Below is the table 3 with the data of 5-X-2-thiophene carboxylic acids. In the figures 7-9, it is clear from the correlation coefficients, the pKa data of 5-X-2-thiophene carboxylic acids correlated well (Hammett ρp = -1.055, R = 0.9939) with Hammett σp values with a deviation of again the lone -COOH substituent. This could be due to the week intramolecular hydrogen bonding interaction of H of COOH with S of thiophene ring as shown below in scheme 7. It is revealed in some protein interactions that though sulfur is a moderately good H-bond donor but also a week H-bond acceptor . Therefore, again the 5 position of the 2-thiophene carboxylic acid is best assumed as para-position.