Phenol, Cresols and Other Alkyl Phenols

Phenol, monohydrobenzene, or carbolic acid discovered in 1834 by F. Runge is the parent compound of a homologous series of compounds with the hydroxy (-OH) group attached to the benzene ring. 

Cresols are an isomeric mixture of the simplest of alkyl phenols, i.e., monomethyl phenol or monohydroxy-toluenes. Similarly xylenols are the next higher homologues of cresols and are known as dimethyl phenols or hydroxy xylenes. 

Sometimes phenols where a hydroxy group replaces one or more of the hydrogen atoms has been compared with alcohols, where the -OH group is attached to a paraffinic carbon atom, whereas in case of phenols, the (-OH) group is attached to a carbon atom in an aromatic system. 

The simplest member of the group, phenol, has no isomer, whereas cresols exist in three isomers namely ortho, meta, and para. Xylenols have six different isomers. Some 

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On distillation of the above mixture in a distillation column, a bottom fraction containing 95-96% p-cresol, 4—5% m-cresol, 1% o-cresol and other alkyl phenols is produced. O-cresol and phenol mixture from the top of the column are further separated into o-cresol (98—99%) and phenol. Further enrichment of p-cresol to 98-99% is done through the process of crystallization and centrifuging. This is discussed in some details in the next chapter. 

I ovolac Synthesis and Properties. Novolac resins used in DNQ-based photoresists are the most complex, the best-studied, the most highly engineered, and the most widely used polymers in microlithography. Novolacs are condensation products of phenoHc monomers (typically cresols or other alkylated phenols) and formaldehyde, formed under acid catalysis. Figure 13 shows the polymerization chemistry and polymer stmcture formed in the step growth polymerization (31) of novolac resins. 

Alkylated phenol derivatives are used as raw materials for the production of resins, novolaks (alcohol-soluble resins of the phenol—formaldehyde type), herbicides, insecticides, antioxidants, and other chemicals. The synthesis of 2,6-xylenol [576-26-1] h.a.s become commercially important since PPO resin, poly(2,6-dimethyl phenylene oxide), an engineering thermoplastic, was developed (114,115). The demand for (9-cresol and 2,6-xylenol (2,6-dimethylphenol) increased further in the 1980s along with the growing use of epoxy cresol novolak (ECN) in the electronics industries and poly(phenylene ether) resin in the automobile industries. The ECN is derived from o-cresol, and poly(phenylene ether) resin is derived from 2,6-xylenol. 

The sulfonic acid resins such as Dowex-50 and Amberlyst-15 have been used to promote the alkylation of the more active aromatic rings but attempts to increase their acidity generally resulted in the degradation of the solid. 2 The more strongly acidic perfluorinated resin sulfonic acid, Nafion-H,2>3 has, however, been used to promote the alkylation of benzene and other aromatic compounds. Nafion-H catalyzed the vapor phase reaction between toluene and methanol. When nm at 185°C a 12% yield of the isomeric xylenes was obtained with the ortho isomer the major product. 0 Methylation of phenol at 205°C over this catalyst gave, at 63% conversion. 37% anisole and 10% of a mixture of the ortho and para cresols in a 2 1 ratio. Reaction of anisole with methanol under these conditions resulted in a 14% selectivity to the methyl anisoles at 40% conversion, with the ortho and para isomers formed in nearly equal amounts. ... 

With the MgAl-COs-LDOs (Mg/Al = 3-10) as catalysts, Velu et al. noted that the major products are anisole, o-cresol, and 2,6-xylenol. The product distribution, and in particular the ratio of O- to C-methylation, is dependent on temperature, phenol/methanol ratio, contact time, pressure, and catalyst composition (549,550). With the calcined MgFe-, MgCr-, and CuAl-LDHs as catalysts, direct C-alkyl-ation is dominant (461,551,552). Kinetic parameters were obtained from the alkylation of phenol with methanol ovct various catalysts derived from MAl-LDHs (M = Mg, Mn, Co, Cu, and Zn divalent ions) (553). The alkylation of phenol with various other reagents, such as Ci-Cg alcohols (554), isobutanol (555a,555b),... 

Various studies have reported the use of zeolite catalysts for the alkylation of alkylbenzenes other than toluene. Studies include the conversion of ethylbenzene and methanol to yield /7-ethyltoluene, which after dehydrogenation yields /7-methylstyrene. The latter compoimd can be polymerized to yield a polystyrene analog [63]. HZSM-5 zeolites prepared with Na, K, and Rb ions were examined by Kolboe et al [63] who reported selectivities of 1050% for ethylbenezene alkylation with methanol Hi para selectivity was observed, particularly for the Rb-containing zeolite that has a low Al content. In addition, the alkylation of phenol with methanol is a commercially viable process for the production of cresols and Tcylenols. The reaction occurs in the liquid phase at about 300°C and 5 MPa pressure. The catalyst is aluminum oxide, but recent work on HY and ZSM-5 zeolites has been reported that increases selectivity to /7-cresol as a result of shape selectivity effects [64]. 

While phenol is the most common monomer for novolac manufacture, it is far more common to see incorporation of other phenolic materials with novolacs than with resoles. Cresols, xylenols, resorcinol, catechols, bisphenols, and a variety of phenols with longer alkyl side chains are often used. While most resoles are made with a single phenolic monomer, two or more phenolic materials are often seen in novolac formulae. These additional monomers may be needed to impart special flow characteristics under heat, change a glass transition temperature, modify cure speed, or to adjust solubility in the application process among others. 

Because of the high antioxidant value of the p-cresol products, the work was extended to study the alkylation with a-olefins of three other readily available phenols phenol, 2,4-xylenol, and 2,6-xylenol. The nominal products were 2,4,6-tri(sec-alkyl)phenols, 6-(sec-alkyl)-2,4-xylends and 4-(sec-alkyl)-2,6-xylenols, as shown in the following reactions. Remember that these structural designations are for convenience the actual products are complex mixtures. Again, yields of the desired alkylated phenol were very high. The trialykylated phenol products were... 

The substituted phenols and cresols constitute about half the total volume of this group. Para-t-butylphenol is produced by the alkylation of phenol with isobutylene. The principal applications for this derivative are in the manufacture of modified phenolic resins for the rubber industry and in surface coatings. BHT is obtained from isobutylene and p-cresol. Technical-grade BHT is an antioxidant for plastics and elastomers, and is a gum inhibitor in gasoline. Food-grade BHT is an antioxidant in edible oils, preserves, and many other foods. 2,6-Di-t-butylphenol is used to produce a wide range of plastics additives, antioxidants, and gasoline additives. 

Friedel-Crafts technology and zeolite- or other solid catalyst-based processes are currently used for other aromatic alkylations, in particular for the manufacture of linear alkylbenzenes (LABs) made from C10-C14 olefins (Equation 8), or from the corresponding chloroparaffins and benzene, and also to make m- and p-cymene (isopropyltoluene Equation 9). LABs are used for the production of sulfonate detergents, while cymenes lead to m- and p-cresols through a procedure analogous to that used for the cumene-to-phenol process. 

Other more prominent process for production of resorcinol is through alkylation of benzene using propylene in such as a way that alkylation is done carefully to introduce isopropyl in 1 and 3 position, i.e., diisopropyl benzene which is then converted to resorcinol using a similar process of phenol from isopropyl benzene or cresols from iosopropyl toluene. 

For most resorcinol plants, sulfonation/caustic fusion is the common route, however, in Japan Sumitomo Chemicals are operating the world s largest resorcinol plant based on alkylation of benzene with propylene using anhydrous AICI3 as the catalyst. Some companies even today use solid phosphoric acid (SPA) as the catalyst for alkylation. Other downstream processes, i.e., oxidation of diisopropy benzene and its clevage to resorcinol are more or less similar to phenol and cresols processes. 

Cresylic acids or tar acids are other names under which mixed cresols containing all the isomers of cresols, some quantities of phenol and mixed xylenols and even some higher alkylated phenols are available in the market. 

The reaction of phenol alkylation with methanol over oxides and zeolites to produce anisol and cresols presents some of the features of the reaction under investigation. Studies of phenol alkylation revealed that the reaction was sensitive to the acid-base properties of the catalyst [5-13]. The catalytic activity increased with acidity, but the selectivity towards O- or C-methylated products did not follow a simple correlation with observed acid-base properties. According to [7,12,16] the catalysts with basic sites favour C-methylation. Other authors [6,11] recently reported that an increase in catalyst acidity promote C-methylation. Therefore, a variance in the results concerning acidity and catalytic properties exists in the literature. 

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