Resonance energy furan

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). 

One of the more useful predicative applications of the relatively crude Hiickel method has been to illustrate quantitatively the effect of benzenoid annelation on the resonance energies of furan and thiophene. The results are summarized in Figure 1. As expected, thiophenes are more stable than the corresponding furans and 3,4-fusion results in less stable compounds than 2,3-fusion (77CR(C)(285)42l). 

In summary, all estimates of resonance energies indicate a decrease in aromaticity in the sequence benzene > thiophene > pyrrole > furan. Similar sequences are also found for the benzo[6] and dibenzo analogues. A somewhat different sequence is found for the benzo[c] fused heterocycles with isoindole > benzo[c]thiophene > benzo[c]furan. As would be anticipated, the resonance energies for the benzo[c] heterocycles are substantially lower than those for their benzo[6] isomers. 

Types i and ii are the most important. An essential difference between them is that in type i both tautomers can be aromatic [e.g., pyrid-2-one (16) and 2-hydroxypyridine (17) both satisfy the criteria for aromaticity and have large resonance energies], whereas in type ii at least one tautomer, that corresponding to 3, is nonaromatic [e.g., 3-hydroxyfuran (18) is aromatic, but furan-3-one (19) is not]. In... 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

The resonance energy of 0.93 v.e. for furane, C4H4O, arises from the same structures as for pyrrole. 

Values of Resonance Energy of Derivatives of Furan and Pyrrole... 

The variety of aromaticity indices based upon structural or magnetic molecular properties along with their values for pyrrole, the other five-membered heterocycles, benzene and cyclopentadiene are summarized in Table 31 in Section 3.01.5.2. Of these indices, the majority predict the aromaticity order benzene >thiophen>pyrrole >furan, in agreement with resonance energy calculations (Section 3.01.5.2). 

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