Sources of Phenols

Phenol was first isolated in the early nineteenth century from coal tar, and a small portion of the more than 4 billion lb of phenol produced in the United States each year comes from this sonrce. Althongh significant quantities of phenol are used to prepare aspirin and dyes, most of it is converted to phenolic resins used in adhesives and plastics. Almost all the phenol prodnced commercially is synthetic, with several different processes in cnrrent nse. These are snmmarized in Table 24.3. 

The reaction of benzenesnlfonic acid with sodium hydroxide (first entry in Table 24.3) proceeds by the addition-elimination mechanism of nucleophilic aromatic substitution (Section 23.6). Hydroxide replaces sulfite ion (SOs ) at the carbon atom that bears the leaving gronp. Thns, p-tolnenesulfonic acid is converted exclusively to p-cresol by an analogons reaction  

On the other hand, C-labeling studies have shown that the base-promoted hydrolysis of chlorobenzene (second entry in Table 24.3) proceeds by the elimination-addition mechanism and involves benzyne as an intermediate. 

The most widely used industrial synthesis of phenol is based on isopropylbenzene (cumene) as the starting material and is shown in the third entry of Table 24.3. The economically attractive features of this process are its use of cheap reagents (oxygen and sulfuric acid) and the fact that it yields two high-volume industrial chemicals phenol and acetone. The mechanism of this novel synthesis forms the basis of Problem 24.29 at the end of this chapter. 

The most important synthesis of phenols in the laboratory is from amines by hydrolysis of their corresponding diazoiuum salts, as described in Section 22.18  

The reaction of benzenesnlfonic acid with sodinm hydroxide (hrst entry in Table 

Multiple substitution by strongly electron-withdrawing groups greatly increases the acidity of phenols, as the values for 2,4-dinitrophenol (4.0) and 2,4,6-trinitrophenol (0.4) in Table 22.2 attest. 

Write a stepwise mechanism for the conversion of p-toluenesulfonic acid to p-cresol under the conditions shown in the preceding equation. 

Reaction of benzenesulfonic acid with sodium hydroxide 

A meta-nitro group is not directly conjugated to the phenoxide oxygen and thus stabilizes a phenoxide ion to a smaller extent. m-Nitrophenol is more acidic than phenol but less acidic than either o- or p-nitrophenol. 

Which is the stronger acid in each of the following pairs Explain your reasoning. 

Sample Solution (a) The best approach when comparing the acidities of different phenols is to assess opportunities for stabilization of negative charge in their anions. Electron delocalization in the anion of p-hydroxybenzaldehyde is very effective because of conjugation. 

A carbonyl group is strongly electron-withdrawing and acid-strengthening, especially when ortho or para to the hydroxyl group. p-Hydroxybenzaldehyde is a stronger acid than phenol. Its pAa is 7.6. 

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The conditions of pyrolysis either as low or high temperature carbonization, and the type of coal, determine the composition of Hquids produced, known as tars. Humic coals give greater yields of phenol (qv) [108-95-2] (up to 50%), whereas hydrogen-rich coals give more hydrocarbons (qv). The whole tar and distillation fractions are used as fuels and as sources of phenols, or as an additive ia carbonized briquettes. Pitch can be used as a biader for briquettes, for electrode carbon after coking, or for blending with road asphalt (qv). 

AH commercial processes for the manufacture of caprolactam ate based on either toluene or benzene, each of which occurs in refinery BTX-extract streams (see BTX processing). Alkylation of benzene with propylene yields cumene (qv), which is a source of phenol and acetone ca 10% of U.S. phenol is converted to caprolactam. Purified benzene can be hydrogenated over platinum catalyst to cyclohexane nearly aH of the latter is used in the manufacture of nylon-6 and nylon-6,6 chemical intermediates. A block diagram of the five main process routes to caprolactam from basic taw materials, eg, hydrogen (which is usuaHy prepared from natural gas) and sulfur, is given in Eigute 2. 

Dihydropyran-4-ones are a source of phenols via an intramolecular [2+2] photocycloaddition reaction and a Lewis-acid catalysed cleavage of the cyclobutane moiety <96TL1663>. 

Since lignins are polymers of phenolics and are major plant constituents with resistance to microbial decomposition, they are the primary source of phenolic units for humic acid synthesis (178, 179). Once transformed, these humic acids become further resistant to microbial attack and can become bound to soils (180) form interactions with other high molecular weight phenolic compounds (ex. lignins, fulvic acids) and with clays (181) and influence the biodegradation of other organic substrates in soils (182, 183). 

These results imply that highly aromatic ether linkages will be considerably broken at coal liquefaction temperatures resulting in a main source of phenolic groups of the dissolved coal. 

Precursors of phenylpropanoids are synthesized from two basic pathways the shikimic acid pathway and the malonic pathway (see Fig. 3.1). The shikimic acid pathway produces most plant phenolics, whereas the malonic pathway, which is an important source of phenolics in fungi and bacteria, is less significant in higher plants. The shikimate pathway converts simple carbohydrate precursors into the amino acids phenylalanine and tyrosine. The synthesis of an intermediate in this pathway, shikimic acid, is blocked by the broad-spectrum herbicide glyphosate (i.e., Roundup). Because animals do not possess this synthetic pathway, they have no way to synthesize the three aromatic amino acids (i.e., phenylalanine, tyrosine, and tryptophan), which are therefore essential nutrients in animal diets. 

There was no information found on the placental transfer of phenol or on the concentrations of phenol present in breast milk. There is evidence that benzene and its (not specifically identified) metabolites do cross the placenta, although there is no evidence of selective accumulation (Ghantous and Danielsson 1986). Additional studies of this issue are needed to determine if phenol and its metabolites are among the metabolites of benzene that cross the placenta, and if so whether phenol behaves like benzene in the lack of accumulation. Information is also needed on the content of phenol in breast milk under various conditions, e.g, smoking versus non-smoking mothers, in order to determine if breast milk could ever be a source of phenol exposure for children. 

The early sources of phenol were the destructive distillation of coal and the manufacture of methyl alcohol from wood. In both cases, phenol was a by-product. Recovered volumes were limited by whatever was made accidentally in the process. Initial commercial routes to on-purpose phenol involved the reaction of benzene with sulfuric acid (1920), chlorine (1928), or hydrochloric acid (1939) all these were followed by a subsequent hydrolysis step (reaction with water to get the -OH group) to get phenol. These processes required high temperatures and pressures to make the reactions go. They re multistep processes requiring special metallurgy to handle the corrosive mixtures involved. None of these processes is in commercial use today. 

Lignin A large polymeric macromolecule synthesized only by woody plants. The degradation of lignin is a unique source of phenolic acids. 

Irrespective of the sources of phenolic compounds in soil, adsorption and desorption from soil colloids will determine their solution-phase concentration. Both processes are described by the same mathematical models, but they are not necessarily completely reversible. Complete reversibility refers to singular adsorption-desorption, an equilibrium in which the adsorbate is fully desorbed, with release as easy as retention. In non-singular adsorption-desorption equilibria, the release of the adsorbate may involve a different mechanism requiring a higher activation energy, resulting in different reaction kinetics and desorption coefficients. This phenomenon is commonly observed with pesticides (41, 42). An acute need exists for experimental data on the adsorption, desorption, and equilibria for phenolic compounds to properly assess their environmental chemistry in soil. 

The major source of phenolic substances in beer is the malt, but the hops, contributes to the total amount as well [321]. Phenolic substances in beer are involved in the physical and chemical stability, as well as in the froth maintenance [322-324]. Despite their important role, the number of scientific papers dealing with phenolic evaluation in beer is not so high [322,325-327]. 

The most prominent wood adhesives used over the last quarter of a century have been aminoplast and polyphenolic types (2). In the United States, polyphenolic adhesives continue to be predominantly used for production of weather-resistant wood products, such as structural plywoods and flake boards (3). Phenolic resin prices have increased over the past decade, generally paralleling phenol prices. This increase has occurred in part due to a continuing erosion of United States phenol manufacturing capacity and the corresponding increase in availability of phenol from other countries. Any significant increase in the price of oil (the source of phenol) itself or interruption in supply will only compound the problem and raise phenol prices even higher. 

There are many phenolic substances in plants and thus in foods. Rich dietary sources of phenolics include fruits, tea, coffee, cocoa, and processed foods derived from these, such as wine. At high levels, and in particular when sugar levels are low, phenols impart an astrin-gency, bitterness, and color to foods. In red wine, unsweetened tea, and chocolate products, the taste is heavily influenced by the presence of phenolics. Therefore, an assessment of phenolic content in food is of great importance. 

Phenolic wastes are one of the most prevalent forms of chemical pollutants in industry today. The major sources of phenolic waste are insulation fiberglass manufacturing, petroleum refineries, textile mills, steel making, plywood, hardboard production, manufacture of organic chemicals, paint stripping, and wood preservatives. Eisenhauer (1964) first studied oxidation of phenolic wastes with Fenton s reagent. It has been demonstrated that the oxidation of phenol involves the intermediate formation of catechol and hydroquinone (Merz and Waters, 1949 Stein and Weiss, 1951 Wieland and... 

Catalysis (27-30) which allows for the direct oxidation of benzene to produce phenol. Economic analyses have shown that these are attractive only in specific instances where, for example, a cheap source of N20 is available. Nevertheless, these developments have shown that direct oxidation is possible and further innovations in this area should probably be expected. The demands for acetone and phenol have generally tended to follow each other. However, as bisphenol A becomes an even more important end use for phenol and acetone, there will be a need for a separate source of phenol. The synthesis of bisphenol A requires two moles of phenol for every one mole of acetone, while the peroxidation of cumene produces one mole of each. Still, processes such as the direct oxidation of benzene are unlikely to have a major impact on cumene demand in the short term since there are competing processes such as Mitsui s for converting acetone back to propylene. 

Pyrolysis of biomass is known to produce a complex mixture of phenolic compounds, which are derived primarily from the lignin fraction of the biomass (1-4) Elder and Soltes (5, 6) have investigated a phenolic fraction obtained from pyrolysis oils made in an updraft gasifier by TECH AIR as a source of phenolic adhesives a phenolics fraction was separated by solubility differences of oil fractions based on solubility of acids in aqueous bicarbonate solutions and... 

Although coal tar is still an industrial source of phenol and the three cresols (methylphenols), e.g. m-cresol (2), and the dimethyl derivatives (xylenols), synthetically manufactured material predominates. 

A crnde extract of sweet potato Ipomoea-batatas (L.) Lam.) was nsed as a source of phenol oxidases (polyphenoloxidase, tyrosinase, catecholoxidase, EC 1.14.18.1). The extract was directly placed in the carrier of a FIA system with UVD, to promote oxidation of phenolic compounds to o-quinones that condense to form melanin-like pigments with a strong absorption at 410 nm. The determination of phenols in industrial wastewaters showed good agreement with conventional methods (correlation coefficient 0.9954) LOD was 10 p,M, with RSD <2.7% (w = 6). Under optimal storage conditions the enzymatic activity did not vary for at least five months . 

Despite its brilliant results, it seems unlikely that the Solutia process can become a major source of phenol. Nitrous oxide availability is quite limited and its production on-purpose (by the conventional ammonium nitrate decomposition, which enables nitrous oxide of high purity to be produced for medical anesthetic applications, or even by selective oxidation of ammonia) would result too expensive. Therefore, the only reasonable scenario to exploit the Solutia process is its implementation close to adipic acid plants, where nitrous oxide is co-produced by the nitric oxidation of cyclohexanol-cyclohexanone mixtures and where it could be used to produce phenol instead of being disposed of However, the stoichiometry of the process is such that a relatively small phenol plant would require a world-scale adipic acid plant for its nitrous oxide supply. In fact, a pilot plant has been operated using this technology, but its commercialization has been postponed. 

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