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ORGANIC LETTERS

Distinguishing the SN2 and the SN2′ Mechanisms in the Gas Phase
Gustavo E. Davico†
Departamento de Quımica Organica, Facultad de Ciencias Quımicas, UniVersidad ´ ´ ´ Nacional de Cordoba, Agencia Postal 4, C.C. 61, 5000 Cordoba, Argentina ´ ´ daVico@jilau1.colorado.edu
Received September 21, 1999

1999 Vol. 1, No. 10 1675-1678

ABSTRACT

The secondary deuterium kineticisotope effects (KIE) for both mechanisms were calculated by using ab initio results and transition state theory. The SN2′ mechanism shows very inverse (kH/kD < 1) KIEs, smaller than those for the SN2 pathway. The difference is primarily originated in the vibrational contribution to the KIE, and more specifically the mid-frequency factor. These results suggest that the reaction mechanism can bedetermined by measuring the KIE.

Determining an ionic reaction mechanism in the gas phase can be very difficult because most of the techniques used to study those reactions detect only the ionic species.1,2 Consider for example the SN2 versus the E2 mechanisms3 (Scheme 1). Since both reactions produce the same ionic product, the mechanism for a particular reaction cannot be discerned.4 However,both mechanisms can be distinguished by measuring the deuterium kinetic isotope effects (KIE).5 While the E2 mechanism exhibits primary, normal (kH/kD >1) KIE; the SN2 mechanism shows secondary, usually inverse (kH/ kD < 1) KIE.5,6 This remarkable difference in the KIEs has been exploited successfully to distinguish these mechanisms
† Present address: JILA, University of Colorado and NationalInstitute of Standards and Technology, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0440. (1) Smith, D.; Adams, N. G. Recent AdVances in Flow Tubes: Measurement of Ion-Molecule Rate Coefficients and Product Distributions; Bowers, M. T., Ed.; Academic: New York, 1979; Vol. 1, pp 1-44. (2) Squires, R. R. Int. J. Mass Spectrom. Ion Process. 1992, 118, 503. (3)March, J. AdVanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; John Wiley & Sons: New York, 1992. (4) DePuy, C. H.; Gronert, S.; Mullin, A.; Bierbaum, V. M. J. Am. Chem. Soc. 1990, 112, 8650. (5) Gronert, S.; DePuy, C. H.; Bierbaum, V. M. J. Am. Chem. Soc. 1991, 113, 4009. (6) O’Hair, R. A. J.; Davico, G. E.; Hacaloglu, J.; Dang, T. T.; DePuy, C. H.; Bierbaum, V. M. J. Am.Chem. Soc. 1994, 116, 3609.

Scheme 1

in the gas phase. Theoretical calculations have been very successful in predicting the KIEs in the SN2 mechanism,7-9 including reactions involving microsolvated nucleophiles.10 In all cases, transition state theory was used even when the reactions showed large negative activation barriers, as in the
(7) Hu, W. P.; Truhlar, D. G. J. Am. Chem. Soc. 1995, 117,10726. (8) Davico, G. E.; Bierbaum, V. M. J. Am. Chem. Soc., submitted. (9) Viggiano, A. A.; Paschkewitz, J.; Morris, R. A.; Paulson, J. F.; Gonzalez-Lafont, A.; Truhlar, D. G. J. Am. Chem. Soc. 1991, 113, 9404. (10) Hu, W.-P.; Truhlar, D. G. J. Am. Chem. Soc. 1994, 116, 7797.

10.1021/ol990290s CCC: $18.00 Published on Web 10/07/1999

© 1999 American Chemical Society

case of thereactions of F- and HO- with methyl chloride. The agreement with experiments was excellent.6,8,10 A similar situation occurs between the SN2 and the SN2′ mechanisms (see Scheme 2). As in the previous case, both

Table 1. Heats of Reaction (∆ER) and Heats of Activation (∆Eq) with Respect to Reactants for the SN2 and SN2′ Mechanisms (MP4(SDTQ)/6-31++G**//MP2/6-31++G**), in kcal/mola
∆Eq SN2′ 9.4 (9.5)-7.8 (-7.6)

Scheme 2

nucleophile ClFa

SN2 4.8 (3.7) -12.1 (-12.4)

∆ER 0 -28.3 (-27.4)

Values in parentheses are corrected for vibrational zero point energies.

reaction mechanisms generate the same ion product and thus cannot be discernible by mass spectrometric methods. In this case, they also give rise to the same neutral product. In addition, both reactions exhibit secondary...