Can Non-bonded Pair of Electrons of Sp3 Nitrogen with Two Single σ-Bonds on Either Side Still Transmit Substituent Electronic Effects to the Reaction Site? Reversal of Attenuation Effect by Sp3 Nitrogen–A Chemical Education Perspective

Abstract

Dependence of reactivity of organic molecules on substituents was a well-established phenomenon in terms of Hammett and Taft equations in physical-organic chemistry. The well-known Hammett and Taft linear free energy relationships were extensively used in elucidating the organic reaction mechanisms. The exponential depletion of Hammett reaction constant (ρ), as a function of distance in terms of increasing the number of sp3 carbon atoms (-CH2-) between the reaction center and the substituent, is understood from our laboratory experiments. But, introduction of sp2 carbon atoms (-CH=CH-) between the reaction center and the substituent enhances the Hammett reaction constant (ρ). In the present work, we have tried and observed the same and even little more effective transmission of substituent effect through sp3 nitrogen (-NH-). However, the presence of a sp3 carbon by the side of sp3 nitrogen (-NH-CH2-) depletes the substituent effect in the usual manner in N-phenyl glycines. Probable explanations were presented in support of our observation. In the present work, the observations were manifested in terms of Taft ρ* values instead of Hammett ρ value as the pKa values of only 4-nitopheylcarbamic acid and carbamic acids and the Taft σ* values of 4-nitrophenyl and H are available from literature.