Resonance StructuresResonance structures are needed when the line structure for a compound doesn't accurately portray the bonds present in the molecule. This most commonly occurs when the p orbital on a charged atom or radical overlaps with one or more pi bonds next to it, spreading the charge around and stabilizing the molecule.Resonance structures are an approximation of the actual molecular orbitals; the true characteristics of the compound are a blending of the individual resonance structures.Resonance structures are always shown in brackets, and have a double-headed arrow between them.Resonance structures Actual compoundthe pi bond is actually spread over all three atoms+1/2 +1/2the + charge is shared by both of the atoms on the endWhen to use them and what they mean+ +three original p orbitalsthree new pi orbitalsSince there is a positive charge, one electron has been lost.both electrons go to the lowest energy pi orbital which includesall three atomsthe nonbonding pi orbital where the third electron should be is empty - the positive charge is on the two end atomsthe top (antibonding) pi orbital is emptyCompounds with resonance structures are always more stable than similar compounds that don't have resonance structures.no resonance structuresless stablestabilized by resonanceResonance structures may only occur when the p orbitals are next to each other. If there is an sp3 atom in between, no overlap can happen.no resonancep orbitals can't overlapResonance structures most often occur cations, radicals, and anions which are allylic or benzylic, in aromatic reaction intermediates, or in enolates.OHBrNO2OHClallylic Ch 15benzylic Ch 16electrophilicaromatic substitutionCh 16nucleophilic aromatic substitutionCh 16enolatesCh 18The true picture involves molecular orbitals in which pi bonds are spread over three or more atoms. As a result, pi bonds and lone electron pairs (and the resulting positive or negative charges) may also be spread over more than one atom.If all of the resonance structures are equal in energy, then they all contribute equally to the actual compound.OCOHOCOHboth have a C=Oboth have a negatively charged OOCCHHHHCCOHHgreaterresonance contributorlesserresonance contributorEqual, greater, or lesser resonance contributorsIf one or more of the structures are lower in energy (more stable), they will contribute more to the actual compound; that is to say, the actual compound will be more like them. These are called "greater resonance contributors". The structures which are higher in energy (less stable) will contribute less to the actual compound; it will be less like them. They are called "lesser resonance contributors".equal resonance contributorsbond shared equally, giving 1 1/2 bonds between both C-O'sOCOH-1/2-1/2Resonance structures Actual compoundHCCOHHless than 1 1/2 bondmore that 1 1/2 bondnegatively charged O more stablenegatively charged Cless stableResonance structuresActual compoundC=O about the same as C=Celectron pair spread acrossboth O atoms, both have -1/2 chargeelectron pair less here;less than -1/2 chargeelectron pair more here;more than -1/2 chargepi bond with lone electron pair is larger on Opi bond across all three is larger on the two C'sDifference between resonance and equilibriumThe compound represented by two or more resonance structures is not going back and forth between these structures; rather it is a blending of the structures. On the other hand, an equilibrium involves an actual change in the molecule; it is represented by two arrows in opposite directions, rather than a double-headed arrow, and has no brackets.OCCCHHHHHHCCCOHHHHHHEquilibriumIn an equilibrium, atoms change positions; in a set of resonance structures, only electrons appear to move (they are actually spread out). Atoms cannot occupy two places at once, while electrons are waves which can be spread over several nuclei.Resonance