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Thiophene 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). [Pg.354]

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). [Pg.3]

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. [Pg.28]

The thermodynamic properties of thiophene,2-methylthiophene, ° and 3-methylthiophene have been computed from careful measurements of the heat capacity of the solid, liquid, and vapor states, the heat of fusion, the heat of vaporization, and the heat of combustion. From the heat of combustion of thiophene and from thermochemical bond energies, the resonance energy of thiophene has been re-estimated to be only 20 kcal/mole. [Pg.19]

A. Hydroxyfuran (also analogous thiophene, pyrrole) - Tautomer non aromatic but some additional resonance energy arises from ester etc. group... [Pg.7]

Resonance energies have been reported164 (see also ref. 154a) for a few conjugated sulfur compounds thiolacetic acid (4-5 kcalmole-1), thiourea (27 kcalmole-1), thiosemicarbazide (28 kcalmole-1), thiophene (20 kcalmole-1), and thianthrene (17 kcalmole-1 difference between the resonance energy of thianthrene and that of two benzene molecules). These values were calculated from the heats of combustion and bond energies, which are 61.5 kcal for the C—S, 115 kcal for the C=S, 87.5 kcal for the S—H, and 67 kcal for the S—S bonds.156 Calorimetry of sulfones has been studied intensively.156... [Pg.42]

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). [Pg.192]

These differences in magnitudes and even sequences of estimated resonance energies are symptomatic of the difficulties involved in defining quantitatively what constitutes a resonance energy. Regardless of these problems, in terms of general characteristics, i.e. spectroscopy (see Section 3.13.3) and reactions, thiophenes with only rare exceptions exhibit... [Pg.716]


See other pages where Thiophene resonance energy is mentioned: [Pg.67]    [Pg.3]    [Pg.28]    [Pg.30]    [Pg.36]    [Pg.38]    [Pg.615]    [Pg.635]    [Pg.542]    [Pg.384]    [Pg.666]    [Pg.666]    [Pg.668]    [Pg.52]    [Pg.132]    [Pg.526]    [Pg.27]    [Pg.20]    [Pg.303]    [Pg.79]    [Pg.80]    [Pg.86]    [Pg.87]    [Pg.89]    [Pg.89]    [Pg.28]    [Pg.30]    [Pg.36]    [Pg.38]    [Pg.716]    [Pg.808]    [Pg.946]    [Pg.1040]    [Pg.1040]    [Pg.1206]    [Pg.945]    [Pg.309]    [Pg.387]   
See also in sourсe #XX -- [ Pg.206 ]




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