Hammett s constant

It is perhaps worth mentioning that substitution of other groups for the C-methyl group in amidinium ions has an effect on their pA a-values, which is best correlated with Hammett s -constants (p = —1T98), and the same is true of C-substituted N-phenylami-dinium ions (p = —12-08) (Charton, 1965). This is in line with the suggestion made on page 286, that -constants are a measure of the inductive and mesomeric interactions of groups with a carbon in the sp state of hybridization. 

Fig. 1. Relationship between F CF3 chemical shifts of S-, Se-, and 7e-(trifluoromethyl)-dibenzothiophenium, -selenophenium, and -tellurophenium triflates and Hammett s constants Om or (Tp for the ring substituents S, Se, and Te refer to substituted and unsubstituted S-, Se-, and Te-(trifluoromethyl)dibenzothiophenium, -selenophenium, and -tellurophenium triflates, respectively. The numbers on the lines are the compound numbers shown in the text. Substituents and their substitution positions are shown in the parentheses. The smaller the F NMR chemical shift is, the more downfield is the resonance (CFCI3 served as an internal standard in CD3CN).

Astatophenols, 31 64-65 dissociation constants, Hammett s-constants, field, and resonance effects, 31 66... 

Hammett s-constants, of astatophenols, 31 66 Hansenula anomala, flavocytochrome bj from, 36 260-261... 

From the correlation of Hammett s constant and P, we might expect the fluorinated sulfone to be a more effective acceptor than the nitro group (Gp(N02)=0.78 vs Gp(S02CF3)= 0.93) (JJJ. A comparison between 4-N, N-dimethylamino-nitrobenzene,6, with 4-N,N-diethylaminophenylperfluoropro-pyl sulfone,5, as indicated in Table II suggests that the use of the fluorinated sulfone leads to comparable values of p (9.0 x 10-30 esu vs 12 x 10 30 esu in this case). More... 

Halogenated phenols correlate well with Hammett s constant (a). Figure 6.24 shows the linear trend of the c values of 3-chlorophenol, 4-chlorophenol, 3,4-dichlorophenol, and 3,5-dichlorophenol with respect to their first-order reaction rate constants. Aromatic compounds with substituents in the meta and para positions have pronounced steric differences and result in a well-defined trend. Table 6.9 summarizes all the QSAR models discussed above. 

Usually the most convenient way to use Hammett s equation is to plot log k/k0 or just log k of the reaction of interest on the vertical axis and o values for the substituents on the horizontal axis. The slope of the plot of log k vs. o may change as a result of changes in the mechanism and in response to the variable electron demand of each substituent. It is anticipated that the various substituents will exert the same kinds of effects on the rate constants as they do on the benzoic acid rate constants, but the greater separation between substitution site and reaction site in the same chemical family makes the reaction less sensitive to the substituent effects. Many different oxidation reactions have been treated using Hammett s equation. The consistent trend of linear relationship with all of the Hammett s constants holds for all chemical classes except halogenated phenols. 

Kinetic rate constants, EHomo/ and Hammett s constants can be correlated with reaction rate constants. Table 8.6 summarizes the QSAR models for these volatile compounds. 

Correlation between kinetic rate constant and Hammett s constant of different chlorophenols. Experimental conditions T = 25°C, pH = 7, ozone concentration = 16.8 mg/L. (Data from Abe, K. and Tanaka, K., Chemosphere, 35, 2837-2847, 1997.)... 

The degradation of the anisoles was found to follow pseudo first-order degradation. The apparent first-order rate constant (/c,. ) was compared to Hammett s constant to determine if a correlation existed between the degradation of mete-substituted and para-substituted anisoles. Removal rates were greater that 80%, and for fluoroanisole and aminoanisole they were 100%. 

Hammett correlations were developed from experimental data for substituted phenols studied under the UV/Ti02 process (D Oliviera et al., 1993). The mechanism for this reaction is understood to proceed via the hydroxyl radical. Experimental data from the study of dichlorophenols and trichlo-rophenols under UV/Ti02 were used for QSAR analysis (D Oliviera et al., 1993). Figure 9.13 demonstrates the QSAR model for substituted phenols formulated from experimental data. The QSAR model developed for substituted phenols shows a goodness of fit of 0.9766. A good correlation was also established for substituted phenols using Hammett s constant, o the correlation coefficient is 0.987 (D Oliviera et al., 1993). Similar correlation coefficients for the constants o and ores demonstrate that the descriptor ores can be used to accurately predict kinetic rate constants for substituted phenols. 

The apparent first-order degradation rate constants of six dichlorophenols and three trichlorophenols were studied by D Oliviera et al. (1993). ft was determined that the first-order rate constant, k, correlated with the Hammett s constant o and the octanol/water partition coefficient. The Hammett correlation for the cholorophenols was developed as follows ... 

This linear relationship was used to predict first-order rate constants based on log Kow and Hammett s constants for chlorophenols. The experimental and calculated values for the chlorophenols studied are listed in Table 9.5. 

Various sets of aromatic compounds and three different molecular descriptors such as EHOmo/ Elumo, and Hammett s constants have been used for other QSAR models. First-order kinetic rates, pseudo first-order kinetic rates, and activation energy were used to correlate with different molecular descriptors. 

Hammett s constants can also be used to correlate with the previous kinetic parameters. The aromatic compounds that were analyzed by Vaca (1999) using Hammett s constant as the molecular descriptor and the first-order kinetic rate as the predicted variable include 2,4-dichlorophenol, 1,4-dichlorobenzene, 1,3-dichlorobenzene, pyridine, and phenol. The correlation of Hammett s constant and first-order kinetic rates of this compound reflected a coefficient of r2 = 0.718. Figure 10.21 shows the correlation of this set of aromatic compounds. From the figure, it can be seen that the kinetic rate decreases as the Hammett s constant increases. The F test showed that the level of significance was 10%. Because F(14)o9o was 7.76 and is larger than 5.54, it can be concluded that there is a relationship between the two variables. 

For aromatic compounds, activation energy increases as Hammett s constants decrease, as shown in Figures 10.21 and 10.22. Figure 10.21 shows the relation between these two parameters. The dataset of 2,4-dichlorophenol, ///-xylene, and phenol were used. 

Hammett s constant represents 89.82% of the variance in the linear regression equation. The F test showed that the level of significance of ELUMO and activation energy with a degree of freedom of 1,2 was 0.01. Because the calculated F is 105.95 and is larger than the F(1 2)0995 value of 98.5 from the F distribution table, the null hypothesis is rejected therefore, the chance of erroneously concluding that they are related is only 1% ... 

E vs. Hammett s constant (o) for aromatic compounds has the following correlation ... 

Chemical Kinetic Class Parameters n Ehomo ft Elumo ft Hammett s Constant ft Taft s Constant... 

Taft s constant87 CJ Hammett s constant IV Hammett s constant 88) CTp 88> e value85)... 

The major electronic factor influencing the activity has been Hammett s constant a. In some cases, the steric factors were also found to be important, e.g., Bs the Sterimol parameter for the largest width of substituents and L the Sterimol parameter for the length of the substituents. The positive coef-Acients of these parameters give an important positive contribution of width and length of substituents to the inhibition. The values of Taft steric param-... 

Hammett s constant, a satisfactory straight line was observed with a slope of 0.74. From this one would expect both the resonance and inductive mechanisms to play a role in the transmission of the effect from the substituent to the acid site. Similarly, the kinetics of the reaction of several heteroannularly disubstituted ferrocenoic acids with diphenyldiazo-methane led the same authors to conclude that Up seemed to give the best estimate of the effect transferred from one ring to the other 42). 

The retention of polar compounds (mono-, di-, and tri-substituted benzenes) on PGC, silica-based, and apolar copolymer supports was performed using unbuffered methanol water as eluent. The relationship between log k and the volume fraction of methanol was calculated separately for each solute. It was found that porous graphitic carbon retains polar compounds fairly well under reversed-phase conditions, while the retention factor increased with an increase in the number of polar substituents. In particular, the retention behavior of polar solutes on PGC supports is mainly governed by several polarity parameters (Hammett s constant, proton-donor capacity, and steric effects of substituents) and is quite different from that observed with other reversed-phase supports. Thus it was concluded that charge-induced interactions between the graphite surface as well as steric... 

Potency of the developed candidates was measured by the log of the concentration causing a 50% inhibition in asthmatic activity. This measure is represented as pl5o- To relate pijo to molecular structure Cramer et al. (1980c) followed the two step QSAR approach. Nineteen compounds were synthesized with varying substituents, pi 50 was measured for each analog. Values for the octanol-water partition logarithm, it, and Hammett s constant, [Pg.302]

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