Mechanism Phenol-methanal reaction

The rate of substitution correlates with the electron density at the aromatic nucleus. Several quantum mechanical calculations (CNDO [1] and AM 1 [52], where CNDO stands for Complete Neglect of Differential Overlap and AMI stands for Austin Model 1) show that the position of the methylol group on the ring influences electron density. The position reactivities determined by Freeman and Lewis [75] were later confirmed by Zsavitsas [76], who did extensive gas chromatography studies on the separation of the substituted phenols formed. Also, the pK -value of these compounds correlates closely with the kinetic data determined. Another group studied the influence of the metal ion in the metal hydroxide catalyst on kinetics and product stmcture [73]. The rate constants evaluated for the phenol methanal reaction under alkaline... 

The mechanism and phenol-formaldehyde reaction under acidic conditions is different from that under basic conditions described previously. In the presence of acid the products o- and p-methylol phenols, which are formed initially, react rapidly with free phenol to form dihydroxy diphenyl methanes (Figure 4.25). The latter undergo slow reaction with formaldehyde and phenolic species, forming polynuclear phenols by further methylolation and methylol link formation. Reactions of this type continue until all the formaldehyde has been used up. The final product thus consists of a complex mixture of polynuclear phenols linked by o- and p-methylene groups. 

In aqueous alkaline solution, phenol reacts with formaldehyde (methanal) at low temperatures to form a mixture of 2- and 4-hydroxy-benzyl alcohols. This hederer-Manasse reaction is another example of electrophilic attack which results in the formation of a new C-C bond. The mechanism is illustrated in Scheme 4.14. These products readily lose water to form quinomethanes (methylenecyclohexadienones), which react with more phenoxide. This process is repeated over and over again to produce a cross-linked polymer or phenol-formaldehyde resin (e.g. Bakelite) in which the aromatic rings are linked to methylene bridges. 

Resoles are formed in a mechanistically complex reaction dependent on the amount of catalyst, temperature, molar ratio of the starting materials, the purity of the taw materials, and the nature of the reaction vessel. A very complete study of this mechanism was published by Freeman and Lewis [75] who investigated the reaction between substituted phenols and methanal under the influence of sodium hydroxide at 30°C. The kinetics of this reaction system were exarttined by measuring the quantity of the different hydroxymethylated phenols formed as a function of concentration and reaction time. The concentration versus time profiles for the products formed in this system are shown in Figure 5. The data of Figure 5 allow the calculation of the rate constants for the formation of different methylol substituted phenols shown in equation 13. The rate constants are ... 

The formation of methylene bridges is slow for the first group, faster for group 2. The following mechanism can be written for the reaction between a phenolic compound and methanal in the presence of alkaline and alkaline earth hydroxides (Scheme 5) [56,73]. 

Reaction Mechanism of the Sodium Hydroxide Catalyzed Reaction between Phenol and Methanal. This Mechanism also Occurs in the Presence of other Alkaline and Alkaline Earth Hydroxides, (source ref [73])... 

Polycondensation products by reaction of melamine, phenol and methanal have been used as moldings, lanunates, eoatings, and adhesives because of their improved chemical, mechanical, electrical, and optical properties relative to pure phenolics and to pure melamine resins. 

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