Cyclopentadiene is not Aromatic and not Planar Like Benzene: Yet Deprotonation (pKa) from its Lone sp3 Carbon Obeys Linear Free Energy Relationships (LFER)

Abstract

Sanjeev Rachuru, D. A. Padmavathi, V. Jagannadham and Adam A. Skelton

Cyclopentadiene (CP) and its seven Cyano substituted derivatives obeyed Linear Free Energy Relationship (LFER) with an excellent correlation for pKa s of deprotonation equilibrium from the lone sp3 carbon with Hammett σm values for the substituents at 2 and 3 positions, Taft σ* values for 1 and 4 positions of CP; for disubstituted derivatives at 1or 4 and 2 or 3 positions we applied the sum of Taft σ* and Hammett σm values respectively. Deprotonation is facilitated by electron withdrawing capacity of the Cyano group with a positive Hammett-Taft reaction constant (ρHT ; HT stands for Hammett-Taft) of 2.75. To the best of our knowledge, we are the first ones to apply the summation of Hammett and Taft σ*. Further, we determined the pKa of the deprotonation equilibrium of unknown 1,4-dicyanoCP using the Hammett-Taft plot; the pKa of this compound in question was also determined theoretically.

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