Pyridines resonance energy

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

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... 

The low yields of 6,6 -disubstituted-2,2 -bipyridincs recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine. On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one 71-electron would be smaller in a quinoline than in a pyridine derivative. 

Data are given in Table IV for heterocyclic compounds. For piperidine there is no difference between E and E, showing that the bond energies used are applicable to saturated heterocyclic molecules. Pyridine and quinoline differ from benzene and naphthalene only by the presence of one N in place of CH and, as expected, the values 1.87 v.e. and 3.01 v.e., respectively, of the resonance energy are equal to within 10 percent to the values for the corresponding hydrocarbons. 

Pyridine and benzene conform to Hiickers rule, which predicts that planar cyclic polyenes containing (4n + 2) -electrons ( = 0, or an integer) should show added stability over that anticipated for theoretical polyenes composed of formal alternate single and double bonds. This difference is sometimes called the empirical resonance energy. For example, benzene, where n = 1, is estimated to be 150 kJ moT more stable than the hypothetical molecule cyclohexatriene (Box 1.8) for pyridine, the empirical resonance energy is 107 kJ mol . ... 

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. 

For pyridine, pyrazine, and related six-membered heterocyclic molecules Kekul6 resonance occurs as in benzene, causing the molecules to be planar and stabilizing them by about 40 kcal/mole. The interatomic distances observed in these molecules,106 C—C = 1.40 A, C—N = 1.33 A, and N—N 1.32 A, are compatible with this structure. The resonance energy found for quinoline, 69 kcal/mole, is about the same as that of naphthalene. 

Chloride ions are comparatively weak nucleophiles, and do not react with pyridines. In general, there is also no interaction with pyridinium and pyrylium compounds, but xanthylium chloride is in equilibrium with an appreciable amount of (269), this being a particularly favorable case with little loss of resonance energy on adduct formation. 

The behavior of pyridine in substitution reactions can be understood on the basis of its resonance structures (la-d) and on the basis of the electron-density distribution at the various ring positions as derived from molecular-orbital-theoretical calculations, An example of the published pi-electron density distribution is shown in II, The resonance energy of pyridine is 35 kcal/mole (versus 39 kcal/mole for benzene). 

For l[2-pyridyl]2-propanone it is quite possible that the resonance energy of the pyridine nucleus is high enough to stabilize the initiating radical (resulting from the enolic double bond or rather the peroxide or ketohydroperoxide therefrom) to such an extent that it acts as an inhibitor. 

Pyridine shows all the characteristics of aromatic compounds. It has a resonance energy of 113 kJ/mol (27 kcal/mol) and it usually undergoes substitution rather than addition. Because it has an available pair of nonbonding electrons, pyridine is basic (Figure 16-11). In an acidic solution, pyridine protonates to give the pyridinium ion. The pyridinium ion is still aromatic because the additional proton has no effect on the electrons of the aromatic sextet It simply bonds to pyridine s nonbonding pair of electrons. 

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