaldehyde. These reaction mechanisms are illustrated... 

The methylol compounds produced by these reactions are relatively stable under neutral or alkaline conditions, but undergo condensation, forming polymeric products under acidic conditions. Consequently, the first step in making an amino plastic is usually carried out under alkaline conditions. The amino compound and formaldehyde are combined and form a stable resin intermediate that may be used as an adhesive or combined with filler to make a molding compound. The second step is the addition of an acidic substance to catalyze the curing reaction, often with the application of heat to cure the amino resin to the solid cross-linked state. In this reaction, the methylol group is probably protonated and a molecule... 

Tetrakis(hydroxymethyl)phosphonium Salts. The reaction of formaldehyde and phosphine in aqueous hydrochloric or sulfuric acid yields tetrakis-(hydroxymethyl)phosphonium chloride [124-62-1 or the sulfate [55566-30-8]. These are made by Rhodia, Cytex, and Chinese producers. The salts have the general structure (HOCH2)4P" X . They are water-soluble crystalline compoimds sold as concentrated aqueous solutions. The methylol groups are highly reactive (121-125) and capable of being cured on the fabric by reaction with ammonia or amino compounds and post-oxidized to form durable cross-linked phosphine oxide structures. 

Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

Hexamethylolmelamine can further condense in the presence of an acid catalyst ether linkages can also form (see Urea Formaldehyde ). A wide variety of resins can be obtained by careful selection of pH, reaction temperature, reactant ratio, amino monomer, and extent of condensation. Liquid coating resins are prepared by reacting methanol or butanol with the initial methylolated products. These can be used to produce hard, solvent-resistant coatings by heating with a variety of hydroxy, carboxyl, and amide functional polymers to produce a cross-linked film. 

The common procedure for adding reactants consists either in simultaneously mixing all the chemical species involved in the reaction, or in allowing the amine and the aldehyde to react first and then adding the substrate. In some cases, however, the condensation of substrate and formaldehyde is carried out first in order to isolate the corresponding methylol derivative, which is subsequently submitted to react with the amine (X-methylation of amino derivatives see Fig. 12) this is advantageous with several substrates, such as nitroalkanes, ferrocenes,- sulfonic acids, and phosphines. ... 

The manufacturing process for partially alkylated amino formaldehyde resins consist of two consecutive reactions. The first reaction is the methylolation (hydroxy-methylation) of an amino compound such as urea, melamine, or benzoguanamine with formaldehyde and can be carried out under basic or acidic conditions. Hydroxy-methylation under acidic conditions leads to simultaneous alkylation and polymerization of the amino resin. The second reaction, alkylation (or etherification), is carried out under acidic conditions. 

Success in making and using amino resins depends on the precise control of these two reactions. The first is primarily concerned with the manufacture of the resin, while the second is most important for the application in which the resin is used. The reaction of formaldehyde with an amino group will take place under either acidic or basic conditions. The second reaction, however is catalyzed by acid. It is common practice to prepare the resin under fairly neutral conditions. Both reactions can proceed slowly to form the methylol derivatives and advance the polymerization to the desired degree. Some polymerization is needed to... 

Other amino comonomers can be used, but their proportions in the materials are low compared to those of the two preceding molecules. The reaction mechanisms corresponding to the formation of the precursors and their polymerization were presented in Chapter 7. The precursors are generally prepared in batches from the comonomers in aqueous solution. The structure and the properties of the final material are determined by the composition of the reaction medium, its pH, and the time and temperature of reaction. The first step of the process is the formation of methy-lol derivatives, and the second step involves their condensation with elimination of formaldehyde or water. An acidic medium does not permit an easy control of the formation of methylol compounds, and polycondensations are generally carried out in slightly basic medium (pH 7.5 to 9.0). 

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