Orientation activation energy

Similar difficulties arise in the nitrations of 2-chloro-4-nitroaniline and /)-nitroaniline. Consideration of the rate profiles and orientation of nitration ( 8.2.5) these compounds suggests that nitration involves the free bases. However, the concentrations of the latter are so small as to imply that if they are involved reaction between the amines and the nitronium ion must occur upon encounter that being so, the observed activation energies appear to be too high. The activation energy for the simple nitration of the free base in the case of/>-nitroaniline was calculated from the following equation ... 

The most complete discussion of the electrophilic substitution in pyrazole, which experimentally always takes place at the 4-position in both the neutral pyrazole and the cation (Section 4.04.2.1.1), is to be found in (70JCS(B)1692). The results reported in Table 2 show that for (29), (30) and (31) both tt- and total (tt cr)-electron densities predict electrophilic substitution at the 4-position, with the exception of an older publication that should be considered no further (60AJC49). More elaborate models, within the CNDO approximation, have been used by Burton and Finar (70JCS(B)1692) to study the electrophilic substitution in (29) and (31). Considering the substrate plus the properties of the attacking species (H", Cl" ), they predict the correct orientation only for perpendicular attack on a planar site. For the neutral molecule (the cation is symmetrical) the second most reactive position towards H" and Cl" is the 5-position. The activation energies (kJmoF ) relative to the 4-position are H ", C-3, 28.3 C-5, 7.13 Cr, C-3, 34.4 C-5, 16.9. 

In 6j5-hydroxy-19-iodo steroids (Figure 12-2) the orientation of the reacting centers O—CH2—I resembles the arrangement in the transition state of an S 2 displacement reaction and consequently the activation energy for the transition (3) (4) is low. It has been suggested that the conformation... 

FIGURE 13.28 Whether a reaction takes place when two species collide in the gas phase depends on their relative orientations. In the reaction between a Cl atom and an HI molecule, for example, only those collisions in which the Cl atom approaches the HI molecule from a direction that lies inside the cone indicated here lead to reaction, even though the energy of collisions in other orientations may exceed the activation energy. 

According to the collision theory of gas-phase reactions, a reaction takes place only if the reactant molecules collide with a kinetic energy of at least the activation energy, and they do so in the correct orientation. 

The value of the rate constant for a particular reaction depends on the activation energy of the reaction, the temperature of the system, and how often a collision occurs in which the atoms are in the required orientation. All these factors can be summarized in a single equation, called the Arrhenius equation k — A ... 

In conclusion, the steady-state kinetics of mannitol phosphorylation catalyzed by II can be explained within the model shown in Fig. 8 which was based upon different types of experiments. Does this mean that the mechanisms of the R. sphaeroides II " and the E. coli II are different Probably not. First of all, kinetically the two models are only different in that the 11 " model is an extreme case of the II model. The reorientation of the binding site upon phosphorylation of the enzyme is infinitely fast and complete in the former model, whereas competition between the rate of reorientation of the site and the rate of substrate binding to the site gives rise to the two pathways in the latter model. The experimental set-up may not have been adequate to detect the second pathway in case of II " . The important differences between the two models are at the level of the molecular mechanisms. In the II " model, the orientation of the binding site is directly linked to the state of phosphorylation of the enzyme, whereas in the II" model, the state of phosphorylation of the enzyme modulates the activation energy of the isomerization of the binding site between the two sides of the membrane. Steady-state kinetics by itself can never exclusively discriminate between these different models at the molecular level since a condition may be proposed where these different models show similar kinetics. The II model is based upon many different types of data discussed in this chapter and the steady-state kinetics is shown to be merely consistent with the model. Therefore, the II model is more likely to be representative for the mechanisms of E-IIs. 

Rate constants governing re-orientation of the glucose transporter, and their activation energies, determined from steady-state and pre-steady-state measurements... 

Equation 4.5 shows that the rate constant, k, is related to the activation energy, Ea, of the reaction by an inverse exponential operation. This means that the greater the activation energy, the smaller the rate constant, i.e., it is difficult to get the reactants to meet at high enough energies for the reaction to progress. The pre-exponential factor is a constant that includes information about how orientation of the reactant species to one another and the... 

The results of the experimental estimation of rate constants for all these reactions prove that larger the volume V4 of TS, lower the rate constant and higher the activation energy for reconstruction of the shape of the cage to form an appropriate orientation of polymer segments around TS. An empirical linear correlation between AEot = RT ln(/ci//cs) and the volume Vu of TS was found [8] as follows ... 

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