A New Insight into Using Chlorine Leaving Group and Nucleophile Carbon Kinetic Isotope Effects To Determine Substituent Effects on the Structure of SN2 Transition States
Chlorine leaving group k35/k37, nucleophile carbon k11/k14, and secondary MG SRC="/images/gifchars/alpha.gif" BORDER=0>-deuterium [(kH/kD)mages/gifchars/alpha.gif" BORDER=0>] kinetic isotopeeffects (KIEs) have been measured for the SN2 reactions between para-substituted benzyl chlorides andtetrabutylammonium cyanide in tetrahydrofuran at 20 mages/entities/deg.gif">C to determine whether these isotope effects can beused to determine the substituent effect on the structure of the transition state. The secondary mages/gifchars/alpha.gif" BORDER=0>-deuteriumKIEs indicate that the transition states for these reactions are unsymmetric. The theoretical calculations at theB3LYP/aug-cc-pVDZ level of theory support this conclusion; i.e., they suggest that the transition states forthese reactions are unsymmetric with a long NC-Cmages/gifchars/alpha.gif" BORDER=0> and reasonably short Cmages/gifchars/alpha.gif" BORDER=0>-Cl bonds. The chlorine isotopeeffects suggest that these KIEs can be used to determine the substituent effects on transition state structurewith the KIE decreasing when a more electron-withdrawing para-substituent is present. This conclusion issupported by theoretical calculations. The nucleophile carbon k11/k14 KIEs for these reactions, however, donot change significantly with substituent and, therefore, do not appear to be useful for determining how theNC-Cmages/gifchars/alpha.gif" BORDER=0> transition-state bond changes with substituent. The theoretical calculations indicate that the NC-Cmages/gifchars/alpha.gif" BORDER=0>bond also shortens as a more electron-withdrawing substituent is placed on the benzene ring of the substratebut that the changes in the NC-Cmages/gifchars/alpha.gif" BORDER=0> transition-state bond with substituent are very small and may not bemeasurable. The results also show that using leaving group and nucleophile carbon KIEs to determine thesubstituent effect on transition-state structure is more complicated than previously thought. The implicationof using both chlorine leaving group and nucleophile carbon KIEs to determine the substituent effect ontransition-state structure is discussed.