Lowering of activation energy

First, the rate of heat production is again related to the sum of the rates of depositional and burning processes, and if the predominant factor affecting the overall rate is temperature, then it does not seem likely that the specific effect of water vapor on the oxidation reported here is chemical catalysis, since a lowering of activation energy for either process would result in an increase in the overall rate relative to dry oxidation. 

In our own studies of the isomerisation of the 75% e/s-polyacetylene produced by the Durham route 347> we have been unable to detect any effect on the isomerisation process of illumination with modest levels of light. We do find that the isomerisation is markedly affected by even trace amounts of oxygen, which lead to a change in the apparent order of reaction and a marked lowering of activation energy, somewhat similar to the observations of Chien and Yang 447) for conventional polymer. However, polymers prepared by the Durham route are very different from Shirakawa polymer and it would be unwise to extrapolate our results to Shirakawa materials. 

The activating cation presumably acts by helping the protein to maintain a productive conformation, and examples are provided by structural studies of enzymes that exhibit this property. Pauling suggested that complementarity between the stracture of the enzyme active site and the transition state of the reaction is responsible for the lowering of activation energies of enzyme-catalyzed reactions. Cation activation may aid this by ensuring that the active site has the correct... 

A) Schemactic showing of the lowering of activation energy in the favored cross event... 

It is also important to note that the most active catalyst of a group is only that with the lowest activation energy when measurements are made below Ti above this temperature, the opposite is tme, thus negating the commonly held (and taught) correlation of activity with a lowering of activation energy, which is (as the Arrhenius equation shows) not the sole determining factor. 

In primary and secondary nitroparaffins, hydrogen transfer reactions take place involving a 1,3-H shift [11,130]. This produces acf-forms of these compounds, which are also called nitronic acids. According to Manehs et al. [ 11 ], the existence of the aci-form of NM represents a more probable cause of lowering of activation energy of thermal decomposition of hquid NM as compared with the gas-phase thermolysis (i.e. in relation to the theories of bi-... 

It may be noted that enhancement of a chemical reaction by the molecular anvil action is due to elevation of the starting level of the reactant system but not the lowering of activation energy at the top of the reaction intermediate state as shown in Figure 6. It also may be noted that molecular anvil action is useful in discriminating a substrate smaller than the size of the cleft of the enzyme. 

The depletion of ozone in the stratosphere by Cl atoms provides an example of the lowering of activation energy by a catalyst. Ozone is normally present in the stratosphere and provides protection against biologically destructive, short-wavelength ultraviolet radiation from the sun. Some recent ozone depletion in the stratosphere is believed to result from the Cl-catalyzed decomposition of O3. Cl atoms in the stratosphere originate from the decomposition of chlorofiuorocarbons (CFCs), which are compounds manufactured as refrigerants, aerosol propellants, and so forth. These Cl atoms react with ozone to form CIO and O2, and the CIO reacts with O atoms (normally in the stratosphere) to produce Cl and O2. 

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. 

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

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,... 

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. 

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