Rate Enhancement and Activation Energy

Question Some biochemicals are stable when in pure form on a shelf and yet in the presence of an enzyme break down rapidly. Why  

There is an important distinction to be made between thermodynamic stability (expressed in terms of the equilibrium constant of the reaction) and the kinrrc stability of a substance the latter merely refers to how fast the action proceeds, the former to the final position of the reaction in terms of the relat amounts of substrate and product at equilibrium. (See Example 8.18.) Enzymes affect the kirn. liability of a substance. 

Most reduced organic molecules, such as glucose, are thermodynamically unstable in our oxidizing atmosphere. 

oxidation is very exergonic (heat-producing), and the reaction is favored by the large —ACT0 (Chap. 10) of the reaction. But we are all aware that glucose is stable on the shelf. Thus, it is thermodynamically unstable but kinetically stable. 

The distinction between kinetic and thermodynamic stability is important and is explained by the concept of the free energy of activation necessary to convert the substrate to its transition state. In order for the substrate to form products, its internal free energy must exceed a certain value i.e., it must surmount an energy barrier. The energy barrier is that of the free energy of the transition state, AG. The transition-state theory of reaction rates introduced by H. Eyring relates the rate of the reaction to the magnitude of AG.  

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Rate constants and activation energies for cyclization of substituted hexenyl radicals 76 have been measured (Scheme 14) [49]. The highest rates (275- and 415-fold acceleration for 76b and 76d, respectively) can mostly be attributed to enhanced interaction of the radical SOMO and the double bond LUMO. The activation energies show however that the transition state for 76d has a small extra stabilization compared to that for 76b. 

In this chapter we have seen that enzymatic catalysis is initiated by the reversible interactions of a substrate molecule with the active site of the enzyme to form a non-covalent binary complex. The chemical transformation of the substrate to the product molecule occurs within the context of the enzyme active site subsequent to initial complex formation. We saw that the enormous rate enhancements for enzyme-catalyzed reactions are the result of specific mechanisms that enzymes use to achieve large reductions in the energy of activation associated with attainment of the reaction transition state structure. Stabilization of the reaction transition state in the context of the enzymatic reaction is the key contributor to both enzymatic rate enhancement and substrate specificity. We described several chemical strategies by which enzymes achieve this transition state stabilization. We also saw in this chapter that enzyme reactions are most commonly studied by following the kinetics of these reactions under steady state conditions. We defined three kinetic constants—kai KM, and kcJKM—that can be used to define the efficiency of enzymatic catalysis, and each reports on different portions of the enzymatic reaction pathway. Perturbations... 

Chen et al. [70] suggested that temperature gradients may have been responsible for the more than 90 % selectivity of the formation of acetylene from methane in a microwave heated activated carbon bed. The authors believed that the highly nonisothermal nature of the packed bed might allow reaction intermediates formed on the surface to desorb into a relatively cool gas stream where they are transformed via a different reaction pathway than in a conventional isothermal reactor. The results indicated that temperature gradients were approximately 20 K. The nonisothermal nature of this packed bed resulted in an apparent rate enhancement and altered the activation energy and pre-exponential factor [94]. Formation of hot spots was modeled by calculation and, in the case of solid materials, studied by several authors [105-108],... 

As Elumo increases, the ability of compounds to undergo reduction decreases. Therefore, the kinetic rates and activation energy increase with Elumo. In other words, the increase in kinetic rates and activation energy increases the reactivity of the molecule and enhances the possibility of oxidizing organic pollutant compounds, thus the destruction efficiency of these compounds will be higher. 

The 97 1 reactivity ratio for bromination is much larger than the 4.5 1 ratio for chlorination. We say that bromination is more selective than chlorination because the major reaction is favored by a larger amount. To explain this enhanced selectivity, we must consider the transition states and activation energies for the rate-limiting step. 

The data in Fig. 28 clearly show that intermediate values of x, which limit olefin removal and enhance secondary readsorption reactions but still permit unrestricted and rapid access of CO and H2 to reaction sites, lead to maximum C5+ selectivity. They also show that eggshell catalysts allow access to these intermediate values of x for any pellet size. The design of eggshell pellets with values of x between 0.2 and 2.0 x 10 m leads to high C5+ selectivity (Fig. 28a) and maintains catalytic rates and activation energies near their intrinsic kinetic values (Table VII). 

The unsaturation present at the end of the polyether chain acts as a chain terminator ia the polyurethane reaction and reduces some of the desired physical properties. Much work has been done ia iadustry to reduce unsaturation while continuing to use the same reactors and hoi ding down the cost. In a study (102) usiag 18-crown-6 ether with potassium hydroxide to polymerise PO, a rate enhancement of approximately 10 was found at 110°C and slightly higher at lower temperature. The activation energy for this process was found to be 65 kj/mol (mol ratio, r = 1.5 crown ether/KOH) compared to 78 kj/mol for the KOH-catalysed polymerisation of PO. It was also feasible to prepare a PPO with 10, 000 having narrow distribution at 40°C with added crown ether (r = 1.5) (103). The polymerisation rate under these conditions is about the same as that without crown ether at 80°C. 

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