Phenol irradiation time

Figure 11.4. Effect of UV-C irradiation time (0, 1, 3, 5, 10 min) on (I) ascorbic acid (mg/100 g FW), (II) p-carotene (mg/100 g FW), and (III) total phenols (mg/100 g FW) of fresh-cut mango (cv. Tommy Atkins) stored at 5°C. Bars show the final values after treatments. Different letters on top of the bars indicate statistical differences among treatments (p < 0.05).

After an irradiation time of 300 min, (500 kHz, 30 W, 20 °C) the hydroxylated intermediates disappear and the chlorine atoms are completely mineralised to chloride ions. The concentrations of the final products (CO and CO2) rise slowly and after 400 min they represent 21 % when starting with cMorophenol and 18 % when starting with phenol. These results are in agreement with previous studies in which it was demonstrated that the sonochemical reactivity of an organic compound is related to its vapor pressure and hydrophobicity. [26,27]. These studies have been extended to the destruction of a number of chloroaromatics [25] (Tab. 4.3). 

Tschimer and Dence (32) have demonstrated the role of both the phenolic hydroxyl and carbonyl groups in light-induced yellowing of spruce TMP. For up to two hours irradiation time, complete reduction of carbonyl groups in TMP... 

De Ruiter et al. [4] observed that photochemical decomposition by ultraviolet irradiation of dansyl derivatives of chlorinated phenolic compounds in methanol-water mixtures led to the formation of highly fluorescent dansyl-OH and dansyl-OH3 species. The optimal irradiation time was 5.5s. This reaction was utilised in a post-column photochemical reactor in the high performance liquid chromatography determination of highly chlorinated phenols in river water. The method calibration curve (for dansylated pentachlorophenol) was linear over three orders of magnitude. 

Figure 6 Experimental values of phenol concentration vs. irradiation time. Ti02 P25 catalyst amount 0.32 g L , lamp power, 500 W. The empty symbols indicate the concentration of starting solution. The solid lines represent the Freundlich photoadsorption model [Equation (26)].

As it is mentioned, N-oxyl radical concentration changes through a maximum as a function of the irradiation time. Higher amounts of phenolic antioxidant result in lower concentration of the N-oxyl radicals (Figure 7). This is in accordance with the above results, namely with the lower photostability of the polymer in the presence of higher amount of antioxidant. This can be explained by presuming an interaction between the HALS, or rather its oxidation product, the N-oxyl radical and the phenolic antioxidant. 

In the presence of the tertiary hindered amine (StNCH,), the hindered N-oxyl (StNO-) and the phenol (AOX), the extent of oxidation and embrittlement were sharply reduced, with AOX being most effective (Table 1). The thioester (DSTDP) had little effect on the 3400 cm-1, -OH absorption but appeared to accelerate embrittlement. Further study of this additive was abandoned. The y-irradiation of an unstabilized PP sample wrapped in a thin, carefully extracted film of natural rubber showed a lower overall oxidation level by IR than a PP film irradiated alone (Table 1). However, when working at the higher dose rate (and so much shorter irradiation times), this effect was barely detectable. 

It can also be observed in Figure 9.5 that at 150 min of irradiation time and for the initial concentrations of 50 ppm of silver and 10 ppm-C of phenol the photoconversion... 

Trimethylsilyl ethers of alcohols and phenols cleave in 15 min in a cleaning bath. The more stable t-butyldimethylsilyl ethers of primary alcohols undergo cleavage even at - 40°C. Ethers of secondary and tertiary alcohols and phenols remain intact. The discrimination between f-butyldimethylsilyl and f-butyldiphenylsilyl groups is also possible, only the former being cleaved after 3 h at room temperature. These reactions require longer irradiation times than for TMS groups due to steric hindrance. 

FIGURE 3.17 Change in relative concentration of phenol c/Cq with irradiation time of ultraviolet (UV) rays for sample Ti/PVB-S-700 prepared through loading of TiOj on poly(vinyl hutyral) (refer to Table 3.5) (a) first cycle (b) second cycle. 

FIGURE 3.33 Changes in relative concentration c/cq of phenol (Ph) with irradiation time for carhon-coated Ti02 (refer to Table 3.6) (a) and dependences of rate constant for Ph decomposition on carbon content (b). 

FIGURE 3,59 Changes in relative concentration c/Cq of phenol in the solution with ultraviolet (UV) irradiation time for Ti02 co-modified by carbon and iron (refer to Table 3.9) without H2O2 addition (a) and with H2O2 addition (b). 

FIGURE 3.60 Changes in concentration of phenol and its decomposition products, hydroquinone (HQ), benzoquinone (BQ), and catechol, in the solution with ultraviolet (UV) irradiation time for Ti02 (ST-01) (a), Ti02 co-modified by carbon and iron FS-550 (refer to Table 3.9) (b,c), and without any photocatalyst (HjOj addition only) (d). Starting Ph concentration was 2.1 x 10 mol/L, and the HjOj concentration was 0.03 mol/L. 

Bicyclo[3,l,0]hex-2-en-3-ones related to 6 (lumiketones) are normally prepared by irradiation of dilute solutions of dienones in dioxane or toluene. Best results are obtained with a low-pressure mercury lamp that emits most of its radiation at 253.7 nm. Control of the irradiation time and of the wavelength are important to avoid further rearrangement of the lumiketones to dienones and phenolic products, which are formed on irradiation with light of wavelength greater than 300 nm. °... 

Photochemistry of Model Compounds. Preliminary photochemical studies have been carried out on l,3-diphenoxy-2-propanol (3)8 as a model compound for bisphenol A-epichloro-hydrin condensates 1. The utilization of 3 as a model compound for thermal degradation of 1 has been reported. Irradiation (254 nm) of 3 in acetonitrile (N2 purge) provides two major volatile products, which have been identified as phenol and phenoxyacetone (4), by comparison of retention times (gas chromatography) with known samples. A possible mechanism for... 

The importance of phenol formation by the proposed pathway was probed by irradiating l,3-diphenoxy-2-methyl-2-propanol (5) under the same conditions. Compared to 3, the rate of phenol formation was approximately 2 times slower. Since the 11-transfer step in Scheme II is not available to 5, the results provide support for the scheme as an important, but not sole, pathway for phenol formation. Irradiation of and 5 with an air purge resulted in faster rates of phenol formation (ca. 5-fold) relative to N2. These findings parallel the accelerated fluorescence intensity loss from polymer 1 films in air as compared to the results in vacuo (see Table I). 

The highly potent anti-HIV natural product daurichromenic acid (10-100) was synthesized by Jin and coworkers [36] using a microwave-assisted reaction of the phenol derivative 10-97 and the aldehyde 10-98 (Scheme 10.25). Normal heating gave the desired benzo[b]pyran 10-99 by a domino condensation/intramolecular SN2 -type cyclization reaction only in low yield. However, when the reaction mixture was irradiated twenty times in a microwave for 1-min intervals, 10-99 was obtained in 60% yield. This compound was then transformed into 10-100 by cleavage of the ester moiety. 

For this purpose, perfluorooctanesulfonyl-tagged benzaldehydes were reacted with 1.1 equivalents of a 2-aminopyridine (or 2-aminopyrazine), 1.2 equivalents of an isonitrile, and a catalytic amount of scandium(III) triflate [Sc(OTf)3] under micro-wave irradiation in a mixture of dichloromethane and methanol (Scheme 7.85). A ramp time of 2 min was employed to achieve the pre-set temperature, and then the reaction mixture was maintained at the final temperature for a further 10 min. The fluorous tag constitutes a multifunctional tool in this reaction, protecting the phenol in the condensation step, being the phase tag for purification, and serving as an acti-... 

An 8000-member library of trisamino- and aminooxy-l,3,5-triazines has been prepared by use of highly effective, microwave-assisted nucleophilic substitution of polypropylene (PP) or cellulose membrane-bound monochlorotriazines. The key step relied on the microwave-promoted substitution of the chlorine atom in monochlorotriazines (Scheme 12.7) [35]. Whereas the conventional procedure required relatively harsh conditions such as 80 °C for 5 h or very long reaction times (4 days), all substitution reactions were found to proceed within 6 min, with both amines and solutions of cesium salts of phenols, and use of microwave irradiation in a domestic oven under atmospheric reaction conditions. The reactions were conducted by applying a SPOT-synthesis technique [36] on 18 x 26 cm cellulose membranes leading to a spatially addressed parallel assembly of the desired triazines after cleavage with TFA vapor. This concept was later also extended to other halogenated heterocycles, such as 2,4,6-trichloropyrimidine, 4,6-dichloro-5-nitropyrimidine, and 2,6,8-trichloro-7-methylpurine, and applied to the synthesis of macrocyclic peptidomimetics [37]. 

Figure 3. Time-resolved resonance Raman spectrum of the phenoxyl radical observed 1 ls after pulse irradiation of a 2 mM phenolate solution (N20 saturated) at pH 11 (excitation at399nm). [Adapted from (18b).]...

While this reaction to form coumarin derivatives can be completed in mineral acids, research shows that the reaction was much faster in ILs even at room temperature. The same group used l-butyl-3-methylimidazolium hexafluoro-phosphate IL at high temperatures without employing any acid catalyst. The yields were comparable to chloroaluminate ILs with catalytic amounts of acid at room temperature. They also concluded that Bronsted acidity (produced by HF when [bmimJIPF ] contacts water) was not responsible for the observed activity. Singh et al have used l-butyl-3-methylimidazolium hydrogen sulfate IL in combination with microwave irradiation. They were able to synthesize coumarins in quantitative yields with drastic reduction in reaction times. Soares et al have used [bmim][NbCl6] IL to perform the Pechmann reaction using various phenols with ethyl acetoacetate to produce coumarin in moderate yields (-35%). 

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