Trends in properties of <i>para</i>‐substituted 3‐(phenylhydrazo)pentane‐2,4‐diones

Trends between the Hammett's σ p and related normal , inductive σ I , resonance σ R , negative and positive polar conjugation and Taft's σ p ° substituent constants and the distance, δ NH NMR chemical shift, oxidation potential ( E p/2 °x, measured in this study by cyclic voltammetry (CV)) and thermodynamic parameters (p K , Δ G 0 , Δ H 0 and Δ S 0 ) of the dissociation process of unsubstituted 3‐(phenylhydrazo)pentane‐2,4‐dione (HL 1 ) and its para ‐substituted chloro (HL 2 ), carboxy (HL 3 ), fluoro (HL 4 ) and nitro (HL 5 ) derivatives were recognized. The best fits were found for σ p and/or in the cases of , δ NH and E p/2 °x, showing the importance of resonance and conjugation effects in such properties, whereas for the above thermodynamic properties the inductive effects ( σ I ) are dominant. HL 2 exists in the hydrazo form in DMSO solution and in the solid state and contains an intramolecular H‐bond with the distance of 2.588(3) Å. It was also established that the dissociation process of HL 1–5 is non‐spontaneous, endothermic and entropically unfavourable, and that the increase in the inductive effect ( σ I ) of para‐ substitutents (H &lt; Cl &lt; COOH &lt; F &lt; NO 2 ) leads to the corresponding growth of the distance and decrease of the p K and of the changes of Gibbs free energy, of enthalpy and of entropy for the HL 1–5 acid dissociation process. The electrochemical behaviour of HL 1–5 was interpreted using theoretical calculations at the DFT/HF hybrid level, namely in terms of HOMO and LUMO compositions, and of reactivities induced by anodic and cathodic electron‐transfers. Copyright © 2010 John Wiley &amp; Sons, Ltd.