Benzene derivatives orientation effects

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

The problems encountered in any attempt to treat the transmission of the effects of one substituent in a disubstituted heterocycle through the heterocyclic nucleus to a reaction site in the other substituent (i.e. the side-chain) are enormous, and it is consequently not surprising that relatively little work has been done in this area. First, while in benzene derivatives there are three positions, i.e. three relations between substituent and reacting side-chain to be considered, the number of complexities is much greater in heterocycles. Thus, e.g., in pyridine alone, after elimination of the orientations involving a vicinal relationship between substituent R and the side-chain Y to which no Hammett-type relation is likely to be applicable, the following cases should be considered ... 

The isolated-molecule method stems from the pioneering work of Wheland and Pauling (1935) on the orientation effects of substituents in benzene derivatives. It assumes that electrophilic and nucleophilic reagents attack preferentially at positions of high and low charge (qf) respectively, thus providing a more precise formulation of earlier... 

Orientation effects in benzene derivatives operate in two ways. If the substituent is inductive there are large first order charge displacements at the ortho and para positions, and these can be estimated approximately using the atom polarizabilities (which is very small at the meta position). The changes of bond order, however, and consequently of free valence, vanish in first order and hence depend on Sa. The charge g g at position s therefore increases or decreases from the value unity in the... 

The nitration of aniline in the presence of a large amount of strong sulfuric acid results wholly in the formation of m-nitro-aniline, but the similar nitration of dimethylaniline gives principally a mixture of the ortho- and para-derivatives. Mono-methylaniline stands between aniline and dimethylaniline in respect to the orienting effect of its amino group it yields a considerable amount of the m-nitro- compound—and dimethylaniline is preferred for the preparation of tetryl. Commercial dimethylaniline contains a certain amount of monomethylaniline, from which it is extremely difficult to free it, and this in the manufacture of tetryl is converted in part into 2,3,4,6-tetranitro-phenylmethylnitramine, or m-nitrotetryl, pale yellow, almost white, crystals from benzene, m.p. 146-147.087... 

Resonance and induction have a marked effect on orientation in aromatic substitution. One would therefore expect some correlation between orientation in substitution and the electronic absorption spectra of benzene derivatives. In Table 5.4 the position of the 200 mp benzene band10 and the spectroscopic moments14 have been compared with the percentage of the meta isomer produced in nitration. A fair parallelism is apparent in the three columns. 

Let us calculate the area that should be occupied by OP-10 molecules in an extremely saturated adsorption layer. The effective area of the normal cross-section of the hydrocarbon chains for benzene derivatives (according to structural analysis) is 23.8 x 10 °m, and the distance between the axes of two oriented chains in the saturated layers is not less than (4.5-6) x 10 m. Consequently the effective cross-section for the OP-10 molecule with two hydrocarbon chains is about (52.1-53.6) X 10 °m. This value is less than that determined experimentally and practically coincides with the area occupied hy the L-19 molecule in the adsorption layer. Thus, L-19 does not form asso-... 

It is difficult to treat the effect of a heteroatom on the localization energies of aromatic systems, but Brown has derived molecular orbital parameters from which he has shown that the rates of attack of the phenyl radical at the three positions of pyridine relatively to benzene agree within 10% with the experimental results. He and his co-workers have shown that the formation of 1-bromoisoquinoline on free-radical bromination of isoquinoline is in agreement with predictions from localization energies for physically reasonable values of the Coulomb parameters, but the observed orientation of the phcnylation of quinoline cannot be correlated with localization ener-... 

However, there are some contradictory reports on the composition of the products of toluene alkylation or benzene dialkylation at high conversions. In some cases, compositions corresponding to the thermodynamic equilibrium between ortho, meta and para isomers were found, and in other cases, kinetic control of orientation, giving mostly the ortho + para substitution, prevailed. Consecutive isomerisation of the ortho and para isomers to the more stable meta isomer seems to be the cause of the disagreement. More active catalysts gave more meta derivatives than the less active ones [343] and increasing the temperature has the same effect [351]. 

Partial benzylation with powdered potassium hydroxide as a base and toluene as a solvent was used some 50 years ago for the preparation of 1,6-anhydro-2,4-0-benzyl-P-D-glucopyranose [79]. Since that time, other solvents, such as benzene [80-82], 1,4-dioxane-toluene mixtures [83, 84], or excess benzyl chloride [82, 85] were used as well, with apparent effects on the regioselectivity. Thus, the axially oriented secondary hydroxyl group of lL-l,2,3,4-tetra-0-benzyl-c/i ro-inositol is more reactive than the equatorial one using benzyl chloride alone (ratio of 79 21), whereas the opposite is true (35 65) in benzene as a solvent [82]. Benzylation of myo-inositol derivatives in the latter solvent was also described [80, 81, 86]. 

An extended study of the optical behavior of this class of products has been performed in connection with investigations on the conformations of lycorine (1) and related compounds. An empirical rule, similar to the octant rule, which allows the prediction of both the sign and magnitude of the Cotton effect at 290 nm from the steric orientation of the atoms about the aromatic chromophore, has been deduced from considerations on the ORD and CD curves of lycorine (1) and derivatives. When the benzene ring is viewed along the — z —+ z axis (the coordinates are fixed on the aromatic ring as depicted in Fig. 1), four back octants are defined which contribute, with the signs indicated, to the Cotton effect. 

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