Significance of Hammett and Taft substituent constants on bonding potential of organic corrosion inhibitors: Tailoring of reactivity and performance

By changing the electron density at donor sites (also called adsorption or coordination sites), the substituents significantly impact the coordination proficiency of organic inhibitors. The effects of the substituents can be inductive or resonance depending on where they are located in the aromatic ring. The substituents at the p-position can produce either inductive (I–) or resonance (R–) or a combined effect of both, and the substituents at the m-position can manifest the inductive effect only. The substituents effect on the inhibition potential of aromatic and aliphatic compounds can be described with the help of Hammett (σ) and Taft (σ*) constants, respectively. The polar substituents such as –OH, –OCH3, –NH2, –NMe2, –CN, –NO2, –COOH etc. at p-position develop resonance effect. Because of the potential chelation and substituent effects of polar substituents at the o-position, the corrosion inhibition efficiency (%IE) may occasionally be improved by e-withdrawing polar substituents at the o-position. In general, e-withdrawing substituents like –NO2, –CN, –COOH, etc. have a negative effect on the %IE because of their –R-effect, but in macromolecules (polymers), they may improve %IE by making the inhibitor more soluble. The present article describes the substituents effect on organic inhibitors' coordination ability and binding potential based on the information available in the literature. In addition to the inductive and resonance effects, other factors have been explored, including steric hindrance, the molecular size effect, solubility, and chelation. This paper discusses the significance of the Hammett (σ) and Taft (σ*) constants on the %IE of organic compounds, but it is also possible to link them to their various other applications, including tribological, pollutant decontamination, medicinal, and catalytic.