Phenols with formaldehyde

The main processes for the manufacture of hydroxybenzaldehydes are based on phenol. The most widely used process is the saligenin process. Saligenin (2-hydroxybenzyl alcohol [90-01-7]) and 4-hydroxybenzyl alcohol [623-05-2] are produced from base-catalyzed reaction of formaldehyde with phenol (35). Air oxidation of saligenin over a suitable catalyst such as platinium or palladium produces sahcylaldehyde (62). 

Uses of Formaldehyde. Formaldehyde is the simplest and most reactive aldehyde. Condensation polymerization of formaldehyde with phenol, urea, or melamine produces phenol-formaldehyde, urea formaldehyde, and melamine formaldehyde resins, respectively. These are important glues used in producing particle hoard and plywood. 

Acid catalysts, such as metal oxides and sulfonic acids, generally catalyze condensation polymerizations. However, some condensation polymers form under alkaline conditions. For example, the reaction of formaldehyde with phenol under alkaline conditions produces methy-lolphenols, which further condense to a thermosetting polymer. 

Thermosets are formed by crosslinking (curing) of reactive linear and branched macromolecules and can be manufactured by polycondensation, polymerization and polyaddition. Thermosets can therefore be processed once only with the application of heat and pressure to form semi-finished products or finished articles and cannot be recovered their processing is irreversible. Amongst the most familiar thermosets are the combinations of formaldehyde with phenol, resorcinol etc. (phenolics), urea, aniline, melamine and similar combinations (aminoplastics). 

Technical Importance of the Aldehydes.—Formalin solution is used as a disinfectant and as a preservative. Caseinogen hardened with formaldehyde is a widely used substitute for vulcanite, as is also the synthetic resin bakelite, which is produced by the condensation of formaldehyde with phenol (p. 243). 

Baekeland recognized that the trifunctional phenol would produce network polymers and therefore used difunctional ortho- or para-substituted phenols to produce linear paint resins. Linear thermoplastic products are formed by alkaline or acid condensation of formaldehyde with phenol derivatives such as /r-cresol (structure 4.81). 

The polymerization products of propylene have been observed to be saturated hydrocarbon polymers and terpenelike unsaturated hydrocarbons (Kuhn, 64). The condensation of formaldehyde with phenols and cyclohexanols by means of aqueous hydrogen fluoride has also been observed (Badertscher el al., 65). 

Drumm, M. F. and J. R. LeBlanc, The Reactions of Formaldehyde with Phenols, Melamine, Aniline and Urea, Chap. 5 in Step-Growth Polymerizations, D. H. Solomon, ed., Marcel Dekker, New York, 1972. 

Bakelite, A synthetic resin obtained by the condensation of formaldehyde with phenols. 

Another example of the use of deoxidation of oxygen-containing compounds for their structural analysis was given by Wi jnands et al.68, who investigated the structure of novolaks, prepared by polycondensation of formaldehyde with phenol, />-cresol and w-cresol. The novolaks were transformed into saturated hydrocarbon mixtures by direct hydrogenation. Ultimate analysis of the hydrocarbons confirmed the linear structure of the novolaks ... 

The first completely synthetic ion exchange resins were prepared by B. A. Adams and E. L. Holmes in 1935. The basis of their synthesis was the condensation polymerization of methanal (formaldehyde) with phenol or polysubstituted benzene compounds to give, after... 

Calixarenes are macrocyclic molecules synthesized with high yield by condensation of appropriate arenes and aldehyde derivatives. Calix means bowF in Latin and Greek, and this phrase reflects the shape of the tetramer, which usually adopts a bowl or beaker-like conformation. Gutsche first introduced the name calixarene for this class of compounds [38]. Several authors have exhaustively reviewed the chemistry and synthetic procedures, which lead to different structural modifications of calixarenes [39-42]. In general, three types of calixarenes derivatives are known first, metacyclophanes (type 1) second, those obtained by condensation of formaldehyde with phenol (type 11), and third, those obtained by reaction with resorcinol (type III) (Scheme 6). The latter modifications are also called resorcarenes to distinguish calixarenes of type III from those of type II. 

Finally, the overall behavior of urea toward formaldehyde is much different than is the behavior of, for example, formaldehyde with phenol. Mixing phenol and formaldehyde at a ratio of 4 1 in an acidic medium will result in a reaction of impressive vigor. At ratios of 4 1 urea and formaldehyde are not capable of advancing under acidic conditions even with the application of heat. Urea-formaldehyde concentrate, a stable mixture of urea and formaldehyde at a mole ratio of 4.8 1 and concentrations of as high as 85% solids, is a common material of commerce. These observations, taken together, are not consistent with the orderly formation of a urea-formaldehyde condensation polymer. 

Among hydroxybenzaldehydes, the o- and p- hydroxy isomers are the most important for commercial applications in agricultural, flavor and fragance, pharmaceutical or polymer fields (ref. 1). The two main processes for the manufacture of hydroxybenzaldehydes are both based on phenol. The most widely used process is the saligenin process. Hydroxybenzyl alcohols (o- and p- isomers) are produced from base - catalyzed reaction of formaldehyde with phenol (ref. 2). Air oxidation of these alcohols over a suitable catalyst (based on palladium or preferentially on platinum) produces hydroxybenzaldehydes (ref. 3). The Reimer -Tiemann process allows the coproduction of o- and p- hydroxybenzaldehydes (ref. 4). Treatment of phenol with aqueous chloroform and sodium hydroxide leads to benzal chlorides which are rapidly hydrolyzed by alkaline medium to aldehydes. The previous processes need two chemical steps and produce salt effluents. 

Novolaks are condensation products of formaldehyde with phenols, obtained by acidic catalysis (usually oxalic acid [27]) ... 

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

Bifunctional monomers, such as A-A, B-B and A-B, yield linear polymers. Branched and crosslinked polymers are obtained from polyfunctional monomers. For example, polymerization of formaldehyde with phenol may lead to complex architectures. Formaldehyde is commercialized as an aqueous solution in which it is present as methylene glycol, which may react with the trifunctional phenol (reactive at its two ortho and one para positions). The type of polymer architecture depends on the reaction conditions. Polymerization imder basic conditions (pH = 9-11) and with an excess of formaldehyde yields a highly branched polymer (resols. Figure 1.8). In this case, the polymerization is stopped when the polymer is still liquid or soluble. The formation of the final network (curing) is achieved during application (e.g., in foundry as binders to make cores or molds for castings of steel, iron and non-ferrous metals). Under acidic conditions (pH = 2-3) and with an excess of phenol, linear polymers with httle branching are produced (novolacs). 

Thermosets can be divided into several classes depending on the chemical composition of the monomers or pre-polymers (resins). Important thermosetting resins in current commercial applications are the condensation products of formaldehyde with phenol (phenolic resins), urea or melamine (amino resins). Other major classes are epoxy resins, unsaturated polyester resins, allyl resins and isocyanate resins. 

Aldehydes can be condensed with materials such as phenols or alcohols to form gels. The history of condensation of formaldehyde with phenol dates back to 1872 by Adolf Baeyer. Later (1910), Leo H. Baekeland used this reaction to produce a product which he named Bakelite. The condensation reactions of formaldehyde with phenolic compounds have recently been used for diversion of flow from high permeability regions in porous... 

The successful use of resins produced by the condensation of formaldehyde with phenol catalyzed the development of other condensation products of formaldehyde with urea and melamine. The urea resins were described by Tollens in 1884 and were patented by John in 1918. 

The most commonly known phenolic composite group is phenol formaldehyde polymers (phenoplasts). They are produced by polycondensation of a phenol and a mixture of phenols (phenol and phenol derivatives like cresol-resorcinol or para tertiary butyl phenol) with an aldehyde, usually formaldehyde and hexamethylene tetramine. Reaction of formaldehyde with phenol (up to 3 moles of formaldehyde can react with one mole of phenol - phenol acts as a three functional monomer) yields methylol groups in the ortho and para positions of the phenol molecule. In a further reaction, the methylol groups condenses with another molecule of phenol to form a methylene bridge. In practice, a prepolymer (usually a powder) is prepared first which is then cured later to the shape of the article in the mould. 

Phenolic resins are the reaction products of formaldehyde (usually in the form of aqueous formaldehyde), with phenol. Sometimes alkyl phenols, such as cresol and xylenol, are incorporated to reduce crosslink density and brittleness, although reactivity is also reduced. Phenolic resins are offered in two forms, novolacs and resols, characterised by their different methods of manufacture and curing. 

Resins. Most attention has focused on C spectra of the basic urea-formaldehyde system, " though resins of formaldehyde with phenols, resorcinol, melamine, and furfuryl alcohol have also been investigated. Because of the extremely irregular structure of these systems, well-resolved spectra are often obtainable only in the early stages of reaction, but even so a large number of signals have been assigned, particularly methylol, often with the aid of model compounds. ... 

This chapter is concerned with the practical aspects of reaction of formaldehyde with phenols to give both methylolated and cross-linked products. The reaction of other aldehydes to give resinous products will be briefly mentioned. The mechanism of these reactions will not be discussed. 

A widely used industrial process for the production of salicylaldehyde is the Saligenin process that produces hydroxybenzyl alcohols (o- and p-isomers) from the base-catalyzed reaction of formaldehyde with phenol, followed by oxidation using a palladium or platinum catalyst to produce the hydroxybenzaldehyde (Eq 1 44) 129,130.131... 

Draw the structure of the addition product of formaldehyde with phenol at the ortho position. Draw the structure of the Michael product of this compound with phenol at the ortho position. 

The phenolics were the first wholly synthetic plastic materials commercialized. The most important phenolic plastics are derived from the condensation of formaldehyde with phenol... 

Phenol Alcohols. As has been previously pointed out, the simplest defimte reaction products of formaldehyde with phenols are the methylol-phenols or phenol alcohols. Although some of these derivath es are too reactive to be isolated, a number have been obtained as pure crystalline products. In some respects they are analogous to the methylol derivatives of aldehydes and ketones, a similarity which is readily demonstrated when the keto- or ortho- and para-quinoidal forms of the phenolic nucleus are designated in the structural formula. The mechanism of their formation from the primary phenolic hemifoniLals may involve tautomeric rearrangements of the sort indicated below ... 

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