有机化学英文课件chapter6

1、Organic ChemistryWilliam H. BrownChristopher S. FooteBrent L. IversonOrganic ChemistryWilliam H. BrReactions of AlkenesChapter 6Reactions of AlkenesChapter 6Characteristic ReactionsCharacteristic ReactionsCharacteristic ReactionsCharacteristic ReactionsReaction MechanismsA reaction mechanism describ

2、es how a reaction occurswhich bonds are broken and which new ones are formedthe order and relative rates of the various bond-breaking and bond-forming stepsif in solution, the role of the solventif there is a catalyst, the role of a catalystthe position of all atoms and energy of the entire system d

3、uring the reactionReaction MechanismsA reaction Gibbs Free EnergyGibbs free energy change, DG0: a thermodynamic function relating enthalpy, entropy, and temperatureexergonic reaction: a reaction in which the Gibbs free energy of the products is lower than that of the reactants; the position of equil

4、ibrium for an exergonic reaction favors productsendergonic reaction: a reaction in which the Gibbs free energy of the products is higher than that of the reactants; the position of equilibrium for an endergonic reaction favors starting materialsGibbs Free EnergyGibbs free enGibbs Free Energya change

5、 in Gibbs free energy is directly related to chemical equilibriumsummary of the relationships between DG0, DH0, DS0, and the position of chemical equilibriumGibbs Free Energya change in GEnergy DiagramsEnthalpy change, DH0: the difference in total bond energy between reactants and productsa measure

6、of bond making (exothermic) and bond breaking (endothermic)Heat of reaction, DH0: the difference in enthalpy between reactants and productsexothermic reaction: a reaction in which the enthalpy of the products is lower than that of the reactants; a reaction in which heat is releasedendothermic reacti

7、on: a reaction in which the enthalpy of the products is higher than that of the reactants; a reaction in which heat is absorbedEnergy DiagramsEnthalpy changeEnergy DiagramsEnergy diagram: a graph showing the changes in energy that occur during a chemical reactionReaction coordinate: a measure in the

8、 change in positions of atoms during a reactionEnergy DiagramsEnergy diagram:Activation EnergyTransition state: an unstable species of maximum energy formed during the course of a reactiona maximum on an energy diagramActivation Energy, G: the difference in Gibbs free energy between reactants and a

9、transition stateif G is large, few collisions occur with sufficient energy to reach the transition state; reaction is slowif G is small, many collisions occur with sufficient energy to reach the transition state; reaction is fastActivation EnergyTransition stEnergy Diagrama one-step reaction with no

10、 intermediateEnergy Diagrama one-step reactEnergy DiagramA two-step reaction with one intermediateEnergy DiagramA two-step reactDeveloping a Reaction MechanismHow it is donedesign experiments to reveal details of a particular chemical reactionpropose a set or sets of steps that might account for the

11、 overall transformationa mechanism becomes established when it is shown to be consistent with every test that can be devisedthis does mean that the mechanism is correct, only that it is the best explanation we are able to deviseDeveloping a Reaction MechanisWhy Mechanisms?they are the framework with

12、in which to organize descriptive chemistrythey provide an intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systemsthey are tools with which to search for new information and new understandingWhy Mechanisms?they are the frElectrophilic Additi

13、onshydrohalogenation using HCl, HBr, HIhydration using H2O in the presence of H2SO4halogenation using Cl2, Br2halohydrination using HOCl, HOBroxymercuration using Hg(OAc)2, H2O followed by reductionElectrophilic AdditionshydrohaAddition of HXCarried out with pure reagents or in a polar solvent such

14、as acetic acidAddition is regioselective regioselective reaction: an addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formedMarkovnikovs rule: in the addition of HX, H2O, or ROH to an alkene, H adds to the carbon of t

15、he double bond having the greater number of hydrogensAddition of HXCarried out withHBr + 2-ButeneA two-step mechanismStep 1: proton transfer from HBr to the alkene gives a carbocation intermediateStep 2: reaction of the sec-butyl cation (an electrophile) with bromide ion (a nucleophile) completes th

16、e reactionHBr + 2-ButeneA two-step mechaHBr + 2-ButeneAn energy diagram for the two-step addition of HBr to 2-butenethe reaction is exergonicHBr + 2-ButeneAn energy diagraCarbocationsCarbocation: a species in which a carbon atom has only six electrons in its valence shell and bears positive chargeCa

17、rbocations areclassified as 1, 2, or 3 depending on the number of carbons bonded to the carbon bearing the positive chargeelectrophiles; that is, they are electron-loving Lewis acidsCarbocationsCarbocation: a speCarbocationsbond angles about a positively charged carbon are approximately 120carbon us

18、es sp2 hybrid orbitals to form sigma bonds to the three attached groupsthe unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electronsCarbocationsbond angles about Carbocation Stabilitya 3 carbocation is more stable than a 2 carbocation, and requires a lower acti

19、vation energy for its formationa 2 carbocation is, in turn, more stable than a 1 carbocation,methyl and 1 carbocations are so unstable that they are never observed in solutionCarbocation Stabilitya 3 carbCarbocation Stabilityrelative stabilitymethyl and primary carbocations are so unstable that they

20、 are never observed in solutionCarbocation Stabilityrelative Carbocation Stabilitywe can account for the relative stability of carbocations if we assume that alkyl groups bonded to the positively charged carbon are electron releasing and thereby delocalize the positive charge of the cationwe account

21、 for this electron-releasing ability of alkyl groups by (1) the inductive effect, and (2) hyperconjugationCarbocation Stabilitywe can acThe Inductive Effectthe positively charged carbon polarizes electrons of adjacent sigma bonds toward itthe positive charge on the cation is thus localized over near

22、by atomsthe larger the volume over which the positive charge is delocalized, the greater the stability of the cationThe Inductive Effectthe positiHyperconjugationinvolves partial overlap of the -bonding orbital of an adjacent C-H or C-C bond with the vacant 2p orbital of the cationic carbon the resu

23、lt is delocalization of the positive chargeHyperconjugationinvolves partiAddition of H2Oaddition of water is called hydrationacid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bondHOH adds in accordance with Markovnikovs

24、 ruleAddition of H2Oaddition of watAddition of H2OStep 1: proton transfer from H3O+ to the alkeneStep 2: reaction of the carbocation (an electrophile) with water (a nucleophile) gives an oxonium ionStep 3: proton transfer to water gives the alcohol+intermediateA 2o carbocation+HOHHOHHCH3CH=CH2CH3CHC

25、H3slow, ratedetermining:+An oxonium ionHOHHCH3CHCH3O-HCH3CHCH3fast:Addition of H2OStep 1: proton Carbocation RearrangementsIn electrophilic addition to alkenes, there is the possibility for rearrangementRearrangement: a change in connectivity of the atoms in a product compared with the connectivity

26、of the same atoms in the starting materialCarbocation RearrangementsIn eCarbocation Rearrangementsin addition of HCl to an alkenein acid-catalyzed hydration of an alkeneCarbocation Rearrangementsin aCarbocation Rearrangementsthe driving force is rearrangement of a less stable carbocation to a more s

27、table onethe less stable 2 carbocation rearranges to a more stable 3 one by 1,2-shift of a hydride ion+A 3 carbocationCH3HCH3HCH3C-CHCH3CH3C-CHCH3fastCarbocation Rearrangementsthe Carbocation Rearrangementsreaction of the more stable carbocation (an electrophile) with chloride ion (a nucleophile) co

28、mpletes the reaction-Cl:2-Chloro-2-methylbutane+CH3CH3CH3C-CH2CH3CH3C-CH2CH3fastCl:Carbocation RearrangementsreacAddition of Cl2 and Br2carried out with either the pure reagents or in an inert solvent such as CH2Cl2addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkaneaddit

29、ion occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bondwe will discuss this selectivity in detail in Section 6.7Addition of Cl2 and Br2carriedAddition of Cl2 and Br2Step 1: formation of a bridged bromonium ion intermediateAddition of Cl2 and Br2Step 1:Addi

30、tion of Cl2 and Br2Step 2: attack of halide ion (a nucleophile) from the opposite side of the bromonium ion (an electrophile) opens the three-membered ring to give the productAddition of Cl2 and Br2Step 2:Addition of Cl2 and Br2for a cyclohexene, anti coplanar addition corresponds to trans diaxial a

31、dditionthe initial trans diaxial conformation is in equilibrium with the more stable trans diequatorial conformationbecause the bromonium ion can form on either face of the alkene with equal probability, both trans enantiomers are formed as a racemic mixtureAddition of Cl2 and Br2for a cAddition of

32、HOCl and HOBrTreatment of an alkene with Br2 or Cl2 in water forms a halohydrinHalohydrin: a compound containing -OH and -X on adjacent carbonsAddition of HOCl and HOBrTreatAddition of HOCl and HOBrreaction is both regiospecific (OH adds to the more substituted carbon) and anti stereoselectiveboth s

33、electivities are illustrated by the addition of HOBr to 1-methylcyclopenteneto account for the regioselectivity and the anti stereoselectivity, chemists propose the three-step mechanism in the next screenAddition of HOCl and HOBrreactAddition of HOCl and HOBrStep 1: formation of a bridged halonium i

34、on intermediateStep 2: attack of H2O on the more substituted carbon opens the three-membered ringAddition of HOCl and HOBrStepAddition of HOCl and HOBrStep 3: proton transfer to H2O completes the reactionAs the elpot map on the next screen showsthe C-X bond to the more substituted carbon is longer t

35、han the one to the less substituted carbonbecause of this difference in bond lengths, the transition state for ring opening can be reached more easily by attack of the nucleophile at the more substituted carbonAddition of HOCl and HOBrStep Addition of HOCl and HOBrbridged bromonium ion from propeneA

36、ddition of HOCl and HOBrbridgOxymercuration/ReductionOxymercuration followed by reduction results in hydration of a carbon-carbon double bondoxymercurationreductionOxymercuration/ReductionOxymerOxymercuration/Reductionan important feature of oxymercuration/reduction is that it occurs without rearran

37、gementoxymercuration occurs with anti stereoselectivityOxymercuration/Reductionan impOxymercuration/ReductionStep 1: dissociation of mercury(II) acetateStep 2: formation of a bridged mercurinium ion intermediate; a two-atom three-center bondOxymercuration/ReductionStep 1Oxymercuration/ReductionStep

38、3: regioselective attack of H2O (a nucleophile) on the bridged intermediate opens the three-membered ringStep 4: reduction of the C-HgOAc bondOxymercuration/ReductionStep 3Oxymercuration/ReductionAnti stereoselectivewe account for the stereoselectivity by formation of the bridged bromonium ion and a

39、nti attack of the nucleophile which opens the three-membered ringRegioselectiveof the two carbons of the mercurinium ion intermediate, the more substituted carbon has the greater degree of partial positive characteralternatively, computer modeling indicates that the C-Hg bond to the more substituted

40、 carbon of the bridged intermediate is longer than the one to the less substituted carbon therefore, the ring-opening transition state is reached more easily by attack at the more substituted carbonOxymercuration/ReductionAnti sHydroboration/OxidationHydroboration: the addition of borane, BH3, to an

41、 alkene to form a trialkylboraneBorane dimerizes to diborane, B2H6BoraneHBHH3CH2=CH2CH3CH2BCH2CH3CH2CH3Triethylborane(a trialkylborane)+BoraneDiborane2BH3B2H6Hydroboration/OxidationHydroboHydroboration/Oxidationborane forms a stable complex with ethers such as THFthe reagent is used most often as a

42、commercially available solution of BH3 in THFHydroboration/Oxidationborane Hydroboration/OxidationHydroboration is both regioselective (boron to the less hindered carbon) and syn stereoselectiveCH3HBH3BR2HH3CH+1-Methylcyclopentene (Syn addition of BH3)(R = 2-methylcyclopentyl)Hydroboration/Oxidation

43、HydroboHydroboration/Oxidationconcerted regioselective and syn stereoselective addition of B and H to the carbon-carbon double bondtrialkylboranes are rarely isolatedoxidation with alkaline hydrogen peroxide gives an alcohol and sodium borateHydroboration/OxidationconcertHydroboration/OxidationHydro

44、gen peroxide oxidation of a trialkylboranestep 1: hydroperoxide ion (a nucleophile) donates a pair of electrons to boron (an electrophile)step 2: rearrangement of an R group with its pair of bonding electrons to an adjacent oxygen atomHydroboration/OxidationHydrogeHydroboration/Oxidationstep 3: reac

45、tion of the trialkylborane with aqueous NaOH gives the alcohol and sodium borateHydroboration/Oxidationstep 3:Oxidation/ReductionOxidation: the loss of electronsalternatively, the loss of H, the gain of O, or bothReduction: the gain of electronsalternatively, the gain of H, the loss of O, or bothRec

46、ognize using a balanced half-reaction1. write a half-reaction showing one reactant and its product(s)2. complete a material balance; use H2O and H+ in acid solution, use H2O and OH- in basic solution3. complete a charge balance using electrons, e-Oxidation/ReductionOxidation: Oxidation/Reductionthre

47、e balanced half-reactionsOxidation/Reductionthree balanOxidation with OsO4OsO4 oxidizes an alkene to a glycol, a compound with OH groups on adjacent carbonsoxidation is syn stereoselectiveOxidation with OsO4OsO4 oxidizOxidation with OsO4OsO4 is both expensive and highly toxicit is used in catalytic

48、amounts with another oxidizing agent to reoxidize its reduced forms and, thus, recycle OsO4Oxidation with OsO4OsO4 is botOxidation with O3Treatment of an alkene with ozone followed by a weak reducing agent cleaves the C=C and forms two carbonyl groups in its place Propanal(an aldehyde)Propanone(a ke

49、tone)2-Methyl-2-penteneCH3OOCH3C=CHCH2CH31. O32. (CH3)2SCH3CCH3 + HCCH2CH3Oxidation with O3Treatment of Oxidation with O3the initial product is a molozonide which rearranges to an isomeric ozonideAcetaldehyde2-ButeneOCH3CH=CHCH3O3(CH3)2SCH3CHCH3CH-CHCH3OOOOOCOCHCH3HH3CA molozonideAn ozonideOxidation

50、 with O3the initial pReduction of AlkenesMost alkenes react with H2 in the presence of a transition metal catalyst to give alkanescommonly used catalysts are Pt, Pd, Ru, and Nithe process is called catalytic reduction or, alternatively, catalytic hydrogenationaddition occurs with syn stereoselectivi

51、ty+ H2PdCyclohexeneCyclohexane25C, 3 atmReduction of AlkenesMost alkenReduction of AlkenesMechanism of catalytic hydrogenationReduction of AlkenesMechanism Reduction of Alkeneseven though addition syn stereoselectivity, some product may appear to result from trans additionreversal of the reaction af

52、ter the addition of the first hydrogen gives an isomeric alkene, etc.Reduction of Alkeneseven thougH0 of HydrogenationReduction of an alkene to an alkane is exothermicthere is net conversion of one pi bond to one sigma bondH0 depends on the degree of substitutionthe greater the substitution, the low

53、er the value of H H0 for a trans alkene is lower than that of an isomeric cis alkenea trans alkene is more stable than a cis alkeneH0 of HydrogenationReduction H0 of HydrogenationH0 of HydrogenationReaction StereochemistryIn several of the reactions presented in this chapter, chiral centers are crea

54、tedWhere one or more chiral centers are created, is the productone enantiomer and, if so, which one?a pair of enantiomers as a racemic mixture?a meso compound?a mixture of stereoisomers?As we will see, the stereochemistry of the product for some reactions depends on the stereochemistry of the starti

55、ng material; that is, some reactions are stereospecificReaction StereochemistryIn sevReaction StereochemistryWe saw in Section 6.3D that bromine adds to 2-butene to give 2,3-dibromobutanetwo stereoisomers are possible for 2-butene; a pair of cis,trans isomersthree stereoisomers are possible for the

56、product; a pair of enantiomers and a meso compoundif we start with the cis isomer, what is the stereochemistry of the product?if we start with the trans isomer, what is the stereochemistry of the product?Reaction StereochemistryWe sawBromination of cis-2-Butenereaction of cis-2-butene with bromine f

57、orms bridged bromonium ions which are meso and identicalBromination of cis-2-ButenereaBromination of cis-2-Buteneattack of bromide ion at carbons 2 and 3 occurs with equal probability to give enantiomeric products as a racemic mixtureBromination of cis-2-ButeneattBromination of trans-2-Butenereactio

58、n with bromine forms bridged bromonium ion intermediates which are enantiomersBromination of trans-2-ButenerBromination of trans-2-Buteneattack of bromide ion in either carbon of either enantiomer gives meso-2,3-dibromobutaneBromination of trans-2-ButeneaBromination of 2-ButeneGiven these results, w

59、e say that addition of Br2 or Cl2 to an alkene is stereospecificbromination of cis-2-butene gives the enantiomers of 2,3-dibromobutane as a racemic mixturebromination of trans-2-butene gives meso-2,3-dibromobutaneStereospecific reaction: a reaction in which the stereochemistry of the product depends

60、 on the stereochemistry of the starting materialBromination of 2-ButeneGiven tOxidation of 2-ButeneOsO4 oxidation of cis-2-butene gives meso-2,3-butanediolOxidation of 2-ButeneOsO4 oxidOxidation of 2-ButeneOsO4 oxidation of an alkene is stereospecificoxidation of trans-2-butene gives the enantiomers