Taft inductive factor

The rates of H-D exchange at the a-positions have been determined for a series of N- substituted pyridinium salts and pyridine 1-oxides in D20 at 75 °C (Scheme 197) (70JA7547). The rates give a good correlation with the Taft inductive parameter <77 pi = 15). The positively charged nitrogen in a ring has been estimated to activate the a-position towards deprotonation and ylide formation by a factor of 1014 16. 

The rates of HD exchange at the -positions for a series of N-substituted pyridinium cations and pyridine 1-oxide derivatives in D20 at 75 C (Figure 2) <1970JA7547> correlate well with the Tafts inductive parameter ] ( = 15). A positively charged nitrogen in a ring is estimated to activate the -position toward deprotonation and ylide formation by a factor of 101S. 

Taft (21) has suggested that the electrical effect of a substituent is composed of localized (inductive and/or field) and delocalized (resonance) factors. Thus we may write the substituent constant of the group X as... 

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 . ... 

Hammett s success in treating the electronic effect of substituents on the equilibria rates of organic reactions led Taft to apply the same principles to steric, inductive, and resonance effects. The Hammett o constants appear to be made up primarily of two electronic vectors field-inductive effect and resonance effect. For substituents on saturated systems, such as aliphatic compounds, the resonance effect is rarely a factor, so the o form the benzoic acid systems is not applicable. Taft extended Hammett s idea to aliphatics by introducing a steric parameter ( .). He assumed that for the hydrolysis of esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base. Rate differences would be caused only by the field-inductive effects of R and R in esters of the general formula (XCOOR), where X is the substituent being evaluated and R is held constant. Field effects of substituents X could be determined by measuring the rates of acid and base catalysis of a series XCOOR. From these rate constants, a value a could be determined by Equation (5.9) ... 

Integrating various factors, namely Taft s steric factor, resonance, inductive, Verloop steric parameters with the partition behaviour of drug molecules Hansch and Fuj ita exploited these principles in determining the establishing quantitative structure-activity relationship (QSAR) of drugs, which has rmdergone a sea change both in expansion and improvement with the help of computer researched softwares. 

A new structure-activity relationship, Xjj =yS+l, where y is a negative constant, S is the total steric effect, and 4 is the total inductive effect, correlated strongly with available measurements of ozonolysis. New rate coefficients were measured for ozonolysis of a number of unsaturated heteroatomic compounds and it has been emphasized that the inductive effect rather than the steric effect is important in predicting their reactivity %, the inductive effect index, was compared with the Taft a constant and rates of reaction of hydroxyl radical with a given species it correlated strongly in both cases (which should be unaffected by steric factors) suggesting a universal response by olefinic species towards electrophilic addition. ... 

Taft and Topsom broke the substituent constants into four parameters (Eq. 8.30), reflecting contributions from field (f), induction (x), polarizability (a), and R (resonance), along with appropriate sensitivity factors (these substituent constants are also given in references at the end of the chapter). 

A comparison of these scales with Taft s induction constants a and Taft s steric constants for different alkyl groups leads to the conclusion that both electronic and steric factors influence the doublebond activity in stereospecific catalysis but that steric factors seem to be more important. An especially significant decrease in the olefin activity was found for olefins branched vicinally to the double bond (3-methyl-pentene-1,3-methylbutene-l, vinylcyclohexane, styrene). This is protebly connected with the space limitations for monomer coordination in the stereospecific active sites (IS, 16). 

The dependence has the form log( x/ o)" p o. Later this equation was modified. Taft introduced the principle of additivity of the influence of different ctors on the reactivity of a compound each factor, being it inductive, conjugation or steric, affects AG independently, so that for each reaction series log( / o) or logfX/ATp) can be presented as the sum 2p,aliphatic compounds the Taft equation has the form... 

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