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  Home > JCE Print > Journal of Chemical Education > Issues > 1998  > November  >
Research: Science and Education
Inadequacies of the SN1 Mechanism
Johannes Dale
Department of Chemistry, Kjemisk Institutt, Universitetet i Oslo, 0315 Oslo, Norway
Cover
November 1998
Vol. 75 No. 11
p. 1482
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.
More Information
*  Citation
 Dale, Johannes. J. Chem. Educ. 1998 75 1482.
*  Keywords
 organic chem, physical chem, mechanisms
*  History
 Created:
Last Updated:		 June 18, 1999
June 24, 2005
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