Isotope effects in nucleophilic substitution reactions. IV. The effect of changing a substituent at the α carbon on the structure of S_N2 transition states

Abstract

Kinetic studies, secondary α-deuterium kinetic isotope effects, primary chlorine kinetic isotope effects (1), Hammett ρ values determined by changing the substituent in the nucleophile, and activation parameters have been used to determine the detailed (relative) structures of the transition states for the SN2 reactions between para-substituted benzyl chlorides and thiophenoxide ion. A rationale for the U-shaped Hammett ρ plots observed when para-substituted benzyl compounds react with negatively charged nucleophiles is also presented.

Isotope effects in nucleophilic substitution reactions. VIII. The effect of the form of the reacting nucleophile on the transition state structure of an SN2 reaction

“…Secondary a-deuterium kinetic isotope effects and Hammett p values can be used to elucidate the structure of the transition states in the SN2 reactions (7,8). The initial state for the secondary a-deuterium kinetic isotope effect in both the contact ion pair complex and the solvent-separated ion pair complex reactions is the same, i.e., n-butyl chloride.…”

“…Since adding three hydrogen bonds (altering the reduced mass of the developing chloride ion) has a negligible effect on the secondary a-deuterium kinetic isotope effect, the different isotope effects observed in 1% and 4% aqueous diglyme must be due to a change in transition state structure. The magnitude of the secondary a-deuterium kinetic isotope effect is primarily dependant on the C,-H(D) out-ofplane bending vibrations and, therefore, on the steric crowding around the C, H(D) bonds in the SN2 transition state (7,8,10). In fact, the C,-H(D) out-of-plane bending vibrations will be of higher energy (the secondary a-deuterium kinetic isotope effect will be smaller) when the a-carbon-nucleophile and a-carbon-leaving group bonds are short in the transition state (8).…”

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on SN2 transition state structure

“…This suggests that the free ion transition states are looser (have longer nucleophile-a-carbon and (or) a-carbon -leaving group bonds) than the ion pair transition states, Fig. 3.6 This is reasonable because the higher charge on the free ion nucleophile q h e length of the a-carbon -leaving group bonds has not been changed in these transition states when the substituent on the nucleophile is changed because other work in our laboratory has suggested that the length of the stronger transition state bond does not change significantly when a substituent is changed in the substrate, the nucleophile, or the leaving group of an SN2 reaction (3,4 would distort the a-carbon -leaving group bond and cause reaction at a greater distance from the a carbon ( 14). Unfortunately, not all of the free ion transition states are looser than the ion pair transition states.…”

“…In fact, this change in the form of the reacting nucleophile seems to be a general phenomenon because the secondary a-deuterium kinetic isotope effect changes in at least three other SN2 reactions when the form of the nucleophile is altered ( I ) .~ These results clearly demonstrate that the magnitude of the secondary a-deuterium kinetic isotope effect and transition state structure can be strongly dependent on the form of the reacting anion in an SN2 reaction. Our interest in the substituent effects on the structure of S N~ transitions states (3)(4)(5) and the many cases where the substituent effects found experimentally do not show a consistent pattern (6) made it important to determine how the form of the reacting anion in an SN2 reaction affects the substituent effects on transition state structure. 'Author to whom correspondence may be addressed.…”

Solvent effects on SN2 transition state structure. II: The effect of ion pairing on the solvent effect on transition state structure

“…These identical kinetic isotope effects are impressive because the rate constant changes by 1400 times when the solvent is changed from DMF to methanol. A change in the length of either the nucleophile-a-carbon and (or) a-carbon -leaving group bonds in the SN2 transition state would change the steric crowding around the a carbon and alter the frequencies of the C,-H(D) out-of-plane bending vibrations, which primarily determine the magnitude of the secondary a-deuterium kinetic isotope effect (9,10). Thus, the simplest explanation is that the transition states do not change when the solvent is altered in the free ion SN2 reaction, i.e., the identical isotope effects indicate that the transition states are identical in all three solvents.…”