Summary of Relative Rates

The previously described reactivity data for the five-membered heterocycles are gathered (in terms of o+ values) in Table 6.12 no data are given for nitration because the rates are encounter controlled and meaningless in terms of electronic effects. Among the other data, those for mercuriation, protiodemercuriation, and protiodeboronation are doubtful, and other qualifying aspects are noted in the table footnotes. The following main features are noteworthy. 

Nevertheless, it is approximately true that in reactions of average demand for resonance, cr furan) - crt(thiophene) is 0.1, cr - cr, (furan) is 0.45, and crt - o, (thiophene) is 0.40. Reasonably consistent values of a2 for furan, thiophene, selenophene, and tellurophene are -0.91, -0.81, -0.88, and -0.90, respectively the best 0-3 values for furan and thiophene are -0.45, and -0.43, respectively. No data are available for the relative reactivities of the 3-positions of selenophene or tellurophene. The 3-position of pyrrole is slightly more reactive than the 2-position in hydrogen exchange, but the reactivities are so close that under different demands for resonance, the reactivity order can be inverted. 

Activation Parameters for Vilsmeier-Haack Formylation in CHC1, 

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