Taft polar substituent constants

It was found (32) that in the acid range (pH 4—6) the alkyl group does not influence the rate of decomposition, which is similar for all xanthates. In the alkaline range the rates are markedly influenced by the substitutional group, and the rates could be correlated with the Taft polar substituent constants estabhshed for the various groups. 

Many other linear Gibbs energy relationships have been proposed for example, the acid strengths of aliphatic compounds can be correlated using the "Taft polar substituent constants" o. ... 

Hoyle et al.39 have found good linear correlations between <5(33S) in substituted alkyl sulphones and the Taft polar substituent constant, a ... 

FIG. 25. Plot of one-bond silicon-hydrogen coupling constants vs. the sum of the Taft polar substituent constants for compounds of structure R R R SiH, where R = alkyl, phenyl, or hydrogen.(190. 191)... 

A number of correlations have been drawn between 1 JSiH and parameters related to bonding and reactivity. The magnitude of 1 JSiH was found to correlate with Hammett a constants in phenyl and benzyl substituted silanes and disilanes144,145, and aryloxy- and (arylthio)dimethylsilanes146 and with Taft polar substituent constants a in silanes R1R2R3SiH (R = alkyl, Ph, H)147. 

An experimental kinetic study has shown that the influence of the a-cyano group on the rate of solvolysis of 2-propyl sulphonates, namely the k(HCMe20Ts)//c(NCCMe20Ts) ratio (extrapolated from 80-170 °C temperature interval to 25 °C) in 100% 2,2,2-trifluoroethanol buffered with 2,6-lutidine, is only 3.5 x 103. This is considerably smaller than the retardation factor expected on the basis of the Taft polar substituent constant. To evaluate the possibility that the investigated methacrylonitrile formation reaction proceed by a rate-limiting concerted E2 elimination, the effect of a-deuterium substitution on the rate of... 

The J(HH) coupling constants are within the narrow range of 5.75-5.97 Hz and tend to increase on increasing the value of Taft polar substituent constants . This was explained by a decrease in the dihedral angles HCCH on shortening the Si- -N bond. Precise determination of 7(HH) in the skeleton of some silatranes made it possible to estimate the NCCO torsion angles, which appeared to be comparable to the solid-state angles. This means that in solution the silatrane molecules have a Si- -N bond and a conformation similar to that in the solid state. 

Calculated values. Varies with pH. Taft polar substituent constant. 

Some of these triply bridged complexes have been investigated extensively by cyclic voltammetry (CV) and Mo NMR spectroscopy (978). In the former, two successive one-electron reductions are observed, potentials correlating well with the Taft polar substituent constants for the dithiocarbamates, while Mo chemical shifts vary between 5 329 (R = Me) and 354 (R = i-Pr), the line width increasing as a function of dithiocarbamate steric bulk. 

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.

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 difference 8pAG ti.,n . - 8RAG cid for an ester hydrolysis should, if these assumptions are valid, be a measure of the polar effect in the substituent. Taft defined a polar substituent constant o based on these ideas ... 

In the early 1950s Taft (10) outlined a sound quantitative basis for the estimation of steric effects and for separating them from polar and resonance effects. This derivation follows the standard extrathermodynamic approach and is therefore empirical. The definition of steric substituent constants is closely related to polar substituent constants, for they are obtained from the same reference system (10). The polar substituent constants, however, have been shown to arise from electronic effects, and they strongly correlate with the inductive substituent constants (10, 43, 61). 

Taft equation Various equations are associated with R.W. Taft, but the term is most often used to designate the family of equations that emerged from Taft s analysis of the reactivities of aliphatic esters and that involved the polar substituent constant a and the steric substituent constant Es... 

Taft s polar substituent constant, a, applicable to aliphatic systems is based on the notion that free energy relationships can exist between rate processes as well as between rate and equilibrium processes (Equations 3-5, Chapter 1). A rate process can thus be employed as a reference reaction. A simple example of such a Class II rate-rate free energy relationship is shown in Figure 3 where the reaction of phenoxide ion with 4-nitrophenyl-substituted benzoate esters is compared with the corresponding reaction of hydroxide ion, which therefore becomes the reference free energy change. 

For alifatic systems an equation proposed by Taft is often applied that involves the polar substituent constant a and the steric substituent constant E... 

Pentadiene and allylbenzene are about 10 times as reactive as 1-butene. The effect on reactivity of an alkyl group, relative to a methyl group at the same position, is nearly independent of whether the alkyl group is bonded to the 2, 3, or 4 position of the 1-alkene chain. The effects of unbranched alkyl substituents or reactivity correlate well with Taft s polar substituent constants , but isopropyl and tertiary butyl groups have much larger deactivating effects... 

Other workers [161] found that the distribution coefficients of the barbiturates are affected not only by the mass of the substituents, but by their inductive effects on the hydrophilic portion of the molecule, as defined by Taft s polar substituent constants. 

It is claimed that multiple regression analyses, involving Taft s polar substituent constant or and Dubois steric parameter E, can satisfactorily account for the properties of alkyl cobaloximes (R = alkyl) and alkyl cobalamines in terms... 

Substituents can be introduced not only into the betaine solute, but also into the surrounding solvent molecules. Let us consider a betaine dye solution in various alcohols R-OH with different alkyl groups R. An enhanced hypsochromic shift of the solvatochromic absorption band with increasing hydrogen-bond donor ability of the alcohols should be obtained. This is indeed the case(25). As Figure 9 shows, even exists a linear correlation between the modified E.p-values, measured in nine different alcohols, and Taft s polar substituents constants for the alkyl groups R of these alcohols(25). 

Clayton and Purcell have reported excellent correlations using dipole moments, amide group moments, and Taft s polar substituent constants a ) along with hydrophobic parameters when applied to an homologous series of molecules.A regression of the butyrylcholinesterase inhibitory potencies of these mono(carbamoylpiperidino)decanes gave eq 5 in which... 

The Taft sigma constant and a, constants are inductive or polar substituent constants (Hansch and Leo 1979) derived from hydrolysis rates of substituted esters (Taft 1956) and ionization of substituted acetic acids (Charton 1981), respectively. Either constant could be useful in describing anionic potential. Therefore, they were evaluated as descriptors for the above mentioned data sets to determine if the outliers could be predicted. The following data indicate that either constant is a good descriptor. 

Taft suggested equation (12) for evaluating the polar effect of a substituent R in the ester RCO2R, where a is a polar substituent constant for R. 

Finally, in this account of multiparameter extensions of the Hammett equation, we comment briefly on the origins of the a, scale. This had its beginning around 1956 in the a scale of Roberts and Moreland for substituents X in the reactions of 4-X-bicyclo[2.2.2]octane-l derivatives. However, at that time few values of o were available. A more practical basis for a scale of inductive substituent constants lay in the o values for XCHj groups derived from Taft s analysis of the reactivities of aliphatic esters into polar, steric and resonance effects . For the few o values available it was shown that o for X was related to o for XCHj by the equation o = 0.45 <7. Thereafter the factor 0.45 was used to calculate c, values of X from o values of XCH2 . ... 

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