Correlation, effects activation energies

The correlation between activation energy and preexponential factor is known as the compensation effect. 

Taft s constants were used as molecular descriptors to correlate with activation energy and kinetic rates, respectively. They were used because they describe the inductive effects of the substituents in aliphatic compounds. Taft s constant showed a poor correlation (r2 = 0.68) for aliphatic compounds however, the correlation between activation energy and Taft s constant was... 

A large value for the activation energy is correlated with a large prefactor and all lines in the Arrhenius plot intersect in a single point, the isokinetic point. The correlation between activation energy and preexponential factor is known as the compensation effect (Figure 3.24). 

The correlation with cr parameters gives the value = -1.5. The same sequence of rates and value have been obtained for the y-induced process. The isotope effect for the photoreaction was kn/kry = 2. The effective activation energy of the photo-induced reaction is ca. 5 kca moU. ... 

Sterically compact ortho substituents such as Me, OH, anti-OMe, F, Cl, and CN destabilize the ground and transition states to a similar degree. As a result, changes in the activation energy are minor. The net effect of these substituents on the cyclization rate is similar to that of the para substituents, and the corresponding computational data fit well into the correlation in Fig. 12.49... 

Electronic effects. Nucleophilic attack is favoured by electron-withdrawing groups on the amide and the acyloxyl side chains. Interpolated bimolecular rate constants at 308 K for the series of para-substituted /V-acetoxy-/V-butoxybenzamides 25c, 26b-g and 26i (Table 5) gave a weak but positive Hammett correlation with a constants ip = 0.13, r = 0.86).42,43 These Sn2 reactions are analogous to those of aniline and substituted pyridines with phenacyl bromides, which have similar Arrhenius activation energies and entropies of activation in methanol (EA= 14-16 kcal mol-1, AS = — 27 to —31 calK-1 mol-1) and 4-substituted phenacyl halides afforded a similar Hammett correlation with pyridine in methanol (cr, p — 0.25).175... 

---