Pathway of a chemical reaction

Figure 2.1 Free energy diagram for the reaction pathway of a chemical reaction, and the same reaction catalyzed by an enzyme. Note the significant reduction in activation energy (the vertical distance between the reactant state and the transition state) achieved by the enzyme-catalyzed reaction.

These theories assert that the pathway of a chemical reaction accessible to a compound is controlled by its highest occupied molecular orbital (HOMO). For the thermal reaction of butadiene, which is commonly called ground-state chemistry, the HOMO is 2 and lowest unoccupied molecular orbital (LUMO) is photochemical reaction of butadiene, which is known to be excited-state chemistry, the HOMO is 1//3 (Fig. 3.5.6). 

Topography of the PES and Properties of a Reacting System 1.3.4 Pathway of a Chemical Reaction... 

The location and magnitude of barriers (i.e., energy maxima in Figure 3) on ground and excited state surface will control the specific pathway of a chemical reaction. 

Clearly then, the understanding of chemical reactions under such a variety of conditions is still in its infancy and the prediction of the course and products of a chemical reaction poses large problems. The ab initio quantum mechanical calculation of the pathway and outcome of a single chemical reaction can only be... 

What a catalyst does is change the reaction pathway to one with a lower energy however, one must remember that the rate of a chemical reaction depends on two things the rate constant, which contains energy terms (both enthalpy and entropy), and concentration terms. 

In the final analysis, basic understanding of chemistry will require successful theoretical approaches. For example, in our picture of the exact pathways involved in a chemical reaction there is no current hope that we can directly observe it in full molecular detail on the fast and microscopic scale on which it occurs. As discussed in Chapter 4, our ability to make a detailed picture of every aspect of a chemical reaction will come most readily from theories in which those aspects can be calculated, but theories whose predictions have been validated by particular incisive experiments. 

Janda, K. D. Shevlin, C. G. Lemer, R. A Oxepane Synthesis Along a Disfavored Pathway The Rerouting of a Chemical Reaction Using a Catalytic Antibody J. Am Chem. Soc 1995,117, 2659-2660. 

Recall from Section 12.12 that a catalyst increases the rate of a chemical reaction by making available a new, lower-energy pathway for conversion of reactants to products. Because the forward and reverse reactions pass through the same transition state, a catalyst lowers the activation energy for the forward and reverse reactions by exactly the same amount. As a result, the rates of the forward and reverse reactions increase by the same factor (Figure 13.14). 

The quantitative prediction of the stereochemistry of a chemical reaction by strain energies requires knowledge of the reaction mechanism, i.e., the selective intermediates and/or transition states involved, and an accurate force field for the transient species. As discussed above, these are two demanding problems and so far there are no reports of studies in this area that have used molecular mechanics for quantitative predictions at the same level of accuracy as for conformational analyses. The application of empirical force field calculations to the design of asymmetric transformations clearly is a worthy task, and some examples of studies in this area have been discussed above. On the basis of two examples we will now discuss some general aspects highlighting the limitations of the qualitative considerations emerging horn molecular mechanics calculations for the interpretation and support of assumed reaction pathways. 

Some 50 years ago, Paul Flory chose the terms step-growth and chain-growth polymerization to describe the processes by which many monomers are converted to polymer (Flory 1953). Although not perfect, the terms are still commonly used and can help us understand the major mechanisms of polymerizations. A mechanism for a reaction describes the processes and pathways by which that reaction proceeds. Mechanisms are important because they help us understand the details of a chemical reaction as well as help us predict the outcome of new reactions. 

This is the study of the speed and mechanism of a chemical reaction. A cyclic system and set of chemical pathways within cells to make and use energy. Also known as the citric acid cycle. 

The simplest example of a chemical reaction with rate-controlling step is the pathway 4.1 with very different rate coefficients of the two steps. Solving the rate equations for A, K, and P in a constant-volume batch reaction with only A present at start and without assumptions about the rate coefficients, one obtains for the rate of product formation ... 

Metal surfaces have long been appreciated for their catalytic properties the ability to alter the pathways of important chemical reactions so as to increase reaction rates and selectivity for desired products well above the rates occurring in a homogeneous phase. Catalytic metals are widely utilized in a variety of chemical processes, and have been critical for processes that have had worldwide energy and health benefits, such as the petroleum refining process for fuel production, the Haber-Bosch... 

Other forms of composition function h(C/s) are used for different types of homogenous and heterogeneous reactions. A written chemical reaction is merely a reaction pathway presentation of many elementary reaction steps that usually involve unstable intermediate species (e.g., free radicals). Hence, the rate of a chemical reaction depends on the rates of the individual steps and results in different forms. For example, the rate of many biological and enzymatically catalyzed reactions is described by an expression of the form (known as the Michaelis-Menten expression). 

The basis for the iron sulfide system as a first and archaic version of a primordial forerunner of a metabolic pathway is a chemical reaction. Under standard conditions, FeS and H2S from submarine geochemical sources provide reducing power of about —620 mV by forming pyrite FeS2. 

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