Using isotope effects to model transition states for carbocation-nucleophile combination reactions

The transition state structures for the carbocation-nucleophile combination reactions of ('4'-substituted-'4'' -methoxydiphenyl)methyl cations with water, chloride and bromide ions in various acetonitrile-water (AN:W) mixtures at 25°C have been investigated by measuring the secondary alpha deuterium kinetic isotope effects (s<math> <f> <g>a</g></f> </math>dKIEs, <math> <f> <rm>R<sup>+</sup></rm></f> </math> + Nu <math> <f> &rarrr;</f> </math> RNu) and secondary alpha deuterium equilibrium isotope effects (s<math> <f> <g>a</g></f> </math>dEIEs, RNu <math> <f> &rlhar2;</f> </math> <math> <f> <rm>R<sup>+</sup></rm></f> </math> + Nu). The s<math> <f> <g>a</g></f> </math>dKIE and s<math> <f> <g>a</g></f> </math>dEIE results for the three nucleophiles with ('4' '-methoxydiphenyl)methyl cation in 20:80 AN:W show that the transition state becomes looser when the nucleophile is changed from water to chloride and to bromide ions. Thus, with increased nucleophilicity, the transition state occurs earlier along the reaction coordinate, in accordance with the Hammond postulate. The identical s<math> <f> <g>a</g></f> </math>dKIEs of 0.89 for the reactions of '4'-methyl, ' 4'-hydrogen and ('4'-trifluoromethyl-'4' '-methoxydiphenyl)methyl cations with water in 20:80 and 90:10 AN:W indicate that the <math> <f> <rm>C<inf><g>a</g></inf></rm></f> </math>--OH2 transition state bond is constant when either the substituent in the carbocation or the solvent is altered. This is not in accordance with the Hammond postulate which predicts an early transition state, i.e., a longer <math> <f> <rm>C<inf><g>a</g></inf></rm></f> </math>--OH2 transition state bond, for a change in substituent in the carbocation from '4'-methyl to '4'-trifluoromethyl. A normal secondary alpha deuterium EIE of 1.20 was found for these reactions, written in the direction <display-math> <fd> <fl><rm>ROH+H<sup>+</sup></rm>&rlhar2;<rm>R<sup>+</sup></rm>+ <rm>H<inf>2</inf></rm><rm>O</rm></fl> </fd> </display-math>A comparison with the secondary alpha deuterium KIE shows that there is approximately 60 percent of <math> <f> <rm>C<inf><g>a</g></inf></rm></f> </math>--OH2 bond making in the transition state. The s<math> <f> <g>a</g></f> </math>dKIEs for the reactions of '4'-methyl and (' 4'-hydrogen-'4''-methoxydiphenyl)methyl cations with bromide ion in 20:80 AN:W also indicate that the transition states for these reactions are insensitive to a change of substituent in carbocation. With the comparison between s<math> <f> <g>a</g></f> </math>dKIE of 0.97 (<math> <f> <rm>R<sup>+</sup></rm></f> </math> + Br- <math> <f> &rarrr;</f> </math> RBr) and s<math> <f> <g>a</g></f> </math>dEKIE of 1.215 (for RBr <math> <f> &rlhar2;</f> </math> <math> <f> <rm>R<sup>+</sup></rm></f> </math> + Br-) proves that the <math> <f> <rm>C<inf><g>a</g></inf></rm></f> </math>--Br bonds were advanced to 20 percent in the transition state for these reactions. The s<math> <f> <g>a</g></f> </math>dKIE of 0.94 and s<math> <f> <g>a</g></f> </math>dEIE of 1.15 (for RCl <math> <f> &rlhar2;</f> </math> <math> <f> <rm>R<sup>+</sup></rm></f> </math> + Cl-) were measured for the reaction between chloride ion and ('4''-methoxydiphenyl)methyl cation in 20:80 AN:W. This also indicates that there was 40 percent of <math> <f> <rm>C<inf><g>a</g></inf></rm></f> </math>--Cl bond forming in the transition state. Finally, when the solvent was altered from 20:80 to 100:0 AN:W the s<math> <f> <g>a</g></f> </math>dKIE for the bromide reaction changed from 0.97 to 1.00. This indicates that when the reaction reaches the diffusion controlled rate in the latter solvent there is no KIE, i.e., no <math> <f> <rm>C<inf><g>a</g></inf></rm></f> </math>--Nu bond forming in the transition state.