Para electron acceptor

Shifts for meta carbon atoms remain almost unaffected by all kinds of substituents, unlike shifts for ortho and para carbons. Electron-releasing substituents (electron donors) increase tt electron densities in ortho and para positions and thereby induce a shielding relative to benzene (Sc(o,p) < 128.5 ppm)22. Electron-withdrawing groups (electron acceptors), on the other hand, decrease the ortho and para jr electron densities and lead to a deshielding relative to benzene (Sc(o,p) > 128.5 ppm). 

When (5(15N) values for the previously used series of 13 para-substituted anilines were measured in acetone42, a significantly weaker hydrogen-bond acceptor solvent than DMSO, a smaller shift dependence on para TT-electron-acceptor substituent solvation (SSAR) effects in acetone was observed (Table 10). This reduction42 was expressed by the (rather unsatisfactory) forms, 4 and 5. 

This initial attack of the ozone molecule leads first to the formation of ortho- and para-hydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron acceptor groups. In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups. 

The reaction of Scheme 4.10 yields only products of ortho and para substitutions the meta isomer is lacking. If it were a standard radical substitution, the meta-isomer would obviously be formed in a certain amount (i.e., in the same amount as that for ortho-substituted product). Introduction of electron-acceptor substituents enhances stability of the substrate to oxidation and prevents electron transfer to benzoyloxy radical. As a result, phenylation takes place instead of benzoyloxylation, and the phenyl radical enters into any free position. 

Anaerobic CP degradation involves sequential reductive dehalogenations with MCPs or DCPs as final metabolites, or degradation may proceed to complete dechlorination to phenol, further transformation to benzoate and, ultimately, conversion to methane and carbon dioxide. Reductive dechlorination of PCP, for example, results in the formation of meta- or para-CPs as the end-products (e.g., Woods et al., 1989 Madsen Aamand, 1992), or anaerobic degradation may continue to complete mineralization (Boyd Shelton, 1984 Mohn Kennedy, 1992 Wu et al., 1993). In reductive dechlorinations, CPs serve as electron acceptors and need a suitable electron donor. 

Silver acetylides have also been used to form bonds with non-carbon centers to produce synthetically useful compounds. Sladkov and coworkers report the reaction of silver acetylides to arenediazonium chlorides to give areneazoethynes.125,126 A necessary condition for the reaction was the presence of para or ortho electron acceptor substituents on the aromatic ring of the diazonium salt (Scheme 1.56). 

The vast majority of reactivities and regioselectivities observed in the reaction with electrophiles on monosubstituted benzenes (Table 5.2) are in agreement with the preceding generalizations (columns 2 and 4). The very few substituents that are not in agreement (column 3) deactivate the aromatic compound as do electron acceptors, but they are para- > ortho-directing as are electron donors. 

The reason for both trends is the same the benzyl cations that attack the toluene or the benzene have different stabilities depending on the nature of their para substituent X. When X is the electron acceptor NOz, we have the most electron-deficient cation, whereas when X is the electron donor MeO, we have the most electron-rich cation. The formation of Wheland complexes from p-02N—C6H4—CHj should therefore be exothermic and exergonic. Conversely, the formation of Wheland complexes... 

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