Lowering the activation energy

There is, however, no direct evidence for the formation of Cl", and it is much more likely that the complex is the active electrophile. The substrate selectivity under catalyzed conditions ( t j = 160fcbenz) is lower than in uncatalyzed chlorinations, as would be expected for a more reactive electrophile. The effect of the Lewis acid is to weaken the Cl—Cl bond, which lowers the activation energy for o-complex formation. 

FIGURE 16.1 Enzymes catalyze reactions by lowering the activation energy. Here the free energy of activation for (a) the uncatalyzed reaction, AGu, is larger than that for (b) the enzyme-catalyzed reaction, AG,". 

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

In thermodynamic terms, a spontaneous reaction AG < 0) may proceed only slowly without enzymes because of a large activation energy (EJ. Adding enzymes to the system does not change the free energy of either the substrates or products (and thus does not alter the AG of the reaction) but it does lower the activation energy and increase the rate of the reaction. 

An enzyme—usually a large protein—is a substance that acts as a catalyst for a biological reaction. Like all catalysts, an enzyme doesn t affect the equilibrium constant of a reaction and can t bring about a chemical change that is otherwise unfavorable. An enzyme acts only to lower the activation energy for a reaction,... 

Catalysis and activation energy. By changing the path by which a reaction occurs, a catalyst can lower the activation energy that is required and so speed up a reaction... 

Enzymes, like all other catalysts, lower the activation energy for reaction. They can be enormously effective it is not uncommon for the rate constant to increase by a factor of... 

A catalyst lowers the activation energy of a reaction from 215 kj to 206 kj. By what factor would you expect the reaction-rate constant to increase at 25°C Assume that the frequency factors (A) are the same for both reactions. (Hint Use the formula In k = In A — EJRT.)... 

Catalysts increase the rate of reactions. It is found experimentally that addition of a catalyst to a system at equilibrium does not alter the equilibrium state. Hence it must be true that any catalyst has the same effect on the rates of the forward and reverse reactions. You will recall that the effect of a catalyst on reaction rates can be discussed in terms of lowering the activation energy. This lowering is effective in increasing the rate in both directions, forward and reverse. Thus, a catalyst produces no net change in the equilibrium concentrations even though the system may reach equilibrium much more rapidly than it did without the catalyst. 

A transition state is an unstable, high-energy configuration assumed by reactants in a chemical reaction on the way to making products. Enzymes can lower the activation energy required for a reaction by binding and stabilizing the transition state of the substrate. 

The presence of the zeolite cavity dramatically lowers the activation energy for the protonation of toluene. It is mainly due to screening of the charges in the transition state due to the polarizable lattice oxygen atoms. In the transition state, a positive charge develops on protonated toluene. 

Enzymes Lower the Activation Energy Barrier for a Reaction... 

By increasing the rate of attainment of equilibrium through lowering the activation energy, catalysts reduce the energy requirement of a process and... 

The electronic, rotational and translational properties of the H, D and T atoms are identical. However, by virtue of the larger mass of T compared with D and H, the vibrational energy of C-H> C-D > C-T. In the transition state, one vibrational degree of freedom is lost, which leads to differences between isotopes in activation energy. This leads in turn to an isotope-dependent difference in rate - the lower the mass of the isotope, the lower the activation energy and thus the faster the rate. The kinetic isotope effects therefore have different values depending on the isotopes being compared - (rate of H-transfer) (rate of D-transfer) = 7 1 (rate of H-transfer) (rate of T-transfer) 15 1 at 25 °C. 

The effect of a conjugating substituent in the monomer may be summarized by observing that its influence is much greater in the product radical than in the monomer. In the activated complex, which is intermediate in character between reactants and product, resonance stabilization is substantially greater than in the monomer reactant, though less than in the product radical. The substituent therefore lowers the activation energy for the process, and enhances thereby the reactivity of the monomer. 

The energy for the fission of the covalent bond in organic contaminants is normally supplied thermally using thermodynamically accessible chemical or biochemical reactions, or by the introduction of catalysts to lower the activation energy of the reactions. There has been interest, however, in using electrical energy in a number of forms to carry out these reactions. A selection of processes for the destruction of contaminant is noted with some illustrative examples. 

The third characteristic refers to the fact that a catalyst does not influence the value of the equilibrium constant because it lowers the activation energy of the forward and backward reactions by the same amount and therefore changes the rates of the forward and backward reactions by the same amount. A catalyst only accelerates the attaining of equilibrium it does not exert any influence whatsoever on the quantitative yield of the products. 

The collective set of energetic advantages that result from productive substrate binding to the enzyme active site is known as the approximation effect. In concert, these effects can provide an important means of at least partially lowering the activation energy for transition state formation. 

As recently as 1965, Thoma and Stewart predicted that alterations in reaction rates [in the presence of the cycloamyloses] should be anticipated whose magnitude and sign will fluctuate with the reaction type, and added that at the present juncture, it is impossible to sort out confidently. . . which factors may contribute importantly to raising or lowering the activation energy of the reaction. In the short interval between 1965 and the present, a wide variety of cycloamylose-induced rate accelerations and decelerations have, indeed, been revealed. More importantly, rate alterations imposed by the cycloamyloses can now be explained with substantially more confidence. The reactions of derivatives of carboxylic acids and organo-phosphorus compounds with the cycloamyloses, for example, proceed to form covalent intermediates. Other types of reactions appear to be influenced by the dielectric properties of the microscopic cycloamylose cavity. Still other reactions may be affected by the geometrical requirements of the inclusion process. 

The catalytic specificity of the cycloamyloses has led to their utilization as a model for understanding enzymatic catalysis. It is the authors expectation that the cycloamyloses will continue to serve as an enzyme model as well as a model for designing more efficient catalytic systems. Toward this end, it would seem profitable to pursue the idea that the cycloamyloses may lower the activation energy of a chemical reaction by inducing strain into the substrate. 

Later on12, Koshland proposed the induced fit model of the active site action that considers that during the formation of the enzyme-substrate complex, the enzyme can change its conformation so as to wrap the substrate like it happens when a hand (substrate) fits in a globe (enzyme). This flexing puts the active site and bonds in the substrate under strain, which weakens the bonds and helps to lower the activation energy for the catalyzed reaction. 

Since catalysis is simply making a reaction go faster, it follows that the activation energy of a catalyzed (faster) reaction is lower than the activation energy of an uncatalyzed reaction. It s possible to say, then, that enzymes work by lowering the activation energy of the reaction they catalyze. This is the same as saying that enzymes work because they work. The question is how they lower the activation energy. 

During the last two decades, more studies have been conducted to explore the catalytic effects of different redox mediators on the bio-transformation processes. Redox mediators, also referred to as electron shuttles, have been shown to play an important role not only as final electron acceptor for many recalcitrant organic compounds, but also facilitating electron transfer from an electron donor to an electron acceptor, for example, azo dyes [8, 11, 12], Redox mediators accelerate reactions by lowering the activation energy of the total reaction, and are organic molecules that can reversibly be oxidized and reduced, thereby conferring the capacity to serve as an electron carrier in multiple redox reactions. 

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