Reactions and Energy Relationships

Reaction coordinate diagrams (Fig. 1) are important tools for analyzing the structure and energy relationships that describe a reaction mechanism. The following statements may prove helpful for eonstrueting or interpreting reaction coordinate diagrams. 

Reactions between an atomic nucleus and another particle are called nuclear reactions. In some such reactions, new nuclei are formed nuclear transmutations) in others the original nucleus is excited to a higher energy state (inelastic scattering) in a third class, the nucleus is unchanged (elasticscattering). Spontaneous nuclear transformations, which are involved in the radioactive decay of unstable nuclei, have be discussed in Chapter 4. In this chapter the enqrhasis is on the mass and energy relationships when a projectile interacts with a nucleus. 

State the significance of the/ree-energy change (AG) of reactions and the relationship of AG to AG°, the equilibrium constant, and the concentrations of reactants and products of the reaction. 

If the HOMO of the olefin is higher in energy than that of the diene and the LUMO of the diene is lower in energy than that of the olefin, the reaction is termed an inverse electron-demand Diels-Alder reaction. The energy relationships of these frontier orbitals are shown in Figure 11.96(c). Spino and co-workers found that the HOMO-LUMO energy gap did not correlate well with reactivity for inverse electron-demand Diels-Alder reactions. ... 

See footnote on p. 43 (Chapter 2) for a discussion of mass and energy relationship in chemical reactions. 

To understand the relationship between a chemical reaction and energy, think of a chemical bond as a spring. As a spring is stretched from its resting position, its energy increases. As... 

Alkynes, compounds containing carbon-carbon triple bonds, are similar to alkenes in their physical properties and chemical behavior. In this chapter, we will examine the structure and chemical reactions of these two classes of compounds. We will also examine briefly the relationship between chemical reactions and energy. 

My background led me to view chemical kinetics as closely related to transport phenomena. While the relationship of these topics is well known, it is often ignored, except for brief discussions of irreversible thermodynamics. In fact, the physics underlying such apparently dissimilar processes as reaction and energy transfer is not so very different. The intermolecular potential is to transport what the potential-energy surface is to reactivity. 

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

This shows that Eqs. (1) and 2) are basically relationships between the Gibbs free energies of the reactions under consideration, and explains why such relationships have been termed linear free energy relationships (LEER). 

Solvents exert their influence on organic reactions through a complicated mixture of all possible types of noncovalent interactions. Chemists have tried to unravel this entanglement and, ideally, want to assess the relative importance of all interactions separately. In a typical approach, a property of a reaction (e.g. its rate or selectivity) is measured in a laige number of different solvents. All these solvents have unique characteristics, quantified by their physical properties (i.e. refractive index, dielectric constant) or empirical parameters (e.g. ET(30)-value, AN). Linear correlations between a reaction property and one or more of these solvent properties (Linear Free Energy Relationships - LFER) reveal which noncovalent interactions are of major importance. The major drawback of this approach lies in the fact that the solvent parameters are often not independent. Alternatively, theoretical models and computer simulations can provide valuable information. Both methods have been applied successfully in studies of the solvent effects on Diels-Alder reactions. 

The applicability of the two-parameter equation and the constants devised by Brown to electrophilic aromatic substitutions was tested by plotting values of the partial rate factors for a reaction against the appropriate substituent constants. It was maintained that such comparisons yielded satisfactory linear correlations for the results of many electrophilic substitutions, the slopes of the correlations giving the values of the reaction constants. If the existence of linear free energy relationships in electrophilic aromatic substitutions were not in dispute, the above procedure would suffice, and the precision of the correlation would measure the usefulness of the p+cr+ equation. However, a point at issue was whether the effect of a substituent could be represented by a constant, or whether its nature depended on the specific reaction. To investigate the effect of a particular substituent in different reactions, the values for the various reactions of the logarithms of the partial rate factors for the substituent were plotted against the p+ values of the reactions. This procedure should show more readily whether the effect of a substituent depends on the reaction, in which case deviations from a hnear relationship would occur. It was concluded that any variation in substituent effects was random, and not a function of electron demand by the electrophile. ... 

Faraday s law states that 96,487 coulombs (1 C = 1 A-s) are required to produce one gram equivalent weight of the electrochemical reaction product. This relationship determines the minimum energy requirement for chlorine and caustic production in terms of kiloampere hours per ton of CI2 or NaOH... 

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