Cyclopentadiene: Explaining Its Deviation from the Correlation of pK_a versus Number of Nitrogen Atoms of the Pyrrole to Pentazole Series and Its Closeness to the Correlation of pK_a versus Number of Phosphorous Atoms of the Phosphole to Pentaphosphole Series

Abstract

A good graphical correlation is observed between the number of nitrogen atoms and pK_a for the deprotonation of N(1)–H of azoles (pyrrole to pentazole). Similarly, there is a good correlation between the number of phosphorous atoms and P(1)–H of phospholes (phosphole to pentaphosphole). Using this empirical Katritzky type correlation technique with our DFT data, this paper presents a comparison of the pK_a of cyclopentadiene (in which the number of nitrogen or phosphorous atoms is zero), by interpolation of the locus to zero, of the pK_a versus number of nitrogen atoms correlation for the azole series and similarly for the pK_a versus number of phosphorous atoms correlation for the phosphole series. A value of pK_a = 22.5 for cyclopentadiene is obtained from the correlation with the azoles and of 13.5 from the correlation with phospholes. The literature value of pK_a for cyclopentadiene, however, is 15. The actual pK_a value for cyclopentadiene differs from that predicted by the correlation by 7.5 pK_a units for azoles, while it deviates by a much smaller 1.53 pK_a units from the value predicted by interpolation from the phospholes. Candidate explanations are considered.