Aromatic species formation

Role of C4 Hydrocarbons in Aromatic Species Formation in Aliphatic Flames... 

Aromatic species formation in aliphatic flames, 3-16 Aromatic species formation in... 

Where a starting material may be converted into two or more alternative products, e.g. in electrophilic attack on an aromatic species that already carries a substituent (p. 150), the proportions in which the alternative products are formed are often determined by their relative rate of formation the faster a product is formed the more of it there will be in the final product mixture this is known as kinetic control. This is not always what is observed however, for if one or more of the... 

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. 

The detailed modeling of soot formation in the shock tube pyrolysis of acetylene [106] and other fuels [107] provides the central basis for the fuel-independent general mechanisms suggested here. It must be noted, as well, that a large body of work by Howard et al. [108, 109] on premixed flames with regard to formation of aromatic species provides direct tests of the proposed mechanisms and are key to understanding and modeling soot formation. 

Even comprehensive mechanisms, however, must be utilized with caution. The GRI-Mech fails, for instance, under pyrolysis or very fuel-rich conditions, because it does not include formation of higher hydrocarbons or aromatic species. Its predictive capabilities are also limited under conditions where the presence of nitrogen oxides enhances the fuel oxidation rate (NO f sensitized oxidation), a reaction that may affect unbumed hydrocarbon emissions from some gas-fired systems, for example, internal combustion engines. 

Formation of Aromatic Compounds A scientific challenge comparable to that of developing oxidation mechanisms for the large hydrocarbon fuels is understanding and describing quantitatively the formation and oxidation of aromatic and polycyclic aromatic compounds (PAH) formed in combustion processes. Aromatic compounds are known to be harmful to the environment, and the emission of these species from a number of combustion systems is a significant concern. Furthermore aromatic species are important pre-... 

The next section makes use of the much more recent observation19 that there is a nearly constant difference of the enthalpies of formation of corresponding vinyl and phenyl derivatives. If vinyl relates to cyclopropyl, and vinyl also relates to phenyl, then how do corresponding cyclopropyl and phenyl derivatives relate Conceptually, vinylcyclopropane (10), also identified as 1, X = Cypr and 2, X = Vi) and styrene (11, X = Vi, also identified as 1, X = Ph) are thus relatable. Likewise, relatable are cyclopropylamine (2, X = NH2) and aniline (11, X = NH2)18. This thermochemical comparison of benzene and cyclopropane derivatives is not merely a check of two purported identities in terms of a third, arithmetically derivable, identity. Benzene is the archetypical 7i-delocalized aromatic species from which understanding of this widespread phenomenon evolves. Cyclopropane is the paradigm of cr-aromatic species from which understanding of this more exotic phenomenon evolves20. Benzene and cyclopropane are thus naturally paired as conceptual models for delocalization and aromaticity. Section III discusses these and related issues. 

Methoxy-substituted aromatic compound 4 is lithiated metalation with Buli in THF, a step in which it proves useful to include lithium chloride. Because of the greater basicity of /t-butyllithium relative to 4. direct metallation is in fact possible thermodynamically, but /i-butyllithium is generally present in solution as a tetra-mer, and this reduces its reactivity. Addition of lithium chloride destroys these aggregates, and that eliminates the kinetic inhibition. Lithiated aromatic species 18 is further stabilized through chelate formation between lithium and the orr/icr-methoxy groups (ortho effect).8... 

The TV-methoxy-jV-methylamide of tiglic acid (17) is used as an aeylating agent in a procedure developed by Weinreb.9 Lithiated aromatic species 18 attacks Weinreb amide 17 with formation of the chelate 19. which is hydrolyzed to ketone 5. Use of Weinreb amide 17 circumvents the primary threat here multiple addition and formation of a tertiary alcohol. Since complex 19 decomposes only in the course of workup, the ketone 5 itself is protected against further nucleophilic attack.10 "BuLi, LiCl. THF. 0 C->RT 17. 77%. 

The two free hydroxy groups are First protected with acetic anhydride. In a second step the acetyl group is reductively cleaved by a Birch reduction with lithium in liquid ammonia.19 Lithium dissolves in the ammonia with the formation of solvated electrons. Stepwise electron transfer to the aromatic species (a SET process) leads first to a radical anion, which stabilizes itself as benzylic radical 38 with loss of the oxygen substituent. A second SET process generates a benzylic anion, which is neutralized with ammonium chloride acting as a proton source (see Chapter 12). 

Other elements can also participate in the formation of aromatic species. Furan, pyrrole, and thiophene are all aromatic molecules. This is due to the fact that if the heteroatom is sp2 hybridized, then a doubly occupied p orbital interacts with the carbon 2p orbitals to give an MO array which contains six it electrons and is aromatic. Note that in the development of the MO diagram for these systems the identity of the heteroatom is not important. It is only important in determining the magnitude of the aromatic stabilization. 

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