Relative energies, disubstituted

A variety of electronic effects control the relative energies of the disubstituted benzenes. For the difluorobenzenes, the meta and para isomers are of comparable energy the ortho isomer is clearly less stable. This is most likely due to the electrostatic repulsions of the two C-F bond dipoles which are aligned in the same direction. This is just like the difluoroethylenes where the 1,2-cis isomer is much less stable than the 1,1-isomer.poj. he dihydroxybenzenes. 

Table 52. Relative energies of the conformations of 1,2-disubstituted cyclohexanes, CgH 10XY...

Table II. Relative Electronic Energies of Disubstituted Benzenes in kcal/mol...

We first consider the relative electronic transition energies in cis and trans 1,2-disubstituted ethylenes. From Fig. 28 we can clearly see that the pi HOMO-LUMO energy gap is larger for the case of the cis isomer relative to the trans isomer. Hence, the mr transition is expected to occur at shorter wavelengths in cis 1,2-disubsti-tuted ethylenes. 

The ene reaction has proved to be particularly powerful in synthesis when carried out intramolecularly. The usual increase in rate for an intramolecular reaction allows relatively unreactive partners to combine. Thus the diene 6.13 gives largely (14 1) the cis disubstituted cyclopentane 6.15 by way of a transition structure 6.14. It is important to recognize that the selective formation of the ci j-disubstituted cyclopentane has nothing to do with the rules for pericyclic reactions. It is a consequence of the lower energy when the trimethylene chain spans the two double bonds in such a way as to leave the hydrogen atoms on the same side of the folded bicyclic structure. This... 

The 17-electron species see Seventeen Electron Configuration) formed can undergo rapid substitution since associative pathways of low activation energy exist for them. Recombination of the substituted flagments can yield the monosubsti-tuted, disubstituted, or more fully substituted complexes. The rate-determining step would be cleavage of the metal-metal bond since all other steps are relatively fast. This pathway may be preferred for photochemical substitution. Clearly, this pathway is not open to mononuclear analogs. The reactivity of low valent metal radical complexes has been reviewed. ... 

In contrast to the protonation of the disubstituted alkenes mentioned above, high ktranJkcis ratios of 9 x 108 and 3 x 103, respectively have been observed in the acid-catalyzed addition of methanol to the trans- and cis-isomers of cycloheptene (39a and 38a) and cyclooctene (37a and 36a) (114a). The rate constants reflect partially the release of strain in the transformation of the cyclic olefins to the appropriate cycloalkyl cations. Comparison of the relative activation energies for these addition reactions with the difference of strain release leads to the estimate that the response to strain effects is about 60%. In a more recent study of the acid-catalyzed hydration of cis- (36b) and trans- 1-methylcyclooctene (37c), it was concluded that two conformationally different 1-methyl-carbocationic intermediates are... 

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