Taft equation polar effects

Taft was also able to isolate steric effects. For the acid-catalyzed hydrolysis of esters in aqueous acetone, long (kJh was shown to be insensitive to polar effects. In cases where resonance interaction was absent, this value was proportional only to steric effects (and any others that are not field or resonance). The equation is... 

In the nineteen sixties, there was some confusion about the value of p, the reaction constant for polar effects of alkyl groups on bromination rates, as calculated according to Taft s equation, log (k/k0) = p a. For 22 alkenes substituted by one, two or three linear alkyl groups, there is a satisfactory relation (16) between their reactivities, log k, in methanol and the sum of their... 

The decrease in hydrogenation rate with increasing size of the alkyl group (Table VI) (93-96) can be correlated by the Taft equation. However, the correlation of the data by Smith and Pennekamp (93) (series 70) using the polar parameter Taft equation is applied that also includes steric constants. Similarly, Kieboom (34) has discussed Yoshida s correlation of series 73 based on main conclusion has been that the data do not allow a clear distinction between steric and polar effects. It seems that both operate in the same direction. Series 72 and 73, in which the rate data have been separated into the rate constants and adsorption coefficients, show opposite trends with the latter parameter. A similar problem has been encountered by Volter, Hermann, and Heise (100) and by Najemnik and Zdrazil (103) in the series of methylbenzenes (Table VII) and is discussed in this connection. 

Correlation analysis of solvent effects on the heterolysis of p-methoxyneophyl tosyl-ate has been performed by using the Koppel-Palm and Kamlet-Taft equations. The reaction rate is satisfactorily described by the electrophilicity and polarity parameters of solvents, but a possible role for polarizability or nucleophilicity parameters was also examined. 

A quantitative correlation of structural effects of four esters and four alcohols in the vapour phase transesterification on a macroreticular ion exchanger at 120°C was made using the Taft equation [441]. The authors found that rate coefficients [from eqn. (27)] yielded better correlation with steric (Es) than with polar (a ) parameters, while there was no significant difference between the correlations of the adsorption coefficients of alcohols, Kb, with both parameters. The correlations with Es yielded the slopes 1.4 and 0.6 for the reactivity of the esters and the alcohols, respectively, and —0.4 for the adsorptivity of the alcohols. The observed... 

Charton used statistical methods to investigate the contributions to Taft s s values of the size of the groups concerned and their polar effects. As a measure of group size he took the van der Waal s radius, rv, and he used o-, and crR as measures of inductive and mesomeric effects. Correlations of steric substituent constants were then carried out250 using the equation (for group x)... 

Roberts269 has studied the hydrolysis of series of ethyl esters271 272 and alkyl benzoates273 in a limited range of water-dimethyl sulphoxide mixtures in some detail, and finds that the data for the hydrolysis of aliphatic ethyl esters (at 35°C in 85% dimethyl sulphoxide-water) fit the modified Taft equation (eqn. 5, p. 132). The values of p = 1.88 and 8 = 0.88 can be compared with p = 2.39 and 8 = 1.04 for 85% EtOH-water at the same temperature. The polar reaction constant is reduced in the dipolar aprotic solvent, consistent with a reduced degree of bond formation in the transition state, expected if the activity of the hydroxide ion is increased. However, Roberts considers that the sensitivity to steric effects, as measured by 8, would be reduced more substantially if bond formation were less advanced. It is difficult to accept this argument, since we... 

The hydroxide anion catalyzed rate constants for the scries of alkyl tris-(2-methoxyethoxy)silanes obtained by Fohl were used to define a modified Taft equation, log(/cHO//cHO ) = 2.48cr +1.67 Es [42], A good correlation was obtained, except for vinyl and phenyl substituents. The anomalous behavior observed for phenyl and vinyl tris-(2-methoxyethoxy)silanes may have resulted because the steric parameter or the polar parameter may be influenced by the carbon-carbon double bond. The steric parameter for a,yS-unsaturated substituents may include an appreciable resonance effect. The polar parameter values may be influenced by the ability of silicon to back-bond through d orbitals with the a,/J-unsaturated system [37,49]. 

Pohl and Osterholtz studied the acetate anion catalyzed condensation of aqueous solutions of a series of alkyl substituted silanetriols [26]. The modified Taft equation for a limited number of alkyl groups (methyl, ethyl, M-propyl, n-butyl, and /-butyl) indicated a large dependence on polar effects [26]. The modified Taft equation was log( kAc/ Ac ) = 6.7 o - 0.07 Es. 

Nucleophilic substitutions of cycloalkylmethyl arenesulfonates C H2n-iCH20S02C6H4Z with anilines XCgH4NH2 in methanol at 65 °C have been studied.140 The reactivity order (given Z and X) n = 4>6>7>5 largely reflects the influence on steric effects of ling size, but the behaviour for n = 5 is anomalous. Application of the Taft equation in a form involving both polar and steric terms suggests that the retardation for n = 5 may be due to an enhanced polar (p cr ) term. 

The effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

The participation of polar effects can be studied adequately by Tafts equation (14) ... 

Taft [109] postulated that the rate of aliphatic ester hydrolysis is a function of substituent type. Using methyl as a standard substituent, he compared, for example, the effects of larger alkyls, and interpreted the results by means of two constants characterizing the steric and polar behaviour of the alkyls. In copolymerization theory, the Taft equation was used many times to describe the relative reactivities (See Chap. 5, Sect. 5.2) in homologous polymer series... 

The Dewar-Grisdale Equation An approach to field and resonance transmission of the polar effect essentially the same as that of Taft s group was suggested by Dewar and Grisdale. The localised and delocalised polar effects are represented by F and M which are taken to be additive in any given a value. The value a,y is related to the sum of... 

Despite their associated problems, attempts have been made to incorporate results from ortho substituents in free energy correlations. It might be necessary, for example, to predict physico-chemical properties of a compound with an ortho substituent which acts as a drug or is a putative intermediate in a proposed mechanism. Quantitation of the ortho substituent effect requires that both steric and polar effects be considered. The ortho effect can be fitted to a modified version of the Pavelich-Taft equation (Chapter 2, Equation 15) where the values... 

The data fit the Taft-Pavelich equations where the polar effect (p CT ) is constant. 

In 1969, Clayton and Purcell (26) studied the structure-activity relationships of some l-decyl-3-carbamoylpiperidines against butyryl-cholinesterase. The equations in Figure 25 were found. In this case the activity is presented as the p/50, the negative logarithm of the concentration of derivative which inhibits butyrylcholinesterase 50%. The parameter o- is the Taft substituent constant which depends only on the net polar effect of the substituent. One compound of moderate activity was omitted from the calculations. The calculated equation was then used to predict the activity of this compound. The predicted p/50 was 5.00, that observed was 5.01 =t. 03. In fact, the activity of these compounds depends so strongly on the partition coefficient that the use of the much simpler equation (Equation 2) predicts the activity nearly as well. 

The rates of epoxidation of cyclododecene with a series of aliphatic peroxy-acids have been correlated, using the Taft equation. The reaction constant (p ) was + 2.0 and the steric constant (6) was found to be essentially zero. A two-parameter correlation has been found for the effect of basicity and polarity of the solvent on the rate of epoxidation of propene with peracetic acid. Rate constants and activation parameters for the epoxidation of a number of cycloalkenes, including (11 R = H or COOMe), (12 R = H, Ph, or 2-furyl), (13), (14), and cyclo-octa-l,5-diene, have been measured. An isokinetic relationship was demonstrated, with the isokinetic temperature of 3 C. There was only a weak dependence of the rate on the structure of the alkene. 

Taft equation. Taft (1956) has extended the Hammett-type correlation to aliphatic systems. Because steric effects of substituents in aliphatic systems cannot be ignored as they were for m- and / -substituted benzene compounds, Taft recognized the need to develop separate terms for the polar and steric effects for substituent constants. Based on the observation that the acid-catalyzed hydrolysis of meta- and para-substituted benzoic acid esters are only slightly affected by the electronic nature of the substituent group (p values are near 0), Taft concluded that the acid-catalyzed hydrolysis of aliphatic esters would also be insensitive to polar effects of substituent groups. Any effect on rate due to substituent groups could therefore be attributed to steric effects. Taft defined a steric substituent constant, E, by ... 

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