Hammett principle

The product ratio is therefore not determined by AG, but by the relative energy of the two transition states A and B. The conclusion that the ratio of products formed from conformational isomers is not determined by the conformational equilibrium ratio is known as the Curtin-Hammett principled Although the rate of the formation of the products is dependent upon the relative concentration of the two conformers, because AGb is decreased relative to AG to the extent of the difference in the two conformational energies, the conformational preequilibrium is established rapidly, relative to the two competing product-forming steps. The position of the conformational equilibrium cannot control the product ratio. The reaction can proceed through a minor conformation if it is the one that provides access to the lowest-energy transition state. 

The same arguments can be applied to other energetically facile interconversions of two potential reactants. For example, some organic molecules can undergo rapid proton shifts (tautomerism) and the chemical reactivity of the two isomers may be quite different. However, it is not valid to deduce the ratio of two tautomers on the basis of subsequent reactions that have activation energies greater than that of the tautomerism. Just as in the case of conformational isomerism, the ratio of products formed in subsequent reactions is not controlled by the position of the facile equilibrium, but by the of the competing steps. 

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Basic Mechanistic Concepts Kinetic versus Thermodynamic Control, Hammond s Postulate, the Curtin-Hammett Principle... 

SECTION 4.4. BASIC MECHANISTIC CONCEPTS KINETIC VERSUS THERMODYNAMIC CONTROL, HAMMOND S POSTULATE. THE CURTIN-HAMMETT PRINCIPLE... 

In a simple statement of the Curtin-Hammett principle, we know that the rate will depend not only on the separate reactivities of the two pathways, but also on the relative populations of the catalyst species. In the following table we highlight the parameters referring to reactivity and to the population. 

Does the Curtin-Hammett principle apply to photoelectrocyclic reactions when more than one conformation of reactant is present in solution Why ... 

The mechanism of this interconversion is known as apparent allyl rotation and results in a switching of the allylic termini in addition to the movement of the central allyl carbon from one side of the coordination plane to the other although, for situations with symmetrically substituted allyl units, only this latter change is noticeable As the name suggests, the process is non-trivial and, in reality, the mechanism is dependent on the specific conditions.1161 Nevertheless, in most cases, the interchange is rapid compared with the rate of nucleophilic attack and, therefore, the product outcome does not necessarily depend on the intermediate isomer ratio (Curtin-Hammett principle). 

If it is assumed that the Curtin-Hammett principle applies, one need only to compare the energies of the minima on the solid and dashed curves to be able to predict the structure of the major product. These curves also allow a direct comparison of Cram s, Cornforth s, Karabatsos s and Felkin s model for 1,2 asymmetric induction. Both Figures show the Felkin transition states lying close to the minima. The Corn-forth transition states (Fig. 3) are more than 4 kcal/mol higher and should contribute little to the formation of the final products assuming a Boltzmann distribution for the transition states, less than one molecule, out of a thousand, goes through them. Similarly, Fig. 4 shows the Cram and Karabatsos transition states to lie more than 2.7 kcal/mol above the Felkin transition states, which means that they account for less than 1% of the total yield. 

Cumyl propyl peroxide, determination, 708 Curing processes, 692, 707 Curtin-Hammett principle, electron-poor alkene photooxygenation, 877 Cyanine dye, TBARS assay, 667 Cyanohydrins, bis(trimethylsilyl) peroxide reactions, 797... 

In both of these situations, the reaction actually observed does not occur from the lowest-energy conformation of the reactants. That this need not be the case is a direct consequence the Curtin-Hammett principled This recognizes that some higher-energy reactive conformation , will be in rapid equilibrium with the global minimum and, assuming that any barriers which separate these conformations are much smaller than the barrier to reaction, will be replenished throughout the reaction. 

Curtin-Hammett Principle. The idea that reactive conformer in a Kinetically-Controlled Reaction is not necessarily the lowest-energy conformer. The rationale is that the energy barriers separating conformers are typically much smaller than barriers to chemical reaction, and that any reactive conformers will be replenished. 

The quaternization of amines is an extensively studied reaction in organic chemistry. The quaternization of piperidine can follow two distinct stereochemical pathways, axial or equatorial alkylation (70MI20700). Since the Curtin-Hammett principle is valid in this reacting system it is important to recognize that the product ratio of (87) to (89) does not directly reflect the ratio of (86) to (88). 

Because of the observation of a fast equilibrium between the C-titanium complexes 19A and 19C and the N-titanium ylides 19B, the reactivity model depicted in Scheme 1.3.16 was proposed in order to account for the regio- and diastereose-lectivity observed in the reaction of 19 with aldehydes. This model, which is based on the assumption of the operation of the Curtin-Hammett principle (that is, the reactions of 19A, 19B, and 19C with aldehydes are significantly slower than their isomerization), features the six-membered cyclic chair-like transition states... 

Application of the Curtin-Hammett Principle would suggest that the different ground state conformers have minimal influence on the product composition. It is the difference in activation energies for the two different isomers that controls the reaction, and the diastereomeric transition states would be attained from either ground state conformation. 

Curtin-Hammett principle, 23 industrial applications, 8, 26 mechanism, 21 phosphine ligands, 7, 18 reaction conditions, 18 scope and limitations, 27 Wilkinson complex, 17 Rhodium-catalyzed olefin isomerization ab initio calculations, 110... 

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