Bond cleavage reactions heterolytic

The nature of the initial bond cleavage (homolytic, heterolytic, or by a concerted pathway) cannot be generalized because it depends on substituent effects and/or reaction conditions. 

FIGURE 2.3. The energetics of a heterolytic bond cleavage reaction in a polar solvent. The specific example shown corresponds to the CH3OCH3— CH3 + CH30 reaction in water. The energy of the covalent state does not include the effect of the solvent on this state, but a more consistent treatment (e.g., eq. (2.21) should account for the polarization of the solvent toward the charges of the ionic state. This would result in destabilization of H31. 

Aryl and aliphatic amines are perhaps the best known D molecules primarily due to the availability of the nonbonded electrons on nitrogen for PET processes. The simplest bond cleavage reaction for amine cation-radicals is heterolytic C—H cleavage or deprotonation. It is well known that the acidity of protons on a species after loss of an electron, i.e., after one-electron oxidation, is greatly enhanced [7]. For example, the pKa of the benzylic protons in the toluene cation-radical have been estimated to be —10 [7] compared to pKa value of 35 before oxidation. The process of proton loss can be extremely fast, on the time scale of picoseconds [8]. 

Heterolytic versus Homolytic Bond-Cleavage Reactions... 

Scheme 5. Heterolytic and homolytic C-S bond-cleavage reactions.

Additional information comes from the study of the heterolytic C-C bond cleavage reactions of 4-cyanodiphenylethane derivative radical anions (4 ) generated by PET in MeCN (Scheme 84) [304]. The rate constants for fragmentation are collected in Table 17. 

The simplest are the heterolytic and homolytic bond cleavage reactions/ equations (11) and (12)... 

Scheme 2 Interrelation between homolytic and heterolytic bond cleavage reactions. Adapted from Wayner, D. D. M. ...

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

The broadest classification of reactions is into the categories of heterolytic and homolytic reactions. In homolytic (free radical) reactions, bond cleavage occurs with one electron remaining with each atom, as in... 

Chemical reaction rate, see Rate of reaction Chemical reactions condensed phases, 42-46 enzymatic, see Enzymatic reactions gas phase, see Gas-phase reactions heterolytic bond cleavage, 46, 47, 51,... 

Excited states can also decay by means of chemical reaction via heterolytic bond cleavage, leading to ions, or by homolytic bond cleavage generating free radicals. 

In contrast to the abundant examples of radical cleavage, only a few proposals of ionic cleavage of carbon-carbon a bonds have been put forward in the long saga of mechanistic studies on heterolytic cleavage reactions. 

DR. JAMES ESPENSON (Iowa State University) I should like to raise a slightly different aspect of substitution reactions, an aspect which relates to a different kind of mechanism. This involves a change from the conventional heterolytic process of metal ligand bond cleavage to one dealing with homolytic bond cleavage. 

Indeed, it was shown that aposcopolamine was not formed by direct dehydration of scopolamine, but via the conjugate scopolamine O-sulfate generated by a sulfotransferase [127]. This explains the species differences observed, and indicates a mechanism of heterolytic C-0 bond cleavage made possible by the electron-withdrawing capacity of the sulfate moiety. The reaction is also facilitated by the acidity of the departing proton carried by the vicinal, stereogenic C-atom. This acidity also accounts for the facile base-catalyzed racemization of scopolamine and hyoscyamine [128]. 

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