Aromatic reactivity

The strength of this theory is the generality of its concepts. Its weaknesses are its inability to yield more than qualitative information and its incapacity for dealing with free-radical substitution. The polar requirements of the latter are small, and their attacks upon aromatic nuclei are not to be understood in terms of the gross electronic distribution in the ground state, or of the redistribution permitted by polarizability. 

The picture of aromatic substitution given by qualitative resonance theory is roughly equivalent to the above treatment, but, as usually presented, less flexible in failing to stress the influence of the approaching reagent.  

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The electronic theory of organic chemistry, and other developments such as resonance theory, and parallel developments in molecular orbital theory relating to aromatic reactivity have been described frequently. A general discussion here would be superfluous at the appropriate point a brief summary of the ideas used in this book will be given ( 7- )-... 

Quantitative eomparisons of aromatic reactivities were made by using the competitive method with solutions of nitronium tetrafluoroborate in sulpholan, and a concentration of aromatic compounds 10 times that of the salt. To achieve this condition considerable proportions of the aromatic compoimds were added to the medium, thus depriving the sulpholan of its role as true solvent thus, in the nitration of the alkyl- and halogeno-benzenes, the description of the experimental method shows that about 50-60 cm of mixed aromatic compounds were dissolved in a total of 130 cm of sulpholan. 

It is the purpose of this chapter to provide a resume of theoretical concepts which are used in discussing aromatic reactivity. Extended discussion is imnecessary, for many of the concepts are the common currency of organic chemistry, and have been frequently expounded. - ... 

There are certain limitations to the usefulness of nitration in aqueous sulphuric acid. Because of the behaviour of the rate profile for benzene, comparisons should strictly be made below 68% sulphuric acid ( 2.5 fig. 2.5) rates relative to benzene vary in the range 68-80% sulphuric acid, and at the higher end of this range are not entirely measures of relative reactivity. For deactivated compounds this limitation is not very important, but for activated compounds it is linked with a fundamental limit to the significance of the concept of aromatic reactivity as already discussed ( 2.5), nitration in sulphuric acid cannot differentiate amongst compounds not less than about 38 times more reactive than benzene. At this point differentiation disappears because reactions occur at the encounter rate. 

The limit to the significance of aromatic reactivity set by reaction at the encounter rate is reached at different levels of reactivity in different conditions. As already seen, for nitration with nitric acid in organic... 

As we have seen, the important zero energy difference which measures aromatic reactivity contains a term involving rr-electron energies, and rvith the transition state model there will also be a contribution from... 

Brown developed the selectivity relationship before the introduction of aromatic reactivities following the Hammett model. The former, less direct approach to linear free-energy relationships was necessary because of lack of data at the time. 

Nitration and aromatic reactivity C. The nitration of monocyclic compounds... 

Nitration and aromatic reactivity D The nitration of hi- and poly-cyclic compounds... 

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