Taft-Topsom equation

Multiparametric equation An equation which uses three or more parameters all of which may be either pure or composite. The LDR equation used in this work is an example of a triparametric equation. The Taft-Topsom equation is an alternative triparametric equation. ... 

Gas-phase elimination kinetics of alkyl A,iV-diethylcarbamates (Et2NCOOR) are affected markedly by branching of R and fall in the rate order t-Bu > i-Pr > Et the effects of heteroatom substituents Z on pyrolysis of ZCOOEt have been correlated using the Taft-Topsom equation, log(i /i H) = —(0.68 0.12)cTa -l- (2.57 0.12)ctf — (1.18 0.27)ctr, which reveals that the field effect outweighs steric and resonance factors. ... 

While, as noted earlier, vinylamine is protonated preferentially at its carbon site, the reason why this is so remains unclear. The question therefore arises as to how an ethylene molecule (AG + = 155.6 kcalmol-1 4) and an ammonia molecule (AG + = 195.6 kcal -mol 1 4, i.e. 40 kcal mol 1 more basic than the former) contribute to making the / carbon the more basic site when incorporated into vinylamine. In order to provide an answer within the context of the Taft-Topsom model, let us consider the equilibrium of equation 15. 

Substituent effects as evaluated on the basis of the Hammett equation and its extended forms, are - this has to be emphasized again — empirical results. Nevertheless, it is very soothing to know that theoretical approaches, i. e., calculations of substituent effects using ab initio molecular orbital theory (Topsom, 1976, 1981, 1983 Taft and Topsom, 1987, STO-3G and 4-31G level), give results that are consistent with the experimental data. However, it is not recommended to use only theoretically calculated substituent constants and values for F, R, and other parameters for the interpretation of experimental data. 

This equation is an elaboration of the dual substituent-parameter (DSP) equation. Its development has been relatively recent, but Taft and Topsom, who have been closely associated with it, have already written a long review article151 involving the equation, and this article will probably acquire the status in respect of the PSP equation that the article of Ehrenson, Brownlee and Taft92 has in connection with the DSP equation. The name Poly Substituent-Parameter Equation was devised by the present author in a short... 

Taft and Topsom s article151 and also Topsom s171 should be consulted for details of the setting up of the scales of substituent parameters. The equation has been applied to a wide range of gas-phase reactivities. (In the multiple regressions an intercept term is often permitted, but usually this turns out to be indistinguishable from zero, as it should be if equation 20 is valid.) For aliphatic and alicyclic saturated systems the resonance term is duly negligible. The roles of field, resonance and polarizability effects are discussed and the interpretat of the various p values is attempted. 

Derived from the Hammett equation, a electronic constants are calculated for different molecular substituents from the rate or equilibrium constant of specific reactions, with respect to a reference compound [Topsom, 1976 Charton, 1981 Taft and Top-som, 1987 Topsom, 1987b]. 

While developing a simple model to describe the effect of substituents (X) on the properties of a molecular system (M) has been one of the most earnestly pursued goals by chemists ever since the Hammett equation was reported, it was not accomplished until Taft and Topsom succeeded in providing a description for the SAG° values of gas-phase acid-base equilibria shown in equation 13,... 

The PSP equation is written by Taft and Topsom in various forms. Equation 24 is a convenient form with which to begin this discussion ... 

Taft and Topsom developed a detailed analysis of the role that substituent effects play in properties such as acidity and basicity. The four primary kinds of substituent effects were considered to be electronegativity (induction), field, resonance, and polarizability effects. As in the Hammett equation, the a values are substituent properties, while the p values represent the sensitivity of the reaction to each of these properties. The general form of such an equation is given in equation 6.95, where the symbols F, x, / and R represent field, electronegativity, polarizability, and resonance effects, respectively. 

Taft and Topsom s article and also Topsom s should be consulted for details of the setting up of the scales of substituent parameters. The equation has been applied to a wide range of gas-phase reactivities. For aliphatic and alicyclic saturated systems the resonance term is duly negligible. 

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