Activation energy alternative mechanisms

This particular reaction has quite a high activation energy, which suggests that a nonconcerted mechanism might be involved. An alternative explanation for the observed stereochemistry has been advanced. 

A catalyst speeds up a reaction by providing an alternative pathway—a different reaction mechanism—between reactants and products. This new pathway has a lower activation energy than the original pathway (Fig. 13.34). At the same temperature, a greater fraction of reactant molecules can cross the lower barrier of the catalyzed path and turn into products than when no catalyst is present. Although the reaction takes place more quickly, a catalyst has no effect on the equilibrium composition. Both forward and reverse reactions are accelerated on the catalyzed path, leaving the equilibrium constant unchanged. 

Other theoretical approaches to the problem of predicting reaction activation energies exist (21-23). For our purposes, however, it is sufficient to recognize that ball-park estimates are the best one can expect. Such estimates are often adequate for purposes of differentiating between alternative mechanisms on the basis of a comparison of predicted and actual activation energies. 

Catalysts are substances which increase the reaction rate when added to a system by providing an alternative mechanism with a lower activation energy. Although a catalyst may enter into a reaction, it does not appear in the overall balanced equation it is a reactant in one step and a product in another. It is not consumed during a reaction. 

Homogeneous catalysts are catalysts that are in the same phase or state of matter as the reactants. They provide an alternate reaction pathway (mechanism) with a lower activation energy. 

In conclusion, under the hypothesis that the reaction has no barrier in excess of its endoergicity, Att/°j(0) = 0, the enthalpy of reaction 3.10 is given by the Arrhenius activation energy for the forward reaction minus a heat capacity term. This term can be estimated by using statistical mechanics, provided that a structure for the activated complex is available. It is often found that T A Cjj > is fairly small, ca. — 1 kJmol-1 at 298.15 K [60], and therefore, the alternative assumption of a,i Ar//" is commonly accepted if T is not too high. Finally, note that either 3,1 Ar//." or Atf/°j(0) = 0 are not equivalent (see equation 3.22) to another current (but probably less reliable) postulate, Ea- = 0. 

Kinetic studies on the thermal racemization of the sulfonium salt 211 revealed that the process requires much lower activation energies when compared with thermal racemization of sulfoxides. Comparison of the relative rates for racemization of sulfonium salts 211, 212, and 213 was taken (151) as evidence that racemization of 211 is the result of pyramidal inversion, not of an alternative dissociation mechanism. On the other hand, Brower and Wu (249) concluded that the volume of activation for the racemization of sulfonium ion 211, AF = +6.4 ml/mol, is more compatible with a transition state in which partial dissociation has occurred. 

Enzymes bind to the reactants and provide an alternative mechanism of lower activation energy for the reaction to proceed. Hence, enzymes speed up biochemical reactions that are otherwise too sluggish to advance. 

A catalyst lowers the activation energy of a reaction by providing an alternative mechanism. A catalyst also increases the rate of the reverse reaction. What effect does a catalyst have on AH of a reaction ... 

A catalyst can increase the rate of this reaction by providing an alternative mechanism with lower activation energy. A possible mechanism for the catalyzed reaction is shown below. 

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