Aromatic resonance energy

It is understandable that dihydro adducts should be formed by polycyclic compounds and not by benzene or pyridine, because the loss of aromatic resonance energy is smaller in the former than in the latter process, (c) When dibenzoyl peroxide is decomposed in very dilute solution (0.01 Af) in benzene, 1,4-dihydro biphenyl is produced as well as biphenyl, consistent with addition of the phenyl... 

In line with Beak s finding, pyridin-2-one was estimated to be 31 kJ mol-1 less aromatic than the pyridine, and a similar figure of 25 was estimated for pyridine-2-thione. Subsequent results (73JCS(P2)1080, 76AHC(S1)71) on the pyridin-4-one, quinolin-2-one and isoquinolin-1-one series showed that aromatic resonance energy difference for the pyridin-4-one/4-hydroxypyridine system was very similar to that for the 2-substituted compounds, in contrast to Beak s findings. 

Although iV-hydroxypyrroles possess in principle several tautomeric forms, e.g. (76), (77) and (78), only the A-hydroxy form (76) has been observed for l-hydroxy-2-cyanopyrrole (73JOC173). In the case of 1-hydroxyindoles, where the potential loss of aromatic resonance energy will be much less, both tautomers (79) and (80) coexist in solution with the relative proportions being dependent on the solvent... 

To explore this possibility further, quantum chemical calculations were carried out for the isodesmic reactions of the model compounds 66 and 67 with dihydrogen to give the corresponding dihydrides (Scheme 17). These calculations showed that the reaction of 66 with H2 is about 14 kcalmol-1 less exothermic than that of 67. This difference may reflect aromatic resonance energy in the unsaturated molecule 66, reducing the enthalpy of the hydrogenation reaction. 

The Crick-Watson base pairing with the complementarity C-G and A-T (for DNA) or A-U (for RNA) is a consequence of the delicate balance between aromatic resonance energies and bond energies for CC, CO and CN single and double bonds. 

This carbocation is called an arenium ion. Although it has considerable resonance stabilization, it has lost the aromatic resonance energy of benzene because the cycle of orbitals does not extend entirely around the ring. Note that the resonance structures have the positive charge located on the carbons ortho and para to the carbon that is bonded to the electrophile. 

Polycyclic aromatic compounds also undergo electrophilic aromatic substitution reactions. Because the aromatic resonance energy that is lost in forming the arenium ion is lower, these compounds tend to be more reactive than benzene. For example, the brotni-nation of naphthalene, like that of other reactive aromatic compounds, does not require a Lewis acid catalyst ... 

The Diels-Alder reaction is certainly one of the most important reactions in organic chemistry. A few other interesting examples are provided in the following equations. Benzene is not very reactive as a diene because the product would not be aromatic. However, reactive dienophiles do add to the central ring of anthracene. In this case the product, with two benzene rings, has not lost much aromatic resonance energy... 

Hydroxy derivatives. 2-Hydroxy derivatives usually exist as the oxo tautomers, unless the hydroxy tautomer is appreciably stabilized by electron-withdrawing or chelating substituents. The tendency for enolic hydroxy compounds to revert to the oxo form can be understood by reference to simple aliphatic ketones where the keto-enol equilibrium constants are of the order of 108. In the five-membered heterocycles under consideration, this tendency will be in opposition to the loss of aromatic resonance energy that increases in the order furan << thiophene < pyrrole. For the 2-hydroxy compounds 217 some extra stabilization of the oxo tautomers 218 and 219 is derived from the resonance energy of the X-C=0 group, which by analogy with open-chain compounds should increase in the sequence thiolester, ester << amide. 

The major confusion as to the existence of neutral homoaromatic systems results from the fact that homoaromatic resonance energies, contrary to aromatic resonance energies, are normally less than 10 kcalmoF 225,226 general it is difficult to separate homoaromatic... 

The h NMR spectrum provides some evidence for aromatic stabilization in 5. The important number is the chemical shift for the ring C-H protons, which falls at 6.75 ppm. This is significantly deshielded compared with the same protons in the precursor 4, 5.73 ppm, or the corresponding dihydride (LSiH2), 6.00 ppm. Theoretical calculations also support the idea that 5 is an aromatic molecule, with aromatic resonance energy of 12 s kcal mol . Particularly convincing, however, is a comparison of the properties of 5 with those of its saturated analog [22]. Silylene 13 was made by a reaction sequence similar to that for 5 in Eq. 4 and Eq. 5, except that for the final step it was necessary to use the dibromo rather than the dichloro compound (Eq. 13). For 13 an X-ray crystal structure could be determined it is shown in Fig. 5. 

An essential part of the driving force of the elimination step is the recovery of the aromatic resonance energy in going from the cyclohexadienyl to the benzene system. 

Tautomerism of unsubstituted 3-pyridazinone in low temperature inert gas matrix has been investigated by means of IR spectroscopy (90SA(A)1087, 92JPC6250). In accordance with the theoretical calculations, only oxo tautomer 42a was observed after the deposition of the matrix. The hydroxy tautomer, however, was produced photochem-ically by UV irradiation of the matrix. Aromatic resonance energy difference for tautomers of 3-hydroxypyridazine was calculated to be 4.7 kcal/mol (92T7857). 

Chin RM, Dong L, Duckett SB, Partridge MG, Jones WD, Perutz RN (1993) Control of if-coordination vs C-H bond activation by rhodium the role of aromatic resonance energies. J Am Chem Soc 115 7685-7695... 

The quantitative comparison of some aromatic properties of all the four congener rings, under strictly homogeneous conditions, has recently been carried out by us.12,16 Owing to experimental difficulties, it was not possible to determine the aromatic resonance energy directly from measurements of heats of combustion or hydrogenation. 

Aromaticity Resonance energy, the stabilizing energy dill crence between benzene and a nonaromatic cyclic triene. 

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