Additivity rules benzenes

Section 11 19 An additional requirement for aromaticity is that the number of rr elec Irons m conjugated planar monocyclic species must be equal to An + 2 where n is an integer This is called Huckel s rule Benzene with six TT electrons satisfies Huckel s rule for n = 1 Square cyclobutadiene (four TT electrons) and planar cyclooctatetraene (eight rr electrons) do not Both are examples of systems with An rr electrons and are antiaromatic... 

Methyl cation affinities of benzene and some substituted benzenes have been calculated. These follow a simple additivity rule and the value for benzene shows good agreement with the experimental estimate. Conclusive evidence is presented that these values are linearly related to the corresponding proton affinities. The competition between deuteriation and alkylation in the reaction of radiolytically formed perdeuterio ethyl cations with iV-methylpyrrole and with thiophene has been studied. Deuteriation, the Brpnsted acid pathway, predominates and intramolecular selectivities have been determined for each reaction. ... 

The quantum-chemical calculation of charge-transfer states as possible intermediates in electrophilic aromatic substitution reactions, making allowance for solvation effects, has been reviewed.6 It has been shown that a simple scaled Hartree-Fock ab initio model describes the ring proton affinity of some polysubstituted benzenes, naphthalenes, biphenylenes, and large alternant aromatics, in agreement with experimental values. The simple additivity rule observed previously in smaller... 

Hence JR T In it = AAF. The quantity RTlnit is equal to the difference in the free energies of activation of the reaction with benzene and its derivative in the given (tth) position of the ring. According to the additivity rule, when there are a number of substituents the differences in the free energies of activation can be simply added, and this is equivalent to multiplying the partial rate factors ... 

On the basis of these observations, Bryce-Smith et al. [115] introduced a rule stating that for addition to benzene, Pmeta when 9.6 eV < IP (alkene) <8.65 eV. They concluded that if this rule is correct, ortho addition of ethylenes to Si benzene necessarily involves an element of charge transfer to or from the ethylene. Indeed, a marked effect of polar solvents (methanol or acetonitrile) in promoting the ortho addition of benzene to ethyl vinyl ether and tetramethylethene was observed (portho increased by 20-50%, whereas cpmeta was unaffected. One exception to this rule was found by Heine and Hartmann [10], who discovered that vinylene carbonate (IP = 10.08 eV) undergoes mainly meta photocycloaddition to benzene, accompanied by some para addition. Bryce-Smith and Gilbert [46] commented that their rule referred to quantum yields and not chemical yields, whereas no quantum yields were given for the vinylene carbonate additions. Moreover, quantum yield measurements should be made at low conversions because most ortho cycloadducts are photolabile. 

In the nitrobenzonitriles, such resonant structures will be mutually suppressed and consequently no large deviations from the additivity rule occur, such as exist when the two groups are of opposite charge so that the resonance is augmented by additional valence bond structures. The dipole moment of />-nitrobenzonitrile is zero, of o-nitrobenzonitrile, 6 19, D and m-nitro-benzonitrile, 3-78, D. Similar behaviour is observed with the di-substituted halogen derivatives of benzene and with the phenylenediamines. 

It is shown that the MP2(/c)/6-31G7//fF/6-31G + ZPE (HF/6-31G ) model reproduces very well the experimental proton affinities in a large number of substituted benzenes and naphthalenes. Extensive applications of this model revealed that the proton affinity of polysubstituted aromatics followed a simple additivity rule, which have been rationalized by the ISA (independent substituent approximation) model. Performance of this model is surprisingly good. Applications of proton affinities, obtained by the transparent and intuitively appealing ISA model, in interpreting directional ability of substituents in the electrophilic substitution reactions of aromatics are briefly discussed. 

Proton transfer reactions play very important role in chemistry and biochemistry [1-3]. Considerable attention has been focused on the gas phase reactions in the last decades, since they are free of the solvent pollution thus being related to the intrinsic reactivity [4 6]. In particular, investigations of gas-phase acidities and basicities were some of the major undertakings in the field [7,8]. The proton affinity (PA), on the other hand is an interesting thermodynamic property by itself. It gives useful information on the electronic structure of base in question and serves as an indicator of the electrophilic substitution susceptibility of aromatic compounds [9]. It is the aim of this article to describe some recent advances in theoretical calculations of the proton affinities of substituted aromatics. We shall particularly dwell in more detail on the additivity rules, which enable simple and quick estimates of PAs in heavily substituted benzenes and naphthalenes. Some prospects for future developements will be briefly discussed too. 

Proton Affinities in Polysubstituted Benzenes-The Additivity Rule 2.4.1. Increments... 

Proton affinities of some disubstituted benzenes obtained by the MP2(I) model (f T[MP2(I)]) and by the additivity rule (PT(add.)). Deviations from the additivity are given by A (in kcal/mol)... 

The ipso protonation deserves a special scrutiny is discussed earlier. Here we show that the additivity rule is operative for the ipso protonation too, if proper reference level is found. Let us consider multiply substituted fluorobenzenes. They are schematically depicted in Fig.7. It is obvious that the proton affinity of benzene cannot serve as a gauge value for the ipso protonation. Instead, we shall employ once again homodesmic reactions and proceed as follows. Protonation at position 1 of 1,2,3-trifluorobenzene will provide an illuminating example in this respect. The corresponding homodesmic reactions read ... 

We have conclusively shown that the ISA model was very useful in predicting and interpreting proton affinities in polysubstituted benzenes and naphthalenes. A compelling evidence is provided which documents that the PA values obtained by the additivity rule of thumb are in very good accordance with available experimental data and/or the accurate theoretical results offerred by the ab initio models at the MP2 level of sophistication. Analogous formulas should work in larger aromatic systems too. If there are some... 

The 7t-electron correlation energy E. in planar heteroatomic molecules follows simple additivity rules. The nondynamical component (ND),i of aromatic compounds is lower than that of their open-chain counterparts and (in comparison with benzene) regarded as another useful aromaticity index <2001GPH(269)11>. 

The shift to slightly higher wavelengths described for the alkyl substitution of benzene is much more generic. It applies for all compound types and a large number of substituents. The classic Woodward-Hoffman additivity rules describe... 

Stuart has made very interesting calculations with a number of disubstitution products of benzene he starts from the various possible interactions of the substituents and calculates the internal molecular potential of the whole molecule this is then compared with the deviations of the heats of combustion of the compounds from the additivity rule, with the result that considerable repulsion potentials are shown by the ortho-compounds, which practically disappear in the meta- and para-derivatiyes. 

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