Transition endothermic reaction

Many programs allow the user to input a weighting factor (i.e., to give a structure that is 70% of the way from reactants to products). This allows the application of the Hammond postulate that the transition structure will look more like the reactants for an exothermic reaction and more like the products for an endothermic reaction. 

Atomic metal ion-hydrocarbon reactions bond dissociation energies for fragments, 15,16t endothermic reactions, 13,15,17f Atomic transition metal ion reactions development of approach for real-time measurements of dissociation kinetics, 39 ion beam apparatus, 12,14f studies of... 

The activation energy, Ea, is the additional energy that must be absorbed by the reactants in their ground states to allow them to reach the transition state. Consider the following three endothermic reactions they are identical except that they occur in different phases ... 

In highly endothermic reaction, the transition-state (TS,) resembles product B. Case II. Highly Exothermic Reaction... 

Equation (82) predicts that for reactions with zero free energy change a = 0.5, while for exothermic reactions, a < 0.5 and for endothermic reactions, a > 0.5. Since according to both Marcus theory and the Bell— Evans-Polanyi model early transition states are related to exothermic reactions and late transition states to endothermic reactions, a may be interpreted as a relative measure of transition state geometry. However, in our view even this interpretation should be treated with a measure of healthy scepticism. Even if one accepts Marcus theory without reservation, the a... 

The Hammond postulate states that in endergic reactions, features which stabilize and thus lower the energy of a product lower the energy of the transition state leading to diat product. This is shown in Figure 5.12. If product 2 (P2) is lower in energy than product 1 (Pi), then transition state 2 ( 2) will be lower than transition state 1 ( 1). It will also be earlier. As a consequence, P2 will have a lower activation barrier and be formed faster than Pi. A simplified restatement of the Hammond postulate is that more stable products are formed faster. It must be remembered that this analysis is for endothermic reactions and assumes that the reactants have the same or similar energies. 

NICS, HOMA, and Bird indices were also calculated for the transition states of the reactions of 61H-0 and 61H-S with a series of carbanions. The results are reported in Table 23. The trends in these parameters show a clear increase as the transition state becomes more product-like with increasing endothermicity, indicating an increase in transition state aromaticity. Even more revealing is the % progress at the transition state which indicates that this progress is >50% not only for the endothermic reactions (product-like transition states) but even for most of the exothermic reactions (reactant-like transition states) except those with strongly negative AH° values. 

According to the Hammond postulate, the transition state for abstraction by chlorine resembles the reactant because this is an exothermic reaction. In contrast, the transition state for abstraction by bromine resembles the product because it is an endothermic reaction (see Figure 21.2). In the case of abstraction by chlorine the carbon-hydrogen bond is only slightly broken in the transition state, and the stability... 

One of the focal points of mechanistic interest has been into the nature of the transition state. A postulate which bears heavily on this topic and which is now most commonly referred to as the Hammond postulate (Hammond, 1955) has become central in the study of transition state structure. Hammond s postulate may be stated as follows the interconversion of two states of similar energy on a reaction pathway will involve only a small amount of structural reorganization. A precise interpretation of this postulate leads only to the limited conclusion that transition states of highly exothermic reactions are similar in structure and energy to reactants, while for strongly endothermic reactions transition states resemble products. 

Leffler (1953, 1963a) has proposed a more general relationship (hence referred to as the Leffler Hammond postulate) which represents an extension of the Hammond postulate since it treats the whole spectrum of reaction types. Thus the transition state is viewed as changing gradually from reactant-like in highly exothermic reactions, to intermediate in character for thermoneutral reactions, to product-like for endothermic reactions. In addition to this proposal, which relates the transition state structure to that of the products and reactants, a free energy relationship (1) which relates changes in... 

Since (16) is a rate-equilibrium relationship [equivalent to the relationship (2) discussed earlier], a is considered to reflect the degree of proton transfer in the transition state and hence is a measure of selectivity. Values of a close to 0 are associated with exothermic reactions in which the degree of proton transfer in the transition state is as yet small. Similarly, values of a close to 1 are associated with endothermic reactions in which the degree of proton transfer in the transition state is almost complete. 

In an endothermic reaction, the transition state is closer to the products in energy and in structure. In an exothermic reaction, the transition state is closer to the reactants in energy and in structure. 

The order of nucleophilicity, graphically illustrated in Fig. 4.1, matches well with expectation—the better donor substituents are the more effective at increasing the rate of reaction. However, since this is an endothermic reaction, the transition... 

Non-additivity of substituent effects has been proposed as a criterion for the operation of the RSR so the linearity argues against its applicability in this system. In a description of transition states by structure-reactivity coefficients (Jencks and Jencks, 1977), two alternative types of behaviour were discussed. In Hammond -type reactions the more endothermic reactions have later transition states, whereas anti-Hammond behaviour is characterized by an adjustment of the transition-state structure to take advantage of favourable substituent effects. These results illustrate that different systems can display quite different behaviour in linear free energy correlations. Thus, in alkene protonations, such correlations cover vast ranges in reactivity with only modest changes in sensitivities, while in solvolytic reactions the selectivity p varies depending on the electron supply at the electron-deficient centre (Johnson, 1978). 

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