THE PROCESS OF BOND FORMATION

Energetically Favorable Reactions Usually Break Weak Bonds and Form Strong Ones the Products of Reversible Reactions Are Controlled by Thermodynamics, Those of Irreversible Reactions Are Controlled by Kinetics 

4 POLARIZABILITY AND HARD AND SOFT ACID-BASE THEORY 

Polarizability HSAB Principle Hard with Hard, Soft with Soft Transmetallation HSAB from a HOMO-LUMO Perspective (Supplementary) Two Attractive Forces Lead to Reaction Hard-Hard Attraction and Soft-Soft Attraction 

Energy Surfaces and Diagrams Transition State, AG° = -RT r Kgq, Every 1.36 kcal/mol (5.73 kJ/mol) Increase in AG° Decreases the Equilibrium Constant by a Factor of 10 at Room Temperature Driving Force AG° = AH° - 7AS° Enthalpy, Calculation of the Heat of Reaction from Bond Strengths Entropy, Disorder 

Free Energy of Activation Rate Constant Upper Limit on Concentration Diffusion-Controlled Limit Dropping the AG by 1.36 kcal/mol (5.73 kJ/mol) Increases the Rate of Reaction Tenfold at Room Temperature Reasonable Rate at 25°C Half-Life Lifetime of an intermediate Rate-Determining Step Transition State Position Reactivity vs. Selectivity Thermodynamic vs. Kinetic AG = AH -TAS, Enthalpy of Transition Entropy of Transition Stabilization of Intermediates Stabilization of Reactants 

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In the valence bond theory, hybridization of orbitals is an integral part of bond formation. As we shall see, the concept need not be explicitly considered in molecular orbital theory but may be helpful in visualizing the process of bond formation. 

The process of bond formation is poorly modeled with a point-charge description that contains the vital information about the relative valence states of dissimilar atoms in terms of ionization radii only. The predicted course of reaction is outlined in figure 4. Atomic promotion is once more modeled [136]... 

Fig. 3.10. Disposition of wave functions of neighboring atoms during the process of bond formation.

As a natural extension of the force and density study of the diatomic binding problem outlined briefly in the previous discussion, the processes of bond formations have been studied for several diatomic systems as a function of the internuclear separation R. 

The curved arrow is used to describe the flow of electrons in the process of bond formation. Since electrons have flowed away from one of the former doubly bonded carbons, that carbon is left deficient in electrons and is now shown as carrying a + charge. The positively charged species is referred to as a carbocation. Third, there is an attraction followed by bond formation between the positive carbocation and negative chloride ion ... 

In a calculation such as that of Weissmann and Cohan it is not really meaningful to split the total binding energy into contributions from different kinds of forces. It is, however, worth while to note that the molecular orbitals characteristic of isolated molecules are considerably distorted in the process of bond formation so that the contribution of delocalization or polarization forces to the total bond energy may be quite large. The effect of this polarization is also seen in the change which it makes in the dipole moment of the molecules involved. For a free water molecule this moment was computed in the present approximation to be i 68 debyes (experimental value I "84 D) while that of a completely hydrogen-bonded molecule was calculated as 2-40 D, an increase of 50 per cent. We shall return to a different treatment of this effect later. 

Two reactive groups diffuse toward each other to form an activated complex with possible creation of a stable bond. As long as diffusion in and out of the complex occurs more if equently than the process of bond formation, the reaction is controlled by chemical kinetics. 

Various explanations have been put forth for the alpha effect. One view is that the ground state of the nucleophile is destabilized by lone pair-lone pair repulsions, which decrease in going to the transition state. Another view is that the transition state is stabilized by electron release from the hetero atom. According to this reasoning, the process of bond formation is akin to that of a two-electron transfer ... 

Cioslowski J, Stefanov BB (1993) Electron flow and electronegativity equalization in the process of bond formation. J Chem Phys 99(7) 5151-5162... 

Several common thermodynamic and kinetic parameters are involved in the process of bond formation. The adhesive may be applied either... 

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