Valence bond structure reactions

Here (in contrast to the approach taken in Chapter 2) we do not assume that the energy of each valence bond structure is correlated with its solvation-free energy. Instead we use the actual ground-state potential surface to calculate the ground-state free energy. To see how this is actually done let s consider as a test case an SN2 type reaction which can be written as... 

With the valence bond structures of the exercise, we can try to estimate the effect of the enzyme just in terms of the change in the activation-free energy, correlating A A g with the change in the electrostatic energy of if/2 and i/r3 upon transfer from water to the enzyme-active site. To do this we must first analyze the energetics of the reaction in solution and this is the subject of the next exercise. 

In this book the discussion has been restricted to the structure of the normal states of molecules, with little reference to the great part of chemistry dealing with the mechanisms and rates of chemical reactions. It seems probable that the concept of resonance can be applied very effectively in this field. The activated complexes which represent intermediate stages in chemical reactions are, almost without exception, unstable molecules which resonate among several valence-bond structures. Thus, according to the theory of Lewis, Olson, and Polanyi, Walden inversion occurs in the hydrolysis of an alkyl halide by the following mechanism ... 

It is wrong (but common) to see a reversible reaction written with a double-headed arrow, as A B. Such an arrow implies resonance, e.g. between the two extreme valence-bond structures of Kekule benzene. 

The high reactivity of allylic halides in SN2 reactions indicates some special stabilization of the transition state ascribable to resonance involving the adjacent tt bond. We can express this in terms of the valence-bond structures, 2a-2c, for the transition state of the reaction of iodide ion with 3-chloropropene (Section 8-7A). The extra stabilization over the corresponding transition state for the reaction of iodide with a saturated chloride (e.g., CH3CH2CH2C1 + 1 ... 

The two possible valence-bond structures of the enolate anion, 7a and 7b, show that the anion should act as an ambident nucleophile—a nucleophile with nucleophilic properties associated with both carbon and oxygen. The addition step in the aldol reaction therefore may be expected to take place in either of two ways The anion could attack as a carbon nucleophile to form a carbon-carbon bond, 8, leading ultimately to the aldol 9, or it might attack as an oxygen nucleophile to form a carbon-oxygen bond, thereby leading to the hemiacetal 10. By this reasoning, we should obtain a mixture of products 9 and 10. However, the aldol 9 is the only one of these two possible products that can be isolated ... 

The combination of neutral non-aromatic and zwitterionic aromatic contributing valence bond structures confers a distinctive chemical reactivity to quinone methides, which has attracted the interest of a tremendous number of chemist and biochemists. This chapter reviews reactions that generate quinone methides, and the results of mechanistic studies of the breakdown of quinone methides in nucleophilic substitution reactions. The following pathways for the formation of quinone methides are discussed (a) photochemical reactions (b) thermal heterolytic bond... 

R. D. Harcourt, in Valence Bond Theory, D. L. Cooper, Ed., Elsevier, Amsterdam, The Netherlands, 2002, pp. 349-378. Valence Bond Structures for Some Molecules with Four Singly-Occupied Active-Space Orbitals Electronic Structures, Reaction Mechanisms, Metallic Orbitals. 

Valence bond structures for some molecules with four singly-occupied active-space orbitals electronic structures, reaction mechanisms, metallic orbitals... 

Valence-bond structure XXVI is an example of an increased-valence structure when LMOs are used to accommodate the three (fractional) bonding electrons [38], Elsewhere, it has been deduced that the A-atom valence is able to exceed unity in each of the VB structures XX [46] and XXVI [38], Increased-valence structures of the type XXVI will be used in the discussion of Sn2 reactions which is provided in the next section. 

Correlation diagram A diagram which shows the relative energies of orbitals, configurations, valence bond structures, or states of reactants and products of a reaction, as a function of the molecular geometry, or another suitable parameter. An example involves the interpolation between the energies obtained for the united atoms and the values for the separated atoms limits. 

The implication of the multiple possible reaction pathways shown in Scheme 4.6 is that any computational approach must allow for the possible contribution of at least these three valence bond structures. " The simplest approach to the nature of the wavefunction for the Cope rearrangement is to just account for the correlation of the active orbitals of the reactants with those of the products. The o-bond between C3 and C4 of the reactant correlates to a(Ci-C ) in the product. Assuming that 1,5-hexadiene has C2 symmetry, both of these orbitals have a synunetry. The in-phase mixing of the two jc-bonds of the reactant (it(Cx-C2)-l-Jc(C5-Cg)) has b synunetry and correlates with (jc(C2-C3)-l-Jt(C4-C5)) of the product. The out-of-phase combination of the reactant Jc-bonds (it(Ci-C2) - it(C5-Cg)) has a synunetry and correlates with (jc(C2-C3) - Jc(C4-C5)) of the product. If the reaction proceeds through a C211 geometry, orbital symmetry demands that these active orbitals of must become Ug aJbJ. So, we may take as the aromatic ... 

The first condition is satisfied automatically with all reactions containing closed shell molecules only. A systematic examination for this type of reactions was performed by Snyder and Basch The theoretical (SCF) heats of reactions were claimed to be more accurate than those obtained using seraiempirical relations of bond energies for reactions of strained molecules, or those not well represented by a single valence-bond structure. However, Snyder and Basch concluded... 

Other measures of the reaction coordinate have been proposed. Jones and Kirby suggest that (within a reaction series) the longer the bond, the more readily it will cleave heterolytically. This idea is based on the premise that a longer covalent bond carries a greater contribution from the charge-separated valence bond structure, as in... 

It is important to stress the difference between resonance and tauto-merism. The concept of resonance has been introduced to describe the delocalization of electrons in all the different forms of the molecule as represented by valence bond structures, among which resonance is said to occur, the configuration of the nuclei is the same. In tautomerism, on the other hand, the atoms are arranged in different ways and tautomeric changes, in which one form is changed into the other, occur as the result of chemical reactions. The tautomeric forms are in fact different chemical entities and in theory and frequently in practice, each can be isolated. 

Figure 8.1. Energetics of the proton transfer between an acid (A) and a base (B) by valence bond resonance structures. The reaction is described in terms of a covalent resonance structure (A-H B) and an ionic resonance structure (A H-B+). The energies of the valence bond structures are as given in the text and depend on the coordinates of the reacting atoms, R, and the coordinates of the rest of...

Our current pictme of the reaction of carbon atoms is given in Table 17. Much of what is known about the reactions of atomic carbon in alkanes and other simple molecules, particularly in the gas phase, is consistent with this picture. The initial reactive encounter can be assumed to be an insertion reaction. The intermediate or complex that is formed need not have a well-defined valence bond structure. Once the complex is formed, the product spectrum is defined by the total energy of the complex, how this energy is distributed (it is not necessary to assume equilibration) and the modes of decay available to the complex. 

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