Electrophilic substitution aromatic, thermal

In our research, three chemical modification approaches were investigated bromination, sulfonylation, and acylation on the aromatic ring. The specific objective of this paper is to present the chemical modification on the PPO backbone by a variety of electrophilic substitution reactions and to examine the features that distinguish modified PPO from unmodified PPO with respect to gas permeation properties, polymer solubility and thermal behavior. 

Aromatic compounds have a special place in ground-state chemistry because of their enhanced thermodynamic stability, which is associated with the presence of a closed she of (4n + 2) pi-electrons. The thermal chemistry of benzene and related compounds is dominated by substitution reactions, especially electrophilic substitutions, in which the aromatic system is preserved in the overall process. In the photochemistry of aromatic compounds such thermodynamic factors are of secondary importance the electronically excited state is sufficiently energetic, and sufficiently different in electron distribution and electron donor-acceptor properties, ior pathways to be accessible that lead to products which are not characteristic of ground-state processes. Often these products are thermodynamically unstable (though kinetically stable) with respect to the substrates from which they are formed, or they represent an orientational preference different from the one that predominates thermally. 

Although dibenzenechromium is thermally quite stable, it is less so than ferrocene and melts with decomposition at 285° to give benzene and metallic chromium. Furthermore, it appears to lack the aromatic character of either benzene or ferrocene as judged by the fact that it is destroyed by reagents used for electrophilic substitution reactions. 

Silver(diazomethyl)phosphoryl compounds may also be synthesized by treatment of the corresponding diazophosphoryl compound with silver oxide. The thermally stable phosphoryl derivatives undergo electrophilic substitution with alkyl iodides73 but, unlike the carbonyl-substituted derivatives, also undergo electrophilic diazoalkane substitution with a variety of Hiickel aromatic salts (Scheme 1.31).74—79... 

Aromatic substrates are by far the most commonly used substrates in the rapidly expanding area of photoinduced electron transfer [1,2]. This is obviously due to the favourable location of the frontier molecular orbitals in such compounds. The same factor facilitates the formation of electron transfer donor-acceptor (EDA) complexes both in the ground state (these possibly are intermediates in some thermal reactions, e.g. selected electrophilic substitutions), and in the excited state (exciplexes). 

Electronegative groups in the aromatic are necessary for reaction the polyhalo groups deactivate the aromatic for electrophilic substitution by SO 3, increase the thermal stability and also block potential sites of substitution. 

Ferrocene, Fe(Ti5-C5H5)2, and related cyclopentadienyl complexes of transition metals in fact are far more thermally stable, less reactive substances than ionic cyclopentadienides, and have an extensive derivative chemistry that is typically aromatic in that their C-H bonds can undergo such electrophilic substitution reactions as Friedel-Crafts alkylation or acylation, nitration, and so on. Moreover, as a substituent, the ferrocenyl group (ri -f sl l5)Fc(ri -( 5l I4) (=R) is even more effective than a phenyl substituent in stabilizing carbenium ions [RCH2]+. The redox and photochemical properties of many metaUocenyl residues make them versatile substituents with many chemical and materials applications. ... 

Polycyclic arene(tricarbonyl)chromium complexes.h These complexes arc best prepared by treatment of polycyclic arcnes with (NHj),CT(CO)37 and BFj ethcratc. As in complcxation with Cr(CO), the terminal or most aromatic ring is complexed selectively. However, the lower temperatures used in the newer method are advantageous with thermally labile polycyclic arcnes. These complexes are useful for substitution reactions at positions that arc not available by electrophilic substitution of the arenc directly. One such reaction is hydroxylation effected by simultaneous reaction with a base (BuLi or I. DA) and tributoxyborane (excess) followed by H2O2/HOAC workup. Rcgiosclectivc silylation is effected by reaction of the complex with LiTMP and (CHj SiCI with... 

Compound (808) undergoes a thermal retrocycloaddition of N O about 10 times faster than (809), strongly indicative of a concerted reaction for the former. The situation is less clear for (810) and (811), for which intermediate rates were obtained. Electrophilic substitution in the aromatic ring of 2-aryl-gfem-dichlorocyclopropanes takes place mainly in the para-position in the case of nitration or bromination of the... 

One report describes a study of HNaY zeolite for the catalysis of tritium exchange into simple aromatic compounds. In analogy with the use of the water-sensitive EtAlCl2 (Section 3.1.2), the anhydrous active centers of the thermally activated zeolite were exposed to small quantities of tritiated water, and subsequent heating to 175 °C with substrate induced the exchange-labeling of the latter. Success was limited to aromatic compounds without bulky substituents or electron-withdrawing substituents, and the pattern of label incorporation was that of electrophilic substitution. 

This process depends on the reaction of phenol with formaldehyde at the ortho- and j ra-positions. The OH group activates the aromatic ring toward electrophilic substitution. Once benzylic alcohols have been produced, these too can act as electrophiles, and a rigid, cross-linked structure is produced. Bakelite was first produced in 1907 and is regarded as the first artificial polymer. It has high thermal and electrical resistivity and is still widely used in electrical fittings, chipboard, and decorative laminates. 

In agreement with the theory of polarized radicals, the presence of substituents on heteroaromatic free radicals can slightly affect their polarity. Both 4- and 5-substituted thiazol-2-yl radicals have been generated in aromatic solvents by thermal decomposition of the diazoamino derivative resulting from the reaction of isoamyl nitrite on the corresponding 2-aminothiazole (250,416-418). Introduction in 5-position of electron-withdrawing substituents slightly enhances the electrophilic character of thiazol-2-yl radicals (Table 1-57). 

Successful thermal decarboxylation of metal arenoates other than poly-halogenoarenoates are restricted to mercury compounds and fall into three categories, namely (i) those where electron-withdrawing substituents other than halogens are present in the organic groups, (ii) those where substituents and/or conditions are used which favor a different mechanism, e.g., classic electrophilic aromatic substitution, or (iii) those where the conditions are sufficiently forcing for both mercuration and decarboxylation to occur. 

Thiadiazolines and thiadiazolium salts can undergo a thermally promoted rearrangement to yield 2-guanidinoben-zothiazoles. Thus the thiadiazoline 42 when heated in ethanol at reflux affords the benzothiazole 43 (Equation 11). There is evidence to suggest that this could be an electrophilic aromatic substitution reaction but a free radical mechanism was also proposed <2003SC2053>. 

Finally, we ask, if the reactive triads in Schemes 1 and 19 are common to both electrophilic and charge-transfer nitration, why is the nucleophilic pathway (k 2) apparently not pertinent to the electrophilic activation of toluene and anisole One obvious answer is that the electrophilic nitration of these less reactive [class (ii)] arenes proceeds via a different mechanism, in which N02 is directly transferred from V-nitropyridinium ion in a single step, without the intermediacy of the reactive triad, since such an activation process relates to the more conventional view of electrophilic aromatic substitution. However, the concerted mechanism for toluene, anisole, mesitylene, t-butylbenzene, etc., does not readily accommodate the three unique facets that relate charge-transfer directly to electrophilic nitration, viz., the lutidine syndrome, the added N02 effect, and the TFA neutralization (of Py). Accordingly, let us return to Schemes 10 and 19, and inquire into the nature of thermal (adiabatic) electron transfer in (87) vis-a-vis the (vertical) charge-transfer in (62). 

In the thermal decomposition of 45 in aniline and A -methylaniline, the carbene 54 showed electrophilic reactivity together with hydrogen abstraction ability (84JOC62). In fact, 48 [R = NHPh, N(Me)Ph], products of A -pyrrylation of anilines, together with 48 (R = H), a product of hydrogen transfer to 54, were obtained. In this case, aromatic substitution, typical of electron-rich benzene derivatives, was not observed. The formation of both A -pyrrylated anilines, evidence of involvement of 54s, and the... 

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