Dimethylphenol atmospheres

Chemical/Physical. Under atmospheric conditions, the gas-phase reaction of o-xylene with OH radicals and nitrogen oxides resulted in the formation of o-tolualdehyde, o-methylbenzyl nitrate, nitro-o-xylenes, 2,3-and 3,4-dimethylphenol (Atkinson, 1990). Kanno et al. (1982) studied the aqueous reaction of o-xylene and other aromatic hydrocarbons (benzene, toluene, w and p-xylene, and naphthalene) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride. The amount of cyanogen chloride formed increased at lower pHs (Kanno et al., 1982). In the gas phase, o-xylene reacted with nitrate radicals in purified air forming the following products 5-nitro-2-methyltoluene and 6-nitro-2-methyltoluene, o-methylbenzaldehyde, and an aryl nitrate (Chiodini et ah, 1993). 

A. 3,5-Dimethylphenyl 1-bromo-2-naphthoate (3). Under a nitrogen atmosphere, a 250-mL, oven-dried, round-bottomed flask containing anhydrous dichloromethane (100 mL) is charged with 1-bromo-2-naphthoic acid (1, 2.51 g, 10.0 mmol), 3,5-dimethylphenol (2, 1.23 g, 10.1 mmol), dicyclohexylcarbodiimide (DCC, 2.26 g. 11.0 mmol), and 4-(dimethylamino)pyridine (DMAP, 244 mg, 2.00 mmol) (Note 1). After the mixture is stirred for 12 hr at room temperature, the white precipitate that forms (Note 2) is discarded by filtration through a Buchner funnel. From the clear filtrate, the solvent is removed by rotary evaporation (35°C, 720 mbar, 540 mm) to give a colorless solid. RItration through a short silica gel column (5 x 40-cm column, silica gel 0.063 - 0.2 mm, 150 g eluent hexane / diethyl ether 5 1) delivers 3.35 g (94%) of the ester 3, which Is recrystallized from diethyl ether / hexane to give 3.28 g (92%) of a colorless solid (Note 3). 

The ability to polymerize readily via selective oxidation utilizing the abundant and cheap oxidant 02 often represents a desirable low-cost method for upgrading the value of a raw material. The most successful example is the oxidative polymerization of 2,6-dimethylphenol to yield poly(2,6-dimethyl-l,4-phenylene ether) with copper-amine catalysts under an 02 atmosphere at room temperature. Thiophenol also has a labile hydrogen but is rapidly oxidized to yield thermodynamically stable diphenyl disulfide. This formation is based on the more facilitated formation of S—S bond through radical coupling [82] in comparison with the formation of C—S—C bond through the coupling with the other molecules in the para position (Eq. 9). 

A study was carried out for LEE by the Soxhlet method and microwave-assisted extraction for the determination of the priority phenols in soil samples. Recoveries varied from 67 to 97% with RSD between 8 and 14% for LEE, and >70% for the MAP, except for nitrophenols that underwent degradation when the latter method was applied. LOD was from 20 ngg for 2,4-dimethylphenol to 100 ngg for pentachlorophenol. The best detection method for EC was atmospheric pressnre chemical ionization MS (APCI-MS). The most abnndant ions obtained by this detection method were [M — H] for the lowly chlorinated phenols and [M — H — HCl] for tri-, tetra- and pentachlorophenols . 

Phenol, the cresols, and the dimethylphenols are formed from the atmospheric degradation of benzene, toluene, and the xylenes, respectively, and data are available concerning the atmospheric reactions of these compounds. The potential atmospheric loss processes of phenolic compounds are reaction with N03, OH radicals, and 03, together with wet and dry deposition (these compounds are readily incorporated into rain- and cloud-water and fog). The OH radical reactions proceed by OH radical addition to the aromatic ring and by H-atom abstraction from the substituent -OH and -CH3 groups (Atkinson, 1989) ... 

The copolymer exhibits a higher thermal air stability of about 70 °C along with improved mechanical properties in comparison to a conventional poly(2,6-dimethyl-1,4-phenylene ether). However, PPE with an end-capped methyl end group shows a higher thermal stability. This indicates that the thermal degradation mainly occurs from the polymer end group in air atmosphere. Thus, the higher thermal stability of the copolymer can be attributed to the 2,5-dimethylphenol unit which is found at the end of the copolymer [38]. 

Example Understanding the Reactivity of 2,3-Dimethylphenol with OH Radical in the Atmosphere... 

The seeondary reaetions of peroxy radieal with HO2 slow down the free radical driven photochemical oxidation reactions and reduce the formation of ozone. In addition, these reactions represent an important chemical sink for HOx radicals in the troposphere. Hence, the reactions of peroxy radical with HO2 are of comparable importance in the atmospheric fate of dimethylphenols. Previous studies on the reactions of dimethylphenol with OH radical have focused only on the initial H-atom abstraction step and its kinetics. Hence, this work focused mainly on the study of possible secondary reactions of the reaction between 2,3-dimethylphenol and OH radical. Theoretical calculations assess the feasibility of different reaction channels and provide thermochemical data for the reaction system. 

The results discussed in the previous section reveal that the reaction rate corresponding to the formation of major by-products of the oxidation reaction is important to determine the lifetime of dimethylphenol in the atmosphere. The rate constants are calculated using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) corrections over the temperature range of 278-350 K. As described in Figure 19.2, the formation of product channels consists of four reaction channels. The rate constants for the formation of alkyl radical (11), peroxy radical (12), m-cresol and the product channels are designated as k, ki2, and kp, respectively, and are summarized in Tables 19.2 and 19.3. The reaction path properties and rate constant obtained for the most favorable product channels, P5 and P6, are discussed in detail. 

The potential energy surface, thermochemical and kinetic data for the reaction of 2,3-dimethylphenol with OH radical reveal several important aspects of alkylated aromatic compounds in the atmospheric chemistry. The reaction between 2,3-dimethylphenol and OH radical is initiated by H-atom abstraction from a methyl group and the resulting alkyl radical further reacts with O2 to form a peroxy radical, a key intermediate in the reaction mechanism. This peroxy radical has excess energy to undergo further reaction with the atmospheric species. 

In the atmosphere, VOCs are emitted as pollutants. The degradation of an aromatie VOC, 2,3-dimethylphenol by reaetion with OH radical is studied as a special case. The secondary reactions from the initial oxidation reaction are studied in detail and the lifetime of w-cresol resulting from the secondary reaction is determined. 

The kinetics of the reaction of NO3 with dimethylphenols have been investigated by Thiiner et al. (2004a) using a relative rate method at atmospheric pressure and 295 K. The data obtained are summarized in table n-E-14. 

The gas-phase reactions of OH and NO3 radicals with dimethylphenols are rapid as described in this section and are expected to be the major atmospheric removal processes. The estimated atmospheric lifetimes with respect to reactions with OH and NO3 are approximately 1 h. 

As for the reactions of OH with phenol and cresols, it is expected that the reaction of OH with dimethylphenols proceeds predominantly by OH addition to the aromatic ring at the temperatures relevant to the atmosphere. 

Using the kinetic data given in this section, the atmospheric lifetimes are estimated to be less than 1 h for dihydrobenzenes with respect to the reaction with OH and NO3. Thus, both processes are important in the removal of dihydrobenzenes from the atmosphere. Over the temperature range relevant to the atmosphere, the reaction of OH with dimethylphenols is expected to proceed predominantly through the OH addition to the aromatic ring. The reaction with NO3 may proceed via an overall H-atom... 

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