Electron displacement effects

Inductive effect: a permanent shift of σ-bond electrons towards a more electronegative atom, polarising the chain; it weakens quickly and is usually negligible beyond three bonds.

−I (electron-withdrawing) groups include –NO₂, –CN, –COOH, –COOR, halogens and aryloxy (–OAr); among halogens the −I effect is F > Cl > Br > I.

+I (electron-releasing) groups are mainly alkyl groups such as –CH₃ and –C₂H₅.

Resonance (mesomeric) effect: a permanent polarity caused by delocalisation of π electrons or lone pairs through a conjugated system.

+R groups push electron density into the conjugated system: halogens, –OH, –OR, –OCOR, –NH₂, –NHR, –NR₂, –NHCOR.

−R groups pull electron density out of it: –COOH, –CHO, >C=O, –CN, –NO₂.

Electromeric effect: a temporary, complete transfer of a π-electron pair to one atom of a multiple bond, occurring only while an attacking reagent is present and disappearing when it is removed.

+E: π electrons move to the atom to which the reagent attaches; −E: they move to the atom away from the attacking reagent.

Hyperconjugation: delocalisation of σ electrons of a C–H bond on the carbon next to (alpha to) a carbocation or a π system; more alpha hydrogens mean more stabilisation.

Hyperconjugation is also called no-bond resonance, and it adds to the explanation of the 3° > 2° > 1° > CH₃⁺ carbocation order and of the stability of more substituted alkenes.