Mass-spectral and n.m.r. analysis of thiophenes labeled by exchange with deuteriosulfuric acid establishes that exchange at the 2 and 5 positions is essentially complete before any deuterium is incorporated at positions 3 and 4. Thus, such exchange is a satisfactory procedure for positionspecific labeling. Mass spectra of the labeled thiophenes show that about 60% of the [CHS]+ ion yield is derived from molecular ions that have not undergone prior rearrangement. The remaining 40% arises by a path or paths in which the four hydrogen atoms lose position identity. Other decomposition paths contributing to the mass spectrum are characterized by more nearly complete scrambling of hydrogens.
DURING studies of gas-phase reactions of aromatic compounds at high temperature and under electron-impact,1-4 a need arose for thiophene-d,. The attempted preparation by exchange with deuteriosulfuric acid in deuterium oxide, stirred at reflux for 16 hours, gave a product consisting chiefly of di-and inonodeuteriothiophenes, with but little of the tri-and tetradeuteriated species. We had naively imagined that the reaction conditions were vigorous enough to attain nearly complete exchange. Evidently, the rate of exchange at the last two positions, presumably 3 and 4, was far lower than at the first two. This conclusion is in accord with reports,"S6 which we had missed earlier, that the exchange rate with D,S0,3 or the tritiated acid at positions 2 and 5 is about 1000 times as great as at positions 3 and 4.* More generally, molecular-orbital calculations and considerable experimental data indicate that the 2 and 5 positions are substantially more reactive than the 3 and 4 in nucleophilic, free-radical, and electrophilic substitution reaction^.^ An attempt to account for a parallel difference in reactivity in platinum-catalyzed deuteriation of thiophene led to a rather detailed mechanistic proposal for the interaction between the molecule and the catalyst surface.8 The far smaller reactivity difference with platinums than with the acid would seem, in retrospect, more in need of a special rationale.
In view of these preliminary results, we extended the reaction time ultimately to 1212 hours, adding fresh D,SO, at intervals. The isotopic compositions of the thiophene samples so obtained cover a sufficiently wide range to permit derivation from their mass spectra of the spectra corresponding to each of the pure isotopic species. Such calculation is based on the assumption that the number of deuterium atoms in the thiophene molecule defines the chemical species: that is, that the isomer distribution of thiophene-d,, for example, is the same in all the samples in which it * Although these kinetic s t ~d i e s ' ~~ established a large difference between the exchange rates at the two positions, the composition of the labeled thiophenes, in the absence of n.m.r. and mass spectral analysis, is not entirely clear.