Bell-Evans-Polanyi relationship

Apparently, these results implied an inverse relationship between reactivity and selectivity, with the reactivity of the carbocation measured by the inverse of the rate constant for solvolysis. This indeed was not unexpected in the context of a general perception that highly reactive reagents, especially reactive intermediates such as carbocations, carbanions, or carbenes are unselective in their reactions.257 259 Such a relationship is consistent with a natural inference from the Hammond postulate258 and Bell-Evans-Polanyi relationship,260 and is illustrated experimentally by the dependence of the Bronsted exponent for base catalysis of the enolization of ketones upon the reactivity of the ketone,261,262 and other examples21,263 including Richard s careful study of the hydration of a-methoxystyrenes.229... 

The thermolyses presented in this chapter are one example of a series of analogous reactions. The Bell-Evans-Polanyi relationship of Equation 1.3 also holds for many other series of analogous reactions. The general principle that can be extracted from Equation 1.3 is that, at least for a reaction series, the more exothermic the enthalpy of reaction, the faster it will be. But this doesn t mean that all reactions that are exothermic are fast, so be careful. 

The thermolyses presented in this chapter are one example of a series of analogous reactions. The Bell-Evans-Polanyi relationship of Equation 1.3 also holds for many other series of analogous reactions. 

The Bell-Evans-Polanyi relationship, Hammond s postulate, and the Marcus equation are all approaches to analyzing, understanding, and predicting relationships between the thermodynamics and kinetics of a series of closely related reactions. This is an important issue in organic chemistry, where series of reactions differing only in peripheral substituents are common. Each of these approaches provides a sound basis for the intuitive expectation that substituents that favor a reaction in a thermodynamic... 

The Bell-Evans-Polanyi relationship and the Hammond postulate (see Section 3.3) provide a basic framework within which to discuss structure-reactivity relationships. The Bell-Evans-Polanyi equation implies that there will be a linear relationship between and the C-H BDE. 

An empirical extension of the Bell—Evans—Polanyi relationship... 

This final point signifies that the value of a in the rate-equilibrium relationship (2) is not constant but decreases as the reaction becomes increasingly exothermic. It should be noted however that since the Bell- Evans-Polanyi model and the Hammond postulate are couched in energy terms the assumption that free energy changes (AG°) are proportional to energy changes (A °) is inherent in eqns (1) and (2). 

The Bell-Evans-Polanyi (BEP) Principle Relationships between Rates of Reactions and Corresponding Equilibrium Constants... 

More complete lists of calculated energies of the gas-phase Sn2 reactions of Y -h CH3X general type are available in Refs. [30,37] (the >i-INDO method) and [26,34] (4-3IG). The data obtained make it possible to check how the well-known relationships among the structural, kinetic and thermodynamic characteristics, such as the Bell-Evans-Polanyi principle, the Hammond postulate, etc., are obeyed in the case of the reactions which proceed without a solvent. 

One of the older reactivity principles relates the activation energy to the reaction enthalpy (A /°). This principle was formulated independently by Bell, and by Evans and Polanyi [34,35]. The Bell-Evans-Polanyi (BEP) relationship is... 

The relationship between rates and equilibria was first proposed by Bronsted and Pedersen (1924) and subsequently treated by Bell (1936) and Evans and Polanyi (1938) it is shown in (108). Some 30 years ago Leffler (1953) provided (108) with a molecular basis by suggesting that the parameter a reflects... 

---