Chemical concerted reaction

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

It has also been shown that sufiBcient surface self-diflfiision can occur so that entire step edges move in a concerted maimer. Although it does not achieve atomic resolution, the low-energy electron microscopy (LEEM) technique allows for the observation of the movement of step edges in real time [H]. LEEM has also been usefiil for studies of epitaxial growth and surface modifications due to chemical reactions. 

The concentration dependence of iron corrosion in potassium chloride [7447-40-7] sodium chloride [7647-14-5] and lithium chloride [7447-44-8] solutions is shown in Figure 5 (21). In all three cases there is a maximum in corrosion rate. For NaCl this maximum is at approximately 0.5 Ai (about 3 wt %). Oxygen solubiUty decreases with increasing salt concentration, thus the lower corrosion rate at higher salt concentrations. The initial iacrease in the iron corrosion rate is related to the action of the chloride ion in concert with oxygen. The corrosion rate of iron reaches a maximum at ca 70°C. As for salt concentration, the increased rate of chemical reaction achieved with increased temperature is balanced by a decrease in oxygen solubiUty. 

Hilvert D. Antibody-Catalyzed Concerted Chemical Reactions in Biotechnol Bridging Res. AppL, Proc. U.S.-lsr. Res. Conf. Adv. Appl Biotechnol. 1991 413, Ed. Kamely D., Chakrabarty A. M. and Kornguth S. E., Pb. Kluwer Boston... 

As described in the first part of this chapter, chemical thermodynamics can be used to predict whether a reaction will proceed spontaneously. However, thermodynamics does not provide any insight into how fast this reaction will proceed. This is an important consideration since time scales for spontaneous reactions can vary from nanoseconds to years. Chemical kinetics provides information on reaction rates that thermodynamics cannot. Used in concert, thermodynamics and kinetics can provide valuable insight into the chemical reactions involved in global biogeochemical cycles. 

The chemical reactions through cyclic transition states are controlled by the symmetry of the frontier orbitals [11]. At the symmetrical (Cs) six-membered ring transition state of Diels-Alder reaction between butadiene and ethylene, the HOMO of butadiene and the LUMO of ethylene (Scheme 18) are antisymmetric with respect to the reflection in the mirror plane (Scheme 24). The symmetry allows the frontier orbitals to have the same signs of the overlap integrals between the p-or-bital components at both reaction sites. The simultaneous interactions at the both sites promotes the frontier orbital interaction more than the interaction at one site of an acyclic transition state. This is also the case with interaction between the HOMO of ethylene and the LUMO of butadiene. The Diels-Alder reactions occur through the cyclic transition states in a concerted and stereospecific manner with retention of configuration of the reactants. 

We have just discussed several common strategies that enzymes can use to stabilize the transition state of chemical reactions. These strategies are most often used in concert with one another to lead to optimal stabilization of the binary enzyme-transition state complex. What is most critical to our discussion is the fact that the structures of enzyme active sites have evolved to best stabilize the reaction transition state over other structural forms of the reactant and product molecules. That is, the active-site structure (in terms of shape and electronics) is most complementary to the structure of the substrate in its transition state, as opposed to its ground state structure. One would thus expect that enzyme active sites would bind substrate transition state species with much greater affinity than the ground state substrate molecule. This expectation is consistent with transition state theory as applied to enzymatic catalysis. 

The most exciting application of bond order indices concerns the description of chemical reactions involving the simultaneous change of several bonds. An example is the unimolecular decomposition of ethanol, which can happen at high temperature or IR multiphoton excitation of the molecule. Out of the possible dissociation channels, the lowest barrier characterizes the concerted water loss of the molecule, yielding ethene and H20 [30]. 

Reactions in which two o bonds terminate at a single atom or made or broken in a concerted chemical reaction are called cheletropic reactions ... 

A concerted reaction is a chemical reaction in which all bond breaking and bond formation occurs in a single step in which reactive intermediates are not involved. Concerted photoreactions tend to be stereospecific, occurring from the vertical excited state. 

It was Woodward and Hoffmann who first introduced organic chemists to the idea that so-called frontier orbitals (the HOMO and LUMO) often provide the key to understanding why some chemical reactions proceed easily whereas others do not. For example, the fact that the HOMO in cw-1,3-butadiene is able to interact favorably with the LUMO in ethylene, suggests that the two molecules should readily combine in a concerted manner to form cyclohexene, i.e., Diels-Alder cycloaddition. 

Concerted chemical reactions involving simultaneous bond breaking and forming, because to do so would require the force-field parameters to evolve from those of the reactant bonding to those for the product bonding as the reaction proceeds ... 

Biradicals are frequently postulated to arise as intermediates in a number of chemical reactions and unimolecular isomerizations. Sometimes there are reasonable alternative concerted mechanisms in which the intermediate (or transition-state complex) is not a biradical. Such a case of much interest37,61 involves the reactions of singlet [5] and triplet [7] methylenes with olefins. We note that the permutational symmetry does not determine whether or not a reaction is concerted rather it is determined by the shapes of the intermolecular potential surfaces.78 The lowest 1Ai methylene is expected to react by a concerted mechanism, since it correlates with the ground state of the product cyclopropane higher excited singlets need not react via a concerted mechanism. 

Hydrogenation of olefins is a good example for demonstrating the roles of the surface atoms in catalysis. The orbital symmetry rule in chemical reactions suggests that the highest occupied molecular orbital (HOMO) of one reaction partner and the lowest unoccupied molecular orbital (LUMO) of the other should meet the symmetry requirements. In this respect, a concerted addition of an H2 molecule to the double bond of an olefin, that is, a molecular addition reaction, is a forbidden process. Adsorption of olefin on transition metal surfaces undoubtedly changes the population of electrons in the HOMO (7tu) and the LUMO (re ) as shown schematically in Fig. 1. In spite of such perturbation of the electron densities of the HOMO and the... 

If the concerted four-center mechanism for formation of chloromethane and hydrogen chloride from chlorine and methane is discarded, all the remaining possibilities are stepwise reaction mechanisms. A slow stepwise reaction is dynamically analogous to the flow of sand through a succession of funnels with different stem diameters. The funnel with the smallest stem will be the most important bottleneck and, if its stem diameter is much smaller than the others, it alone will determine the flow rate. Generally, a multistep chemical reaction will have a slow rate-determining step (analogous to the funnel with the small stem) and other relatively fast steps, which may occur either before or after the slow step. 

Loss of stereochemical integrity during a chemical reaction is usually taken as good evidence that the products have been formed via an intermediate incapable of sustaining stereochemical information, but there are alternative explanations (competing concerted processes or equilibrations of stereoisomeric products under the reaction conditions) and these should be considered and excluded where possible by other experiments. 

A chemical reaction always involves bond-breaking/making processes or valence electron rearrangements, which can be characterized by the variation of VB structures. According to the resonance theory [1, 50], the evolution of a system in the elementary reaction process can be interpreted through the resonance among the correlated VB structures corresponding to reactant, product and some intermediate states. Because only symmetry-adapted VB structures can effectively resonate, all VB structures involved in the description of a reaction will thus retain the symmetry shared by both reactant and product states in the elementary process. Therefore, we postulate that the VB structures of the reactant and the product states for concerted reactions should preserve symmetry-adaptation, called the VB structure symmetry-adaptation (VBSSA) rule. 

One conceivable extrapolation of this definition of concertedness is that every elementary chemical reaction is concerted. Every chemical process may be at least conceptually broken up into a sequence of one, two, or more elementary steps or concerted subprocesses. Therefore a reaction may be termed one-step, two-step, or multi-step, and the term "concerted" becomes a redundancy simply equivalent to "one-step . The frequent use of "concerted and "one-step as interchangeable adjectives is one consequence of this extrapolation. 

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