Atomic aromaticity constant

Table 2. Atom Types and Corresponding Aromaticity Constants Ah for R = H...

Table 2 Aromaticity constants (relative electronegativities) of first-row atoms...

The ever-present problem as to whether nucleophilic addition has occurred a or y to the heteroatom in a six-atom aromatic ring is best solved via PMR spectral analysis, considering both chemical shift and coupling constant data. In particular, the 1,2- and 1,4-dihydroquinoline derivatives, 3 and 4, which are potential methoxide adducts from quinolinium cations are readily... 

Balaban and Simon159 have argued that an index based on the relative electrophilicity and nucleophilicity of a ring compared with benzene is a useful means of expressing in numerical terms the aromaticity of a ring system. The authors derive an aromaticity constant K from a summation of k values for each ring position, where k is an expression of the tendency of the atom to release or attract -electrons from the delocalized 7r-cloud and is defined by... 

Picosecond spectroscopy has been used to study the photodissociation of Ij-aromatic complex and the formation of iodine atom-aromatic complexes. Shizuka, Nakamura, and Morita have studied the anion-induced fluorescence of aromatic molecules. Electron transfer from the anion to the excited aromatic seems to be the key step (Table 20). Triplet formation has also been studied by nanosecond laser spectroscopy. The rate constants are shown in Table 21. 

The correction term in Eq. (9) shows that the basic assumption of additivity of the fragmental constants obviously does not hold true here. Correction has to be appHed, e.g., for structural features such as resonance interactions, condensation in aromatics or even hydrogen atoms bound to electronegative groups. Astonishingly, the correction applied for each feature is always a multiple of the constant Cu, which is therefore often called the magic constant . For example, the correction for a resonance interaction is +2 Cj, or per triple bond it is -1 A detailed treatment of the Ef system approach is given by Mannhold and Rekker [5]. 

It has been proposed that aromatic solvents, carbon disulfide, and sulfur dioxide form a complex with atomic chlorine and that this substantially modifies both its overall reactivity and the specificity of its reactions.126 For example, in reactions of Cl with aliphatic hydrocarbons, there is a dramatic increase in Ihe specificity for abstraction of tertiary or secondary over primary hydrogens in benzene as opposed to aliphatic solvents. At the same time, the overall rate constant for abstraction is reduced by up to two orders of magnitude in the aromatic solvent.1"6 The exact nature of the complex responsible for this effect, whether a ji-coinplex (24) or a chlorocyclohexadienyl radical (25), is not yet resolved.126- 22... 

There is one other substituent which is comparable with the diazonio group in the sense that it is cationic and that it has, in one of its mesomeric structures, a triple bond between the atom attached to the aromatic system and the second atom. It is the acylium group in 7.9. However, no substituent constants are known for this group, obviously because this cation is detectable in measurable concentrations only in superacidic media (see review by Olah et al., 1976). 

Further lowering the dielectric constants has been achieved by preparing highly fluorinated polyethers without any sulfone, ketone, or other polarizable groups.239 241 Typically, the /jara-lluorinc atoms on highly fluorinated aromatic compounds, such as hexafluorobenzene and decafluorobiphenyl, are activated and thus can go through aromatic nucleophilic substitution with HFBPA under typical reaction conditions (Scheme 6.31).217... 

The effect of the substitution of a heavy-atom directly onto the nucleus of aromatic compounds (internal heavy-atom effect) on intercombinational radiative and nonradiative processes can be seen by examination of experimental data obtained for naphthalene and its derivatives. The data obtained by Ermolaev and Svitashev<104) and analyzed by Birks(24) to obtain individual rate constants for the various processes are collected in Table 5.9. 

The Claisen-Schmidt condensation of 2 -hydroxyacetophenone and different chlorinated benzaldehydes over MgO has been investigated through kinetic and FTIR spectroscopic studies. The results indicate that the position of the chlorine atom on the aromatic ring of the benzaldehyde substantially affects the rate of this reaction. In particular, the rate increases in the following order p-chlorobenzaldehyde < m-chlorobenzaldehyde < o-chlorobenzaldehyde. The difference between the meta and para-substituted benzaldehyde can be attributed to electronic effects due to the difference in the Hammett constants for these two positions. Steric effects were found to be responsible for the higher rate observed with the o-chlorobenzaldehyde. 

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