Hammond’s postulate

Hammond s postulate (Section 4 8) Pnnciple used to deduce the approximate structure of a transition state If two states such as a transition state and an unstable intermediate de rived from it are similar in energy they are believed to be similar in structure... 

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... 

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. ... 

Fig. 4.3. Some typical potential energy diagrams that illustrate the application of Hammond s postulate.

The significance of the concept incorporated in Hammond s postulate is that, in appropriate cases, it permits discussion of transition-state structure in terms of the reactants, inteimediates, or products in a multistep reaction sequence. The postulate indicates that the cases in which such comparison is appropriate are those in which the transition state is close in energy to the reactant, intermediate, or product. Chemists sometimes speak of early or late transition states. An early transition state is reactant-like whereas a late transition state is product-like. 

Application of Hammond s postulate indicates that the transition state should resemble the product of the first step, the carbocation intermediate. Ionization is facilitated by factors that either lower the energy of the carbocation or raise the energy of the reactant. The rate of ionization depends primarily on how reactant structure and solvent ionizing power affect these energies. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

Hammett correlation, 225-228 Hammond s postulate, 242 Heat capacity of activation, 160-161 Hydrated electrons, 266-267 Hydroxyl radicals, 266-268... 

The initial quantum yields for cis- to tam-stilbene isomerization (O0 T) and for trans to cis isomerization (4>T-.C) are consistent with Hammond s postulate that isomerization takes place from a common state, most likely the twisted or phantom triplet state ... 

It is known that in the vast majority of cases the activation energy E,. of the reverse reaction is very small or even negligible. Using Hammond s postulate [3], it is possible to assume that in the case of endothermic fragmentation the transition state will be much closer to the products than to the initial particle (Fig. 5.14). Thus, the stability of the products influences significantly the efficiency of fragmentation. It is important to consider stability of both products a neutral and a daughter ion. 

With the help of Hammond s postulate the transition state can be assumed to be reasonably approximated by the structure of either the reactant, intermediate or product. 

According to Hammond s postulate in highly exothermic reaction, the transition state (TS2) resembles reactant A. 

Ab initio calculations on the equilibrium between (14a) and (14b) are carried out with the 3-21G basis set. Figure 4 shows a plot of the calculated activation energy E vs. the reaction energy AE for the cyclization reactions. The plot is linear and provides a striking confirmation of Hammond s postulate. The cyclic structures (14b) are found to be planar, while the terminal =NH group in (14a) is ca. 50° out of the plane formed by the other atoms in (14a) <90CC882>. The gas phase basicity and acidity of 1,2,3-triazole have been calculated by ab initio methods (6-31G //6-31G) and compared with experimental values <89MI 401-01 >... 

Ab initio calculations on the equilibrium between (102) and (103) are carried out with the 3-21G basis set. The plot of the calculated activation energy Ea vs. the reaction energy AEr for the cyclization reactions is linear and that provides a striking confirmation of Hammond s postulate. 

According to Figure 1.20, analogous propagation steps possess the same heat of reaction, independent of the degree of chlorination. With the help of Hammond s postulate, one concludes from this that the associated free activation energies should also be independent of the degree of chlorination. This means that the monochlorination of methane and each of the sub-... 

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