Benzenes from decomposition reaction

As in steam cracking, a large number of by-products is produced. Some of them result from the consecutive reactions of the chlorination of vinyl chloride and of its derivatives obtained by dehydrochlorination (tri-, tetra-, pentachloroethane, perchloro-ethane, di-, trichloroethylene. perchloroethyleneX and the others from the hydrochlorination of vinyl chloride il.l-dichloroethane), while others result from decomposition reactions (acetylene, cokei or conversion of impurities initially present (hydrocarbons such as ethylene, butadiene and benzene, chlorinated derivatives such as chloroprene, methyl and ethyl Chlorides, chloroform, carbon tetrachloride, eta, and hydrogen) ... 

The results obtained from the decomposition reaction of (triphenylmethyl)-methyldichlorosilane to (diphenylmethyl)methyldichlorosilane in benzene solvent in the presence of aluminum chloride are summarized in Table XV. 

To confirm the production of benzene from the decomposition reaction of methyl(triphenylmethyl)dichlorosilane, the decomposition reaction of methyKdi-phenylmethyl)dichlorosilane in the presence of aluminum chloride was carried out in toluene solvent at 80 C. In this reaction, the exchange reaction between phenyl groups on the methyl group of (diphenylmethyl)(inethyl)dichlorosilane and toluene occurred to give [phenyl(tolyl)methylJ(methyl)dichlorosilane and (di-tolylmethyl)(methyl)dichlorosiIane (Scheme 1). " ... 

Separate experiments in which tert.-butoxy radicals were produced thermally in benzene from di-tert.-butyl peroxyoxalate failed to reveal any direct reaction of these radicals with amine II. Even at higher temperatures (A/ 150°C, dichlorobenzene, +00+ decomposition), the +0 radicals attacked neither amine II nor nitroxide I. The earlier described experiments of ketone photooxidation showed additionally that amine II displays no specially marked reactivity towards peroxy radicals. 

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

Dioxins, 1,4-oxathiins, and 1,4-dithiins have often been prepared by elimination reactions from saturated analogs as described in CHEC-II(1996) <1996CHEC-II(6)447>. Since then, a synthesis of tetramethyl l,4-dithiin-2,3,5,6-tetracarboxylate 241 has been reported in low yield (12%) by thermal decomposition of the 1,4,2,5-dithiadiazine system 240 in refluxing o-dichlorobenzene in the presence of DMAD <1997J(P1)1157>. Recently, 2,6-divinyl-l,4-dithiin 68 has been isolated from the reaction of l,4-bis(4-bromobut-2-ynyloxy)benzene with an excess of alumina-supported sodium sulfide. The formation of 68 has been presumed to take place via cyclic sulfide 242 <2003S849>. 

In the presence of approximately 1% cobalt naphthenate in benzene, only 4 hours at 100°C. were required to decompose the hydroperoxide almost completely. The yields of products from decompositions catalyzed by some commonly used cobalt and vanadium (1) compounds are given in Table I. Polymerization appears to be the major reaction. 

Figure 16.15—Electron ionisation (El). The collision of an electron with a sample molecule m produces ionisation that leads to formation of a parent ion and fragment ions. Ions that result from the reaction m/ and raj are also called secondary or daughter ions. Since they carry no charge, neutral fragments produced during decomposition, (ra, m[ and m ), are not detected. An illustration of electron ionisation of benzene is shown. Also shown is a schematic of the ionisation chamber (ion source). Using a parallel magnetic field can increase the effective path of an electron in the ion source, which increases ionisation efficiency.

Ammonia decomposes on zeolites (9), and the effect of this decomposition on the chlorobenzene reaction may be important. Thus, the activity of CuY zeolite for ammonia decomposition was studied. Helium was used as a carrier gas, 1 ml of ammonia was injected, and the extent of ammonia decomposition was determined as a function of temperature. The decomposition was 2.4% at 350°C, 7.8% at 450° C, and 24% at 550° C. The apparent activation energy of ammonia decomposition was estimated at 13 kcal/mole. The activation energy of ammonia decomposition is close to that of benzene formation from chlorobenzene and ammonia. Thus, benzene formation results from the reaction of chlorobenzene and hydrogen formed by the decomposition of ammonia. 

OH radical formed from OH- and h+ rapidly adducts benzene to form a cyclohexadienyl radical, which is subsequently oxidized to a peroxy radical in the presence of 02. The peroxy radical transforms to the various intermediates including phenol, hydroquinone and to CO and C02. In the presence of H20, the surface hydroxyl groups consumed in the photoreaction are regenerated, leading to a successive catalytic cycle. Although the mechanism for the decomposition of the polymeric compounds is not well understood, it can be deduced that the OH radicals produced from the hydroxyl groups play a very significant role in the decomposition reaction. 

Solvent polarity also affects the rate of peroxide decomposition. Most peroxides decompose faster in more polar or polarizable solvents. This is true even if the peroxide is not generally susceptible to higher order decomposition reactions. This phenomenon is illustrated by various half-life data for tert-butyl peroxypivalate [927-07-1]. The 10-h half-life temperature for tert-butyl peroxypivalate varies from 62°C in decane (nonpolar) to 55°C in benzene (polarizable) and 53°C in methanol (polar). 

Biphenyl, benzocyclooctatetraene (9), and benzobicyclo[2,2,2]octatriene (10) resulted from the reaction of benzyne (by decomposition of benzene-diazonium carboxylate) with benzene at 45° (Miller and Stiles, 1963). Both 9 and 10 have been found to go to naphthalene and acetylene 9 on photolysis (Fonken, 1963), and 10 in a sealed tube at 300° (Miller and Stiles, 1963). 

An organocopper intermediate was detected by Lewin and Cohen in the reaction of / -iodotoluene with copper in a good complexing solvent (184). Analysis of protonated aliquots from a reaction performed in quinoline indicated an accumulation of />-tolylcopper to a maximum of 43% after 95 hours, at which point the iodide was consumed, and then a slow decrease to by dimerization. Other experiments also indicate the formation of an arylcopper compound in Ullmann reactions (127,141, 210). The isolation of deuterated products, presumably from the decomposition of an intermediate organocopper species in deuterated benzene and cyclohexane, suggested decomposition to free radicals (127). Decompositions of certain o-haloarylcopper intermediates by a benzyne mechanism cannot be totally excluded. The formation of a dichlorobenzene and by-products such as dibenzofuran and triphenylene from only the ortho isomer of the chloroiodobenzenes in Ullmann coupling reactions (210)... 

The thermal and mass spectral reactions of the dinitrobenzenes are even more interesting. ) The primary ionic and thermal decomposition reaction is in every case loss of NO2 to give a nitrophenyl cation or radical respectively. In both systems the nitrophenyl radicals may lose another NO2 fragment to give phenylene diradicals. In the case of o-dinitrobenzene loss of two NO2 fragments leads to the formation of benzjme. Benzofurazan, 44, is one of the minor products obtained from thermolysis of o-dinitro-benzene, it is also a minor ion in the electron impact mass spectrum of the compound. 

Moreover, studies of products formed during the smog-chamber oxidations of multialkyl benzenes invariably indicate the presence of ring decomposition reactions (leading, for example, to the production of per-oxyacetyl nitrate from m-xylene or mesitylene). Chemical reactivity of aromatic hydrocarbons is enhanced by an increasing number of alkyl groups, especially those in meta positions on the benzene ring. 

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