Hydrogen Chlorine Chapter 10 Radical Radical Reactions

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What are radicals?Radicals are intermediates with an unpaired electronChapter 10Radical radical Often called free radicals Formed by homolytic bond cleavage Radicals are highly reactive, short-lived speciesz Half-headed arrows are used to show movement of single electronsProduction of radicalsReactions of radicals Usually begins with homolysis of a relatively weak bond Radicals seek to react in ways that lead to pairing of theirsuch as O-O or X-X Initiated by addition of energy in the form of heat or light Reaction of a radical with any species that does not haveunpaired electron and completing a full octet.an unpaired electron will produce another radicalz Hydrogen abstraction is one way a halogen radical can reactto pair its unshared electron1

Bond Dissociation EnergiesElectronic structure of methyl radicalAtoms have higher energy (are less stable) than themolecules they can formBreaking covalent bonds requires energy (i.e., it is endothermic)Example of using Bond Dissociation EnergiesConsider the possible reaction of H2 with Cl2Table of bonddissociationenergies in text,p. 430Reaction is exothermic, more energy is released in forming the 2 H-Cl bonds ofproduct than is required to break the H-H and Cl-Cl bonds of reactants2

Relative Stability of organic radicalsRelative stability of organic radicalsCompare the Bond Dissociation Energies for theprimary and secondary hydrogens in propaneUsing the same table, the tert-butyl radical ismore stable than the isobutyl radicalDiff 10 kJ/molDiff 22 kJ/molSince less energy is needed to form the isopropyl radical (fromsame starting material), the isopropyl radical must be more stable.Relative Stability of Free RadicalsEnergy diagrams for formation of radicalsThe relative stabilities of carbon radicals follows thesame trend as for carbocationsz The more substituted radical is the more stable.z Radicals are electron deficient, as are carbocations, andare therefore also stabilized by hyperconjugation.3

The Reactions of Alkanes with HalogensAlkanes undergo substitution reactions with halogens(fluorine, bromine and chlorine) initiated by heat or lightChlorination Chlorination of higher alkanes leads to mixtures ofmonochlorinated product (and more substituted products) Free radical halogenation can yield a mixture of halogenatedcompounds because all of the hydrogen atoms in an alkane arecapable of substitution. Monosubstitution can be achieved by using a large excess of thealkane - so long as all hydrogens are equivalent.For example in CH4, CH3CH3, cyclopentane (BUT NOT IN, for example, CH3CH2CH3)Chlorine is relatively unselective and does not greatly distinguishbetween the type of hydrogen it replaces.If there were no selectivity, the t-butyl chloride account for 1/9 of the product,whereas it is actually 2/3 meaning that there is a preference of about 5-fold. Mechanism of Chlorination: a Chain Reaction The reaction mechanism has three distinct aspects:1. Initiation2. Propagation3. TerminationInitiation – Step 1Chlorination of Methane: Mechanism of ReactionPropagation (2 steps which are repeated many times)z A chlorine radical reacts with a molecule of methane togenerate a methyl radicalz The methyl radical reacts with a molecule of chlorine to yieldchloromethane and regenerates chlorine radicalz The new chlorine radical reacts with another methanemolecule, continuing the chain reactionz Chlorine radicals form when the reaction mixture issubjected to heat or light.Recall that the Cl-Cl bond is relatively weakA single initiation step can lead to thousands ofpropagation steps, hence the term chain reaction4

Electron flow in the mechanismTerminationOccasionally, the reactive radical intermediates are quenchedby reaction pathways that do not generate new radicals.Therefore, the reaction of chlorine with methane requires constantirradiation to replace radicals quenched in chain-terminating steps.Reaction of Methane with Other HalogensHalogenation of Higher Alkanes Monochlorination of alkanes proceeds with limited selectivity.z Tertiary hydrogens are roughly 5 times more reactive than primary.The order of reactivity of methane substitution withhalogens is: fluorine chlorine bromine iodinez Secondary hydrogens are roughly 3.5 times more reactive thanprimary.5

Useful ChlorinationsChlorination occurs so rapidly it cannot distinguish well between types ofhydrogen and so is not very selective.Chlorination is synthetically useful only when molecularsymmetry limits the number of possible substitution products.ClCl2 ionIs Fluorination Selective?Bromination is the only halogenation that is controllable and selective. Fluorine shows almost no discrimination inreplacement of hydrogens because it is soreactiveTherefore, free radical bromination is the only practical method forhalogenating alkanes.CH3 It is so reactive that only perfluoro compounds(all H replaced by F) are formed via directfluorination.CH3 CCH3CH3Br2H CH3 CCH3BrCH3 99%CH3Br2 CH3 C HHBrCH2Br 1%CH3Br HBr 6

Reactions that Generate Tetrahedral Stereogenic Carbons A reaction of achiral starting materials which produces a productwith a stereogenic carbon will produce a racemic mixtureGeneration of a Second Stereogenic Carbon When a molecule with one or more stereogenic carbons reactsto create another stereogenic carbon, two diastereomericproducts are produced.z The intermediate radical is chiral and and reactions on the twofaces of the radical lead to two diastereomers.Radicals are PLANAR(like carbocations)Anti-Markovnikov Addition of HBr to AlkenesA Second Way That Radicals ReactAddition of hydrogen bromide in the presence ofperoxides gives anti-Markovnikov additionBesides abstracting a hydrogen atomabstractionBrHBrRH RRadicals can add to the pi bond of an alkeneWorks only for HBr: the other hydrogen halides do notgive this type of anti-Markovnikov additionBradditionBr7

Mechanism for the Anti-Markovnikov Addition of HBrA free radical chain mechanismIn step 3, the first step of propagation, a bromine radical adds tothe double bond to give the most stable of the two possiblecarbon radicals (in this case, a 2o radical)z Attack at the 1o carbon is also less sterically hinderedSteps 1 and 2 of the mechanism are chain initiation steps whichproduce a bromine radicalBrBrStep 3CH2CHCH3CH2CHCH32ÞRadicalStep 4 regenerates a bromine radicalBrStep 4CH2BrCHCH3 HBrCH2HCHCH3 BrThe new bromine radical reacts with another equivalent of alkene, andsteps 3 and 4 repeat in a chain reactionControlling Addition of HBr to AlkenesEarly studies of HBr addition gave contradictory results –sometimes Markovnikov addition and sometime anti-MarkovnikovRadical Polymerization of AlkenesPolymers are macromolecules made up of repeating subunitsz The subunits used to synthesize polymers are called monomersPolyethylene is made of repeating subunits derived from ethylenez Polyethylene is called a chain-growth polymer or addition polymern large number Polystyrene is made in an analogous reaction using styrene as theTo favor “normal” addition, remove possible traces of peroxides from thealkene and use a polar, protic solventmonomerTo favor anti-Mark, add peroxide and use non-polar solventVery useful for your synthetic tool box8

Initiator used to start a chain reaction mechanismChain terminationA very small amount of a diacyl peroxide is added to initiate thereaction so that few, but very long polymer chains are obtainedChain growth can terminate by combination of two radicals.Produces an alkylradical to initiate chainThe propagation step simply adds more ethylene molecules to agrowing chainChain TerminationStep 42RCH2CH2CH2CH2CombinationnR CH2CH2Some other addition polymers from common alkenesCH2CH2 CH2CH2nCH2CH2nRSuperglue Some monomers canalso be polymerizedby nucleoophilesNote the regular alternation of the X groups, since theaddition step always produces the more stable radical9

Controlling Addition of HBr to Alkenes Early studies of HBr addition gave contradictory results – sometimes Markovnikov addition and sometime anti-Markovnikov To favor “normal” addition, remove possible traces of peroxides from the alkene and use a polar, protic solvent To favor anti-

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