Endothermic reaction Hammond postulate

Enantiotopic (NMR), 455 Endergonic. 153 Endergonic reaction, Hammond postulate and, 197-198 Endo stereochemistry, Diels-Alder reaction and, 495 Endothermic, 154 -ene, alkene name ending, 176 Energy difference, equilibrium position and, 122... 

Many programs allow the user to input a weighting factor (i.e., to give a structure that is 70% of the way from reactants to products). This allows the application of the Hammond postulate that the transition structure will look more like the reactants for an exothermic reaction and more like the products for an endothermic reaction. 

Obtain the energies of propene, dimethylborane, and 1-propyldimethyl borane, and calculate AH n for dimethylborane addition. Is this reaction exothermic or endothermic Use this result and the Hammond Postulate to predict whether the transition state will be more reactant like or more product like . Compare the geometry of the transition state to that of the reactants and products. Does the Hammond Postulate correctly anticipate the structure of the transition state Explain. 

Therefore, for a thermoneutral reaction, the intersection point between the bond order profiles for the bond making and the bond breaking processes coincides with the TS the reactivity of the two reacting atoms also equalizes at the TS, as can be seen from the intersection of their IT profiles. These intersection points of the associated bond orders and condensed FFs he toward the left (right) of the TS for an endothermic (exothermic) reaction, in agreement with the Hammond postulate. 

Figure 2.9 In an exothermic reaction (a), the Hammond postulate assumes that the transition state should resemble the starting material, whereas in an endothermic process (b), it should resemble the product.

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. 

According to the Hammond postulate, the transition state for abstraction by chlorine resembles the reactant because this is an exothermic reaction. In contrast, the transition state for abstraction by bromine resembles the product because it is an endothermic reaction (see Figure 21.2). In the case of abstraction by chlorine the carbon-hydrogen bond is only slightly broken in the transition state, and the stability... 

One of the focal points of mechanistic interest has been into the nature of the transition state. A postulate which bears heavily on this topic and which is now most commonly referred to as the Hammond postulate (Hammond, 1955) has become central in the study of transition state structure. Hammond s postulate may be stated as follows the interconversion of two states of similar energy on a reaction pathway will involve only a small amount of structural reorganization. A precise interpretation of this postulate leads only to the limited conclusion that transition states of highly exothermic reactions are similar in structure and energy to reactants, while for strongly endothermic reactions transition states resemble products. 

Leffler (1953, 1963a) has proposed a more general relationship (hence referred to as the Leffler Hammond postulate) which represents an extension of the Hammond postulate since it treats the whole spectrum of reaction types. Thus the transition state is viewed as changing gradually from reactant-like in highly exothermic reactions, to intermediate in character for thermoneutral reactions, to product-like for endothermic reactions. In addition to this proposal, which relates the transition state structure to that of the products and reactants, a free energy relationship (1) which relates changes in... 

In contrast, the transition state occurs "late" along the FNSi->NSiF reaction coordinate r(F "Si) in the transition state is elongated to 2.399 A fix)m 1.603 A in FSiN and r(N—F) is relatively short (N F = 1.480 A, 1.346 A in FNSi). This again is in accord with the Hammond postulate as the FNSi->NSiF rearrangement is endothermic. 

Suppose there are two possible products of an endothermic reaction, but one is more stable (lower in energy) than the other (Figure 7.18). According to the Hammond postulate, the transition state to form the more stable product is lower in energy, so this reaction should occur faster. 

In the Sj-jl reaction, the rate-determining step is the formation of the carbocation, an endothermic reaction. According to the Hammond postulate, the stability of the carbocation detennines the rate of its formation. 

Hammond postulate In an endothermic reaction, the more stable product is formed faster. In an exothermic reaction, this is not necessarily true (7.15). 

According to the Hammond postulate, the transition state of an endothermic reaction resembles the produets, so the energy of aetivation to form the more stable 2° radical is lower and it is formed faster, as shown in the energy diagram in Figure 15.5. Because the 2° radical [(CH3)2CH-] is converted to 2-bromopropane [(CH3)2CHBr] in the seeond propagation step, this 2° alkyl halide is the major product of bromination. 

Because of the principle of microscopic reversibility it is appropriate to consider frontier orbital analysis of the reaction in either direction. The Hammond postulate dictates that the more exothermic the reaction the more the transition state will reflect the starting geometry, and frontier orbital analysis of reactant orbitals is expected to be a better predictor of relative transition state orbital interactions than for an endothermic or a less exothermic process. Conversely, frontier orbital analysis of product orbitals in exothermic reactions would be a poorer predictor of transition state energy. 

A transition state contains partially broken and formed bonds, but the degree of this varies and depends on how close to the starting materials or products the point of transition is. The Hammond postulate, illustrated by Figure 2.22, states that the transition states of exothermic reactions resemble starting materials in energy and geometry whereas transition states of endothermic reactions resemble products. Transition states... 

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