Rate-Equilibrium Relationships for Reactions of Families of Carbanion Nucleophiles with N-Benzyl-N,N-dimethylanilinium Cations and with Alkyl Chlorides, Bromides, and Iodides

Article abstract of DOI:10.1021/ja00283a027

Rates of reaction of a family of 9-methylfluorenide anions, 9-MeFl^-, with 15 substituted N-benzyl-N,N-dimethylanilinium cations, GC6H4CH2N(Me)2C6H4Y⁺, were measured in a 0.8 M KNO3 solution of Me2SO at 25 deg C.Substitution in the benzyl moiety produced a U-shaped Hammett plot, with m-CN reacting 9- to 15-fold faster than H and p-MeO reacting 1.5- to 3-fold faster than H, depending on the basicity of the 9-RFl^- ion.Broensted Β_Nu values obtained by varying nucleophile basicity in the 9-MeFl^- ion family were 0.28 for N-(p-methoxybenzyl)-N,N-dimethylanilinium ion, 0.33 for N-benzyl-N,N-dimethylanilinium ion, and 0.39 for N-(m-cyanobenzyl)-N,N-dimethylanilinium ion.These β_Nu values were unaffected by substitution in the aniline portion of the substrate.Hammett p_Lg values were obtained by substitution of m-Cl, p-Br, H, p-Me and p-MeO groups (Y) in the dimethylaniline leaving group; p_Lg values were all near 2.0 and were insensitive to the basicity of the attacking nucleophile and the substituent G in the benzyl group of the substrate. β_Nu values for families of 9-RFl^- ions reacting with ArCH2X, n-BuX, and CNCH2X also remained constant as the leaving group, X, was changed (Cl, Br, and I) for a given substrate and when the donor atom was changed along the series C^-, N^-, O^-, and S^-.These results indicate that there is no direct relationship between the size of β_Nu and the extent of bond making or bond breaking in the transition state for S_N2 reactions, as has been generally assumed in theoretical attempts to rationalize the effects of structural variations on rate.It is suggested that S_N2 reactions occur in two stages.The size of β_Nu is determined in the first stage where an ion-dipole complex is formed.The rate is determined largely in the second stage where bonds are formed and cleaved and where molecular and solvent reorganization occurs.