GAS-PHASE ANALOGS OF SOLVOLYSIS - THE CONJUGATE ACID OF NORMAL-PROPYL PHENYL ETHER

The analogy between first-order decomposition of gaseous ions via ion-neutral complexes and solvolytic elimination is explored using an MS/MS technique, chemical ionization-metastable ion kinetic energy spectroscopy (CI-MIKES). Expulsion of propene from M + 2 conjugate acid ions in the methane-d4 CI-MIKES of n-propyl phenyl ethers shows isotopic exchange that is difficult to explain unless complexes containing propene and PhOHD+ are formed reversibly. Comparison of the CD4 CI-MIKES of CH3CD2CH2OPh (beta-d2), CD3CH2CD2OPh (alpha, gamma-d5), and CH3CH2CH2OPh (d0) shows that further isotopic scrambling takes place, which is interpreted in terms of intermediates of the form [i-Pr+ phenol]. Steady-state kinetic analysis of relative product yields shows that the ion-neutral complex [CH3CHCH3+ PhOD] is at least 5 times more likely to yield free propene than to form a complex of the form [C3H6PhOHD+]. The complex [CH3CDCH3+ PhOH] isomerizes to [CDH2CHCH3+ PhOH] but is less than half as likely to do so as to yield free CH3CD=CH2. If the isotope effect for the proton transfer that yields free propene is assumed to be much larger than any other isotope effect in the reaction sequence, then its value is k(H)/k(D) = 1.5, which is consistent with proton transfer to the phenol ring.