Otheractivation parameters such as free energy of activation ( ?G ), enthalpy of activation ( ?H ), andentropy of activation ( ?S ) are calculated from thefollowing relations equationIn aprotic solvent HMPA cyclic
The threedinitrobenzenes i.e., 1,2-DNB, 1,3-DNB and 1,4-DNB exhibit completely reversible first reduction wave in HMPA at glassy carbon electrode.
Table-7: Activation parameters (G, H, S) calculated from disproportionation constants in HMPA
Table-8: Activation parameters (G, H, S) calculated from disproportionation constants in HMPA
At 5oC, in solvent HMPA with GCE, the observed second order rate constant k2 for 1,2DNB (1.05 A-102 Lmol-1s-1) and 1,3-DNB (1.95A-102 Lmol-1s-1) are comparable but that of 1,4-DNB (2.86A-102Lmol-1s-1) is higher.
When the protonating agent is salicylic acid, the k2 value for 1,3-DNB for the GCE as well as the HMDE is higher than that observed for 1,2-DNB and1,4-DNB in solvent HMPA. However, it should be noted that the rate constant for 1,2-DNB anion radical is comparable to 1,3-DNB at 5oC and 25oC in solvent HMPA and it may be due to the closely placed nitro groups which hinder the attack of bulky protonating agent (benzoic acid).
?H and ?S values are higher than for 1,3-DNB- and 1,4-DNB irrespective of the electrode system in solvent HMPA when benzoic acid is used as protonating agent.
When salicylic acid is used as protonating agent in HMPA the entropy and enthalpy of activation for the homogeneous chemical reaction of1,2-DNB-.