Kinetic Study of the Substitution Reactions of Benzyl Halides and Halide Ions in Acetone

Abstract

AbstractThe kinetics of the Finkelstein reactions of benzyl halides and halide ions in dry acetone was studied by using the GC and HPLC methods. The method of conductivity is used to measure the degree of dissociation of alkali halide in acetone. The dissociation of the salt and the common‐ion effect were used to correct the halide ion concentration. Let kxx′‐ and Kxx′‐ represent the corrected second‐order rate constant and equilibrium constant for the PhCH2X‐X′‐ reaction, respectively, then at 25° kxx‐−= 1.83x×10−3 M−1 s−1, KII‐=4.48×104, kBrcl‐=0.377 M−1 s−1, KBrcl‐=874; and kBr‐=7.88 M−1 s−1, KIBr‐=19.2. The entropy of activation is always negative which implies that the activated complex is more solvated than the reactants. There is a good correlation between the enthalpy of activation and the C‐X bond energy. The enthalpy of activation is much smaller than the corresponding C‐X bond energy which implies that both the bond formation and the bond dissociation are taking place simultaneously in the transition state. A modified Taft equation is used to correlate the Finkelstein reactions of alkyl halides (RCH2X). However, the benzyl halides can not be fit by the correlation equation due to the strong electrical polar effects. Discussion of the Finkelstein reactions of organic halides in terms of rate constants, equilibrium constants, solvent effects, and the thermodynamic parameters is also presented.