Taft correlation

The dehydration rate depends very strongly on substitution on Ca. Large differences in reactivity of primary, secondary and tertiary alcohols over solid catalysts were reported as early as in 1931 by Dohse [90]. Also, substituents on Cp affect the rate. Both influences can be quantitatively described by the Hammett and Taft relationships the published correlations are summarised in Table 4. Of special interest is the extensive set of alcohols of the type R R2R3COH [56], which includes primary, secondary and tertiary alcohols and gives a single Taft correlation with an excellent fit. The values of p and p which can give information about the mechanism and catalyst nature will be discussed in the following sections. 

In some cases, the effect of reactant structure may outweigh the influence of catalyst nature. This is seen by comparison with the dehydration of aliphatic secondary alcohols and substituted 2-phenylethanols on four different oxide catalysts (Table 4). With aliphatic alcohols, the slope of the Taft correlation depended on the nature of the catalyst (A1203 + NaOH 1.2, Zr02 0.3, Ti02—0.8, Si02—2.8 [55]) whereas for 2-phenyl-ethanols, the slope of the corresponding Hammett correlation had practically the same value (from —2.1 to —2.4) for all catalysts of this series [95]. The resonance stabilisation of an intermediate with a positive charge on Ca clearly predominates over other influences. 

Fig. 8. Taft correlation of the cracking of alkylphenols on an aluminium fluoroborate catalyst at 425°C (Data by Schneider et al. [204].)...

The electrophilic displacement mechanism for catalystic cracking is further confirmed by the negative slopes of the Hammett and Taft correlations (Table 14) and has been supported by MO calculations in which electrophilic, nucleophilic and free radical mechanisms were compared [228]. 

Fig. 9. Taft correlation of the dehydrosulphidation of alkanethiols (line 1, p = —33) and of dialkylsulphides (line 2, p = —38). (Data by Sugioka and Aomura [248].)...

Fig. 14. Taft correlation with polar substituent constants (a ) of the vapour phase esterification of acetic acid with alcohols ( ) and of the olefin formation from alcohols (O) over Na-poisoned silica—alumina at 250°C [126]. 1, Methanol 2, ethanol 3, 1-propanol 4, 1-butanol 5, 2-methyl-l-propanol 6, 2-propanol 7, 2-butanol 8, 2-methyl-2-propanol.

Fig. 15. Taft correlation with steric substituent constants (Es) in the vapour phase esterification of carboxylic acids with ethanol over Na-poisoned silica—alumina at 250°C ( ) [126] and in homogeneous acid-catalysed esterification at 40°C (°) [419], Acids 1, acetic, 2, propionic, 3, butyric, 4, isobutyric, 5, isovaleric, 6, pivalic, 7, 2-ethylbutyric.

A common probe of reaction mechanisms used to infer charge distribution in the transition state involves variation of substituent groups near the reaction center. From the variation in reaction rate produced by electron-donating and electron-withdrawing groups or by the steric hindrance of various sized groups, transition state characteristics can be inferred. Two empirical correlations have been proposed and refined which provide a common framework for this process. The Hammett equation is applied to aromatic systems [45]. The Taft correlation is applied to aliphatic systems [45], Definitions of terms, collections of substituent constants (steric and electronic effects for various substituents), and listings of observed reaction response parameters (for typical reaction types) have been collected [45]. 

In a similar fashion, Shirai et al. constructed a modified Taft correlation for the hydrolysis of a series of triaikylalkoxysilanes of R,SiOR structures (in which R was varied to include different alkyl groups) in 55% aqueous acetone using a similar extraction/quenching technique [35]. The resulting modified Taft equation was log(fcHO//cHO ) = 1.50 silane esters and the reaction conditions were very different for the Pohl and Shirai studies, the values of p and s were quite similar, (p and a are empirically derived coefficients for the polar parameter and the steric parameter, respectively.)... 

The face-selectivity of hydride reductions of the conformationally-rigid ketone series (100) has been examined for pure axial and equatorial isomers with four different R groups, viz. Me, Cl, OMe, and SMe.162 The reactivities show Taft correlations with the inductive effects of the substituents. Only through-bond and -space electrostatic interactions are used to explain the results neither Cieplak nor Anh antiperiplanar effects are invoked. 

Summary of QSARs for Hydrolysis Reactions Based on Hammett and Taft Correlations (Harris, 1982). 

Various methods are available for predicting the acid dissociation constant (pKa) within homologous series. Most are based on the Hammett equation (benzene derivatives) and the Taft correlation (aliphatics and alicyclics). A comprehensive review of methods was published by Perrin et al. (1981). 

Estimation methods for the hydrolysis rates of several types of carboxylic acid esters, carbamates, aromatic nitriles, and phosphoric acid esters have been reported. Hydrolysis rates are subject to substituent effects, and consequently LFERs, as represented by Hammett or Taft correlations, have been applied to their estimations. Reviews (e.g., Harris, 1990 Peijnenburg, 1991 Nendza, 1998) reveal that QSARs are available only for a few compound classes and are based mostly on... 

Evidently, the approximately linear Taft correlation obtained for the elimination of / -substituted primary chlorides6 suggested an examination of a possible linear free-energy relationship for aliphatic secondary alkyl chlorides pyrolyses, that is, for a-substituted ethyl chlorides ZCH(C1)CH370 (Table 6). 

The slopes of the lines obtained from the Taft correlations of aliphatic primary, secondary and tertiary chlorides obtained at different temperatures by the extrapolation p%2lP%i T IT2 indicate that the positive nature at the carbon reaction center of the C—Cl bond in the transition state increases from a primary to a tertiary carbon atom (Table 8)70. An additional fact is that for each type of alkyl halide, the degree of positive charge at the carbon reaction center tends to decrease as the temperature increases. This means that the pyrolytic eliminations tend to be more concerted and less polar at very high temperatures. These data support Maccoll s theory on the heterolytic character of the alkyl halides pyrol-yses in the gas phase1. 

Tlie nucleophilic character of these carbenes has been demonstrated by Hammett -Taft correlations (p = +2 in competition experiments, with Ar- N=C=S/Ar-N =C=0 as substrates) (20). The syn-anti distribution of isomers in cycle pro pa nation reactions with carbenes is the result of steric and electronic effects. The dissipation of the charge by secondary interactions of the carbene substituents and the substrate are determining factors. As an example. CCIF reacts according to the orientation of the Cl group syn to the... 

Except for one report, there have been no systematic studies on how the rate of process (2) is affected by the character of the nucleophile. In the reaction of l-halo-2-(2-thienyl)acetyIenes with alkyl thiolates in Me0H-H20, it was established that the rate of halogen attack increases as the basicity of the thiolate increases. Specifically, a good Taft correlation with p was obtained . 

W or as changes in the NMR chemical shifts of the cyclopentadienyl protons in the compounds Cp(CO)3MoSnX3. The Oj parameters turned out to fit the Hammett-Taft correlation best of all hence the electronic effects of X were transferred through the bimetallic covalent chain in the same manner as electronic effects in saturated hydrocarbon chains, via an inductive mechanism (227-229, 236-240). Objects that are of interest for the study of transfer of electronic effects through a chain of atoms of metals were the clusters obtained by us (230-236) from CpM(CO)4 where M = Ta, Nb, V. 

The F nmr spectra of the fluorine-substituted oxiranes have been reasonably well studied, for in the region of high chemical shifts, the fluorine nuclei readily yield spectra that can be interpreted directly. A further principle is that the long-range coupling constants can be measured well because of the large nature of the constants.In the case of a fluorinated substituent, the linear Taft correlation may be used to determine the inductive and resonance substituent constants. ... 

The kinetics of the reaction of fert-BuOK with aryl-substituted oxiranes follow a linear Taft correlation. Steroid 5a,6a- and 5(3,6/3-oxiranes react with pyridine to give 97 (Eq. 113). ... 

Another approach to the quantitative correlation between the structure and the reactivity of hydrocarbons may be the use of the Taft correlation equation. A consideration of the dependence of the exchange rate on the constant a for a number of functionally substituted methane and ethane derivatives has shown [10] that the two-parameter equation is obeyed ... 

Fig. 9. Taft correlation between a -values and the modified transition energies of the longest wavelength n -7r -absorption band of the pyridinium-N-phenoxide betaine, measured at 25°C in different aliphatic alcohols R-OH (Erj,...

Perhydrolysis of carboxylic acids catalyzed either homogeneously or heterogeneously followed the Taft correlation based only on the steric part, which impHes that the steric effect of the substituent on the reaction center governs the reaction, but not the polar or resonance effects. 

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