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1、Organic Chemistry,William H.BrownChristopher S.FooteBrent L.Iverson,Reactions of Alkenes,Chapter 6,Characteristic Reactions,Characteristic Reactions,Reaction Mechanisms,A reaction mechanism describes how a reaction occurswhich bonds are broken and which new ones are formedthe order and relative rate
2、s 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 during the reaction,Gibbs Free Energy,Gibbs free energy change,DG0:a thermodynamic function relating entha
3、lpy,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 equilibrium for an exergonic reaction favors productsendergonic reaction:a reaction in which the Gibbs free energy of the products is higher tha
4、n that of the reactants;the position of equilibrium for an endergonic reaction favors starting materials,Gibbs Free Energy,a change in Gibbs free energy is directly related to chemical equilibriumsummary of the relationships between DG0,DH0,DS0,and the position of chemical equilibrium,Energy Diagram
5、s,Enthalpy change,DH0:the difference in total bond energy between reactants and productsa measure 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
6、 is lower than that of the reactants;a reaction in which heat is releasedendothermic reaction:a reaction in which the enthalpy of the products is higher than that of the reactants;a reaction in which heat is absorbed,Energy Diagrams,Energy diagram:a graph showing the changes in energy that occur dur
7、ing a chemical reactionReaction coordinate:a measure in the change in positions of atoms during a reaction,Activation Energy,Transition 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 ene
8、rgy between reactants and a 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 fast,Energy Diagram,a one-step reaction with no int
9、ermediate,Energy Diagram,A two-step reaction with one intermediate,Developing a Reaction Mechanism,How it is donedesign experiments to reveal details of a particular chemical reactionpropose a set or sets of steps that might account for the overall transformationa mechanism becomes established when
10、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 devise,Why Mechanisms?,they are the framework within which to organize descriptive chemistrythey provide an intellectual satisfaction deriv
11、ed from constructing models that accurately reflect the behavior of chemical systemsthey are tools with which to search for new information and new understanding,Electrophilic Additions,hydrohalogenation using HCl,HBr,HIhydration using H2O in the presence of H2SO4halogenation using Cl2,Br2halohydrin
12、ation using HOCl,HOBroxymercuration using Hg(OAc)2,H2O followed by reduction,Addition of HX,Carried out with pure reagents or in a polar solvent such as acetic acidAddition is regioselective regioselective reaction:an addition or substitution reaction in which one of two or more possible products is
13、 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 the double bond having the greater number of hydrogens,HBr+2-Butene,A two-step mechanismStep 1:proton transfer from HBr to the alkene gives a carbocation in
14、termediateStep 2:reaction of the sec-butyl cation(an electrophile)with bromide ion(a nucleophile)completes the reaction,HBr+2-Butene,An energy diagram for the two-step addition of HBr to 2-butenethe reaction is exergonic,Carbocations,Carbocation:a species in which a carbon atom has only six electron
15、s in its valence shell and bears positive chargeCarbocations 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 acids,Carbocations,bond angles about a positively charged carbon are approxim
16、ately 120carbon uses 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 electrons,Carbocation Stability,a 3 carbocation is more stable than a 2 carbocation,and requires a lower activation ener
17、gy 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 solution,Carbocation Stability,relative stabilitymethyl and primary carbocations are so unstable that they are never observed in solution,Carbocat
18、ion Stability,we 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 for this electron-releasing ability of alkyl groups by(1)the induct
19、ive effect,and(2)hyperconjugation,The Inductive Effect,the positively charged carbon polarizes electrons of adjacent sigma bonds toward itthe positive charge on the cation is thus localized over nearby atomsthe larger the volume over which the positive charge is delocalized,the greater the stability
20、 of the cation,Hyperconjugation,involves 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 result is delocalization of the positive charge,Addition of H2O,addition of water is called hydrationacid-catalyzed hydration of an alk
21、ene is regioselective;hydrogen adds preferentially to the less substituted carbon of the double bondHOH adds in accordance with Markovnikovs rule,Addition of H2O,Step 1:proton transfer from H3O+to the alkeneStep 2:reaction of the carbocation(an electrophile)with water(a nucleophile)gives an oxonium
22、ionStep 3:proton transfer to water gives the alcohol,+,+,+,intermediate,+,H,O,H,H,O,H,H,:,:,:,:,:,+,+,+,An oxonium ion,H,O,H,H,fast,:,Carbocation Rearrangements,In electrophilic addition to alkenes,there is the possibility for rearrangementRearrangement:a change in connectivity of the atoms in a pro
23、duct compared with the connectivity of the same atoms in the starting material,Carbocation Rearrangements,in addition of HCl to an alkenein acid-catalyzed hydration of an alkene,Carbocation Rearrangements,the driving force is rearrangement of a less stable carbocation to a more stable onethe less st
24、able 2 carbocation rearranges to a more stable 3 one by 1,2-shift of a hydride ion,+,+,A 3 carbocation,H,H,fast,Carbocation Rearrangements,reaction of the more stable carbocation(an electrophile)with chloride ion(a nucleophile)completes the reaction,-,:,:,:,:,2-Chloro-2-methylbutane,+,+,fast,:,:,:,A
25、ddition of Cl2 and Br2,carried out with either the pure reagents or in an inert solvent such as CH2Cl2addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkaneaddition occurs with anti stereoselectivity;halogen atoms add from the opposite face of the double bondwe will discuss
26、 this selectivity in detail in Section 6.7,Addition of Cl2 and Br2,Step 1:formation of a bridged bromonium ion intermediate,Addition of Cl2 and Br2,Step 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 product,
27、Addition of Cl2 and Br2,for a cyclohexene,anti coplanar addition corresponds to trans diaxial additionthe 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,bo
28、th trans enantiomers are formed as a racemic mixture,Addition of HOCl and HOBr,Treatment of an alkene with Br2 or Cl2 in water forms a halohydrinHalohydrin:a compound containing-OH and-X on adjacent carbons,Addition of HOCl and HOBr,reaction is both regiospecific(OH adds to the more substituted carb
29、on)and anti stereoselectiveboth selectivities 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 screen,Addition of HOCl and HOBr,Step 1:formation of a bridged haloniu
30、m ion intermediateStep 2:attack of H2O on the more substituted carbon opens the three-membered ring,Addition of HOCl and HOBr,Step 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 than the one to the less su
31、bstituted 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 carbon,Addition of HOCl and HOBr,bridged bromonium ion from propene,Oxymercuration/Reduction,Oxymercuration followed by red
32、uction results in hydration of a carbon-carbon double bondoxymercurationreduction,Oxymercuration/Reduction,an important feature of oxymercuration/reduction is that it occurs without rearrangementoxymercuration occurs with anti stereoselectivity,Oxymercuration/Reduction,Step 1:dissociation of mercury
33、(II)acetateStep 2:formation of a bridged mercurinium ion intermediate;a two-atom three-center bond,Oxymercuration/Reduction,Step 3:regioselective attack of H2O(a nucleophile)on the bridged intermediate opens the three-membered ringStep 4:reduction of the C-HgOAc bond,Oxymercuration/Reduction,Anti st
34、ereoselectivewe account for the stereoselectivity by formation of the bridged bromonium ion and anti 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 positiv
35、e characteralternatively,computer modeling indicates that the C-Hg bond to the more substituted 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 carbon,Hydrobo
36、ration/Oxidation,Hydroboration:the addition of borane,BH3,to an alkene to form a trialkylboraneBorane dimerizes to diborane,B2H6,Borane,H,B,H,H,B,+,Borane,Hydroboration/Oxidation,borane forms a stable complex with ethers such as THFthe reagent is used most often as a commercially available solution
37、of BH3 in THF,Hydroboration/Oxidation,Hydroboration is both regioselective(boron to the less hindered carbon)and syn stereoselective,H,H,H,+,1-Methylcyclopentene,Hydroboration/Oxidation,concerted regioselective and syn stereoselective addition of B and H to the carbon-carbon double bondtrialkylboran
38、es are rarely isolatedoxidation with alkaline hydrogen peroxide gives an alcohol and sodium borate,Hydroboration/Oxidation,Hydrogen 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 wit
39、h its pair of bonding electrons to an adjacent oxygen atom,Hydroboration/Oxidation,step 3:reaction of the trialkylborane with aqueous NaOH gives the alcohol and sodium borate,Oxidation/Reduction,Oxidation:the loss of electronsalternatively,the loss of H,the gain of O,or bothReduction:the gain of ele
40、ctronsalternatively,the gain of H,the loss of O,or bothRecognize using a balanced half-reaction1.write a half-reaction showing one reactant and its product(s)plete a material balance;use H2O and H+in acid solution,use H2O and OH-in basic plete a charge balance using electrons,e-,Oxidation/Reduction,
41、three balanced half-reactions,Oxidation with OsO4,OsO4 oxidizes an alkene to a glycol,a compound with OH groups on adjacent carbonsoxidation is syn stereoselective,Oxidation with OsO4,OsO4 is both expensive and highly toxicit is used in catalytic amounts with another oxidizing agent to reoxidize its
42、 reduced forms and,thus,recycle OsO4,Oxidation with O3,Treatment of an alkene with ozone followed by a weak reducing agent cleaves the C=C and forms two carbonyl groups in its place,2-Methyl-2-pentene,O,O,Oxidation with O3,the initial product is a molozonide which rearranges to an isomeric ozonide,A
43、cetaldehyde,2-Butene,O,O,O,O,O,O,C,O,C,H,H,A molozonide,An ozonide,Reduction of Alkenes,Most 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 hydrogenati
44、onaddition occurs with syn stereoselectivity,Pd,Cyclohexene,Cyclohexane,25C,3 atm,Reduction of Alkenes,Mechanism of catalytic hydrogenation,Reduction of Alkenes,even though addition syn stereoselectivity,some product may appear to result from trans additionreversal of the reaction after the addition
45、 of the first hydrogen gives an isomeric alkene,etc.,H0 of Hydrogenation,Reduction 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 lower the value of H H0 for a trans alkene is low
46、er than that of an isomeric cis alkenea trans alkene is more stable than a cis alkene,H0 of Hydrogenation,Reaction Stereochemistry,In several of the reactions presented in this chapter,chiral centers are createdWhere one or more chiral centers are created,is the productone enantiomer and,if so,which
47、 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 starting material;that is,some reactions are stereospecific,Reaction Stereochemistry,We saw in Section
48、 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 product;a pair of enantiomers and a meso compoundif we start with the cis isomer,what is the stereochemistry of the product?if
49、we start with the trans isomer,what is the stereochemistry of the product?,Bromination of cis-2-Butene,reaction of cis-2-butene with bromine forms bridged bromonium ions which are meso and identical,Bromination of cis-2-Butene,attack of bromide ion at carbons 2 and 3 occurs with equal probability to
50、 give enantiomeric products as a racemic mixture,Bromination of trans-2-Butene,reaction with bromine forms bridged bromonium ion intermediates which are enantiomers,Bromination of trans-2-Butene,attack of bromide ion in either carbon of either enantiomer gives meso-2,3-dibromobutane,Bromination of 2
