Heterolytic bond formation

If one of the bonded elements is positively charged to begin with, it can gain die bonded pair upon bond cleavage and become neutral. Note, however, that net charge is always conserved in any reaction. Moreover, bond cleavages are depicted the same whedier they involve cr or it bonds. 

The formation of a bond between two atoms can proceed by one of the atoms donating an electron pair and die other atom accepting the electron pair. As before, charge must be conserved and die loss and gain in electrons by the donor 

The atom donating the electron pair must obviously have an electron pair that is not tightly bound and thus is available for donation. Commonly, lone pairs and n -bonded electron pairs can be most easily donated, but occasionally electron pairs in o bonds can be donated if the a bond is weak or electron rich. 

The atom accepting the electron pair must have an unfilled orbital available which the donated electron pair can populate. This can be an unfilled valence shell orbital, as is the case if the acceptor atom has a valence sextet, or it can be an accessible antibonding orbital, either a or it.  

Thus the reaction of acetone with BF3 is a Lewis acid-base reaction in which a lone pair of the ketone oxygen atom is donated to an unfilled valence orbital of BF3. Bond formation is accompanied by the development of formal charges on both oxygen and boron. 

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Heterolytic bond formation)cleavage can be treated as a simple reaction dimension. This assumes that with only bond cleavage, energy as a function of r will be described by a Morse curve, or in terms of a bond order-related coordinate energy will be described by a quadratic. 

For an example of heterolytic bond formation, see the SnI reaction (Section 5.3.1.2j... 

The general Lewis-acid-base reaction (3.95) exemplifies the two-electron stabilizing donor-acceptor interaction of Fig. 1.3 (namely the nN->-nB interaction for (3.94)), which may be distinguished from the complementary bi-directional donor-acceptor interactions of covalent-bond formation (Section 3.2.1). However, this leaves open the question of whether (or how) the equilibrium bond reflects the formal difference between heterolytic (3.95) and homolytic (3.96) bond formation. 

It is convenient to separate heterolytic bond cleavage and bond formation (ki and k-i) from the transport steps k-, k and k ) for Scheme 2. The values of... 

Table 2.3. Heterolytic bond dissociation enthalpies, AHnhet, molecules and ions [kJ mol" ] and heats of formation of some ...

If Figure 3.9a represents bond formation between two molecules, the reverse process would correspond to heterolytic bond cleavage. One notes, however, that in the gas phase heterolytic bond cleavage is never observed. As we shall see, the interaction depicted in Figure 3.9a is the primary interaction between any pair of molecules whether it leads to bond formation or not. It is responsible for van der Waals attraction and hydrogen bonding. 

The biocatalyst a-chymotrypsin s ability to hydrolyze 20 is inhibited in the presence of copolymer 19a loaded with 0.2 mol% of the triphenyl carbinol units. 47b Photoirradiation of 19a results in heterolytic bond cleavage and the formation of the cationic copolymer 19b. In this polymer structure, the biocatalyzed hydrolysis of 20 is activated (V = 1.0 pM min-1). The polymer-induced photostimulated activation and deactivation of a-chymotrypsin in the different membrane environments correlates with the permeability and transport properties of the substrate 20 through the different structures of the polymer membranes.1471 Flow dialysis experiments showed that the polymer states 17a, 18a, and 19a are nonpermeable to 20, and hence the biocata-lytic functions of the immobilized enzyme are blocked. The polymer structures 17b,... 

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... 

In polar reactions, heterolytic (unsymmetrical) bond cleavage (heterolysis) and bond formation occur, while homolytic (symmetrical) bond cleavage (homolysis) and bond formation occur in radical reactions as shown below (Scheme a). 

An entirely different description emerges for the two 1,3-dipolar cycloaddition reactions that we have studied [3,4]. For such systems, the bond breaking and bond formation involves instead the shifts of well-identifiable orbital pairs, rather than any spin recouplings. Such heterolytic mechanisms, that do not pass through an aromatic structure, now seem to be a likely outcome of studies on other gas-phase concerted 1,3-dipolar cycloaddition reactions. 

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