Below is the table 2 with the data of 5-X-2-pyrrole carboxylic acids. In the figures 4-6, it is clear from the correlation coefficients, the pKa data of 5-X-2-pyrrole carboxylic acids correlated well (Hammett ρp = -1.539, R = 0.9873) with Hammett σp values with a deviation of again the lone -COOH substituent. This could be due to two types of intramolecular hydrogen bonding interaction as shown below in scheme 6 . Therefore the 5 position of the 2-pyrrole carboxylic acid is best assumed as para-position.