Slide Number 1Slide Number 2Slide Number 3Production of RadicalsSlide Number 5Reactions of RadicalsSlide Number 7Single-Bond Homolytic Dissociation Energies (DH°) at 25°C Single-Bond Homolytic Dissociation Energies (DH°) at 25°C Slide Number 10Slide Number 11Slide Number 12How to Use Homolytic Bond Dissociation Energies to Determine the Relative Stabilities of RadicalsHow to Use Homolytic Bond Dissociation Energies to Determine the Relative Stabilities of RadicalsEnergy DiagramsRadical Relative StabilitySlide Number 17Allylic Radicals Are Stabilized by Electron DelocalizationRelative Stability of Radicals: OverviewSlide Number 20Slide Number 21Multiple Halogen SubstitutionRadical HalogenationsSymmetrySlide Number 25(1) Chain Initiation(2) Chain Propagation(2) Chain Propagation(3) Chain TerminationEnergy of ActivationSlide Number 31Halogenation of Higher AlkanesSlide Number 33Selectivity of BromineMechanismSlide Number 36Slide Number 37Slide Number 38The Stereochemistry of chlorination at C2 of pentane Generation of a Second Chirality Center in a Radical HalogenationSlide Number 41Slide Number 42Slide Number 43Allylic Chlorination (High Temperature)Allylic Bromination with N-BromosuccinimideRole of N-BromosuccinimideSlide Number 47Benzylic Bromination with N-BromosuccinimideSlide Number 49Radical MechanismSynthetic RelevanceSlide Number 52Slide Number 53Radical MechanismSlide Number 55Other Relevant PolymersOther Relevant PolymersSlide Number 58Slide Number 59ReviewSlide Number 61Slide Number 62Slide Number 63Slide Number 64Slide Number 65Slide Number 66Slide Number 67Slide Number 68Slide Number 69Slide Number 70Chapter 10 Radical Reactions 1In this chapter : 1. The properties of radicals, their formation, and their reactivity 2. Significant radical-based reactions in nature 2Introduction: How Radicals Form and How They React ? Heterolysis A Bionsheterolyticbondcleavage+A BHomolysis A Bradicalshomolyticbondcleavage+A B3Production of Radicals Homolysis of covalent bonds relatively weak bonds such as O-O or X-X bonds : • Need heat or light (hν) R O O RheatR O2Cl ClhνCl2(alkoxyl radical) (chlorine radical) 4Practice Problems: “Peroxide" refer to which structure(s)? 1. ROOR 2. ROOH 3. - 4. Two of these choices. 5. Three of these choices. RCOOCROO5Reactions of Radicals 1. Almost all small radicals are short-lived, highly reactive species 2. Radicals tend to react in ways that lead to pairing of their unpaired electron. Hydrogen abstraction is one way a halogen radical can react to pair its unshared. Cl+H CH3ClH+CH3R+C CCRC(alkyl radical) (methyl radical) 6Homolytic Bond Dissociation Energies (∆H°) H+H HHCl+Cl ClCl∆Ho = −436 kJ/mol∆Ho = −243 kJ/molBond formation is an exothermic process. H+H H HCl+Cl Cl Cl∆Ho = +436 kJ/mol∆Ho = +243 kJ/molReactions in which only bond breaking occurs are always endothermic. 71. Energy must be supplied to break covalent bonds 2. The energies required to break covalent bonds homolytically are called homolytic bond dissociation energies, and they are usually abbreviated by the symbol ∆H ° Single-Bond Homolytic Dissociation Energies (∆H°) at 25°C Bond Broken kJ/mol H–H 436 F–F 159 Cl–Cl 243 Br–Br 193 I–I 151 Bond Broken kJ/mol H–F 570 H–Cl 432 H–Br 366 H–I 298 8Bond Broken kJ/mol Bond Broken kJ/mol H3C–H 440 H3C–F 461 H3C–Cl 352 H3C–Br 293 H3C–OH 387 H3C–I 240 H3C–OCH3 348 Single-Bond Homolytic Dissociation Energies (∆H°) at 25°C 9Bond Broken kJ/mol Bond Broken kJ/mol 354 294 355 298 349 292 ClClClBrBrBrSingle-Bond Homolytic Dissociation Energies (∆H°) at 25°C 10Bond Broken kJ/mol Bond Broken kJ/mol 423 369 413 465 400 474 375 547 HHHPh HHHPh HC HHCSingle-Bond Homolytic Dissociation Energies (∆H°) at 25°C 11In Class Problem: Estimate the heat of reaction, ∆Ho, for each of the following reactions. H3C CCH3CH3H+Br2H3C CCH3CH3Br+HBr(1)12How to Use Homolytic Bond Dissociation Energies to Determine the Relative Stabilities of Radicals +H∆Ho = +423 kJ/mol∆Ho = +413 kJ/molHH+HPropyl radical (a __ radical)Isopropyl radical (a __radical)13+H∆Ho = +422 kJ/mol∆Ho = +400 kJ/molH+Htert-Butyl radical (a __ radical)Isobutyl radical (a __ radical)HHow to Use Homolytic Bond Dissociation Energies to Determine the Relative Stabilities of Radicals 14Energy Diagrams 15 Relative Stability ● The relative stabilities of radicals follows the same trend as for carbocations. The most substituted radical is most stable. Radicals are electron deficient, as are carbocations, and are therefore also stabilized by hyperconjugation.  3o > 2o > 1o CH3CCH3CH3HCH HHCCH3HHCCH3CH3> > >(positive inductive effect of alkyl groups stabilize radical)Radical Relative Stability 16In Class Problem: Which free radical is the most stable? 17 III IIIVIVAllylic Radicals Are Stabilized by Electron Delocalization H+X+ HXH+X+ HXAllyl radical Vinylic radical Eact is high Eact is low 18allylic or allyl > 3o > 2o > 1o > vinyl or vinylicContributing ResonanceStructuresResonanceHybridRelative Stability of Radicals: Overview 1920Reactions of Alkanes with Halogens 1. Alkanes have no functional groups, are inert to many reagents, and do not undergo many reactions 2. Halogenation of alkanes is one of the most typical free radical reactions 21 R HX2+R XH X+heatorlight (hν)Multiple Halogen Substitution +Cl2HCH HH+HCH ClH+ClCH ClH+ClCCl ClH+ClCCl ClClH Clheatorlight22Radical Halogenations 1. Chlorination of higher alkanes leads to mixtures of monochlorinated product (and more substituted products). Chlorine is relatively unselective 2. Bromine is less reactive but more selective than chlorine (Sec. 10.6A). 23 + + +HClCllightClPolychlorinatedproducts(23%)IsobutaneIsobutylchloride(48%)tert-Butylchloride(29%)Cl2Symmetry • Molecular symmetry is important in determining the number of possible substitution products 24 +Cl2+H ClheatorlightClNeopentane(excess)Neopentylchloride++HClheatorlightCyclohexane(excess)ChlorocyclohexaneCl2ClChlorination of Methane: Mechanism of Reaction Most radical reactions include 3 stages, or steps (1) chain initiation (2) chain propagation (3) chain termination 25Cl Clhν(homolyticcleavage)2 Cl(1) Chain Initiation • Radicals are formed in this step 26HCH HH+HCl CH3 CH3+Cl ClCH3Cl+(i)(ii)+ClCl(2) Chain Propagation 2. Repeating (i) and (ii) in a chain reaction provides the product CH3Cl 27 1. In chain propagation, one radical