Energy surface addition-elimination

Addition-Elimination Energy Surface Drawing Energy Diagrams the ApKa Rule Approaches to Addition-Elimination Mechanisms Addition-Elimination Flowchart... 

Addition and elimination reactions, being complementary processes, occur on essentially the same energy surface. Conditions that lower the alkene comer favor elimination, whereas conditions that raise it or lower the addition product comer favor addition. A top view of the general addition/elimination energy surface is shown in Figure 4.38. 

We can modify our familiar addition and elimination surfaces to give us a combined simplified addition-elimination energy surface (Fig. 4.46). Although this system is further complicated by additional proton transfer reactions, we can get an overview of the problem space with this simplified surface as a map. The reactants are in the upper left comer. 

Figure 4.46 Top view of the simplified combined energy surface for addition-elimination.

Figure A.4 shows the usefulness of the reaction cube as a data structure. Additions to carbonyls often occur between different charge types, and frequently three-dimensional energy surfaces are used to clarify the various equilibria. We have seen two faces of this cube before as individual energy surfaces. The bottom faee of the cube is Figure 7.16, polarized multiple bond addition/elimination mechanisms in basic media. The back face of the cube is Figure 7.17, polarized multiple bond addition/elimination mechanisms in acidic media.

One obvious use of the CASSCF method is in studies of energy surfaces for chemical reactions. A number of such calculations have been reported in the literature. Some of the studies in transition-metal chemistry have already been mentioned. In this context, a study of the elimination and addition reactions of methane and ethane with nickel is also worth mentioning... 

Thus, a continuum between two mechanistic extremes is probably the best way to view the mechanistic dichotomy between the cr-bond metathesis and the sequence of oxidative addition and reductive elimination. The distinction between the two classes of mechanism would then be made by the amount of M-H bonding in a transition state (more M-H bonding would indicate an oxidative addition/reductive elimination sequence) and the presence or absence of a minimum on the energy surface that would correspond to the dialkyl intermediate of the oxidative addition/reductive elimination pathway. 

ABSTRACT. The calculation and characterization of molecular potential energy surfaces for polyatomic molecules poses a daunting challenge even in the Age of Supercomputers. We have written a program, STEEP, which computes reaction paths (IRCs) for chemical reactions and characterizes the reaction valley centered on the IRC. This approach requires that only a swath of the potential surface be determined, a computationally tractable problem even for many-atom systems. We report ab initio reaction paths/valleys for two abstraction reactions the OH + H2 reaction, which is a simple, direct process and the H + HCO reaction which can proceed along two distinct pathways, a direct pathway and an addition-elimination pathway. We find that the reaction path/valley method provides many insights into the detailed dynamics of chemical reactions. 

The introduction of an >-substituent (CN, Cl, or OH) into a primary n-alkyl chloride considerably enhances the rate of 5 n2 chloride exchange in the gas phase. Reactivity trends suggest that the acceleration is due primarily to through-space solvation of the transition state, especially charge-dipole interactions. Potential-energy surfaces are discussed. In further work by the same group, the translational energy dependence of the rate constants of several gas-phase 5 n2 and carbonyl addition-elimination reactions has been measured by FT-ICR spectroscopy. The results were interpreted by RRKM calculations. 

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