Endothermic reaction transition state

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. 

The ortho-para- versus meta-directing and activating versus deactivating effects of substituents can also be described in terms of PMO theory. The discussion can focus either on the structure of the cr complex or on the aromatic substrate. According to the Hammond postulate, it would be most appropriate to focus on the intermediate in the case of reactions which are relatively endothermic. The transition state should then resemble the a complex in reactions when the initial step has an appreciable activation energy. For more highly reactive electrophiles the transition state may be more reactant-like, in which case consideration of the reactant and application of frontier orbital theory would be more appropriate. Let us examine the effect of substituents from both perspectives. 

Because the first propagation step of the chlorination reaction is very slightly endothermic, the transition state is only moderately past the center of the reaction coordinate axis. In contrast, the transition state for the more endothermic bromination reaction is farther along the reaction coordinate axis (Figure 4.19b). What does this information tell us about the structure of the transition states for the two reactions We recall that the Hammond postulate states that the structures of transition states most closely resemble those species that are most similar in energy (Section 3.11). Thus, the structure of the transition state for an exothermic process is more reactant-like, or early, and the structure of the transition state for an endothermic process is more product-like or late. ... 

Plot energy (vertical axis) vs. carbon-carbon distance (horizontal axis). Is this reaction endothermic or exothermic Is there a point on the diagram that can be identified as a transition state If so, what is the barrier for this 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. 

Holroyd (1977) finds that generally the attachment reactions are very fast (fej - 1012-1013 M 1s 1), are relatively insensitive to temperature, and increase with electron mobility. The detachment reactions are sensitive to temperature and the nature of the liquid. Fitted to the Arrhenius equation, these reactions show very large preexponential factors, which allow the endothermic detachment reactions to occur despite high activation energy. Interpreted in terms of the transition state theory and taking the collision frequency as 1013 s 1- these preexponential factors give activation entropies 100 to 200 J/(mole.K), depending on the solute and the solvent. 

The activation energy, Ea, is the additional energy that must be absorbed by the reactants in their ground states to allow them to reach the transition state. Consider the following three endothermic reactions they are identical except that they occur in different phases ... 

As can be seen from the data presented, the high energies of complex formation decrease sharply the endothermicity of the retro-Wittig type decomposition and, moreover, fundamentally change the reaction mechanism. As has been shown for betaines (")X-E14Me2-CH2-E15( + )Me3 (X = S, Se E14 = Si, Ge E14 = P, As), the reaction occurs as bimolecular nucleophilic substitution at the E14 atom. For silicon betaines, the transition states TS-b-pyr with pentacoordinate silicon and nearby them no deep local minima corresponding to the C-b complexes can be localized in the reaction coordinate. 

In accordance with the increased charge separation in the transition state and products, the gas-phase EA of 22.6 kcal mol-1 for the reaction reduces to just 4.4 kcal mol-1 with inclusion of semiempirical solvation energies in water while the overall reaction, which is very endothermic in the gas phase becomes exothermic by 5.1 kcal mol-1 with solvation.179 The calculated EA is lower than the experimental values for substitution by A-methylaniline in methanol which fall in the range of 6-15 kcalmol-1 (Table 5).42,43 However, in aqueous solution these barriers would be lower than in methanol. 

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. 

In highly endothermic reaction, the transition-state (TS,) resembles product B. Case II. Highly Exothermic Reaction... 

When the protonated amine-arm does not participate in the reaction (TS1-I ), the reaction is very endothermic. However, the activation barrier decreases to 2.6 kcal/mol and the endothermicity of the reaction becomes very small (only 0.2 kcal/mol), if the protonated amine-arm participates in the reaction (TS1-II). In this transition state the O-H distance is 1.709A, typical of a hydrogen bond. The origin of the acceleration was explained in terms of the enhancement of polarization of CO2 induced by the protonated amine arm, which increases the positive charge in the carbon atom. This polarization favors the electrostatic interaction between hydride and C02, and since this polarization increases the contribution of the C p orbital in the it orbital of C02, it favors also the charge-transfer from the H ligand to the 71 orbital of C02. In other paths starting with direct interaction between C02 and Ru the effect of the protonated amine was much smaller. 

Fig. 4.5 Schematic projection of the energetics of a reaction. The diagram shows the Born-Oppenheimer energy surface mapped onto the reaction coordinate. The barrier height AE has its zero at the bottom of the reactant well. One of the 3n — 6 vibrational modes orthogonal to the reaction coordinate is shown in the transition state. H and D zero point vibrational levels are shown schematically in the reactant, product, and transition states. The reaction as diagrammed is slightly endothermic, AE > 0. The semiclassical reaction path follows the dash-dot arrows. Alternatively part of the reaction may proceed by tunneling through the barrier from reactants to products with a certain probability as shown with the gray arrow...

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