Chemical reactions Hammond postulate

Because the rates of chemical reactions are controlled by the free energy of the transition state, information about the stmcture of transition states is crucial to understanding reaction mechanism. However, because transition states have only transitory existence, it is not possible to make experimental measurements that provide direct information about their structure.. Hammond has discussed the circumstances under which it is valid to relate transition-state stmcture to the stmcture of reactants, intermediates, and products. His statements concerning transition-state stmcture are known as Hammond s postulate. Discussing individual steps in a reaction mechanism, Hammond s postulate states if two states, as, for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have neariy the same energy content, their interconversion will involve only a small reorganization of molecular stmcture. ... 

It would be desirable to achieve a quantitative version of the Hammond postulate. For this purpose we need a measure of progress along the reaction coordinate. Several authors have used the bond order for this measure.The chemical significance of bond order is that it is the number of covalent bonds between two atoms thus the bond orders of the C—C, C==C, bonds are 1, 2, and 3,... 

With reference to the occurrence of this type of kinetically determined selectivities of organic chemical reactions, the Hammond postulate now states that ... 

It can be concluded that in analogy with the gas-phase results, the TS structures more closely resemble the products than the reactants which is in good accord with the Hammond postulate. The other related difference between the results of the gas phase and the COSMO approach reflects a partial change of the reaction coordinate in most of the cases of the deammination reactions (cf. Figure 10b). The ion-pair structures in the frame of the supermolecule are formed as the products in both hydration steps. The NH4+ ion is released from the platinum complex instead of the ammonia molecule. The extra proton is taken from the ligated water. The preference for the NH4+ cation is about 2.6 kcal/mol. Consequently, the difference between the gas-phase product conformers P10 and P13 disappears. The formation of the ion-pair structure is in good accordance with the quantum chemical calculations of the amino acids. There, the formation of the NH2-... 

The Hammond postulate [41] is a qualitative assumption that interrelates structural similarities between reactants, transition states, and products with the endo- or exothermicity of chemical reactions. It states that if the transition state is near in the potential energy surface to an adjacent stable complex, then it is also near in structure to the same complex. This postulate, which can be applied to most chemical reactions, has been used for predicting the effects of substituent changes and external perturbations on transition-state geometry [42,43]. It is basically accomplished if slopes and matrices of force constants associated with reactants and products are similar [44,45]. As yet, the unique attempt to quantify the Hammond postulate within a quantum mechanical framework is due to Cioslowski [46]. He has shown that the quantification of the Hammond postulate can be achieved by defining two new parameters, the isosynchronicity... 

If we re-take the TS of chemical bonds as the origin, these facts well explain the shift of TS (from the early TS side to the late TS side) that Hammond s postulate predicts, indicating that the CASVB method is a powerful tool for describing the electronic structure and chemical bond during chemical reactions. 

Energy Barriers in Chemical Reactions The Transition State and Activation Energy Reaction Mechanism The Hammond Postulate Thermodynamics versus Kinetics... 

Bulat FA, Toro-Labbe A (2003) An extension of the Hammond postulate. Structural effects on the classification of chemical reactions. J Phys Chem A 107 3987-3994 Toro-Labbe A, Gutierrerez-Oliva S, Murray JS, Politzer P (2007) A new perspective on chemical and physical processes the reaction force. Mol Phys 105 2619-2625 Toro-Labbe A, Gutierrerez-Oliva S, Politzer P, Murray JS (2009) The reaction force a rigorously defined approach to analyzing chemical and physical processes. In Chattaraj PK (ed) Chemical reactivity theory a density functional view. CRC, Boca Raton, pp 293-302 90. Politzer P et al (2005) The reaction force three key points along an intrinsic reaction coordinate. J Chem Sci 117 467-472... 

Sir Robert Robinson played an early central role in the development of the electronic theoiy of organic chemistry (5S). For example, he was the first to use the now commonplace curly arrow to imply the reorganization of electron density during the course of a chemical reaction (59). The constract of stereoelectronic control (which, in its most literal sense, explains all chemical reactions) underpins many concepts now in the lexicon of mechanistic and synthetic organic chemistry. These include the Hammond postulate, the Curtin-Hammett Principle, the Markovnikov rule (for additions to alkenes), the Thorpe-lngold effect (on rates of cyclization), the Btirgi-Dunitz approach trajectory, Cram/Comforth/Felkin-Ahn controlled additions (to chiral ketones... 

Hammond added that "in highly exothermic steps it will be expected that the transition states will resemble reactants closely and in endothermic steps the products will provide the best models for the transition states." The postulate is a logical consequence of the idea that the energy of a chemical entity is a function of its structure. Therefore, two species that occur consecutively during a reaction and that have very similar energies might be expected to have very similar structures as well. In this context, the phrase "similar structures" means similar coordinates on the horizontal axis of the reaction coordinate diagram. 

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