Kinetics Hammond postulate

According to this very simple derivation and result, the position of the transition state along the reaction coordinate is determined solely by AG° (a thermodynamic quantity) and AG (a kinetic quantity). Of course, the potential energy profile of Fig. 5-15, upon which Eq. (5-60) is based, is very unrealistic, but, quite remarkably, it is found that the precise nature of the profile is not important to the result provided certain criteria are met, and Miller " obtained Eq. (5-60) using an arc length minimization criterion. Murdoch has analyzed Eq. (5-60) in detail. Equation (5-60) can be considered a quantitative formulation of the Hammond postulate. The transition state in Fig. 5-9 was located with the aid of Eq. (5-60). 

Electrophilic nitration of a substituted benzene may lead to ortho, meta or para products, depending on the substituent. According to the Hammond Postulate, the kinetic product will be that which follows from the most stable intermediate benzenium ion, i.e. 

The assumption of a kinetically controlled course of the reaction, however, readily explains the observed results, even though the transition structures have not, as yet, been calculated. Because epoxide opening is exothermic, 39 can be regarded as a simple model for the transition structure according to the Hammond postulate. It is clear from the structure of 39 that the left-hand ethoxy substituent of the epoxide is in close proximity to the ligand of the catalyst, whereas the other substituent hardly encounters any steric interaction. Epoxide opening will release the former interaction. After reduction of the radical, this results in formation of the product with the absolute configuration observed experimentally. 

There is an explicit assumption in the funnel mechanism that there is a kinetic flow down through a series of states that are progressively lower in energy. The assumption that kinetic pathways follow thermodynamic energies downhill is normally unwarranted, because there is not a formal link between kinetics and thermodynamics. (The Hammond postulate and linear free energy relationships... 

The Hammond Postulate applies only if both forward reactions are fast. Obtain energies for the transition states leading to 1 -propyl and 2-propyl radicals (propane+F end andpropane+F center). Draw an energy diagram for each hydrogen abstraction reaction (place the diagrams on the same axes). Do these diagrams indicate that use of the Hammond Postulate is justified Calculate the barrier for each reaction, and calculate the relative concentrations of 1-propyl and 2-propyl radicals that would form at 298 K if each reaction were irreversible. Use equation (2). How does this (kinetic) ratio compare to the equilibrium (thermodynamic) ratio of these radicals ... 

In this section, solvolysis reactions are described which are thought to proceed via silyl-substituted carbocations. The reader should be aware of the fact that nearly all effects which are described here are of purely kinetic origin and therefore refer to energy differences between ground states and transition states. Hence they are not strictly applicable to the intermediate silyl-substituted carbocations, although the Hammond postulate suggests a close structural resemblance between the transition state for the ionization and the formed carbocation. 

In many series of analogous reactions a second proportionality is found experimentally, namely, between the free energy change (AGr a thermodynamic quantity) and the free energy of activation (AG, a kinetic quantity). In a series of analogous reactions, a third parameter besides AH and AG no doubt also depends on the AG and AGr values, namely, the structure of the transition state. This relationship is generally assumed or postulated, and only in a few cases has it been supported by calculations (albeit usually only in the form of the so-called transition structures they are likely to resemble the structures of the transition state, however). This relationship is therefore not stated as a law or a principle but as a postulate, the so-called Hammond postulate. 

For reactions under kinetic control, the Hammond postulate now states that ... 

Finally, because the addition of Br2 to cyclohexene is 27 kcal/mol - 11 kcaFmol = 16 kcal/ mol more exothermic than the substitution of Br2 on cyclohexene, can we conclude that the first reaction also takes place more rapidly Not necessarily The (fictitious) substitution reaction of Br2 on cyclohexene should be a multistep reaction and proceed via a bromonium ion formed in the first and also rate-determining reaction step. This bromo-niurn ion has been demonstrated to be the intermediate in the known addition reaction of Br2 to cyclohexene (Section 3.5.1). Thus, one would expect that the outcome of the competition of substitution vs. addition depends on whether the bromonium ion is converted— in each case in an elementary reaction—to the substitution or to the addition product. The Hammond postulate suggests that the bromonium ion undergoes the more exothermic (exergonic) reaction more rapidly. In other words, the addition reaction is expected to win not only thermodynamically but also kinetically. 

The tetrahedral intermediate is a high-energy intermediate. Therefore, independently of its charge and also independently of the detailed formation mechanism, it is formed via a late transition state. It also reacts further via an early transition state. Both properties follow from the Hammond postulate. Whether the transition state of the formation of the tetrahedral intermediate has a higher or a lower energy than the transition state of the subsequent reaction of the tetrahedral intermediate determines whether this intermediate is formed in an irreversible or in a reversible reaction, respectively. Yet, in any case, the tetrahedral intermediate is a transition state model of the rate-determining step of the vast majority of SN reactions at the carboxyl carbon. In the following sections, we will support this statement by formal kinetic analyses of the most important substitution mechanisms. 

First, it is important to note that most aromatic electrophilic substitution reactions are under kinetic, and not thermodynamic, control. This is because most of the reactions are irreversible, and the remainder are usually stopped before equilibrium is reached. In a kinetically controlled reaction, the distribution of products (or product spread), i.e. the ratio of the various products formed, is determined not by the thermodynamic stabilities of the products, but by the activation energy barrier that controls the rate determining step. In a two-step reaction, it is a reasonable assumption that the transition state of the rate determining step is close in energy to that of the intermediate, which in this case is the Wheland intermediate and so by invoking the Hammond postulate, one may assume that they have similar geometries. 

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