Hammet plot

They demonstrated that electron-deficient R groups and electron-rich R substituents at S accelerated the reductive elimination. They proposed 123 (Lj = DPPE, R = Ph, R = Ar) as a transition state, where R acts as an electrophile and thiolate as a nucleophile. The Hammet plot for the reductive elimination showed that the resonance effect of the substituent in R determines the inductive effect of the R group, and the effect in SR showed an acceptable linear relationship with the standard o-values. The relative rate for sulfide elimination as a function of the hybrid valence configuration of the carbon center bonded to palladium followed the trend sp > sp spl... 

Fig. 19. Hammet plot for the oxidation of benzylic alcohols (X-CeHaCHjOH or X,Y-C6H3CH2OH) by complex 36...

A ] between the cation radical of la and the anion radical of m-DCB were proposed. In the absence of quencher, the quantum yield (cationic species at a nearly diffusion-controlled rate (1 x 10 dm mol s" ). In the presence of Q, the quantum yield (rate constants (kf ) for the nucleophilic addition of RNHj were obtained, as shown in Table 6.8. Hammet plots of Infc vs Taft o gave a linear correlation with a large negative slope (-2.1), showing that a substantial positive charge should be populated on the nitrogen atom of RNHj in the transition state (Figure 6.1). 

The thermodynamic stability of the radical anions has been determined from log K and are displayed in Table 1.11. The tetrasubstituted derivatives showed lower values of K, thus indicating that steric factors are responsible for this behaviour. The substituent effect in positions 2 and 3 on the redox potentials of the A,AT-dicyano-1,4-anthraquinonediimine series can be correlated with the Hammet a constants. The Hammet plot of E i2 v5. (73 [133] gives a p value of 0.123 with a correlation coefficient of r = 0.971. The comparison of this value with that reported by Aiimuller and Hunig [32] for other simpler DCNQI series (p — 0.490) indicates that the substituent effect over the ease of reduction decreases as the conjugation is extended. 

Fig. 56. Plot of Log ku (1) and ai values (2) versus the Hammet s apara constants of apical substituents.

Examination of O chemical shifts of several 2-substituted and 2,5-disubstituted furans <85MRC985> has demonstrated the additivity of substituent effects by plotting the O chemical shifts of 2,5-disubstituted furans versus those of the 2-substituted analogues. Failure of the electronic character of substituents in determining O chemical shifts is found, because they do not correlate with Hammet constants. Such a lack of correlation is known for C ortho chemical shifts in aromatic systems. A good correlation is obtained by plotting O chemical shifts versus a parameter Q of the substituent—a parameter which accounts for the paramagnetic contributions and is a function of the polarizability and ionization potential of the substituent. The cyano- and methylfurans, however, do not follow the Q relationship. 

FIGURE 15 The peak oxidation potentials, p, of several 3-substituted thiophene monomers are plotted against their respective Hammet substituent constants, [Pg.798]

---