Polanyi principle

The Bell-Evans-Polanyi Principle/Hammond Postulate/... 

By the Polanyi principle the activation energy of the initiation reaction, , will be related antibatically to the enthalpy change of the initiation reaction, which can be... 

The basis of this statement is the Polanyi Principle, which correlates enthalpies of reaction AH with activation enthalpies AH ... 

I AHp, both for the three hydrocarbons and for the two vinyl ethers (Figure 8). This is the exact opposite of what is usually found. For normal addition reactions of similar compounds, an increase in AH produces a decrease in the activation enthalpy AH and thus an increase in the rate constant (Polanyi principle). 

BASE PAIRING BECQUEREL BEER-LAMBERT LAW ABSORPTION SPECTROSCOPY Bell-Evans-Polanyi principle,... 

For a reaction adequately described by just two configurations, reactant and product, the analysis of substituents effects is straightforward and was first treated by Horiuti and Polanyi (1935) almost 50 years ago. Subsequent contributions by Bell (1936) and Evans and Polanyi (1938) have led to these general ideas being jointly termed the Bell-Evans-Polanyi principle (Dewar, 1969). The treatment of multiconfiguration reactions is analogous and is illustrated in Fig. 12. Let us discuss this in detail. 

In Section 1.2.1 we discussed the stabilities of reactive radicals. It is interesting that they make an evaluation of the relative rates of formation of these radicals possible. This follows from the Bell-Evans-Polanyi principle (Section 1.3.1) or the Hammond postulate (Section 1.3.2). 

Fig. 1.10. Enthalpy change along the reaction coordinate in a series of thermolyses of aliphatic azo compounds. All thermolyses in this series except the one highlighted in color follow the Bell-Evans-Polanyi principle.

For five out of the six reactions investigated, Figure 1.10 shows a decrease in the activation enthalpy AH with increasingly negative reaction enthalpy AH. Only for the sixth reaction— drawn in red in Figure 1.10—is this not true. Accordingly, except for this one reaction AH and AHr are proportional for this series of radical-producing thermolyses. This proportionality is known as the Bell-Evans-Polanyi principle and is described by Equation 1.3. 

This stabilization of the radical intermediates, arising from a better mesomeric stabilization of radicals in the phthalimide moiety, consequently increase the exoenergicity of reactions and, according to the Bell-Evans-Polanyi principle, lowers the activation barrier and thus enables processes that are unknown from ketones. The unique photochemical reactivity of phthalimides will be demonstrated with some examples. 

This follows from the Bell-Evans-Polanyi principle (Section 1.3.1) or the Hammond postulate (Section 1.3.2). 

The Bell-Evans-Polanyi Principle/Hammond Postulate/ Marcus Theory---------------------------------------------... 

Bell-Evans-Polanyi principle The linear relation between ENERGY of activation (EJ and enthalpy of reaction (A Ht) sometimes observed within a series of closely related reactions. 

The Bell-Evans-Polanyi principle rationalizes the increase in rates of many reactions with increase in exothermicity (the release of heat by a system as a reaction occurs) by describing the transition states as a blend of reactant and product configurations.As shown in Figure 6.12, the... 

Figure 19.1 Intersecting curves to illustrate the Bell-Evans-Polanyi Principle for a general group-transfer reaction (19.1a). Three members of a reaction series involving different acceptor groups B (/ = 1,2, 3) give separate energy curves for step (19.1c) each intersecting the energy curve for step (19.1 b).

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