Hammond postulate features

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. 

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. 

Like an 8 1 reaction, the first step in the electrophilic addition reaction is the rate determining step. According to the Hammond postulate, the transition state for this step resembles the carbocation intermediate. Structural features that stabilize the carbocation intermediate also stabilize the transition state and accelerate the electrophilic addition reaction. 

The equation of Bell-Evans—Polanyi (see eq. (7.18)) implies that exothermic reactions will have lower barriers than the endothermic ones. The view that a transition state has structural and energy features that are intermediate between those of starting materials and products is due to George Hammond [9], who resurrected the view implied by those three authors in 1955. The aim of Hanunond was that of mechanistic interpretations, postulating that the changes in structure of the TS are affected by the manner in which the substituents affect the energies of intermediates on alternate pathways from reactants to products. Since then this assumption has been known as the Hammond postulate. 

The transition state is closer in energy to the carbocation and, according to Hammond s postulate, more closely resembles it than the alkyloxonium ion. Thus, structural features that stabilize carbocations stabilize transition states leading to them. It follows. 

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