有机化学 英文课件 chapter6.ppt

1、Organic Chemistry,William H. Brown Christopher S. Foote Brent L. Iverson,Reactions of Alkenes,Chapter 6,Characteristic Reactions,Characteristic Reactions,Reaction Mechanisms,A reaction mechanism describes how a reaction occurs which bonds are broken and which new ones are formed the order and relati

2、ve 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 and energy of the entire system during the reaction,Gibbs Free Energy,Gibbs free energy change, DG0: a thermodynamic function

3、relating enthalpy, entropy, and temperature exergonic 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 products endergonic reaction: a reaction in which the Gibbs free energy of the p

4、roducts is higher than 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 equilibrium summary of the relationships between DG0, DH0, DS0, and the position of chemical

5、equilibrium,Energy Diagrams,Enthalpy change, DH0: the difference in total bond energy between reactants and products a measure of bond making (exothermic) and bond breaking (endothermic) Heat of reaction, DH0: the difference in enthalpy between reactants and products exothermic reaction: a reaction

6、in which the enthalpy of the products is lower than that of the reactants; a reaction in which heat is released endothermic 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 sh

7、owing the changes in energy that occur during a chemical reaction Reaction 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 reaction a maximum on an energy diagram Activ

8、ation Energy, G: the difference in Gibbs free energy between reactants and a transition state if G is large, few collisions occur with sufficient energy to reach the transition state; reaction is slow if G is small, many collisions occur with sufficient energy to reach the transition state; reaction

9、 is fast,Energy Diagram,a one-step reaction with no intermediate,Energy Diagram,A two-step reaction with one intermediate,Developing a Reaction Mechanism,How it is done design experiments to reveal details of a particular chemical reaction propose a set or sets of steps that might account for the ov

10、erall 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,Why Mechanisms?,they are the framework within which to organize descript

11、ive 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,Electrophilic Additions,hydrohalogenation using HCl, HBr, HI hydration using

12、 H2O in the presence of H2SO4 halogenation using Cl2, Br2 halohydrination using HOCl, HOBr oxymercuration using Hg(OAc)2, H2O followed by reduction,Addition of HX,Carried out with pure reagents or in a polar solvent such as acetic acid Addition is regioselective regioselective reaction: an addition

13、or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed Markovnikovs 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-st

14、ep mechanism Step 1: proton transfer from HBr to the alkene gives a carbocation intermediate Step 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-butene the reaction i

15、s exergonic,Carbocations,Carbocation: a species in which a carbon atom has only six electrons in its valence shell and bears positive charge Carbocations 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

16、electron-loving Lewis acids,Carbocations,bond angles about a positively charged carbon are approximately 120 carbon uses sp2 hybrid orbitals to form sigma bonds to the three attached groups the unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons,Carbocati

17、on Stability,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 carbocation, methyl and 1 carbocations are so unstable that they are never observed in solution,Carbocation Stability,relative

18、stability methyl and primary carbocations are so unstable that they are never observed in solution,Carbocation 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

19、positive charge of the cation we account for this electron-releasing ability of alkyl groups by (1) the inductive effect, and (2) hyperconjugation,The Inductive Effect,the positively charged carbon polarizes electrons of adjacent sigma bonds toward it the positive charge on the cation is thus locali

20、zed over nearby atoms the larger the volume over which the positive charge is delocalized, the greater the stability 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 delocali

21、zation of the positive charge,Addition of H2O,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,Addition of H2O,Step 1: proton tra

22、nsfer from H3O+ to the alkene Step 2: reaction of the carbocation (an electrophile) with water (a nucleophile) gives an oxonium ion Step 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 electr

23、ophilic addition to alkenes, there is the possibility for rearrangement Rearrangement: a change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material,Carbocation Rearrangements,in addition of HCl to an alkene in acid-catalyzed hydration o

24、f an alkene,Carbocation Rearrangements,the driving force is rearrangement of a less 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 carbocation,H,H,fast,Carbocation Rearrangements,reaction of the more stabl

25、e carbocation (an electrophile) with chloride ion (a nucleophile) completes the reaction,-,:,:,:,:,2-Chloro-2-methylbutane,+,+,fast,:,:,:,Addition of Cl2 and Br2,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 t

26、rans-dihalocycloalkane addition occurs with anti stereoselectivity; halogen atoms add from the opposite face of the double bond we will discuss 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:

27、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,Addition of Cl2 and Br2,for a cyclohexene, anti coplanar addition corresponds to trans diaxial addition the initial trans diaxial conformation is in eq

28、uilibrium 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 mixture,Addition of HOCl and HOBr,Treatment of an alkene with Br2 or Cl2 in water forms a halohydrin H

29、alohydrin: a compound containing -OH and -X on adjacent carbons,Addition of HOCl and HOBr,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 account for the regioselect

30、ivity 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 halonium ion intermediate Step 2: attack of H2O on the more substituted carbon opens the three-membered ring,Addition of HOCl and HOBr,Step 3: p

31、roton transfer to H2O completes the reaction As 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 att

32、ack of the nucleophile at the more substituted carbon,Addition of HOCl and HOBr,bridged bromonium ion from propene,Oxymercuration/Reduction,Oxymercuration followed by reduction results in hydration of a carbon-carbon double bond oxymercuration reduction,Oxymercuration/Reduction,an important feature

33、of oxymercuration/reduction is that it occurs without rearrangement oxymercuration occurs with anti stereoselectivity,Oxymercuration/Reduction,Step 1: dissociation of mercury(II) acetate Step 2: formation of a bridged mercurinium ion intermediate; a two-atom three-center bond,Oxymercuration/Reductio

34、n,Step 3: regioselective attack of H2O (a nucleophile) on the bridged intermediate opens the three-membered ring Step 4: reduction of the C-HgOAc bond,Oxymercuration/Reduction,Anti stereoselective we account for the stereoselectivity by formation of the bridged bromonium ion and anti attack of the n

35、ucleophile which opens the three-membered ring Regioselective of the two carbons of the mercurinium ion intermediate, the more substituted carbon has the greater degree of partial positive character alternatively, computer modeling indicates that the C-Hg bond to the more substituted carbon of the b

36、ridged 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,Hydroboration/Oxidation,Hydroboration: the addition of borane, BH3, to an alkene to form a trialkylborane Borane dime

37、rizes to diborane, B2H6,Borane,H,B,H,H,B,+,Borane,Hydroboration/Oxidation,borane forms a stable complex with ethers such as THF the reagent is used most often as a commercially available solution of BH3 in THF,Hydroboration/Oxidation,Hydroboration is both regioselective (boron to the less hindered c

38、arbon) 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 bond trialkylboranes are rarely isolated oxidation with alkaline hydrogen peroxide gives an alcohol and sodium borate,H

39、ydroboration/Oxidation,Hydrogen 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 bonding electrons to an adjacent oxygen atom,Hydroboration/Oxidation,step 3: re

40、action of the trialkylborane with aqueous NaOH gives the alcohol and sodium borate,Oxidation/Reduction,Oxidation: the loss of electrons alternatively, the loss of H, the gain of O, or both Reduction: the gain of electrons alternatively, the gain of H, the loss of O, or both Recognize using a balance

41、d half-reaction 1. 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 solution 3. complete a charge balance using electrons, e-,Oxidation/Reduction,three balanced half-reactions,Oxidation with OsO4,O

42、sO4 oxidizes an alkene to a glycol, a compound with OH groups on adjacent carbons oxidation is syn stereoselective,Oxidation with OsO4,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 OsO4,Oxidation

43、 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,Acetaldehyde,2-Butene,O,O,O,O,O,O,C,O,C,H,H,A m

44、olozonide,An ozonide,Reduction of Alkenes,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 catalytic reduction or, alternatively, catalytic hydrogenation addition occurs with syn stereoselec

45、tivity,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 addition reversal of the reaction after the addition of the first hydrogen gives an iso

46、meric alkene, etc.,H0 of Hydrogenation,Reduction of an alkene to an alkane is exothermic there is net conversion of one pi bond to one sigma bond H0 depends on the degree of substitution the greater the substitution, the lower the value of H H0 for a trans alkene is lower than that of an isomeric ci

47、s alkene a 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 created Where one or more chiral centers are created, is the product one enantiomer and, if so, which one? a pair of enantio

48、mers 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 6.3D that bromi

49、ne 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, what is the stereochemistry of the product? if we start

50、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 give en

51、antiomeric 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-Butene,

52、Given these results, we say that addition of Br2 or Cl2 to an alkene is stereospecific bromination 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 Stereospecific reaction: a reaction in which the stereochemist

53、ry of the product depends on the stereochemistry of the starting material,Oxidation of 2-Butene,OsO4 oxidation of cis-2-butene gives meso-2,3-butanediol,Oxidation of 2-Butene,OsO4 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 oxidati