Furans and thiophens are considerably weaker bases than pyrroles; the same applies to the benzo derivatives. The Hcfunction is obeyed by many of these compounds. Aromatic resonance energies are deduced for pyrrole, furan, and benzopyrroles by comparison of their basicities with those for model nonaromatic compounds. © 1976.
Carmody, M. P., Cook, M. J., & Tack, R. D. (1976). Aromaticity and tautomerism-V. The basicity and aromaticity of pyrrole, furan, thiophen and their benzo derivatives. Tetrahedron, 32(14), 1767–1771. https://doi.org/10.1016/0040-4020(76)85172-1