However, Freeman reported the Hammett plot log k2 versus σ for MnO4 -oxidation of 5-X-furfurals in aqueous neutral and basic medium with substituents X = H, Me, n-Bu, Et, Cl and Br. Though MnO4 -oxidation of 5-X-furfurals with X = NO2 studied, but not shown in the Hammett plot. But the data is not available in this publication except the Hammett plot. Therefore, it was not possible to analyze the data in detail in the present study. The plot of pKa versus Taft σ*ortho (figure ) is not linear. Then we thought that using Swain and Lupton type equation (σ = fF + rR) and plotting pKa versus F (field effect contribution) (figure ) and pKa versus R (resonance effect contribution) (figure ) to see the individual field and resonance effects on the acid dissociation. But none of them are linear. The non-linearity of any of the three plots could possibly be due to the involvement of various types of intramolecular hydrogen bondings in 5-X-1, 2, 3-triazole-4-carboxylic acids as shown below in scheme 11. Due to the presence of these hydrogen bonding any of the substituent does not behave as if it is a pure isolated substituent, hence the non-linearity in the pKa versus Taft σ*ortho, pKa versus F and pKa versus R plots. In the figures 36-38 it is clear from the correlation coefficients, the pKa data of 3-X-pyrroles correlated well (Hammett ρm = -3.91, R = 0.9883) with Hammett σm values. Therefore the 3 position of the 3-X-pyrroles is best assumed as meta-position. In the figures 39-41 it is clear from the correlation coefficients, the pKa data of 3-X-pyrazoles correlated well (Hammett ρmeta = -7.77, R = 0.9943) with Hammett σm values. Therefore the 3 position of the 3-X-pyrazoles is best assumed as meta-position. Therefore the 4 position of the 4-X-imidazoles is best assumed as meta-position. The substituent effect may be better transmitted through the 5 positioned sp2 carbon. Therefore the 3 position of the 1-substituted (N1H) 3-X-1,2,4-triazole is best assumed as meta-position. The situation here is analogous to the structure F of scheme 3 as shown below in scheme 13. Therefore the 4 position of the 1-substituted (N1H) 4-X-1,2,3-triazole is best assumed as meta-position. The situation again here is analogous to the structure F of scheme 3 as shown below in scheme 14 but in the opposite direction via carbon 5. The effect of substituents (X) on the pKa values of 5-X-1, 2, 3, 4-tetrazoles is rather complex . Tetrazole itself is a strange molecule with 80% nitrogen of the total weight of the molecule. It is known that the tetrazole molecule exists in two tautomeric forms as shown in scheme 15 . In a nonpolar medium, both the 1H and 2H tautomers are predicted to exist in comparable amounts. In the solvents like water with high dielectric constant the existence of more polar 1H-tautomer is appreciable . From figures 42-45, the correlation of pKa values with para and meta substituents is good (R = 0.9769 and 0.9931) with a deviation of NH2 group with the para-susbtituents correlation. Though the content of 2H-tautomer is less than the 1H-tautomer in more polar solvents (since the pKa values are from water as the solvent ), yet the para and meta substituent effects originate from less abundant 2H-tautomer. If one looks at the 2H-tautomer (scheme 15), the functional group (N2-H) is at meta to X at position 5 via nitrogen 1 and it will be para to X at position 5 via nitrogen 3 and 4. This is just like the visual observation for defining the para and meta positions of 5 membered heterocycles as given in scheme 2 and scheme 3. This is the reason that the pKa values are well correlated with both Hammett σpara and σmeta substituent constants. The deviation of NH2 in the correlation with para substituents may be due to the intramolecular hydrogen bonding as shown in scheme 16. From Taft plots given in figures 44 and 45 it may be assumed that the ortho-substituent effects originate from 1H-tautomer. If one looks at the 1H-tautomer (scheme 15), the functional group (N1-H) is at ortho to X adjacent to nitrogen 1. This is just similar to the visual observation for defining the ortho position of 5 membered heterocycles as given in scheme 4. Also, it is noteworthy to see the correlation is improved without the bulky groups like CF3, I and NO2 (figure ).