## electronic effect of substituents: symbols and signs

https://doi.org/10.1351/goldbook.E01996
The @[email protected] has universally been represented by the symbol $$\text{I}$$. This is now commonly taken to include both through-bonds and through-space @[email protected], but $$\text{I}$$ is also used specifically for through-bonds @[email protected]; through-space @[email protected] is then symbolized as $$\text{F}$$ (for @[email protected]). The symbols for the influence of substituents exerted through electron @[email protected] have variously been $$\text{M}$$ (@[email protected]), $$\text{E}$$ (@[email protected]), $$\text{T}$$ (tautomeric), $$\text{C}$$ (conjugative), K (konjugativ) and $$\text{R}$$ (@[email protected]). Since the present fashion is to use the term @[email protected], $$\text{R}$$ is the most commonly used symbol, although $$\text{M}$$ is still seen quite often. Both the possible sign conventions are in use. The Ingold sign convention associates @[email protected] (relative to hydrogen atom) with a negative sign, electropositivity with a positive sign. Thus the nitro group is described as electronwithdrawing by virtue of its $$-\,\text{I}$$ and $$-\,\text{M}$$ effects; chloro is described as a $$-\,\text{I}$$, $$+\,\text{M}$$ substituent, etc. For @C[email protected] and @[email protected] this convention has been found inconvenient, for it is in contradiction to the sign convention for polar substituent constants (σ-constants). Authors concerned with these fields often avoid this contradiction by adopting the opposite sign convention originally associated with Robinson, for electronic effects. This practice is almost always associated with the use of R for the electron @[email protected] effect: thus the nitro group is a $$+\,\text{I}$$, $$+\,\text{R}$$ substituent; chloro a $$+\,\text{I}$$, $$-\,\text{R}$$ substituent, etc.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1111 [Terms] [Paper]