Bonds as Electron Donors

Lion between C and the incoming acid. Carried to the extreme, a substitution reaction ensues in which the configuration at C may be retained. If the center is H rather than C, the corresponding reaction is a hydride transfer (abstraction), as in the Cannizzaro or Meerwein-Pondorf-Verley reactions. If the interaction falls short of abstraction, a hydride bridge may be fonned. Both aspects are discussed further in Chapter 10. 

If the LUMO is the sp hybrid orbital at the C end of a polarized a bond, such as to a halide, geometric distortion also occurs, particularly a lengthening of the receiving a bond. Carried to the extreme, an elimination reaction occurs (Elcb, as discussed in Chapter 10). Migration of M to the adjacent group does not occur. 

The donor ability of a cr bond may be improved in two ways, by raising the energy of the orbital and by polarizing the orbital toward one end. The first improves the interaction between it and a potential electron acceptor orbital by reducing the energy difference a — the second by increasing the possibility of overlap and therefore increasing the 

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The triply connected phosphoms compounds have a lone electron pair that dominates much of the chemistry for these compounds. Triply connected compounds typically exhibit pyramidal symmetry arising fromp hybridization. A considerable amount of sp character may be present as well. Bond angles range near 100° vs 90° theoretical. Tricoordinate compounds typically act as electron donors, forming metal coordination compounds and addition compounds such as H P BF [41593-56-0]. 

Deviations from Raonlt s law in solution behavior have been attributed to many charac teristics such as molecular size and shape, but the strongest deviations appear to be due to hydrogen bonding and electron donor-acceptor interac tions. Robbins [Chem. Eng. Prog., 76(10), 58 (1980)] presented a table of these interactions. Table 15-4, that provides a qualitative guide to solvent selection for hqnid-hqnid extraction, extractive distillation, azeotropic distillation, or even solvent crystallization. The ac tivity coefficient in the liquid phase is common to all these separation processes. 

The ability to ionically polymerize apparently correlates in many cases with the capacity of the substituents to act as electron acceptors (anionic polymerizability) or as electron donors (cationic polymerizability) on the rt-bond of the vinyl group. These relationships should be visible in carefully chosen quantum chemical parameters. 

The ability of compounds with double bonds to act both as electron donors and as electron acceptors in charge transfer complex formation is well known (81,82). Hammond (83) has studied the correlations of association constants and of the energy of the charge transfer absorption of 2-substituted-l,4-benzoquinones complexed with hexamethylbenzene with the Hammett equation. Charton (84) has studied the correlation with eq. (2) of association constants of 1-substituted propenes with Ag. ... 

The rapid and reversible formation of complexes between some metal ions and organic compounds that can function as electron donors can be used to adjust retention and selectivity in gas and liquid chromatography. Such coordinative interactions are very sensitive to subtle differences in the composition or stereochemistry of the donor ligand, owing to the sensitivity of the chemical bond towards electronic, steric and strain effects. A number of difficult to separate mixtures of stereoisomers and isotopomers have been separated by complexation chromatography. 

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

Diels-Alder cycloaddition reactions of electron-poor dienophiles to electron-rich dienes, which are generally carried out thermally, afford widespread applications for C—C bond formation. On the basis of their electronic properties, numerous dienes can be characterized as electron donors and dienophiles as electron acceptors. Despite the early suggestions by Woodward,206 the donor/ acceptor association and electron-transfer paradigm are usually not considered as the simplest mechanistic formulation for the Diels-Alder reaction. However, the examples of cycloaddition reactions described below will show that photoirradiation of various D/A pairs leads to efficient cycloaddition reactions via electron-transfer activation. 

The methyl groups on the pyridine ring result in a major difference in the reactivity of lutidines. In 3,5-lutidine the methyl groups act as electron donors tending to increase the stability of the tt-bonds, and activating the ring for electrophilic attack at the a-positions. The MOs in 3,5-lutidine show the it-levels pushed to lower energy... 

In cationic polymerisation monomer molecule acts as electron donor and higher activity is shown by vinyl derivatives which have an increased electron density at the double bond. 

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