Exothermic reaction Hammond postulate

The energy difference of the HOMO of the nucleophile and the LUMO of the carbonyl compared to the LUMO of the electrophile and the HOMO n-orbital of the carbonyl will be a factor in establishing whether the reaction is electrophile or nucleophile driven. In the case of a reaction catalysed by acid the reaction is considered to be electrophile driven and attack of the nucleophile occurs to the protonated carbonyl. A carbonyl, coordinated with a Lewis acid or cation (e.g. H, Li , Na", AIH3 [5-7]) or uncoordinated, can be attacked by a neutral or anionic nucleophile. In the former case the nucleophile must bear an acidic hydrogen to allow for the formation of a neutral product [4]. Since reduction of aldehydes and ketones is exothermic the Hammond postulate dictates that the transition state is closer in energy and structure to the reactants than to the products. 

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. 

The Hammond Postulate implies that the transition stah of a fast exothermic reaction resembles the reactants (se( reaction energy diagram at left). This means that it wil be hard to predict the selectivity of competing exothermi( reactions both barriers may be small and similar even i one reaction is more exothermic than the other. 

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. 

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. 

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. 

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

The assumption of a kinetically controlled course of the reaction, however, readily explains the observed results, even though the transition structures have not, as yet, been calculated. Because epoxide opening is exothermic, 39 can be regarded as a simple model for the transition structure according to the Hammond postulate. It is clear from the structure of 39 that the left-hand ethoxy substituent of the epoxide is in close proximity to the ligand of the catalyst, whereas the other substituent hardly encounters any steric interaction. Epoxide opening will release the former interaction. After reduction of the radical, this results in formation of the product with the absolute configuration observed experimentally. 

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.

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. 

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... 

As each reaction in the series becomes more exothermic, so the transition state increasingly resembles the reactants in accordance with the Leffle Hammond postulate. 

This final point signifies that the value of a in the rate-equilibrium relationship (2) is not constant but decreases as the reaction becomes increasingly exothermic. It should be noted however that since the Bell- Evans-Polanyi model and the Hammond postulate are couched in energy terms the assumption that free energy changes (AG°) are proportional to energy changes (A °) is inherent in eqns (1) and (2). 

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

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