Electron ortho position

The major influence of the methyl group is electronic The most important factor IS relative carbocation stability To a small extent the methyl group sterically hinders the ortho positions making attack slightly more likely at the para carbon than at a single ortho carbon However para substitution is at a statistical disadvantage because there are two equivalent ortho positions but only one para position... 

Substitutions that displace electrons toward the carboxyl group of aromatic acids diminish the rate of the reaction (16). The substitution of fluoromethoxy or ethoxy groups in the ortho position has an accelerating action, whereas iodo, bromo, nitro, or methyl groups produce retardation. The influence of groups in the meta and para positions is not nearly so marked (17). 

As to the electron-withdrawing substituents, the activating effect of a nitro group in the piperidino-dechlorination of 2-chloropyridine involves factors of 7.3 x 10 and 4.6 x 10 from the para and ortho positions, respectively. An ortho-cyano group was found to be... 

In many cases, however, the ortho isomer is the predominant product, and it is the meta para ratio which is close to the statistical value, in reactions both on benzenoid compounds and on pyri-dine. " There has been no satisfactory explanation of this feature of the reaction. One theory, which lacks verification, is that the radical first forms a complex with the aromatic compound at the position of greatest electron density that this is invariably cither the substituent or the position ortho to the substituent, depending on whether the substituent is electron-attracting or -releasing and that when the preliminary complex collapses to the tr-complex, the new bond is most likely to be formed at the ortho position.For heterocyclic compounds such as pyridine it is possible that the phenyl radical complexes with the nitrogen atom and that a simple electronic reorganization forms the tj-complex at the 2-position. 

Using more basic (electron-donating) ligands generally speeds up the reactions while bulky ligands slow it down. The use of tris(o-tolyl)phosphine is an extreme example of the latter as IrCl(CO)[P(o-tolyl)3]2 fails to add 02, H2 or S02 and only adds HCI slowly the reason is that methyls in an ortho-position tend to block the axial positions (Figure 2.77) [126b]. 

The intense activation of ortho positions by strongly electron-withdrawing groups has produced some interesting kinetic results in the base-catalysed exchange of pyridine and its derivatives. For the neutral molecule, exchange occurs... 

The decarboxylation of aromatic acids is most often carried out by heating with copper and quinoline. However, two other methods can be used with certain substrates. In one method, the salt of the acid (ArCOO ) is heated, and in the other the carboxylic acid is heated with a strong acid, often sulfuric. The latter method is accelerated by the presence of electron-donating groups in ortho and para positions and by the steric effect of groups in the ortho positions in benzene systems it is... 

X values clearly reflect the poorer transmission of pi delocalization effects from the meta than para position. For the select meta sets of Table XI, for example, X" is typically about. 4, whereas for corresponding reactions, X is around unity. Evidence is meager with respect to the ortho position, but it appears that in accord with the classical ideas of pi electron transmission in the benzene ring, generally X > X > X"" for corresponding reactions (cf. subsequent discussion). 

The ffg parameter scales of Table V appear to be applicable to substituent effects at the ortho and meta positions. For the latter position, however, the data generally are not capable of discriminating between Og scales (cf. earlier results and discussion). For the ortho position, the great difficulty is in obtaining a data set covering the full range of electronic properties without the incursion of substantial proximity effect (2a) contributions. In a following section are reported some of the results of treatment of ortho data sets by eq.(l). 

Here summation is done over all types of interactions, including inductive, resonance and mixed (which is possible due to nonlinear term). The o constants denote electronic (Hammett-type) constants, whereas 5 and Hb denote steric and H-bonding interaction increments which mostly occur in the ortho position. 

The structure of the products is determined by the site of protonation of the radical anion intermediate formed after the first electron transfer step. In general, ERG substituents favor protonation at the ortho position, whereas EWGs favor protonation at the para position.215 Addition of a second electron gives a pentadienyl anion, which is protonated at the center carbon. As a result, 2,5-dihydro products are formed with alkyl or alkoxy substituents and 1,4-products are formed from EWG substituents. The preference for protonation of the central carbon of the pentadienyl anion is believed to be the result of the greater 1,2 and 4,5 bond order and a higher concentration of negative charge at C(3).216 The reduction of methoxybenzenes is of importance in the synthesis of cyclohexenones via hydrolysis of the intermediate enol ethers. 

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