Methylol derivatives Formaldehyde with phenols

These are a member of the phenolics. In the presence of acid catalysts, and with the mole ratio of formaldehyde to phenol less than 1, the methylol derivatives condense with phenol to form, first, dihydroxydiphenyl methane and, on further eondensation with the methyl bridge formation, fusible and soluble linear low polymers called novolacs are formed, having the following structure ... 

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. 

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 ... 

These materials will then slowly react with further formaldehyde to form their own methylol derivatives which in turn rapidly react with further phenol to produce higher polynuclear phenols. Because of the excess of phenol there is a limit to the molecultir weight of the product produced, but on average there are 5-6 benzene rings per molecule. A typical example of the many possible structures is shown in Figure 23.11. 

In alkaline media, phenolic units may react with formaldehyde, forming methylol derivatives that condense with themselves or with another phenol (J, Fig. 1.4). This formaldehyde condensation reaction forms the basis for using technical lignins in the production of adhesives. 

Urea (NH2CONH2) reacts with formaldehyde similarly to phenol to produce methylol derivatives that then condense further to yield a cross-linked network (Scheme 1). Actually, at a mole ratio of 1.5-2 mol of formaldehyde to urea and a pH of 7.5, a mixture of the monomethylol, dimethylol, trimethylol, and tetramethylol ureas are formed. For further extensive condensation to take place, the pH of the system must be made acidic. Thus, it is possible to concentrate the initial resin solution or spray-dry it to a soluble powder that can be dissolved and mixed with an acid catalyst at the time of application to induce the curing reaction. The ratio of formaldehyde to urea used in commercial resins varies with the manufacturer, but is always less than 2 1. 

Thermosetting phenolic resins include a number of polymers, the most common being obtained from the condensation of phenol with formaldehyde. The OH group on the benzene ring increases the reactivity in the o- and p- positions leading to three reactive centers for the phenolic component, while formaldehyde acts as having two active centers that can lead to a fully crosslinked polymer. The process may take place in neutral or alkaline conditions when in the first stage of the reaction, compounds known as methylol derivatives are formed. The condensation of phenol with formaldehyde occurs randomly at ortho- or para- position of the phenol, as shown below ... 

Novolacs (two-stage). In the presence of acid catalysts, and with a mole ratio of formaldehyde to phenol of less than 1, the methylol derivatives... 

General Chemistry. Phenolic polymers are formed with a phenolic or phenol derivative monomer and formaldehyde crosslinking agent in two discrete steps. The first step, shown in Fig. 2, is an addition or methylolation step. Formaldehyde reacts with an active ortho or para proton to form the addition product which... 

Other important methylol derivatives include saJigenin (methylolphenol) which is formed when alkaline formaldehyde reacts with phenol,... 

XTnder acid conditions, the situation is reversed in that the conversion of methylol derivatives to methylene derivatives is usually more rapid than their formation. Granger suggests that this is because mono-methylol derivatives are converted to methylene derivatives as soon as they are formed.. s a result, the introduction of more than one methylol group in the simple phenol is prevented and the polynuclear phenols thus obtained, as would be expected, react less readily with additional formaldehyde. 

Miscelianeous Substituted Phenols Phenofe containing other substituents than meth l groups and containing free ortho and/or para positions form phenol alcohol derivatives rtth A aiydng degrees of ease vhen subjected to reaction with formaldehyde in the presence of alkah. Numerous examples cd ieolable products obtained by this procedure, such as the 2,6-dimethyloI derivath e of p-ben dphenol and the methylol derivatives of ortho-ehlorophenol, 2,4-dimethylol-l-chlorophenol and 4-methylol-1-clilorophenol, etc., are described. 

Methoxy derivatives of divalent phenols such as guaiacol react like simple monovalent phenols, forming methylol and methylene derivatives. Euler and co-workers- isolated vanillyl alcohol (methjdolguaiacol) and a di-methylol derivative of a methylene-bis-methoxyphenol from the products obtained by reacting guaiacol with formaldehyde under alkaline conditions. Under acidic conditions tw o isomeric methylene-bis-methoxyphenols were obtained. 

The second phase in resole formation is reaction of the activated phenol with the aldehyde to form the phenol alcohol derivative. When the aldehyde is formaldehyde, the derivative is a hydroxymethyl phenol and the process is known as methylolation. Scheme 2 illustrates this reaction. Since resoles are usually made with excess aldehyde, more than one substitution may be made on the ring. When the reactants are phenol and formaldehyde, up to three methylol groups may be substituted. This reaction has been extensively studied and the rates of... 

The reaction of phenol and formaldehyde in alkaline conditions results in the formation of o- and p-methylol phenols. These are more reactive towards formaldehyde than the original phenol and undergo rapid substitution with the formation of di- and trimethylol derivatives. The methylol phenols obtained are relatively stable in an alkaline medium but can undergo self-condensation to form dinuclear and polynuclear phenols (of low molecular weight) in which the phenolic nuclei are bridged by methylene groups. Thus in the base-catalyzed condensation of phenol and formaldehyde, there is a tendency for polynuclear phenols, as well as mono-, di-, and trimethylol phenols to be formed. 

Methylol phenols are sensitizers contained in resins based on phenol and formaldehyde of the resol type. Cross reactivity is possible with other phenol derivative molecules. 

Meta-cresol, which has three positions capable of methylolation, is high reactive with formaldehyde, and the alcohols formed resinify rapidly even under alkaline conditions. As a result, it is extremely difficult to isolate pure phenol alcohols. However, crystalline products beliei ed to be monoalcohols and melting at 105, 110, 117, and 122 C have been reported . The difficulties of isolating pure products are considerable, since three monomethylol derivatives, three dimethylol derivatives and one trimethylol derivative are possible. 

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