Bond cleavage, heterolytic, and

Scheme 1. Relationship between homolytic and heterolytic bond cleavage and electron-transfer reactions.

Benzobenzvalene (100) has been shown to add sulphur dioxide to give the sulphone (101) and the 7-sultine (102) both adducts extrude sulphur dioxide on direct photolysis regenerating benzobenzvalene accompanied by naphthalene. Competing homolytic and heterolytic bond cleavages have been observed in singlet excited triarylsulphonium salts, and intramolecular electron transfer is involved in the photodissociation of p-nitrobenzyl 9,10-dimethoxy-anthracene-2-sulphonate.A photochemical method for the detosyl-ation of sulphonamides has been described,and the thermolysis and photolysis of some thiourea derivatives have been compared. 

We shall postpone further discussions of reactions involving radicals and homolytic bond cleavage until we reach Chapter 10. At this point we focus our attention on reactions involving ions and heterolytic bond cleavage. 

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... 

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. 

In heterolytic bond cleavage, both electrons end up on system B. The energy change associated with the bond dissociation is just the difference between electron energies before (2sl) and after (2 ), equation (4.1) and Figure 4.1 ... 

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,... 

Because of these clear differences of chemical pathway in the enzymatic and model reactions, it is difficult to assess the actual factor by which the apoenzyme enhances the reactivity of reduced flavin with O2. It is fairly clear, however, that the environment of the apoenzyme suppresses homolytic pathways and, presumably by general-acid-base and steric mechanisms, allows only heterolytic bond cleavages. 

The curved arrow notation first discussed in Section 1.5 works fine for heterolytic bond cleavage because it illustrates the movement of an electron pair. For homolytic cleavage, however, one electron moves to one atom in the bond and one electron moves to the other, so a different kind of curved arrow is needed. 

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