Electrophilic aromatic substitution localization energy

Electrophilic aromatic substitution is a typical reaction for BHs. In the MO treatment, some indices such as free valence [40], localization energy [41], and other quantities [42,43] have been introduced to predict the orientation of electrophilic aromatic substitution. Within the VB framework, several indices have also been formulated [44]. Here we introduce an alternative index, which is available from accurate VB wave functions, and demonstrate its applicability in accounting for the electrophilic aromatic substitution. 

In a chemical reaction, a more stable transition state, measured by the magnitude of the activation energy, implies an easier chemical reaction. Aromatic transition states are also known to facilitate the chemical reaction. Zhou and Parr defined the activation hardness as the hardness difference of the products and the transition state and found, in the case of electrophilic aromatic substitution, that the smaller the activation hardness, the faster the reaction is. For this specific reaction they also found a correlation of the activation hardness and Wheland s cation localization energy, also proposed as an indicator of aromaticity. This finding can indeed be interpreted as a manifestation of the maximum hardness principle. A transition state with a high hardness is more stable than one with a smaller hardness and is therefore easier to reach energetically. The same can be said about two transition states with different aromaticity. Again, hardness and aromaticity parallel each other. The activation hardness has been used in numerous applications for the prediction of site selectivity in chemical reac-... 

As discussed in the theoretical section (4.04.1.2.1), electrophilic attack on pyrazoles takes place at C-4 in accordance with localization energies and tt-electron densities. Attack in other positions is extremely rare. This fact, added to the deactivating effect of the substituent introduced in the 4-position, explains why further electrophilic substitution is generally never observed. Indazole reacts at C-3, and reactions taking place on the fused ring will be discussed in Section 4.04.2.3.2(i). Reaction on the phenyl ring of C- and A-phenyl-pyrazoles will be discussed in Sections 4.04.2.3.3(ii) and 4.04.2.3.10(i), respectively. The behaviour of pyrazolones is quite different owing to the existence of a non-aromatic tautomer. 

Both HMO calculations and more elaborate MO methods can be applied to the issue of the position of electrophilic substitution in aromatic molecules. The most direct approach is to calculate the localization energy. This is the energy difference between the aromatic molecule and the n-complex intermediate. In simple Hiickel calculations, the localization energy is just the difference between the energy calculated for the initial n system and that remaining after two electrons and the carbon atom at the site of substitution have been removed from the conjugated system ... 

The reactivity and the orientation effects in aromatic electrophilic substitutions are usually estimated from the relative facility of formation of o-complexes (arenium ions) by comparing such calculation parameters as the index of free valence, the it-electron density, the localization energy of a pair of electrons etc. S. Nagakura and J. Tanaka 27-679) judependraitly, R. Brown have emphasized the role... 

Within an aromatic substitution transform the terms RLENERGY, NLENERGY, and ELENERGY refer to the radical, nucleophilic, and electrophilic localization energies respectively. Several types of statements use these terms, e.g. ... 

Electronic effects in conjugated systems may be computed with the Hiickel HMO method. Electron densities are used to predict aromatic substitution reactions, and it is possible to refer to radical, nucleophilic, and electrophilic localization energies.For the latter, the term ELENERGY is used in a transform, for example IF ELENERGY ON ATOM 1 BETTER THAN ATOM 2 THEN ADD 20. 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

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