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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