Hammond—Leffler postulate

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... 

The rationale for the Leffler-Hammond postulate and the free energy relationship expressed in (1) and (2) may be found in a model... 

On the basis of the Leffler-Hammond postulate the theoretical justification for the reactivity-selectivity principle may be observed. First, let us define the selectivity 5, of a species A, in its reaction with two competing reagents X and Y, as indicated by (4), where k ... 

It is important to note that the above presentation, justifying the reactivity-selectivity principle, is based on a number of fundamental assumptions. First, it is assumed that the Leffler-Hammond postulate is valid, which in turn implies that the reaction under consideration obeys a rate-equilibrium relationship [eqn (2)]. This assumption often cannot be verified since for reactions of highly active species such as carbenes, free radicals, carbonium ions, etc., equilibrium constants are generally not measurable. However it follows that for reactions which do not conform to a rate-equilibrium relationship, no reactivity-selectivity relationship is expected. Also, in Fig. 4, the difference in the free energy of the... 

In line with the Leffler-Hammond postulate, which is a differential analogue of the Hammond postulate, the rationale for a differential reactivity-selectivity principle has been demonstrated, i.e., the marginal stabilization of a particular species will result in a corresponding increase in its selectivity. Recently, the view has been expressed that there is no substantive evidence for such behaviour... 

Reactivity-selectivity relationships are obtainable in carbene chemistry provided an independent selectivity parameter is found. Under such circumstances the relative rate data then serve as a measure of relative reactivity. This has been done with a number of Hammett studies. Thus the addition of dichlorocarbene to compounds [ 1 ] — [3] (listed in Table 15) indicates that a reactivity-selectivity relationship is obtained using p as a measure of selectivity. A more reactant-like transition state is obtained for the more reactive substrate in accordance with the Leffler- Hammond postulate. 

LEE-WILSON TREATMENT LEFFLER S ASSUMPTION HAMMOND POSTULATE Left-to-right convention,... 

The acronym Bemahapothle (Bell, Marcus, Hammond, Polanyi, Thornton, Leffler) is sometimes used in recognition of the principal contributors toward expansion of the original idea of the Hammond postulate. 

These equations are essentially quantitative statements of the considerations of Hammond [92] and Leffler [16bj consideration of the Hammond postulate indicates that as acid strength increases (pATjJ.) the selectivity ( 2) in Eqn. 107 should decrease thus C is negative [93]. The above relationships have been derived by a number of authors [94]. 

Hammond-Leffler postulate The TS is more like the reactant or product that is closer in energy endothermic TS is like the product, exothermic TS is like the reactant. 

In other words, under these restrictive conditions, outer sphere electron-transfer reactions obeying the Marcus-Hush model are typical examples where the Hammond-Leffler postulate and the reactivity-selectivity principle (see, for example, Pross, 1977, and references cited therein, for the definition of these notions) are expected to apply. 

ACIDITY FUNCTION BUNNETT-OLSEN EQUATIONS DECREE OF DISSOCIATION HAMMETT EQUATION HAMMOND PRINCIPLE/POSTULATE LEFFLER S ASSUMPTION REACTING BOND RULES HANES PLOT... 

Hammond-Leffler postulate. States that the structure of the transition state more closely resembles the product or the starting material, depending on which is higher in enthalpy. 

Many reactions exhibit effects of thermodynamics on reaction rates. Embodied in the Bell-Evans-Polanyi principle and extended and modified by many critical chemists in a variety of interesting ways, the idea can be expressed quantitatively in its simplest form as the Marcus theory (15-18). Murdoch (19) showed some time ago how the Marcus equation can be derived from simple concepts based on the Hammond-Leffler postulate (20-22). Further, in this context, the equation is expected to be applicable to a wide range of reactions rather than only the electron-transfer processes for which it was originally developed and is generally used. Other more elaborate theories may be more correct (for instance, in terms of the physical aspects of the assumptions involving continuity). For the present, our discussion is in terms of Marcus theory, in part because of its simplicity and clear presentation of concepts and in part because our data are not sufficiently reliable to choose anything else. We do have sufficient data to show that Marcus theory cannot explain all of the results, but we view these deviations as fairly minor. 

A closely related statement of the correlation of energy barriers with heats of reaction is known as the Bell-Evans-Polanyi (BEP) principle (equation 6.69). ° Note that the BEP principle is concerned with the activation energies, while the Hammond and Leffler postulates are concerned with the structures of transition states. Of course, bonding and energy are inherently related, so the Hammond-Leffler postulate and the Bell-Evans-Polanyi principle are complementary. 

The Hammond-Leffler Postulate If you review the free-energy diagrams that accompany the mechanism for the Sj fl reaction of tert-butyl chloride and water (Section 6.10), you will see that step 1, the ionization of the leaving group to form the carbocation, is uphill in terms of free energy (AG° for this step is positive). It is also uphill in terms of enthalpy (AH° is also positive), and, therefore, this step is endothermic. 

According to the Hammond—Leffler postulate, the transition-state structure for a step that is uphill in enei should show a strong resemblance to the structure of the product of that step. 

The transition state for the reaction leading to 2-methyl-2-butene (Fig. 7.6) has the developing character of the double bond in a trisubstituted alkene. The transition state for the reaction leading to 2-methyl-Tbutene has the developing character of a double bond in a disubstituted alkene. Because the transition state leading to 2-methyl-2-butene resembles a more stable alkene, this transition state is more stable (recall the Hammond—Leffler postulate, Fig. 6.10). Because this transition state is more stable (occurs at lower free energy), the free energy of activation for this reaction is lower and 2-methyl-2-butene is formed faster. This explains why 2-methyl-2-butene is the major product. 

This effect illustrates another application of the Hammond—Leffler postulate (Section 6.13A). The arenium ion is a high-energy intermediate, and the step that leads to it is a highly endothermic step. Thus, according to the Hammond—Leffler postulate, there should be a strong resemblance between the arenium ion itself and the transition state leading to it. 

Hammond—Leffler postulate (Section 6.13A) A postulate stating that the structure and geometry of the transition state of a given step will show a greater resemblance to the reactants or products of that step depending on which is closer to the transition state in energy. This means that the transition state of an endothermic step will resemble the products of that step more than the reactants, whereas the transition state of an exothermic step will resemble the reactants of that step more than the products. 

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