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Inadequacies of the SN1 Mechanism
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Abstract
Although it has been recognized for half a century that alkyl carbocations are extremely reactive and can exist only in the vacuum of the mass spectrometer or in nonnucleophilic solvents like the superacids, textbooks of organic chemistry still present them as well-defined intermediates in solvolytic displacement reactions. A planar free carbocation intermediate is used as a simple model to explain racemization in solvolysis; but the problem is that racemization is never complete, the enantiomer in excess having the inverted configuration. Also, the unimolecularity has never been demonstrated, but is an assumption only.Racemization is commonly observed for reactions that are reversible, as in the hydrolysis of an alkyl chloride. It is also necessary that either the substrate, the product, or both can undergo a degenerate reaction. This is equivalent to saying that both the attacking nucleophile and the leaving nucleofuge can play both roles. Such exchanges were demonstrated by isotopic labeling 40 years ago.A number of examples are given to show that in irreversible solvolysis reactions of stereoisomeric pairs, which by an SN1 mechanism should have a common carbocation intermediate, produce instead different compounds which are those that can be predicted from an SN2-like inversion.
Keywords (Audience):
Second-Year UndergraduateKeywords (Domain):
Organic ChemistryKeywords (Subject):
Mechanisms of ReactionsCiting Articles
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This article has been cited by 3 ACS Journal articles (3 most recent appear below).
How Persistent Is Cyclopropyl upon Nucleophilic Substitution, and Is Frontside Displacement Possible? A Model Study
Einar UggerudThe Journal of Organic Chemistry2001 66 (21), 7084-7089How Persistent Is Cyclopropyl upon Nucleophilic Substitution, and Is Frontside Displacement Possible? A Model Study
Einar UggerudThe Journal of Organic Chemistry2001 66 (21), 7084-7089Quantum chemical model calculations (MP2/6-31G(d,p)) demonstrate that frontside nucleophilic substitution is not possible in the reaction between water and protonated cyclopropanol. Instead, ring opening occurs, in accordance with a well-known disrotary ...
Troposelective Substitutions in Microsolvated Systems
A. Filippi and M. SperanzaJournal of the American Chemical Society2001 123 (25), 6077-6082Troposelective Substitutions in Microsolvated Systems
A. Filippi and M. SperanzaJournal of the American Chemical Society2001 123 (25), 6077-6082The mechanism and the stereochemistry of the intracomplex solvolysis of the proton-bound complexes IX between CH318OH and (R)-(+)-1-aryl-ethanol (1RX; aryl = phenyl (X = H); pentafluorophenyl (X = F)) have been investigated in the gas phase in the 25...
Intermediates, Transition States, Butterflies, and Frogs
Trevor M. KitsonJournal of Chemical Education2001 78 (4), 504Intermediates, Transition States, Butterflies, and Frogs
Trevor M. KitsonJournal of Chemical Education2001 78 (4), 504An analogy that may be useful in the classroom is described. The changes that occur in typical simple SN1 and SN2 reactions are compared to the metamorphoses undergone by caterpillars and tadpoles, respectively.
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- Received: August 03, 2009
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