Quinoline resonance energy

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. 

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. 

Quinazoline is a benzo-fused pyrimidine with a cyclic conjugated tOrr-electron system. It is a bicyclic, n-electron-deficient/ aromatic heterocycle (hetarene) with a resonance energy value of 127.2 kJ mol, and an aromaticity index of 143 (cf. naphthalene 142,- quinoline 134," quinazolin-4(3/7)-one 110.4, 2-methylquinazoline 3-oxide 119.8 ). Unlike pyrimidine, quinazoline has no symmetry. 

Determinations of the ERE of quinoline from thermochemical data have given results which range from 47.3 to 69 kcal mole-1.49-271-272 There appear to be no corresponding estimates for isoquinoline, but a value of 48 9 kcal mole-1 has recently been deduced from a comparison of the equilibrium constants for pseudo-base formation of iV-methylisoquinoline cation and its dihydro analog 73.273 Comparison of these ERE values with those obtained for naphthalene (61-75 kcal mole-1) shows that they parallel the somewhat lower resonance energy of pyridine relative to that of benzene. 

Quinoline is characterized by an empirical resonance energy of 222 kJ mol" and by a Dewar... 

Application of the above-mentioned basicity method is limited, and it can be applied only to systems whose aromaticities are destroyed on protonation. Heteroaromatics such as pyridine and quinoline conserve their aromaticity during protonation therefore a different method is applied, that of considering the basicities of pseudo-bases derived from /V-meth-ylheteroaromatic cations and the corresponding nonaromatic analogues. The principle is shown for isoquinoline in Scheme 14. The model compound used for comparison is 2-methyl-3,4-dihydroisoquinolinium cation. The pAa values for the equilibrium 1 — 2 and 3 — 4 were measured they correspond to an enthalpy difference A/A of 9.2 kcal/mol (X and Y are the differences in the resonance energies between 1 and benzene and 3 and benzene, respectively). 

Combining these results with the aromatic stabilization energies deduced as described above for the parent pyridine, quinoline, and isoquinoline molecules, gives the differences in the aromatic resonance stabilizations for these pyridone-like compounds as compared to the parent heterocycle (Scheme 21) 45,47.48 -piig results show that aromatic resonance energies for pyridines are similar in the 2- and 4-series. For bicylic compounds such as quinolines... 

Likewise, quantum mechanical calculation succeeds in giving a theoretical explanation of some facts that the resonance theory could not explain, for example, why bis(pyridine-2)monomethine cyanine and bis(pyridine-4)monomethine cyanine possess the same lowest energy transition contrary to the 2,2 - and 2,4 -quinoline monomethine dyes, together with a molecular coefficient extinction lower than that of the 4,4 -quinoline dye (11). Calculation shows also that there is no theoretical reason for observing a relationship between and pK in a large series of dyes with different nuclei as it has been postulated, even if limited observations and calculations in short homogeneous series could lead to this conclusion (105). 

Resonance activation in the 8-substituted-isoquinolines (344) or -2-nitronaphthalenes is predicted to be greater than that in 5-substituted-quinolines (345) or -1 -nitronaphthalenes due to the lower energy charge-... 

As a general trend, six-membered mononuclear N-heteroaromatics such as pyridine and derivatives are much less prone to undergo hydrogenation than bi-and trinuclear N-ring compounds (e.g., quinolines, benzoquinolines, acridines) due to their higher resonance stabilization energy. 

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