Electron displacement effects and reaction types

The electrons in a molecule can be pulled or pushed in four ways, called electron displacement effects:

• Inductive effect: a permanent shift of σ electrons along a chain, caused by a polar bond. It fades quickly with distance, becoming negligible after about three carbons. Groups that pull electrons show a –I effect: Cl, NO₂, CN, COOH. Groups that push electrons show a +I effect: alkyl groups.
• Resonance (R or mesomeric, M) effect: the spreading out (delocalisation) of π or lone-pair electrons through a conjugated system (alternating single and multiple bonds, as in benzene). Groups that push electrons into the system show +R: –OH, –OR, –NH₂, halogens. Groups that pull electrons out show –R: –NO₂, –CHO, –CN, –COOH.
• Electromeric effect: a temporary, complete shift of a π pair to one atom, which happens only when an attacking reagent comes near, and disappears when it goes.
• Hyperconjugation: the electrons of C–H σ bonds next to a positive carbon (or a π system) spread into its empty p orbital. The more such C–H bonds, the more stable the species. (CH₃)₃C⁺ has nine of them, so it is far more stable than CH₃CH₂⁺, which has three.

When drawing resonance structures, the atoms must stay in place. The most important structures have the most covalent bonds, full octets, the least separation of charge, and any negative charge on the more electronegative atom. The real molecule, the resonance hybrid, is lower in energy than any single structure; the difference is the resonance energy.

Organic reactions are of four main types: substitution (one atom or group is replaced by another), addition (atoms add across a multiple bond), elimination (atoms are removed, often forming a multiple bond) and rearrangement (the skeleton itself changes).