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 \(\text{R}^{\text{o}}\text{Y}\). These forces may be governed by charge separations arising from differences in the @E01990@ of atoms (leading to the presence of dipoles), the presence of unipoles, or electron @D01583@. It is synonymous with @E01996@ or 'electrical effect' of a substituent as distinguished from other substituent effects, e.g. @S05997@. 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 @D01583@ between a substituent and the molecular framework to which it is attached are excluded. Polar effect is then not synonymous with electronic effect