Activation energy Hammond postulate

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 ... 

An explanation of the relationship between reaction rate and intermediate stability was first advanced in 1955. Known as the Hammond postulate, the argument goes like this transition states represent energy maxima. They are high-energy activated complexes that occur transiently during the course of a reaction and immediately go on to a more stable species. Although we can t... 

Radical additions are typically highly exothermic and activation energies are small for carbon30-31 and oxygen centered32,33 radicals of the types most often encountered in radical polymerization, Thus, according to the Hammond postulate, these reactions are expected to have early reactant-like transition states in which there is little localization of the free spin on C(J. However, for steric factors to be important at all, there must be significant bond deformation and movement towards. sp hybridization at Cn. 

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and fra/w-stilbene. When a sensitizer triplet had insufficient excitation energy to promote fims-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by... 

The position of the transition state along the reaction coordinates in relation to the well-known Hammond postulate [53] will now be considered. If the activation energy, AG+, of a reaction is only small the TS looks like the GS (it is depicted as a reactant-like transition state ). Consequently, the polarity is only slightly modified between the GS and TS during the course of the reaction and only weak specific micro-wave effects can be foreseen under these conditions. 

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. 

Another method for evaluating carbocation stability involves the measurement of solvolysis rates (14,45). Typically, the transition state of the rate-determining step in SN1 reactions is assumed to closely resemble the intermediate ion pair, on the basis of the Hammond postulate (46). Thus, the free energy of activation for this reaction, AG, reflects the relative thermodynamic stabilities of the intermediate carbocations. 

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. 

Equation 8.18 and find relation 8.21 between activation free energy, AG, and standard free energy change, AG°. This equation is equivalent to the Bronsted catalysis law as was shown in Section 3.3 (Equations 3.49 and 3.53), and we may conclude that an interpretation of a as a measure of the position of the transition state is consistent with the Hammond postulate. 

The Hammond postulate states that in endergic reactions, features which stabilize and thus lower the energy of a product lower the energy of the transition state leading to diat product. This is shown in Figure 5.12. If product 2 (P2) is lower in energy than product 1 (Pi), then transition state 2 ( 2) will be lower than transition state 1 ( 1). It will also be earlier. As a consequence, P2 will have a lower activation barrier and be formed faster than Pi. A simplified restatement of the Hammond postulate is that more stable products are formed faster. It must be remembered that this analysis is for endothermic reactions and assumes that the reactants have the same or similar energies. 

However, the Hammond postulate also holds for exergic reactions where the transition state is early and the activated complex more resembles the reactant. For such reactions changes in the energy of the reactants have the greatest effect on the energy of the transition state and thus the rate of the reaction. 

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. 

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-... 

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... 

Activation Energy and theTemperature Dependence of Rates 147 4-10 Transition States 148 4-11 Rates of Multistep Reactions 149 4-12 Temperature Dependence of Halogenation 150 4-13 Selectivity in Halogenation 151 4-14 The Hammond Postulate 157 4-15 Radical Inhibitors 161 4-16 Reactive Intermediates 162... 

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