Acetal formation from formylthiophene-2-carboxylic acids and recalculation of σI and σR substituent constants of the acetal group by 13C NMR chemical shifts

A uv-visible, ¹H and ¹³C nmr study has been carried out to show that 4-, 1, and 5-formylthiophene-2-carboxylic acids, 2, give acetals in methanol. The ms data have confirmed the acetal formation, which has been studied kinetically by the uv technique. The substituent chemical shifts induced on the carboxy carbon atom and on the endocyclic carbon atoms in the acetals formed from 1 and 2 and in the corresponding anions have been used to recalculate σ_I and σ_R values by means of a dual substituent parameter treatment of data.