The Nonchain Electron Transfer Substitution Mechanism

The SrnI mechanism for substitution (Chapter 2) is initiated by electron transfer to the substrate undergoing substitution. A nonchain mechanism for substitution that is initiated by electron transfer can also be envisioned. Electron transfer from the nucleophile to the electrophile occurs to give a radical and a radical anion. The leaving group departs the radical anion to give a new radical. The two radicals then combine to give the product. The overall result is the same as if a simple Sn2 reaction had occurred. 

Certain nucleophiles (e.g., Cgo . some metal complexes) are very prone to transfer electrons to a substrate, and for these nucleophiles the nonchain electron transfer mechanism should be considered. Usually, though, it is better not to propose a nonchain electron transfer mechanism unless there is experimental evidence suggesting that such a mechanism is operative. Some physical organic chemists have made their reputations by showing that the accepted, polar mechanisms for many reactions are in fact incorrect and that electron transfer mechanisms actually operate. These studies show clearly that mechanisms that most chemists accept as reasonable are often incomplete or even incorrect. Still, Occam s razor compels one not to propose mechanistic intermediates unless theoretical or experimental evidence suggests that they exist, and for this reason the nonchain electron transfer mechanism is only occasionally used to explain the outcome of a reaction. 

Methyl tert- mXy ether, MTBE, and ethyl rerf-butyl ether, ETBE, are added to gasoline in order to increase the efficiency of gasoline combustion, which reduces the quantity of volatile organic compounds (VOCs) that escape into the atmosphere and cause smog. The chemical industry is very interested in using MTBE as a substitute for THE and diethyl ether, because autoxidation of THE and diethyl ether is a major safety problem for chemieal companies. The industry is less interested in ETBE as a solvent substitute. 

The production of chlorofluorocarbons, or CFCs, has been banned by international treaty because of their deleterious effect on the ozone layer. The ozone layer absorbs much of the Sun s dangerous UV radiation before it reaches the Earth s surface. CFCs are extremely stable in the lower atmosphere (one reason why they are so useful), but when they reach the stratosphere they decompose, producing potent catalysts of ozone destruction. Ozone destruction is most evident above Antarctica during the spring, when this region is exposed to the Sun for the first time in months. Dichlorodifluoromethane (CF2CI2) is a typical CFC. 

Hydrochlorofluorocarbons, or HCFCs, are being promoted as temporary replacements for CFCs. HCFCs, unlike CFCs, have at least one C—H bond. HCFCs are not less prone than CFCs to decompose in the stratosphere, but there is a pathway by which HCFCs can decompose in the lower atmosphere (where they cannot damage ozone) that is not accessible to CFCs. 2,2,2-Trichloro-l,l-difluoroethane, CHF2CCI3, is a typical HCFC. 

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