有机化学英文课件chapter6.ppt

1、uA reaction mechanism describes how a reaction occurs which bonds are broken and which new ones are formed the order and relative rates of the various bond-breaking and bond-forming steps if in solution,the role of the solvent if there is a catalyst,the role of a catalyst the position of all atoms a

2、nd energy of the entire system during the reaction a thermodynamic function relating enthalpy,entropy,and temperature 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 products a reaction in whic

3、h the Gibbs free energy of the products is higher than that of the reactants;the position of equilibrium for an endergonic reaction favors starting materials a change in Gibbs free energy is directly related to chemical equilibrium summary of the relationships between D DG0,D DH0,D DS0,and the posit

4、ion of chemical equilibriumthe difference in total bond energy between reactants and products a measure of bond making(exothermic)and bond breaking(endothermic)the difference in enthalpy between reactants and products a reaction in which the enthalpy of the products is lower than that of the reactan

5、ts;a reaction in which heat is released:a reaction in which the enthalpy of the products is higher than that of the reactants;a reaction in which heat is absorbed a graph showing the changes in energy that occur during a chemical reaction a measure in the change in positions of atoms during a reacti

6、on an unstable species of maximum energy formed during the course of a reaction a maximum on an energy diagram the difference in Gibbs free energy between reactants and a transition state if D DG is large,few collisions occur with sufficient energy to reach the transition state;reaction is slow if D

7、 DG is small,many collisions occur with sufficient energy to reach the transition state;reaction is fast a one-step reaction with no intermediateuA two-step reaction with one intermediateuHow it is done design experiments to reveal details of a particular chemical reaction propose a set or sets of s

8、teps that might account for the overall transformation a mechanism becomes established when it is shown to be consistent with every test that can be devised this does mean that the mechanism is correct,only that it is the best explanation we are able to devise they are the framework within which to

9、organize descriptive chemistry they provide an intellectual satisfaction derived from constructing models that accurately reflect the behavior of chemical systems they are tools with which to search for new information and new understanding hydrohalogenation using HCl,HBr,HI hydration using H2O in t

10、he presence of H2SO4 halogenation using Cl2,Br2 halohydrination using HOCl,HOBr oxymercuration using Hg(OAc)2,H2O followed by reductionuCarried out with pure reagents or in a polar solvent such as acetic aciduAddition is regioselective an addition or substitution reaction in which one of two or more

11、 possible products is formed in preference to all others that might be formed 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 hydrogensuA two-step mechanismStep 1:proton transfer from HBr to the alkene gives a carbocation intermediat

12、eStep 2:reaction of the sec-butyl cation(an electrophile)with bromide ion(a nucleophile)completes the reactionuAn energy diagram for the two-step addition of HBr to 2-butene the reaction is exergonic a species in which a carbon atom has only six electrons in its valence shell and bears positive char

13、geuCarbocations are classified as 1,2,or 3 depending on the number of carbons bonded to the carbon bearing the positive charge electrophiles;that is,they are electron-loving Lewis acids bond angles about a positively charged carbon are approximately 120 carbon uses sp2 hybrid orbitals to form sigma

14、bonds to the three attached groups the unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons a 3 carbocation is more stable than a 2 carbocation,and requires a lower activation energy for its formation a 2 carbocation is,in turn,more stable than a 1 carboca

15、tion,methyl and 1 carbocations are so unstable that they are never observed in solution relative stability methyl and primary carbocations are so unstable that they are never observed in solution we can account for the relative stability of carbocations if we assume that alkyl groups bonded to the p

16、ositively charged carbon are electron releasing and thereby delocalize the positive charge of the cation we account for this electron-releasing ability of alkyl groups by(1)the inductive effect,and(2)hyperconjugation the positively charged carbon polarizes electrons of adjacent sigma bonds toward it

17、 the positive charge on the cation is thus localized over nearby atoms the larger the volume over which the positive charge is delocalized,the greater the stability of the cation involves partial overlap of the -bonding orbital of an adjacent C-H or C-C bond with the vacant 2p orbital of the cationi

18、c carbon the result is delocalization of the positive charge addition of water is called hydration acid-catalyzed hydration of an alkene is regioselective;hydrogen adds preferentially to the less substituted carbon of the double bond HOH adds in accordance with Markovnikovs rule Step 1:proton transf

19、er from H3O+to the alkene Step 2:reaction of the carbocation(an electrophile)with water(a nucleophile)gives an Step 3:proton transfer to water gives the alcohol+intermediateA 2o carbocation+HOHHOHHCH3CH=CH2CH3CHCH3slow,ratedetermining:+An oxonium ionHOHHCH3CHCH3O-HCH3CHCH3fast:uIn electrophilic addi

20、tion to alkenes,there is the possibility for rearrangement a change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material in addition of HCl to an alkene in acid-catalyzed hydration of an alkene the driving force is rearrangement of a les

21、s stable carbocation to a more stable one the less stable 2 carbocation rearranges to a more stable 3 one by 1,2-shift of a hydride ion+A 3 carbocationCH3HCH3HCH3C-CHCH3CH3C-CHCH3fast reaction of the more stable carbocation(an electrophile)with chloride ion(a nucleophile)completes the reaction-Cl:2-

22、Chloro-2-methylbutane+CH3CH3CH3C-CH2CH3CH3C-CH2CH3fastCl:carried out with either the pure reagents or in an inert solvent such as CH2Cl2 addition of bromine or chlorine to a cycloalkene gives a trans-dihalocycloalkane addition occurs with;halogen atoms add from the opposite face of the double bond w

23、e will discuss this selectivity in detail in Section 6.7 Step 1:formation of a bridged bromonium ion intermediate 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 for a cyclohexene,anti coplanar ad

24、dition corresponds to trans diaxial addition the initial trans diaxial conformation is in equilibrium with the more stable trans diequatorial conformation because the bromonium ion can form on either face of the alkene with equal probability,both trans enantiomers are formed as a racemic mixtureuTre

25、atment of an alkene with Br2 or Cl2 in water forms a halohydrin a compound containing-OH and-X on adjacent carbons reaction is both regiospecific(OH adds to the more substituted carbon)and anti stereoselective both selectivities are illustrated by the addition of HOBr to 1-methylcyclopentene to acco

26、unt for the regioselectivity and the anti stereoselectivity,chemists propose the three-step mechanism in the next screenStep 1:formation of a bridged halonium ion intermediateStep 2:attack of H2O on the more substituted carbon opens the three-membered ring Step 3:proton transfer to H2O completes the

27、 reactionuAs the elpot map on the next screen shows the C-X bond to the more substituted carbon is longer than the one to the less substituted carbon because 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 s

28、ubstituted carbon bridged bromonium ion from propeneuOxymercuration followed by reduction results in hydration of a carbon-carbon double bond oxymercuration reduction an important feature of oxymercuration/reduction is that it occurs without rearrangement oxymercuration occurs with anti stereoselect

29、ivity Step 1:dissociation of mercury(II)acetate Step 2:formation of a bridged mercurinium ion intermediate;a two-atom three-center bond Step 3:regioselective attack of H2O(a nucleophile)on the bridged intermediate opens the three-membered ring Step 4:reduction of the C-HgOAc bonduAnti stereoselectiv

30、e we account for the stereoselectivity by formation of the bridged bromonium ion and anti attack of the nucleophile which opens the three-membered ringuRegioselective of the two carbons of the mercurinium ion intermediate,the more substituted carbon has the greater degree of partial positive charact

31、er alternatively,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 the addition of

32、 borane,BH3,to an alkene to form a trialkylboraneuBorane dimerizes to diborane,B2H6BoraneH BHH3 CH2=CH2CH3CH2BCH2CH3CH2CH3Triethylborane(a trialkylborane)+BoraneDiborane2 BH3B2H6 borane forms a stable complex with ethers such as THF the reagent is used most often as a commercially available solution

33、 of BH3 in THFuHydroboration is both regioselective(boron to the less hindered carbon)and syn stereoselectiveCH3HBH3BR2HH3CH+1-Methylcyclopentene (Syn addition of BH3)(R=2-methylcyclopentyl)concerted regioselective and syn stereoselective addition of B and H to the carbon-carbon double bond trialkyl

34、boranes are rarely isolated oxidation with alkaline hydrogen peroxide gives an alcohol and sodium borateuHydrogen peroxide oxidation of a trialkylborane step 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 bon

35、ding electrons to an adjacent oxygen atom step 3:reaction of the trialkylborane with aqueous NaOH gives the alcohol and sodium borate the loss of electrons alternatively,the loss of H,the gain of O,or both the gain of electrons alternatively,the gain of H,the loss of O,or bothuRecognize using a bala

36、nced 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-three balanced half-reactionsuOsO4 oxidizes an alkene to a,a compound with OH groups on adjacent c

37、arbons oxidation is syn stereoselective OsO4 is both expensive and highly toxic it is used in catalytic amounts with another oxidizing agent to reoxidize its reduced forms and,thus,recycle OsO4uTreatment of an alkene with ozone followed by a weak reducing agent cleaves the C=C and forms two carbonyl

38、 groups in its place Propanal(an aldehyde)Propanone(a ketone)2-Methyl-2-penteneCH3OOCH3C=CHCH2CH31.O32.(CH3)2SCH3CCH3 +HCCH2CH3 the initial product is a molozonide which rearranges to an isomeric ozonideAcetaldehyde2-ButeneOCH3CH=CHCH3O3(CH3)2SCH3CHCH3CH-CHCH3OOOO OCOCHCH3HH3CA molozonideAn ozonideu

39、Most alkenes react with H2 in the presence of a transition metal catalyst to give alkanes commonly used catalysts are Pt,Pd,Ru,and Ni the process is called or,alternatively,addition occurs with syn stereoselectivity+H2PdCyclohexeneCyclohexane25C,3 atmuMechanism of catalytic hydrogenation even though

40、 addition syn stereoselectivity,some product may appear to result from trans addition reversal of the reaction after the addition of the first hydrogen gives an isomeric alkene,etc.uReduction of an alkene to an alkane is exothermic there is net conversion of one pi bond to one sigma bonduD DH0 depen

41、ds on the degree of substitution the greater the substitution,the lower the value of D DHu D DH0 for a trans alkene is lower than that of an isomeric cis alkene a trans alkene is more stable than a cis alkeneuIn several of the reactions presented in this chapter,chiral centers are createduWhere one

42、or more chiral centers are created,is the product one enantiomer and,if so,which one?a pair of enantiomers as a racemic mixture?a meso compound?a mixture of stereoisomers?uAs we will see,the stereochemistry of the product for some reactions depends on the stereochemistry of the starting material;tha

43、t is,some reactions are uWe saw in Section 6.3D that bromine adds to 2-butene to give 2,3-dibromobutane two stereoisomers are possible for 2-butene;a pair of cis,trans isomers three stereoisomers are possible for the product;a pair of enantiomers and a meso compound if we start with the cis isomer,w

44、hat is the stereochemistry of the product?if we start with the trans isomer,what is the stereochemistry of the product?reaction of cis-2-butene with bromine forms bridged bromonium ions which are meso and identical attack of bromide ion at carbons 2 and 3 occurs with equal probability to give enanti

45、omeric products as a racemic mixture reaction with bromine forms bridged bromonium ion intermediates which are enantiomers attack of bromide ion in either carbon of either enantiomer gives meso-2,3-dibromobutaneuGiven these results,we say that addition of Br2 or Cl2 to an alkene is stereospecific br

46、omination of cis-2-butene gives the enantiomers of 2,3-dibromobutane as a racemic mixture bromination of trans-2-butene gives meso-2,3-dibromobutane a reaction in which the stereochemistry of the product depends on the stereochemistry of the starting material OsO4 oxidation of cis-2-butene gives mes

47、o-2,3-butanedioluOsO4 oxidation of an alkene is stereospecific oxidation of trans-2-butene gives the enantiomers of 2,3-butanediol as a racemic mixture(optically inactive)and oxidation of cis-2-butene gives meso 2,3-butanediol(also optically inactive)uWe have seen two examples in which reaction of a

48、chiral starting materials gives chiral products in each case,the product is formed as a racemic mixture(which is optically inactive)or as a meso compound(which is also optically inactive)uThese examples illustrate a very important point about the creation of chiral molecules optically active(enantio

49、merically pure)products can never be produced from achiral starting materials and achiral reagents under achiral conditions although the molecules of product may be chiral,the product is always optically inactive(either meso or a pair of enantiomers)uNext let us consider the reaction of a chiral sta

50、rting material in an achiral environment the bromination of(R)-4-tert-butylcyclohexene only a single diastereomer is formed the presence of the bulky tert-butyl group controls the orientation of the two bromine atoms added to the ringuFinally,consider the reaction of an achiral starting material in