Molecular orbital calculations of the acid-catalysed hydrogen exchange in substituted thiophenes.

Continuing the early w o rk of M e l a n d e r , O l s s o n and coworkers1 who studied the heterogeneous acid catalysed hydrogen exchange in thiophene, determina­ tions of the dedeuteration 2 and detritiation 3 rates of a number of substituted thiophenes in homogeneous con­ ditions were recently carried out. The relative rates are strongly dependent on the substitution in the thio­ phene ring. S h a t e n s h t e in et al. 2 showed that in mix­ tures of acetic and fluoroacetic acid the ratio of the dedeuteration rates of 5-methoxythiophene-2-d and thiophene-3-d is about 1010 under same conditions. The relative exchange rates are not much dependent on the reaction conditions. A ratio of 200 was observed 2 for the dedeuteration rates of 5-methylthiophene-2-d and thiophene-2-d in 33.3 mole-% trifluoroacetic acid, 66.7 mole-% acetic acid at 25°. Detritiation of the tritium analogs at the same temperature in 44.5 mole-% and 27.9 mole-% trifluoroacetic acid gave ratios of 206 and 202, respectively 3. This suggests that the exchange rate is primarily a function of the electronic configura­ tion. Such an assumption was previously made by M e l a n d e r 4 who thus explained the ratio of exchange rates for the 2and 3-position in thiophene. Now having more data available concerning the homo­ geneous acid catalysed hydrogen exchange of different positions in substituted thiophenes, we thought it worth­ while to see whether the measured relative rates cor­ relate with theoretical quantities reflecting the electro­ nic structure and reactivity of these compounds to-