Pyridine, aromaticity dipole moment

As the table indicates, C6o fullerene solubility is practically the same in nonpolar benzene and polar pyridine. Like benzene, pyridine has a pronounced "aromatic" nature, i.e. electron distribution in a pyridine molecule is identical to that in benzene. However pyridine has a reasonable dipole moment as opposed to benzene. 

Table 42 gives an overview of annular tautomerism data for azoles in the gas phase and in solution or crystals. In the gas phase the stability of alternative tautomers largely depends on their relative aromaticities. In Section 2 A.4.2.2 it was noted that 1,2-relationships between pyrrole- and pyridine-type nitrogen atoms favor aromaticity (Figure 21) and this is consistent with the relative stabilities of triazole and tetrazole tautomers in the gas phase (Table 42) <2010T2695>. In solution (and crystals) other factors such as solvent polarity, hydrogen bonding, and temperature become important and the relative stabilities can be reversed. Polar solvents tend to stabilize the tautomer with the largest dipole moment and this probably accounts for the observation of both 2H-1,2,3-triazole (p = 0.12D) and H-1,2,3-triazole (p = 4.55D) in...

The low reactivity of pyridine toward electrophilic aromatic swbetitu lion ia due to a combination of factors. Most important is that the electran density of the ring is decreased by the electron-withdrawing inductive elTect of the electronegative nitrogen atom. Thus, pyridine has a substantial dipole moment (m 2.26Q)> the ring carbons acting as the positive end of... 

Dinur and Hagler propose a novel method to determine atomic point charges and point dipoles from derivatives of the molecular dipole moment and second moments. The method is limited to planar molecules and has been applied to hydrogen fluoride, water, formaldehyde, formamide, ethylene, benzene, and pyridine. As was also noted by Williams, they found that atomic dipoles do not necessarily point along the bond directions. Price proposed a distributed multipole model for several aromatic hydrocarbons using carbon sites only. 

The MM3 force field has been extended by Allinger and co-workers to cover aromatic heterocycles of the pyridine and pyrrole types <93JA11906>. Structures (32 compounds), dipole moments (35 compounds), heats of formation (35 compounds), and vibrational spectra (11 compounds) were examined. The results are good for structure and fair for the other items resonance energies were reported for the series benzene (17.79 kcal mol ), pyridine (17.02 kcal mol ), pyridazine (14.35 kcal mol ), pyrazine (17.01 kcal mol ), pyrimidine (15.60 kcal mol ), 1,3,5-triazine (13.51 kcal mol ), and 1,2,4,5-tetrazine (17.72 kcal mol ). Finally, ab initio studies of the dipole polarizabilities of conjugated molecules have been reported in which monocyclic azines (pyridine, pyridazine, pyrimidine, pyrazine, 5-triazine, and 5-tetrazine) are compared <94JST(304)109>. 

The role of heteroatoms in ground- and excited-state electronic distribution in saturated and aromatic heterocyclic compounds is easily demonstrated by a comparison of a number of heteroaromatic systems with their perhydro counterparts. In Jt-excessive heteroaromatic systems, because of their resonance structures, their dipole moments are less in the direction of the heteroatom than in the corresponding saturated heterocycles furan (1, 0.71 D) vs. tetrahydrofliran (2, 1.68 D), thiophene (3, 0.52 D) vs. tetrahydrothiophene (4, 1.87 D), and selenophene (5, 0.40 D) vs. tetrahydroselenophene (6, 1.97 D). In the case of pyrrole (7, 1.80 D), the dipole moment is reversed and is actually higher than that of pyrrolidine (8, 1.57 D) due to the acidic nature of the pyrrole ring (the N-H bond) In contrast, the dipole moment of n-deficient pyridine (9, 2.22 D) is higher than that of piperidine (10, 1.17 D). In all these compounds, with the exception of pyrrole (7), the direction of the dipole moment is from the ring towards the heteroatom [32-34]. 

This five-membered heterocycle contains one pyrrole-like and two pyridine-like N-atoms in the 1,2,3-positions. It was known as v-triazole (v meaning vicinal). Since all ring atoms are sp -hybridized, its six available electrons are in delocalized r-MOs. Thus, 1,2,3-triazole is aromatic. Its ionization energy, measured by photoelectron spectroscopy, amounts to 10.06 eV, which is greater than that of imidazole (8.78 eV) and pyrazole (9.15 eV), i.e. the HOMO of 1,2,3-triazole is lower [150]. The dipole moment in benzene is 1.82 D and is similar to that of pyrazole. 

Pyridine and pyrrole have contrasting resonance descriptions (Fig. 13.51), and this difference results in some very different physical properties. For example, the direction of the dipole moment is different in the two aromatic molecules. 

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