Exergonic reaction, Hammond

Markovnikov s rule can be restated by saying that, in the addition of HX to an aikene, the more stable carbocation intermediate is formed. This result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an aikene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formation. 

Hammond postulate (Section 6.10) A postulate stating that we can get a picture of what a given transition state looks like by looking at the structure of the nearest stable species. Exergonic reactions have transition states that resemble reactant endergonic reactions have transition states that resemble product. 

Ethynylestradiol, structure and function of, 1083 von Euler, Ulf Svante, 1068 Exergonic reaction, 153 Hammond postulate and,... 

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. 

When AG° < A., the predictions of such an equation are in agreement with the deductions of the Hammond postulate. Indeed, for exergonic reactions, tends to zero when AG° —> — A, which corresponds to an activationless reaction. Conversely, for endergonic reactions, tends to unity when AG° —>X, which features a barrierless reaction. 

The curve acts as a reference line, but it also shows that the slope, a, in Equation (1.30) has a value near one for AG° near zero, and approaches zero as AG ° becomes a large negative number. Such behavior is an expected consequence of the Bell-Evans-Polanyi-Leffler-Hammond principle.It corresponds to a late transition state for the more difficult reactions and a progressively earlier transition state for more exergonic reactions. However, the curve appears to level off at an n value near 8, which corresponds to a second-order rate constant of about 1 s at 25 °C, far from the diffusion-controlled limit. 

According to the Hammond postulate, the transition state is more similar in structure to the species to which it is more similar in energy. In the case of an exergonic reaction, the transition state (I) is more similar in energy to the reactant than to the product (Figure 4.3, curve I). Therefore, the structure of the transition state will more... 

The enhanced selectivity of alkane bromination over chlorination can be explained by turning once again to the Hammond postulate. In comparing the abstractions of an alkane hydrogen by Cl- and Br- radicals, reaction with Br- is less exergonic. As a result, the transition state for bromination resembles the alkyl radical more closely than does the transition state for chlorination, and the stability of that radical is therefore more important for bromination than for chlorination. 

How does this order of the rates v > vtom come about As high-energy species, radical intermediates react exergonically with most reaction partners. According to the Hammond postulate, they do this very rapidly. Radicals actually often react with the first reaction partner they encounter. Their average lifetime is therefore very short. The probability of a termination step in which two such short-lived radicals meet is consequently low. 

The reaction is exergonic because it is spontaneous. According to the Hammond postulate, the transition state should resemble the isobutyl cation. 

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