Reactions of r-Nucleophiles with Alkyl Chlorides: Competition between S N 2 and E2 Mechanisms and the Gas-Phase r-Effect Stephanie M. Villano, Nicole Eyet, W. Carl Lineberger, and Veronica M. Bierbaum* JILA, UniVersity of Colorado and the National Institute of Standards and Technology, Boulder, Colorado 80309, and Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309 Received February 16, 2009; E-mail: veronica.bierbaum@colorado.edu Abstract: Reaction rate constants and deuterium kinetic isotope effects for the reactions of BrO - with RCl (R ) methyl, ethyl, isopropyl, and tert-butyl) were measured using a tandem flowing afterglow-selected ion flow tube instrument. These results provide qualitative insight into the competition between two classical organic mechanisms, nucleophilic substitution (S N 2) and base-induced elimination (E2). As the extent of substitution in the neutral reactants increases, the kinetic isotope effects become increasingly more normal, consistent with the gradual onset of the E2 channel. These results are in excellent agreement with previously reported trends for the analogous reactions of ClO - with RCl. [Villano et al. J. Am. Chem. Soc. 2006, 128, 728.] However, the reactions of BrO - and ClO - with methyl chloride, ethyl chloride, and isopropyl chloride were found to occur by an additional reaction pathway, which has not previously been reported. This reaction likely proceeds initially through a traditional S N 2 transition state, followed by an elimination step in the S N 2 product ion-dipole complex. Furthermore, the controversial R-nucleophilic character of these two anions and of the HO 2 - anion is examined. No enhanced reactivity is displayed. These results suggest that the R-effect is not due to an intrinsic property of the anion but instead due to a solvent effect. Introduction Gas-phase ion-molecule studies provide a route to under- standing the intrinsic factors that affect a reaction in an environment free from solvent. These studies, therefore, provide important insight into the role of the solvent in a given reaction. Several notable differences between the intrinsic gas-phase reactivity and the reactivity in solution have been observed. 1-4 This work focuses on two important areas where solvent effects are significant: (1) the competition between nucleophilic substitution (S N 2) and base-induced elimination (E2) and (2) the R-effect or enhanced reactivity due to a lone pair of electrons that is adjacent to the nucleophilic center. It has been shown, both experimentally 5 and theoretically, 6 that the gas-phase reaction of ClO - with ethyl chloride proceeds through both an S N 2 and an E2 mechanism, as shown in Scheme 1. Since these two mechanisms generate the same ionic product and since the detection of the distinct neutral products presents serious analytical challenges, gas-phase experiments that address this competition are severely limited. 5,7-18 One distinguishing difference between these two mechanisms, however, is that S N 2 mechanisms generally display inverse deuterium kinetic isotope effects (KIE < 1), while E2 mechanisms display normal (1) Mackay, G. I.; Bohme, D. K. J. Am. Chem. Soc. 1978, 100, 327. (2) Olmstead, W. N.; Brauman, J. I. J. Am. Chem. Soc. 1977, 99, 4219. (3) Brauman, J. I.; Blair, L. K. J. Am. Chem. Soc. 1970, 92, 5986. (4) Brodbelt, J. S.; Isbell, J.; Goodman, J. M.; Secor, H. V.; Seeman, J. I. Tetrahedron Lett. 2001, 42, 6949. (5) Villano, S. M.; Kato, S.; Bierbaum, V. M. J. Am. Chem. Soc. 2006, 128, 736. (6) Hu, W. P.; Truhlar, D. G. J. Am. Chem. Soc. 1996, 118, 860. (7) Lieder, C. A.; Brauman, J. I. Int. J. Mass Spectrom. Ion Process. 1975, 16, 307. (8) Wladkowski, B. D.; Brauman, J. I. J. Am. Chem. Soc. 1992, 114, 10643. (9) Lum, R. C.; Grabowski, J. J. J. Am. Chem. Soc. 1988, 110, 8568. (10) Jones, M. E.; Ellison, G. B. J. Am. Chem. Soc. 1989, 111, 1645. (11) Gronert, S.; DePuy, C. H.; Bierbaum, V. M. J. Am. Chem. Soc. 1991, 113, 4009. (12) DePuy, C. H.; Gronert, S.; Mullin, A.; Bierbaum, V. M. J. Am. Chem. Soc. 1990, 112, 8650. (13) Gronert, S.; Pratt, L. M.; Mogali, S. J. Am. Chem. Soc. 2001, 123, 3081. (14) Gronert, S.; Fagin, A. E.; Okamoto, K.; Mogali, S.; Pratt, L. M. J. Am. Chem. Soc. 2004, 126, 12977. (15) Gronert, S. Acc. Chem. Res. 2003, 36, 848. (16) Lum, R. C.; Grabowski, J. J. J. Am. Chem. Soc. 1992, 114, 9663. (17) Noest, A. J.; Nibbering, N. M. M. AdV. Mass Spectrom. 1980, 8, 227. (18) Bartmess, J. E.; Hays, R. L.; Khatri, H. N.; Misra, R. N.; Wilson, S. R. J. Am. Chem. Soc. 1981, 103, 4746. Scheme 1 Published on Web 05/20/2009 10.1021/ja9012084 CCC: $40.75  2009 American Chemical Society J. AM. CHEM. SOC. 2009, 131, 8227–8233 9 8227 Downloaded by UNIV OF COLORADO on August 12, 2009 Published on May 20, 2009 on http://pubs.acs.org | doi: 10.1021/ja9012084