Benzenes proton affinities

Evaluation of the only appropriate Fukui function is required for investigating an intramolecular reaction, as local softness is merely scaling of Fukui function (as shown in Equation 12.7), and does not alter the intramolecular reactivity trend. For this type, one needs to evaluate the proper Fukui functions (/+ or / ) for the different potential sites of the substrate. For example, the Fukui function values for the C and O atoms of H2CO, shown above, predicts that O atom should be the preferred site for an electrophilic attack, whereas C atom will be open to a nucleophilic attack. Atomic Fukui function for electrophilic attack (fc ) for the ring carbon atoms has been used to study the directing ability of substituents in electrophilic substitution reaction of monosubstituted benzene [23]. In some cases, it was shown that relative electrophilicity (f+/f ) or nucleophilicity (/ /f+) indices provide better intramolecular reactivity trend [23]. For example, basicity of substituted anilines could be explained successfully using relative nucleophilicity index ( / /f 1) [23]. Note however that these parameters are not able to differentiate the preferred site of protonation in benzene derivatives, determined from the absolute proton affinities [24],... 

The use of N.M.R. spectroscopy further requires that the proton affinity of the aromatic substances should not be too low. Benzene and toluene, for instance, have such a low proton affinity that no proton addition complex has so far been demonstrated by means of N.M.R. spectroscopy, even at — 100°C. A combination with other methods is therefore required in order to demonstrate proton addition complexes in these cases. 

A resolution into two or more components always occurs if the solvent has a high proton affinity, so that a solvent molecule can form a particularly stable association with a phenol molecule as a result of an energetically favourable mutual orientation. This is the case, for example, if benzene and toluene are used as the solvents. However, this effect is even more pronounced in the case of cyclohexene. Dielectric constant measurements for phenol in various solvents agree with this observation. In particular, the dipole moments in benzene and cyclohexene (1-45 and 1-79 D respectively), are considerably greater than the value of 1-32 in cyclohexane. Liittke and Mecke (1949) attributed this effect to the ability of this unsaturated solvent to act as a proton acceptor, i.e. to form 7r-complexes. 

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... 

On the basis of the evaluation of the proton affinity (860.6 kJmoP for hexa-methylbenzene and 845.6 kJmoP for tetramethylbenzene 148)), the possibility of obtaining hexamethylbenzene and tetramethylbenzene as carbocations in the pores of a zeolite had been excluded. However, Haw and co-workers 146) recently demonstrated by means of NMR spectroscopy that H-heptamethylbenzene may be formed in the cavities of a H(3 zeolite. H-hexamethylbenzene and H-tetramethyl-benzene ions have been observed in zeolite H(3 by a combination of IR and UV-visible spectroscopies 149,150). DRS UV-Vis- and FTIR spectroscopy proved to be techniques well suited to verify, under reaction conditions, the existence of stable H-hexamethylbenzene and H-tetramethylbenzene in the zeolite. Owing to the symmetry properties of H-hexamethylbenzene and H-tetramethylbenzene, characteristic changes of their vibrational features were observed when the aromatic system was perturbed upon protonation. In the same study it was found that the lower polymethylbenzene homologues, such as 1,3,5-trimethylbenzene (PA = 836.2 kJmol ), did not undergo appreciable protonation in H(3 zeolite. On the basis of these results, a proton affinity limit for hydrocarbons that form stable... 

Figure 3. Proton affinity [Ref.lO], vs. Tm of benzene (B), toluene (T), naphthalene (N), m-xylene (mX) (obtained from the second TPD peak) and mesitylene (TMB) (second TPD peak) for H-ZSM-11 zeolite.

Cubane has had an interesting place in the discussion of the correlation between C-H acidity and carbon hybridization. Its acidity was measured by the H exchange NMR technique and found to be about 6.6 x 10 as reactive as benzene. An experimental gas phase measurement of the proton affinity (PA) as 404kcal/mol is available. (See Tables 3.14 and 3.38 for comparable data on other hydrocarbons.) Both of these values indicate that cubane is somewhat more acidic than expected on the basis of the carbon hydridization. There appears to be unusual hybridization of the anion in this case. An AIM analysis suggests that the C—C bond paths in the anion are less than 90°, suggesting that the bonds bend inward toward the center of the ring. Sauers also noted an increase in s character on going from the hydrocarbon to the anion. Of the 28 deprotonations he examined, only cyclopropane and bicyclo[l.l.l]pentane also showed increased s character in the anion. 

A theoretical approach has been used to compare the proton affinities of phospha-, arsa-, and stiba-benzenes with that of pyridine, and other simpler organo-group 15 systems. The electronic excitation spectra of pyridine and phosphabenzene have also been studied by theoretical methods. A route to the... 

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. 

Theoretical absolute proton affinities obtained for monosubstituted benzenes C H X are presented in Table 1. The examined substituents encompass X = C//3, OH, OGH3,... 

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