Reactions of Phenols as Acids

The second and third steps of the mechanism may actually take place at the same time that is, the loss of H2O and the migration of CeHs— may be concerted. 

Although phenols are structurally similar to alcohols, they are much stronger acids. The pKa values of most alcohols are of the order of 18. However, as we see in Table 21.1, the pKa values of phenols are smaller than 11. 

Let us compare two superficially similar compounds, cyclohexanol and phenol  

Although phenol is a weak acid when compared with a carboxylic acid such as acetic acid (p/ a = 4.76), phenol is a much stronger acid than cyclohexanol (by a factor of eight p Tg units). 

We can understand this effect by noting that the carbon atom which bears the hydroxyl group in phenol is sp hybridized, whereas in cyclohexane it is sp hybridized. Because of their greater i character, ip -hybridized carbon atoms are more electronegative than sp -hybridized carbon atoms (Section 3.8A). 

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REACTIONS OF PHENOLS AS ACIDS 21.5A Strength of Phenols as Acids... 

Phenolic Resins. Phenohc resins (qv) are formed by the reaction of phenol [108-95-2] C H O, and formaldehyde [50-00-0] CH2O. If basic conditions and an excess of formaldehyde are used, the result is a resole phenohc resin, which will cure by itself Hberating water. If an acid catalyst and an excess of phenol are used, the result is a novolac phenohc resin, which is not self-curing. Novolac phenohc resins are typically formulated to contain a curing agent which is most often a material known as hexamethylenetetraamine [100-97-0] C H22N4. Phenohc resin adhesives are found in film or solution... 

PhenoHc resins are prepared by the reaction of phenol or substituted phenol with an aldehyde, especially formaldehyde, in the presence of an acidic or basic catalyst. Their thermosetting character and the exotherm associated with the reaction presented technical barriers to commercialization. In 1900, the first U.S. patent was granted for a phenoHc resin, using the resin in cast form as a substitute for hard mbber (10). 

Phenolic Resins. PhenoHc resins [9003-35 ] (qv) are thermosets prepared by the reaction of phenol with formaldehyde, through either the base-cataly2ed one-stage or the acid-cataly2ed two-stage process. The Hquid intermediate may be used as an adhesive and bonding resin for plywood, particle board, ftberboard, insulation, and cores for laminates. The physical properties for typical phenoHc laminates made with wood are Hsted in Table 1. 

The reaction of alcohols and acid chlorides in the presence of magnesium has been described (68). With primary and secondary alcohols the reaction is very smooth, and affords high and sometimes quantitative yields. Difficulty esteritiable hydroxy compounds such as tertiary alcohols and phenols can be esteritied by this method. The reaction carried out in ether or benzene is usually very vigorous with evolution of hydrogen. 

Phenolic resins were discovered by Baeyer in 1872 through acid-catalyzed reactions of phenols and acetaldehyde. Kleeberg found in 1891 that resinous products could also be formed by reacting phenol with formaldehyde. But it was Baekeland who was granted patents in 1909 describing both base-catalyzed resoles (known as Bakelite resins) and acid-catalyzed novolac products.2... 

For many years phenol was made on a large industrial scale from the substitution reaction of benzene sulfonic acid with sodium hydroxide. This produced sodium sulfite as a by-product. Production and disposal of this material, contaminated with aromatic compounds, on a large scale contributed to the poor economics of the process, which has now been replaced by the much more atom economic cumene route (see Chapter 2, Schemes 2.2 and 2.3). 

Thermosetting resin produced by the reaction of phenol and formaldehyde in the presence of either an acid or an alkaline catalyst. In rubber compounding these resins are used as plasticisers and reinforcing materials. Phenyl-a-Naphthylamine... 

The intermolecular reaction of phenols with propiolic esters occurs in the presence of a Pd(OAc)2 catalyst to afford coumarin derivatives directly.48,48a An exclusive formation of 5,6,7-trimethoxy-4-phenylcoumarin is observed in the Pd(OAc)2-catalyzed reaction of 3,4,5-trimethoxyphenol with ethyl phenylpropiolate in TFA (Equation (46)). Coumarin derivatives are obtained in high yields in the cases of electron-rich phenols, such as 3,4-methylenedioxyphenol, 3-methoxyphenol, 2-naphthol, and 3,5-dimethylphenol. A similar direct route to coumarin derivatives is accomplished by the reaction of phenols with propiolic acids (Equation (47)).49 A similar reaction proceeds in formic acid at room temperature for the synthesis of coumarins.50,50a Interestingly, Pd(0), rather than Pd(n), is involved in this reaction. 

An example where electron transfer from PhO- is important comes from a related publication on the reaction of phenol with O2 where [Ru(bpy)3]2+ is used as a photosensitizer (14). In acidic media the reaction involves generation of 02 by quenching of excited [Ru(bpy)3]2+ reaction of 2 with phenol leads to the production of benzoquinone. The quantum yields for benzoquinone production are highly pH dependent, showing a sharp peak at pH 8.4. This unusual pH dependence arises from the competition of several pathways, and one of the most important being the electron-transfer quenching of [ Ru(bpy)3]2+ by PhO- ... 

Kanno et al. (1982) studied the aqueous reaction of phenol and other substituted aromatic hydrocarbons (aniline, toluidine, 1-naphthylamine, cresol, pyrocatechol, resorcinol, hydroquinone, and 1-naphthol) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride (Kanno et al., 1982). The amount of cyanogen chloride formed increased at lower pHs. At pH 6, the greatest amount of cyanogen chloride was formed when the reaction mixture contained ammonium ion and hypochlorous acid at a ratio of 2 3 (Kanno et al., 1982). 

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