Pyrrole ionization constants

Ultraviolet (UV) spectra of some selected furo[3,2-3]- and furo[2,3 ]pyrroles were published and their apparent ionization constants determined by spectrometric titration (Table 5), and correlated with their structures <2001CCC1615>. 

Acid dissociation constants have also been measured for several pyrrole carboxylic acids238-243 (Table X) and attempts have been made, by taking the reaction constant p for ionization as unity, to extrapolate the a values obtained from the ionization constants of... 

Using the data on the ionization constants of the 5-substituted 2-carboxylic acids 30 and 31 (69T5815 72JCS(P2)1738), the transmission of total electronic effects, was shown to increase in the sequence benzene < thiophene < selenophene, tellurophene < furan < pyrrole (Table XXII). 

Interestingly, it has been observed that the ionization potentials (determined by the electron-impact technique) of several substituted five-membered heterocycles correlate with the substituent effects is larger than for benzene (p = —14.7) and larger than that measured for the most sensitive electrophilic substitution (bromination in acetic acid, which gives p = —10.0 for thiophene). As pointed out by Linda et al. [12], the sensitivity to substituent effects... 

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