https://doi.org/10.1351/goldbook.P04709
For a reactant molecule RY, the polar effect of the group R comprises all the processes whereby a substituent may modify the electrostatic forces operating at the reaction centre Y, relative to the standard . These forces may be governed by charge separations arising from differences in the
electronegativity
of atoms (leading to the presence of dipoles), the presence of unipoles, or electron delocalization
. It is synonymous with electronic effect
or 'electrical effect' of a substituent as distinguished from other substituent effects, e.g. steric effects
. Sometimes, however, the term 'polar effect' is taken to refer to the influence, other than steric, that non-conjugated substituents exert on reaction rates, i.e. effects connected with electron delocalization
between a substituent and the molecular framework to which it is attached are excluded. Polar effect is then not synonymous with electronic effectSee also:
field effect
, inductive effect
, mesomeric effect