Electron Delocalization and Resonance> More about Molecular Orbital Theory | CHEM 2261, Study notes of Organic Chemistry

Download Electron Delocalization and Resonance> More about Molecular Orbital Theory | CHEM 2261 and more Study notes Organic Chemistry in PDF only on Docsity! 1 Chapter 7 Electron delocalization and resonance. More about molecular orbital theory 2 Benzene Structure (C6H6) Br2 Br Br Br Br + H+/H2O OH* Hg(OAc)2, H2O OH* 1. BH3 OH* 2. H2O2, OH -, H2O C C C C C C H H H H H H or Br2 H /H2O + Hg(OAc)2 BH3 H2O H2O2, OH -, H2O no reaction no reaction no reaction no reaction C-C 0.154 nm C=C 0.134 nm C-C(benzene) 0.139 nm 5 Electron Delocalization and Resonance (7.1 - 7.6) X : X : X X : XX : + - + Drawing resonance contributors (7.4) The electrons in one resonance contributor are moved to generate the next resonance contributor. Rules for drawing resonance contributors: 1.Only p or nonbonding electrons move. σ electrons do not move. Atoms never move 2.The total number of paired or unpaired electrons does not change 3.Row two elements (B, C, N, O, F) may not carry more than 8 electrons in a resonance structure. 4.Move electrons towards a sp2 hybridized atom (a positive charge, a  bond or a radical), or from a sp3 hybridized atom with nonbonding electrons. Moving x electrons toward a x bond 8 — © resonance contributors resonance hybrid + + aoe CH),—CH=CH—CH, <——> ncton cu ttn, —> CH,—CH=CH—CH, resonance contributors CH)==CH=-CH==CH) resonance hybrid pp 292,295 6 7 Moving a nonbonding pair of electrons toward a  bond p 295 10 Problem: Which of the following molecules have delocalized electrons? O O O 11 Electron Delocalization and Resonance (7.1 - 7.6) Predict the stability of resonance structures 12 O O O : - O : - ? H2C CH2 H2C CH2 H2C CH2 H2C CH2 Relative stability of resonance structures Non-octet on C, Charge separation Non-octet on C, Charge separation stable next stable least stable stable not stable 15 Problem: write legitimate resonance structures for each of the following structures and identify which are significant resonance contributors. CH2 CH CH2 + H2C CH CH2 H2C CH CH2 H2C CH CH2 H2C CH CH2 H2C CH CH2 16 Problem: write legitimate resonance structures for each of the following structures and identify which are significant resonance contributors. CH2 CH CH2 + H2C CH CH2 H2C CH CH2 H2C CH CH2 H2C CH CH2 H2C CH CH2 17 H2C CH CH2 H2C CH CH2 H2C CH CH2 H2C CH CH2 H2C CH CH2 CH2 CH CH2 + H2C CH CH2 stable Problem: write legitimate resonance structures for each of the following structures and identify which are significant resonance contributors 20 Problem: write significant resonance structures for each of the following structures. Cl Cl Cl Cl Cl + : : : Cl 21 Problem: write legitimate resonance structures for each of the following structures. C C H R H C H H C C C H H R H H C C C H H R H H C C C H H R H H 22 Problem: write significant resonance structures for each of the following structures. C C H R H C H H C C C H H R H H C C C H H R H H C C C H H R H H 25 R C O OH R C O OH relatively stable less stable R C O O R C O O equally relatively stable C O O O C O O O C O O O equally relatively stable Stability of Dienes: Isolated, Cumulated and Conjugated (7.7) Dienes are hydrocarbons with 2 double bonds that can be conjugated (separated by one single bond), isolated (separated by more than one single bond) or cumulated (adjacent). CH,CH=CH—CH=CHCH, CH,=CH—CH,—CH=CH, a conjugated diene an isolated diene CH,—CH=C=CH—CH3 a cumulated diene an allene 26 27 54.1 60.2 70.5 E n e r g y p 301 30 Always ask yourself:  How many MOs should there be? The number of MO = The number of AO  How many nodes does each MO have? The number of node: 0, 1,2…n-1 (n = total number of AO). The number of node goes up as the MO goes up in energy.  How many bonding and anti-bonding interaction does each MO have?  Is the MO bonding, anti-bonding (or non-bonding) orbital?  Is the MO symmetrical or asymmetrical? MO Description of Stability of Conjugated Systems 31 C C H HH H p 306 asymmetric symmetric The number of node: 0, 1,2…n-1 (n = total number of AO) p 307 Energy Ya LUMO p atomic orbitals energy of the | Yo — HOMO vy a molecular orbitals energy levels 1,3-butadiene 32 Copyright © 2007 Pearson Prentice Hall, Inc. 35 3 nodal planes 2 nodal planes 1 nodal plane 0 nodes p 311 36 Benzene is unusually stable because of large delocalization energies p 311 37 Electron Delocalization and Resonance (7.7,7.8) Rules for aromaticity (see book pages 640-647): 1. compound must have an uninterrupted cyclic cloud of electrons above and below the plane of the molecule (cyclic, planar molecules) 2. must have an odd number of pairs of electrons. “Hückel’s rule” (book, page 642) says “4n+2 -electrons” (n = 0, 1, 2, 3, 4……). Compounds that fulfill rule 1 but have an even number of pairs of electrons are called antiaromatic compounds N N H O antiaromatic aromatic aromatic aromatic aromatic not aromatic 40 How delocalized electrons affect pKa (7.9) Why is a carboxylic acid a stronger acid than an alcohol? R C O OH + H2O R C O O + H3O Carboxylic acid: R OH + H2O R O + H3O Alcohol: 1. presence of electron-withdrawing group 2. resonance energy of carboxylate ion pKa = 4.8 pKa = 16 ee :0 ae R OH oe relatively stable | C R~ SOH relatively unstable resonance contributors of a carboxylic acid :0 | + Cc . + H RO GF oe relatively stable R~ “SO: relatively stable resonance contributors of a carboxylate ion Copyright © 2007 Pearson Prentice Hall, Inc. Al 42 Why is phenol a stronger acid than ethanol? Phenol: CH3CH2 OH + H2O CH3CH2 O + H3O Ethanol: OH + H2O + H3O O 1. an sp2 carbon is more electronegative than an sp3 2. resonance energy of phenoxide ion pKa = 16 pKa = 10 More about Resonance Structures + We OY ON Or <= <__—_ <= OD — Co — CO— Co 45 46  Stability of allylic and benzylic cations (7.7) Allylic and benzylic cations have delocalized electrons, therefore are more stable than similarly substituted carbocations with localized electrons. CH2 CH CH2 CH2 CH CH2 allyl cation CH2 CH2 CH2 CH2 CH2 benzyl cation 47 Relative Stabilities of Allylic and Benzylic Cations p 304 50 Some Chemical Consequences of Electron Delocalization Watch out for 1,2 shifts! CAREFUL!! 51 O H + HBr ? ? Problems: predict the major reaction product 52 HBr O H + 1,2-shift of hydride Br O H + HBr ? ? Electrophilic Addition Reactions of Isolated Dienes: CH,—CHCH.CH,CH=CH, + HBr —-> CHLCHCH,CHLCHCHE 1,5-hexadiene excess a | mechanism for the reaction of 1,5-hexadiene with excess HBr Fie Br pO cece, ganas ke IER “Br: | CH,=CHCH;CH,;CH=CH, + Bi: _ CH;CHCH,CH,CH= CH; =, CH;CHCH;CH,CH oe H—Br: Br Br a I | 'Br: I ‘Br: + CH;CHCH;CH,CHCH; gots CH,CHCH,CH,CHCH; p 317 55 56 If there is only 1 mole of HBr, it will add preferentially to the more reactive double bond (the one that forms the most stable carbocation) HBr 1 mol Br 57  Reactions of conjugated dienes (7.10) When 1 mol of reagent is present two products are formed: 1,2-addition product from direct addition 1,4-addition product from conjugate addition Br Br 1,2 product 1,4 product HBr 1 mol + 1 2 3 1 2 3 4 4 60 1 mol Br2 HBr 1 mol 1 mol HBr Br Br Br Br Br Br Problems: What are the major products from the following reactions? 61 Electron Delocalization and Resonance (7.9 - 7.10)  Thermodynamic vs kinetic control (7.11) The kinetic product is the one that forms faster. The thermodynamic product is the most stable. At low temperature the reaction is irreversible and the major product is the kinetic product. At high temperature the reaction is reversible (involves an equilibrium) and the major product is the thermodynamic product. 62 kinetic control Thermodynamic or equilibrium control Compare the two addition product distribution at different reaction temperatures pp 322,323 secondary allylic carbon primary allylic carbon \ \ HB + + CH,=CHCH=CH, —— > CH;CHCH=CH, <—> CH,;CH=CHCH; + Bro | + Br ét F st # CH3CHCH=CH), CH;CH=CHCH) Br Br transition state for transition state for formation of the formation of the 1,2-addition product 1,4-addition product Copyright © 2007 Pearson Prentice Hall, Inc. Proximity Effect: 1,2-addition always gives kinetic product 6s 66 Relative Stabilities of Allylic and Benzylic Cations p 304 67 Examples: 1,2 product 1,4 product + Br Br thermodynamic kinetic HBr 1 mol HBr 1 mol vs. non-delocalized & kinetic e Diels-Alder reaction is a1,4-addition (7.12) In a Diels-Alder reaction a conjugated diene reacts with a dienophile to form a cyclic product. gm = A Diels-Alder reaction is a[4 + 2] cycloaddition reaction because of the 6 7 electrons involved, 4 come from the diene and 2 from the dienophile. co M A C¢ 2Me heat CO Me I NG Cc 4 70 71 OH O R O OR O H O C N N O O -COOH -COOR -COR -COH -CN -NO2 EWG: More about Diels-Alder Reactions diene four 7 electrons p 327 DA Mechanism oT transition state six 7 electrons [nysleophie] dienophile two 7 electrons Copyright © 2007 Pearson Prentice Hall, Inc. new o bond A new double bond new a bond 72 75  Stereochemistry of Diels-Alder reactions The configuration of the reactants does not change because the reaction is concerted (syn addition). Therefore, when 2 chirality centers are created, only 2 stereoisomers are formed. COOH + + COOH HOOC CH3 H3C COOH + + COOH HOOC CH3 H3C 76 C COOH COOCH3 heat heat heat O H Problems: predict reaction products and indicate stereochemistry where needed O H+ O H COOCH3 COOH COOH + 77 Predict DA Products when both Reagents are unsymmetrical CN O H3CO O OCH3 OCH3 H O H O OCH3 or ? OCH3 OCH3 H O H O OCH3 H O CH3O H O H H CN O H3CO O OCH3 OCH3 H O H O OCH3 or ? OCH3 OCH3 H O H O OCH3 H O CH3O H O H H + ? CN O H3CO O OCH3 OCH3 H O H O OCH3 or ? OCH3 OCH3 H O H O OCH3 H O CH3O H O H H Which arrangement is preferred? 80 Stereoselectivity of DA for Cyclopentadiene -COOH -COOR -COR -COH -CN -NO2 CO2Me + CO2Me H endo the exo product is not formed. H CO2Me exo Substituents with -electrons Subsituents with - electrons: CO2Me + CO2Me H endo the exo product is not formed. H CO2Me exo OH *  OH *  81 Problems: give appropriate reactants for the following DA reactions CN CN CO2Me CO2Me + + CN NC CN CN + CO2Me CO2Me heat heat 82 Problems: give appropriate reactants for the following DA reactions CO2Me CO2Me CO2Me O O + + + CO2Me CO2Me CO2Me O O 2eqv. heat heat heat