Reactions of α-Nucleophiles with Alkyl Chlorides: Competition between S_N2 and E2 Mechanisms and the Gas-Phase α-Effect

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 alpha-nucleophilic character of these two anions and of the HO(2)(-) anion is examined. No enhanced reactivity is displayed. These results suggest that the alpha-effect is not due to an intrinsic property of the anion but instead due to a solvent effect.

Origin of the α-Effect in Nucleophilic Substitution Reactions of Y-Substituted Phenyl Benzoates with Butane-2,3-dione Monoximate and Z-Substituted Phenoxides: Ground-State Destabilization vs. Transition-State Stabilization

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Thus, the enhanced reactivity shown by these nucleophiles was termed the α-effect. 1 Numerous studies have been performed to investigate the cause of the α-effect.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Many theories have been advanced to explain the α-effect, e.g., destabilization of the ground state (GS) due to electronic repulsion between the nonbonding electron pairs, transition state (TS) stabilization including general acid/base catalysis, thermodynamic stability of products, solvent effects. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] However, none of these theories is conclusive. Particularly, solvent effect on the α-effect remains controversial.…”

“…Particularly, solvent effect on the α-effect remains controversial. [8][9][10][11][12][13][14][15][16][17][18][19] Solvent effect was suggested to be unimportant since the magnitude of the α-effects was found to be similar for reactions performed in H 2 O and in organic solvents such as MeCN and toluene. 8 Besides, it has been reported that α-nucleophiles are intrinsically more reactive than normal nucleophiles of similar basicity in gas-phase reactions.…”

“…12 Furthermore, from recent gas-phase ion-molecule studies, Bierbaum et al have found that α-nucleophiles such as HOO -, BrO -and ClO -do not exhibit enhanced reactivity in gas-phase reactions with alkyl chlorides. 13 Accordingly, they have concluded that the α-effect is due to solvent effect but not due to an intrinsic property. 12,13 We have initiated a systematic study to investigate the effect of solvent on the α-effect.…”

“…13 Accordingly, they have concluded that the α-effect is due to solvent effect but not due to an intrinsic property. 12,13 We have initiated a systematic study to investigate the effect of solvent on the α-effect.…”