Electrostatic Stabilization of the Transition State in Proton Transfer from Carbon

Article abstract of DOI:10.1021/ja00313a037

The base-promoted reactions of 2-(p-nitrophenyl)ethyl chloride (1), 9-(chloromethyl)fluorene (2), 1-(2-chloro-2-propyl)indene (3), and 1-(2-acetoxy-2-propyl)indene (4) have been studied in 28 molpercent Me2SO4-H2O.They all exhibit rate-limiting proton transfer.Quinuclidine (Q) was found to be a more effective general base "catalyst" than hexafluoro-2-propanol anion (P^-) in these reactions.Thus, after correction for the small difference in thermodinamic basicity, the following rate constant ratios k_Q/k_p^- were obtained: 1.2, 4.4, 3.6, and 16, respectively.The small ratio for 1 indicates that partial desolvation of P_- is responsible fo not more than a minor part of the rate differences between Q and P^- in these reactions.It is concluded that the reactions of 2, 3, and 4 are accelerated by electrostatic interaction between the developing positive charge on the tertiary amine and the partial negative charge on the carbon atom in the transition state.This favorable electrostatic interaction seems to be considerably larger in methanol.