Resonance Effect

The existence of resonance within a conjugated system frequently ends up developing a real, ground-state POLARITY in the overall molecule -- and this specific polarity, arising directly from the interaction between conjugated pi bonds (or, equally, from the interaction between one pi bond and a p-orbital sitting on some atom directly attached to it), is called the RESONANCE EFFECT (also known, interchangeably, as the mesomeric effect). Unlike the inductive effect (which weakens rapidly with distance along a sigma-bond chain), the resonance effect is transmitted along the entire length of a chain of conjugated pi bonds, essentially without the same rapid fall-off. There are two distinct directions in which this effect can act. A POSITIVE resonance effect (written +R, or sometimes +M; also called an electron-donating or electron-releasing resonance effect) occurs whenever the attached substituent group itself carries a genuine LONE PAIR of electrons that it is able to donate INTO the pi bond (or conjugated pi system) it is attached to; groups showing this +R behaviour include $-OH$, $-OR$, $-O^{\ominus}$, $-NHR$, and the halogens -- every one of which carries at least one lone pair capable of being donated in this way. This +R donation specifically raises electron density at particular positions within the attached ring system; the chapter's worked example is aniline ($C_6H_5{-}NH_2$), where the nitrogen's own lone pair, donated by resonance into the aromatic ring, specifically raises electron density at the ORTHO and PARA ring positions (relative to the $-NH_2$ group) in particular, leaving the meta positions comparatively unaffected. A NEGATIVE resonance effect (written -R; also called an electron-withdrawing resonance effect), by contrast, occurs whenever the attached substituent group instead has a tendency to actively WITHDRAW electron density FROM the attached pi bond (or conjugated system) toward itself; groups showing this -R behaviour include $-COOH$, $-CHO$, $-CO-$, $-C …