Acids bound phenolic

The Lewis acid complex 4 can cleave into an ion-pair that is held together by the solvent cage, and that consists of an acylium ion and a Lewis acid-bound phenolate. A fr-complex 6 is then formed, which further reacts via electrophilic aromatic substitution in the ortho- or para-position ... 

Procedure A 0.02 ml aliquot of the sample solution for free phenolics and 0.005 ml for bound phenolics in methanol was taken and 7 ml distilled water plus 0.1 ml Folin-Ciocalteu s phenol reagent was added and after 3 min 0.2 ml of 20% Na2C03 was added. After boiling at 90 °C (exactly 5 min) samples were cooled at room temperature and were diluted with H20 to the volume of 10 ml. Only distilled water and reagents were used as a blank. The absorbance of total phenolics (free and bound) was measured at 660 nm spectrophotometrically (a Shimadzu UV 160 spectrophotometer), according to Feldman and Hanks (1968), with a sensitivity of 0.05 pg/g d.w. A standard curve was constructed with different concentrations of ferulic acid (Serva, Germany). 

Amount of Sulfuric Acid (mMole/g lignin) Reaction Period at 170°C (Hour) Bound Phenol (Mole/lignin unit)... 

Such reactions can occur on oxides of Fe, Al, and Ti. Carboxylic acids and phenols are common reactive moieties of the sorbates /. Often the resultant bound species is not really known. Given this additional sorption mechanism, the situation becomes more complicated, and multiple equilibria must be modeled (Stone et al. 1993) to account for all species ... 

These acid-catalyzed reactions occur at temperatures lower than those used in conventional liquefaction processes. Phenol induces acid-catalyzed solvolytic reactions at temperatures as low as 120°C. The liquefied products contain chemically bound phenolic groups, a portion of which can be recovered by cracking. 

Support-bound phenols, oximes, and related compounds yield, upon acylation, esters that are highly susceptible to nucleophilic cleavage. These esters are often used as insoluble acylating agents for the preparation of amides or esters, but only occasionally as linkers for carboxylic acids [113]. These linkers are considered in Sections 3.3.3 and 3.5.1. 

Silyl ethers that have been used in solid-phase synthesis include TES, TIPS, TBS, and TBDPS ethers. Polystyrene-bound phenols can be converted into TIPS ethers by treatment with TIPS-OTf/imidazole in DMF for 5 min [105], These silyl ethers are stable towards bases and weak acids, but can be selectively removed by treatment with TBAF (Entries 6 and 8, Table 7.8) or pyridinium hydrofluoride (THF, 25 °C, 15 h [24,75,106]). 

Support-bound phenols [173-176] or other compounds with an acidic OH group (oximes [177-179], hydroxamic acids, /V-hydroxy benzotriazoles, etc.) undergo rapid esterification with carbodiimides as coupling agents (Entries 13 and 14, Table 13.12). The resulting products are susceptible to nucleophilic attack, and can be used as insoluble acylating agents. 

An HPLC method using a 90-min binary gradient with (a) acidified water, pH 2.4, and (b) acetonitrile on an Adsorbosphere C]8, 3-/zl cartridge (Alltech) was also developed for pheno-lics in barley (127). Seven phenolic compounds, including vanillic acid, p-coumaric acid, ferulic acid, and their derivatives, were separated by HPLC after alkaline hydrolysis in order to evaluate the role of bound phenolic acids in their antioxidant activity in beer. In this method, cis and trans isomers of p-coumaric and ferulic acids are quantified by HPLC, although cls-p-coumaric acid was not well separated from its trans isomer in this analysis. 

MN Maillard, C Berset. Evolution of antioxidant activity during kilning Role of insoluble bound phenolic acids of barley and malt. J Agric Food Chem 43 1789-1793, 1995. 

Silicon bound to a phenyl group can also influence the bond system by additional (p- -d) back donation from carbon to silicon. In agreement with this model, p-trimethylsilyl-substituted benzoic acid shows a greater acidity than expected from inductive effects. Furthermore, p-trimethylsilyl phenol exhibits a greater acidity than phenol itself, and p-trimethylsilyl aniline shows a decreased basicity as compared with that of the nonsubstituted compound. This behaviour can be described by the following resonance structures [Eqs. (4) and (5)] ... 

Rao, M. V. S. S. T. S. and Muralikrishna, G. (2002). Evaluation of the antioxidant properties of free and bound phenolic acids from native and malted finger millet (ragi, Eleusine coracana Indaf-15). J. Agric. Food Chem. 50, 889-892. 

Blum, U., Worsham, A. D., King, L. D. and Gerig, T. M., 1994. Use of water and EDTA extractions to estimate available (free and reversibly bound) phenolic acids in Cecil soils. J. Chem. Ecol. 20, 341-359... 

To analyze free-form phenolic acids in fmit or vegetable juice, the sample preparation is straightforward and simple. The juice can be directly injected into an HPLC system after it is filtered to remove any insoluble particles. However, for a sample with a solid fraction containing both free and bound phenolic acids, the sample preparation is not as simple. A mechanical method is needed to physically break down the sample and release the free phenolic acids, which are blocked in the inner core of the sample matrix. Chemical (acid or alkaline) or enzymatic hydrolysis must be applied to break down linkages in the bound phenolic acids to release free phenolic acids. However, the recovery of total phenolic acids is significantly affected by hydrolysis and other extraction conditions. An intensive hydrolysis condition may increase the release rate of bound phenolic acids to free phenolic acids however, it can also cause degradation of some phenolic acids and lower... 

Holtekjolen, A.K. Kinitz, C. Knutsen, S.H. 2006. Flavanol and bound phenolic acid contents in different barley varieties. J. Agric. Food Chem. 54 2253-2260. 

BCrygier, K. Sosulski, F. Hogge, L. 1982. Free, esterified, and insoluble-bound phenolic acids. 2. Composition of phenolic acids in rapeseed flour and hulls. J. Agric. Food Chem. 30 330-334. 

Nardini, M. Ghiselli, A. 2004. Determination of free and bound phenolic acids in beer. Food Chem. 84 137-143. 

FIGURE 4. Hypothetical structure of humic acid showing free and bound phenolic OH groups, quinone structures, oxygen as bridge units, and carboxyls variously placed on the aromatic ring. From Stevenson (1982). 

---