Electron Delocalization and ResonanceChapter 7 Chapter 7 1 Contents of Chapter 7 Benzene Delocalized Electrons and Resonance Stability of Allylic and Benzylic Cations and Radicals Effect of Electron Delocalization on pKa Molecular Orbital Description of Stability Relative Stabilities of Dienes Reactivity Considerations Electrophilic Addition Reactions Direct (1,2-) and Conjugate (1,4-) Additions Thermodynamic vs. Kinetic Control Diels-Alder Reaction Chapter 7 2 Delocalized Electrons and Resonance Chapter 7 3 Bonding in Benzene (a) (b) (c) (d) Carbon±carbon and carbon±hydrogen W bonds Carbon p-orbitals overlap with neighbors Cloud of T electrons above and below ring Electrostatic potential map for benzene Chapter 7 4 Delocalized Pi Systems Extend 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) H H .. OH N B .. C .. : O: + H Two separate pi systems broken up by saturated tetravalent carbon Chapter 7 5 Resonance Tricks Lone-Pair/Double-Bond Interconversion + O .. H + .. O .. H O: .. + .. O .. : One atom charge goes down, other goes up. Atom receiving e¶s has charge go more negative. Chapter 7 6 Resonance Tricks Double Bond Flip + + .. .. . . Double bond flips like page in book Instability (+, -, or ) jumps 1,3 Electrons move diff. but effect same for +- Chapter 7 7 Resonance Tricks Electron Dominoes .. :O .. O .. : : .. O .. O: .. Combinations of interconversions and flips Instability moves thru pi system in 2-atom increments. Chapter 7 8 Resonance Structures Chapter 7 9 Resonance Structures Chapter 7 10 Resonance Structures Chapter 7 11 Rules for Drawing Resonance Structures Chapter 7 12 The Resonance Hybrid When there is a choice, electrons tend to move toward the more electronegative atom Chapter 7 13 Resonance Energy The resonance energy tells us how much more stable the compound with delocalized electrons is due to that delocalization The greater the number of relatively stable resonance contributors, the greater the resonance energy Chapter 7 14 Resonance Energy The more nearly equivalent the resonance structures, the greater the resonance energy Chapter 7 15 Stability of Allylic and Benzylic Cations Chapter 7 16 Stability of Allylic and Benzylic Cations CHR CHR CHR CHR CHR Chapter 7 17 Stability of Allylic and Benzylic Cations Chapter 7 18 Crude Carbocation Stability Index Add 1 for each attached carbon. Add 1 for adjacent double bond or phenyl ring. Subtract 1 if C+ on double bond with R on other end. Subtract 2 if C+ on double bond without R on other end. } 4 4 " 3 } 3 " " 1 19 } 2 2 " 1 " 0 Chapter 7 } CH 3 0 Chemical Consequences of Electron Delocalization Rearrangements also are driven by electron delocalization The driving force for the rearrangement is the formation of a more stable secondary benzylic cation Chapter 7 20 Effect of Delocalization on pKa Carboxylic acids (RCOOH) are much more acidic than and alcohols (ROH) O CH3COH pKa = 4.76 CH3CH2OH pKa = 15.9 Chapter 7 21 Effect of Delocalization on pKa Chapter 7 22 Effect of Delocalization on pKa A protonated aniline is more acidic than a protonated cyclohexylamine pKa = 4.6 pKa = 11.2 Chapter 7 23 Types of Dienes When double bonds are separated by at least one sp3 carbon, isolated diene Chapter 7 24 Types of Dienes When double bonds are separated by only one single bond (i.e. four sp2 carbons in a row), conjugated diene Chapter 7 25 Types of Dienes When both sets of double bonds emanate from the same carbon, cumulated diene Chapter 7 26 Relative Stabilities of Dienes Chapter 7 27 Relative Stabilities of Dienes Doubly-bonded carbons in isolated and conjugated dienes all are sp2 hybridized The central carbon in a cumulated diene is sp hybridized Chapter 7 28 Electrophilic Addition Reactions of Isolated Dienes Reaction of 1,5-hexadiene with excess HBr adds HBr independently to each double bond Markovnikov¶s Rule is followed Chapter 7 29 Electrophilic Addition Reactions of Conjugated Dienes Conjugated dienes can give both 1,2and 1,4- addition products Chapter 7 30 Mechanism of Addition of HBr to Conjugated Dienes The positive charge on the allylic cation is not localized on C-2, but is shared between C-2 and C-4 Chapter 7 31 Thermodynamic vs. Kinetic Control The product that is formed most rapidly is the kinetic product The most stable product is the thermodynamic product Chapter 7 32 Electrophilic Additions to Conjugated Dienes Electrophile adds to one end of pi system Draw all resonance structures for C+ for adding electrophile to each end of pi system Evaluate which set of structures is more stable (stability index) Add nucleophile to each C+ in this set of structures to get observed products Product with most highly substituted double bonds is thermo prod (count total numbr of R¶s directly attached to alkene carbons). Kinetic product is 1,2 addition product. Chapter 7 33 Electrophilic Addition to Conjugated Dienes Example Br 2 3O add'n of Br to left end of pi system gives resonance structures ith index values of 3, 3, and 2 (less stable) stability index 4 index 3 O 3 Br O 1,2 addition kinetic product 3 Br 4 R groups on alkenes Chapter 7 Br Br Br index 2 Br O 3 5 R groups on alkenes thermodynamic product 34 Retrosynthetic Analysis of 1,4 Addition Products 2 3 HO 4 2 1 3 4 1 Find the group that added as nucleophile in product If it¶s attached to an allylic carbon you can make product from 1,4 addition Number carbon with nucleophile 4 and number downards through double bond to next C (no. 1) Remove added groups and put double bonds between C¶s 1&2, 3&4. Chapter 7 35 Diels±Alder Reaction Regioselectivity Diene is nucleophilic; make it reactive with e donor Dienophile electrophilic; activate by e withdrawing Nucleophile has more pi e¶s (4) than electrophile (2) Donor and withdrawing grp wind up on adjacent C¶s Chapter 7 36 Diels±Alder Reaction Stereoselectivity The Diels±Alder reaction is stereospecific because different stereoisomeric reactants give different stereoisomeric products Chapter 7 37 Diels±Alder Reaction Stereoselectivity When the diene is cyclic, the endo product is preferred Chapter 7 38 Diels±Alder Product Analysis 8 8 9 5 7 6 CN 10 4 3 2 O C H3 1 ( 9 5 4 7 CN 6 O CH 3 10 1 2 3 Number diene and dienophile atoms to facilitate analysis Diene gets numbers 1-4 with end e donor given No. 1 Dienophile gets numbers 5 & 6 with EWG given No. 6 Numbers on other C¶s will show what¶s connected in prod Draw 6-membered ring of prod first and number 1-6 Extra double bond goes between C¶s numbered 2 & 3 Chapter 7 39 Diels-Alder Retrosynthetic Analysis d / d / C C CH CH + CH C CH D-A prod is 6-membered ring with double bond (db) First project db to opposite side of ring Break 2 single bonds adjacent to new db pos¶n Double 2 single bonds adjacent to old db pos¶n Do analysis with scripting notation first, then use eraser Chapter 7 40