Preferred Label : electronic effect of substituents: symbols and signs;
IUPAC definition : The inductive effect has universally been represented by the symbol I. This is now
commonly taken to include both through-bonds and through-space transmission, but I
is also used specifically for through-bonds transmission; through-space transmission
is then symbolized as F (for field effect). The symbols for the influence of substituents
exerted through electron delocalization have variously been M (mesomeric), E (electromeric),
T (tautomeric), C (conjugative), K (konjugativ) and R (resonance). Since the present
fashion is to use the term resonance effect, R is the most commonly used symbol, although
M is still seen quite often. Both the possible sign conventions are in use. The Ingold
sign convention associates electronegativity (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 -I and -M effects; chloro is described as a -I,
M substituent, etc. For correlation analysis and linear free-energy relationships
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 delocalization effect: thus the nitro group is
a I, R substituent; chloro a I, -R substituent, etc.;
Origin ID : E01996;
See also
The inductive effect has universally been represented by the symbol I. This is now
commonly taken to include both through-bonds and through-space transmission, but I
is also used specifically for through-bonds transmission; through-space transmission
is then symbolized as F (for field effect). The symbols for the influence of substituents
exerted through electron delocalization have variously been M (mesomeric), E (electromeric),
T (tautomeric), C (conjugative), K (konjugativ) and R (resonance). Since the present
fashion is to use the term resonance effect, R is the most commonly used symbol, although
M is still seen quite often. Both the possible sign conventions are in use. The Ingold
sign convention associates electronegativity (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 -I and -M effects; chloro is described as a -I,
M substituent, etc. For correlation analysis and linear free-energy relationships
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 delocalization effect: thus the nitro group is
a I, R substituent; chloro a I, -R substituent, etc.