Primary hydrogen isotope effects on eliminations and rearrangements

3 Primary hydrogen isotope effects on eliminations and rearrangements 

Ion abundances for methane loss from metastable (CH3CD2CHD2)t ions have been found to be in the ratios 87 1 12 (CH4 CH3D CH2D2) after correction for the numbers of equivalent pathways [250]. The reaction is a 1, 2 elimination, so these results indicate both a large primary and a large secondary deuterium isotope effect (see Sect. 7.5.4). On the basis of these intramolecular isotope effects, a mechanism involving a non-classical transition state with a CCH three-centre bond has been proposed for this methane elimination [250]. 

Intermolecular isotope effects observed [534] in metastable ion decompositions of (C3H8)t and (C3D8)t again evidence the fact that intermolecular isotope effects on ion abundances are not a reliable guide to kinetic isotope effects. The isotope effect on methane loss (lcH4/fcD ) is 7 cf. 87 for (ICH4 //ch,d) with CH3CD2CHD2 (above). 

An FIK study of n-pentylbenzenes specifically labelled at 3, 7, 5, and e positions showed that at times longer than lOOps, formation of (C7H8)f ions occurred through hydrogen transfers via 3-, 4-, 5- and 6-membered cyclic transition states with an average isotope effect of 1.12 [90]. 

The (C6H7)+ ion observed in the chemical ionization (Cl) mass spectra of alkylbenzenes is attributed to a hydrogen rearrangement within the protonated alkylbenzene. Deuterium isotope effects upon the peak heights have been determined for ethylbenzene [514]. 

6-membered cyclic transition states with an average isotope effect of 

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