Phenols acid-base properties

In the polyazamacrocyclic phenols [64], an acidic function, the phenol group, has an intra-annular orientation while the basic units, the nitrogen atoms of the macrocycles, have no defined orientation. In water at 25°C, the pATa values of the phenols were measured and compared with those of other macrocyclic and non-macrocyclic phenols (Kimura et al., 1987a,b). Because the nitrogen atoms and the phenol function both possess acid-base properties, more than one pXg value could be measured. By the use of UV measurements, the values of the phenol group could be distinguished from those of the amines. 

Phenol methylation to 2,6-xylenol has been widely studied for the past few deeades owing to the room for improvisation from the viewpoint of product selectivity. Generally during phenol methylation to 2,6-xylenol, occurs via sequential methylation of phenol to o-cresol to 2,6-xylenol, various reaction parameters mediate the selectivity between the two. For instance, when the reaetants stoichiometry of methanol to phenol molar ratio > 2, and significant residence time of o-cresol may favor 2,6-xylenol selectivity. However, excess methanol is often used, sinee some amount of methanol tend to undergo oxidation into various reformate produets [71] under vapor phase condition. Similarly, reaction temperature, catalyst acid-base property, and space velocity of the reaetant are the parameters that govern the selectivity to 2,6-xylenol. 

Catalysts Composition (X) Metal content (Wt%) Relative Acid-base property Phenol conversion (Wt%) 2-lP selectivity (Wt%)... 

With respect to the well-known acid/base properties of water, ammonia, and phenol in aqueous solution, one has to conclude that enormous solvation energies must contribute to the differences from the behavior of isolated water molecules. 

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. 

The presence of acidic functional groups, mostly carboxyl and phenolic OH groups, in the molecular structure of soil HS renders them major players in the acid-base buffering capacity of soils and in the fate, bioavailability, and physico-chemical behavior of macro- and micronutrients, toxic metal ions, and several xenobiotic organic compounds in soil (Ritchie and Perdue, 2003 Senesi and Loffredo, 2005). Consequently, the effects of amendment on the acid-base properties of soil HAs and FAs is a subject of considerable interest. 

In contrast to the acid/base behaviour of polymeric bulk water, monomeric water is a relatively weak acid and base in the gas phase compared to its substituted derivatives (R—OH, R—O—R, etc.), whose conjugated base or acid ions are stabilized by polarization of the alkyl groups. The gas-phase basicity of water is 138 kJ/mol (33 kcal/mol) below that of ammonia. Its gas-phase acidity is comparable to that of propene and it is less acidic than phenol by about 167 kJ/mol (40 kcal/mol). With respect to the well-known acid/base properties of water, ammonia, and phenol in aqueous solution, one has to conclude that enormous solvation energies must contribute to the difference from the behaviour of isolated water molecules. See Section 4.2.2 for further discussions and references. 

There are also many well-dociunented cases of excited state proton transfer reactions. It has been known for a long time that the acid-base properties of organic molecules such as phenols are drastically modified upon light absorption. About 60 years ago, Forster suggested a simple method for estimating the excited state pif values of photoactivated species from thermodynamic and spectroscopic data (117), which became very popular (Fig. 12). 

The acid-base properties of DOM are of intrinsic interest because acidic functional groups contribute to the acid-base balance of natural waters, affect complexation and transport of dissolved metals, and interact with mineral surfaces. The concentrations of carboxyl and phenolic functional groups are among the most widely measured and reported properties of DOM. Methodologically, there are two basic approaches for measuring acidic group content—indirect titrations and direct titrations (Perdue et al., 1980 Perdue, 1985 Ritchie and Perdue, 2003). 

Hydrogen-bonding, steric effects, and acid-base properties of phenols are involved in their retention on silica gel and alumina with benzene and isopropylether as eluents. 

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. 

Magnesium-aluminium hydrotalcites are prepared with Mg/Al atomic ratios 2-7 and characterized by XRD, DTA, surface area measurements and acidity-basicity measurements. The tert-butylation of phenol is carried out on the calcined magnesium-aluminium hydrotalcites using isobutanol in the temperature range of350-500°C. The major products of this reaction are O-tert-butyl phenol and 2-tert-butyl phenol. Along with the normal alklyated products 0-butenyl phenol and 2-butenyl phenols are also obtained. A correlation between the product distribution and the nature of acid-base properties of the catalyst is studied. 

Many convention- acid and base catalysts are being replaced by more eco-fnendly solid catalysts. In the present investigation calcined magnesium-aluminium hydrotalcites are used for the tert-butylation of phenol using isobutanol. The aim of the present work is to correlate the acid-base properties of hydrotalcites with the obtained product distribution. 

The main products of t-butylation of phenol on hydrotalcites are O- and 2-butenyl phenols and O- and 2-tert-butyl phenols. The other dialkenyl and alkyl phenols are also formed in trace amounts. However, the nature of the products differed depending on the acid-base properties of hydrotalcites. Figure 4 shows the product distribution at different temperatures. O- and 2-tert-butyl phenols are the only products on AI2O3 at 400"C. On MgO, O-butenyl and 2-butenyl phenols are obtained in equal amount at 400°C and with increase in temperature the formation of 2-butyl phenol is also observed. The product distribution on hydrotalcites differed from that of AI2O3 and MgO. On HT with Mg/Al ratio = 2, only O- alkylation is observed. On HT with Mg/Al ratio = 4-6, C-alkylation is also taking place along with 0-alkyIation. 2-butenyl phenol is formed only above 450 C. There is no pronounced effect of Mg/Al ratio on the selectivity of this product. 

The product distribution in the t-butylation of phenol with isobutanol may be explained based on the nature of acid-base strengths and the mode of adsorption of phenol. It has been reported in the literature that phenol is adsorbed horizontally on acid-catalysts like AI2O3 and a vertical mode of adsorption is proved on basic catalysts like MgO [16,17], The horizontal adsorption of phenol results in O-alkylation and also C-alkylation at ortho and para positions which are close to the surface of the catalyst whereas in the vertical adosrption mode, only the ortho selectivity is observed. The extent of C-alkylation depends on the strength of the acid site. However, a combined participation of acid-base properties is also reported in the methylation of phenol over hydrotalcites [18]. Hence, a scheme depicting the correlation between the acid-base properties of the catalysts with the product distribution in the t-butylation of phenol is shown below as. 

In the tert-butylation of phenol it is found that hydrotalcites are more active than the pure oxides, MgO and y-Al203. The formation of the 0-alkylated products on these basic catalysts indicate the presence of weak acidic sites. It was also quite interesting to observe the formation of the alkenyl phenols along with the normal products whose mass differed by 2 units from the normal alkyl phenols. The distribution of the products varied with reaction temperature and the acid-base properties of the cat ysts. 

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