Isomerism substituted benzenes

The two substituted carbons are connected by a double bond in one structure but by a single bond in the other Because no such cases of isomerism m benzene derivatives were known and none could be found Kekule suggested that two isomeric structures could exist but mterconverted too rapidly to be separated... 

Perfluoroalkylation of substituted benzenes and heterocyclic substrates has been accomplished through thermolysis of perfluoroalkyl iodides in the presence of the appropriate aromatic compound Isomeric mixtures are often obtained W-Methylpyrrole [143] and furan [148] yield only the a-substituted products (equation 128) Imidazoles are perfluoroalkylated under LTV irradiation [149] (equation 129). 4-Perfluoroalkylimidazoles are obtained regioselectively by SET reactions of an imidazole anion with fluoroalkyl iodides or bromides under mild conditions [150] (equation 130) (for the SET mechanism, see equation 57)... 

Regardless of substrate and solvent, isomerization fell in the order 5% Pd-on-C 5% Rh-on-C > 5% Pt-on-C, and, regardless of substrate or catalyst, isomerization fell with solvent in the order ethanol > pentane > 1 1 benzene-ethanol. Benzene is effective as an isomerization inhibitor mixed with other solvents as well 1 20 benzene-acetone showed marked inhibition. Substituted benzenes are less effective than benzene. 

A valence isomerization reaction similar to that encountered with the tropylium ion has been observed when a variety of substituted benzenes are photolyzed in strong acid. The pentamethylbenzenium ion (18) photolyzed at —78° to give a single product, the pentamethylbicyclo-[3,l,0]hexenyl cation (19), in excess of 80% conversion (Childs et cd., 1968). 

The alkylation of halogen-substituted benzenes such as fluorobenzene and dichlorobenzenes with other (dichloroalkyl)silanes in the presence of aluminum chloride catalyst afforded isomeric mixtures of the corresponding (dihalogen-substituted phenyl)alkylsilanes in moderate yields (Eq. (13)). These results are summarized in Table Xll. 

Acetophenones can be obtained by various routes such as by reacting acetic anhydride with the substituted benzene (Shirley-PREPARATION OF ORGANIC INTERMEDIATES, 1951, pg. 190) and may then be isomerized, if necessary, with aluminum chlonde (JCS 232(1944)). 

The second structural property described by the 4ypc index is the substitution pattern on the benzene ring. The value of the 4ypc index increases sharply with the degree of substitution, while in the isomeric classes of substituted benzenes it increases with the proximity of substituents. Thus, this structural parameter has also been found to be very useful in describing activities and properties of polysubstituted benzenes [103], chlorinated benzenes [279], and polychlorinated biphenyls [286]. 

The vapor-phase pyrolysis of 4-hydroxy-1,2,3-triazole and its iV-methyl derivative affords methan-imine and its A-methyl analog. Analysis of the reaction path by the MNDO method shows the presence of two stable or metastable isomers, (liif)-4-hydroxy-l,2,3-triazole and its ketone protomer <89NJC551>. 4-Diazo-1,2,3-triazoles (122) thermolyze or photolyze in benzene to 4//-l,2,3-tri-azolylidenes (123) which convert benzene to 4-phenyl-1,2,3-triazoles and/or isomerize to a-diazo-nitriles (124). Intermediates (124) react with benzene via a carbene to give addition, ring expansion or substitution products (Scheme 17) <82TL5115>. The similar thermolysis of diazotriazoles in substituted benzene gives complex mixtures in which all of the components are sometimes impossible to isolate and identify <90AHC(48)65>. 

IV. EXPERIMENTAL EVIDENCE FOR THE ISOMERIZATION ROUTE FROM BENZENE AND SUBSTITUTED BENZENES... 

As will be seen in a later section, substituted benzenes rearrange photochemically. Thus o-xylene isomerizes to m-xylene and 1,3,5-tri-isobutyl benzene isomerizes to the 1,2,4- and the 1,2,3-triisobutyl benzenes.410 Such isomerizations conceivably could proceed through free-radical intermediates but Wilzbach and Kaplan and their coworkers have shown that the ring carbon to which the moving substituent is attached also changes position with the substituent. These authors offer the very reasonable explanation that the formation of benzvalene followed by rupture of bonds other than the new ones just formed could lead to rearrangements of the type in question. It should be noted that both benzvalene and prismane could serve as intermediates in this way but that Dewar benzene could not. 

The behavior of substituted benzenes does not afford adequate proof that benzene would behave in a similar way. There are two lines of evidence that it does (/) 1,3,5-trideuterobenzene isomerizes to benzenes with different deuterium distributions43 (the status of this work is at present not entirely satisfactory) and (2) benzvalene has been identified by nuclear magnetic resonance in liquid benzene exposed to ultraviolet light.41... 

Nevertheless, eq. (4.16) predicts the signal sequence of multisubstituted benzenes. Straightforward assignment can be achieved frequently, provided signals are separated by at least 2 ppm. - Experimental data of several o-, m-, p-disubstitutcd isomeric series and higher substituted benzene derivatives are collected in Table 4.54 [73 k, 1, m],... 

Earlier in this chapter we talked about the enthalpies of formation of singly and then multiply substituted cyclopropanes and the correspondingly substituted benzenes. It is now time to talk about multiple cyclopropanes and the corresponding benzene derivatives. In Section II.D, the enthalpies of formation of the isomeric (cis)- and (trans)-1,1 2, 1 "-ter-cyclopropyl (9a and 9b) were estimated to be 208 kJ mol 1. It was also asserted that the difference of this value and the enthalpy of formation of bicyclopropyl (8) was nearly identical to the difference of the enthalpies of formation of bicyclopropyl and cyclopropane. Let us now rewrite the oligomerization/dehydrogenation reactions 11a and lib as reactions 31aand31b ... 

Study of isomer distribution in substitution of benzene rings already carrying one substituent presents some potential pitfalls. Inspection of product ratios for ortho, meta, and para substitution, as in investigation of electrophilic substitution (Section 7.4, p. 392), might be expected to give misleading results because of the side reactions that occur in radical substitution. The isomeric substituted cyclo-hexadienyl radicals first formed by radical attack partition between the simple substitution route and other pathways (Equation 9.102). In order for the... 

The SM2/AM1 model performed well in predicting free energies of hydrata-tion for the substituted benzenes [65], The use of the SM2/AM1 model to assess solvation effects allowed to propose a mechanism for the cis-trans isomerization of diazene, which proved consistent with experimental observations [66], The results obtained indicated that trace amounts of acid can rapidly protonate tram... 

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