electron delocalization and resonance chapter 7

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Electron Delocalization and Resonance Chapter 7. Contents of Chapter 7. Benzene Delocalized Electrons and Resonance Stability of Allylic and Benzylic Cations and Radicals Effect of Electron Delocalization on p K a Relative Stabilities of Dienes Reactivity Considerations - PowerPoint PPT Presentation

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  • Electron Delocalization and ResonanceChapter 7

    Chapter 7

  • Contents of Chapter 7BenzeneDelocalized Electrons and ResonanceStability of Allylic and Benzylic Cations and RadicalsEffect of Electron Delocalization on pKaRelative Stabilities of DienesReactivity ConsiderationsElectrophilic Addition ReactionsDirect (1,2-) and Conjugate (1,4-) Additions

    Chapter 7

  • Delocalized Electrons and Resonance

    Chapter 7

  • Bonding in Benzene(a)Carboncarbon and carbonhydrogen s bonds(b)Carbon p-orbitals overlap with neighbors(c)Cloud of p electrons above and below ring(d)Electrostatic potential map for benzene

    Chapter 7

  • Delocalized Pi SystemsExtend continuously through all connected atoms which lack 4 single bonds (except H)Broken up by saturated atoms with 4 single bonds (C).Two double bonds on same carbon break up system (cumulated dienes)Broken by two saturated connected atoms with same Lewis acid/base properties (ie. 2 O atoms or C+ connected to B)

    Chapter 7

  • Resonance TricksLone-Pair/Double-Bond InterconversionOne atom charge goes down, other goes up.Atom receiving es has charge go more negative.

    Chapter 7

  • Resonance TricksDouble Bond FlipDouble bond flips like page in bookInstability (+, -, or ) jumps 1,3Electrons move diff. but effect same for +-

    Chapter 7

  • Resonance TricksElectron DominoesCombinations of interconversions and flipsInstability moves thru pi system in 2-atom increments.

    Chapter 7

  • Resonance Structures

    Chapter 7

  • Resonance Structures

    Chapter 7

  • Resonance Structures

    Chapter 7

  • Rules for Drawing Resonance Structures

    Chapter 7

  • The Resonance HybridWhen there is a choice, electrons tend to move toward the more electronegative atom

    Chapter 7

  • Resonance EnergyThe resonance energy tells us how much more stable the compound with delocalized electrons is due to that delocalizationThe greater the number of relatively stable resonance contributors, the greater the resonance energy

    Chapter 7

  • Resonance EnergyThe more nearly equivalent the resonance structures, the greater the resonance energy

    Chapter 7

  • Stability of Allylic and Benzylic Cations

    Chapter 7

  • Stability of Allylic and Benzylic Cations

    Chapter 7

  • Crude Carbocation Stability IndexAdd 1 for each attached carbon.Add 1 for adjacent double bond or phenyl ring.Subtract 1 if C+ on double bond.Alkene C doesnt count but C attached to either alkene C does.

    Chapter 7

  • Relative Stabilities of CarbocationsVinyl cations are one level less stable than alkyl cations

    Chapter 7

  • Effect of Delocalization on pKaCarboxylic acids (RCOOH) are much more acidic than and alcohols (ROH)pKa = 4.76 pKa = 15.9

    Chapter 7

  • Effect of Delocalization on pKa

    Chapter 7

  • Effect of Delocalization on pKaA protonated aniline is more acidic than a protonated cyclohexylaminepKa = 4.6pKa = 11.2

    Chapter 7

  • Types of DienesWhen double bonds are separated by at least one sp3 carbon, isolated diene

    Chapter 7

  • Types of DienesWhen double bonds are separated by only one single bond (i.e. four sp2 carbons in a row), conjugated diene

    Chapter 7

  • Types of DienesWhen both sets of double bonds emanate from the same carbon, cumulated diene

    Chapter 7

  • Relative Stabilities of Dienes

    Chapter 7

  • Relative Stabilities of DienesDoubly-bonded carbons in isolated and conjugated dienes all are sp2 hybridizedThe central carbon in a cumulated diene is sp hybridized

    Chapter 7

  • Electrophilic Addition Reactions of Isolated DienesReaction of 1,5-hexadiene with excess HBr adds HBr independently to each double bond Markovnikovs Rule is followed

    Chapter 7

  • Electrophilic Addition Reactions of Conjugated DienesConjugated dienes can give both 1,2- and 1,4- addition products

    Chapter 7

  • Mechanism of Addition of HBr to Conjugated DienesThe positive charge on the allylic cation is not localized on C-2, but is shared between C-2 and C-4

    Chapter 7

  • Electrophilic Additions to Conjugated DienesElectrophile adds to one end of pi systemDraw all resonance structures for C+ for adding electrophile to each end of pi systemEvaluate which set of structures is more stable (stability index)Number most stable C+ positions with even numbers, end of pi system #1Electrophile (H) adds to position #1Add nucleophile to each C+ (even-numbered C atom) in most stable set of resonance structures to get observed products

    Chapter 7

  • Electrophilic Addition to Conjugated Dienes Example

    Chapter 7

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