有机化学课件3The Nature of Organic Reactions

1、3环戊烯5-甲基-1,3-环己二烯CH32-hexene2-methyl 3-hexene5-methyl 1,3-cyclohexadienecyclopenteneCH3CH2CH2CH CHCH3(CH3)2CHCH CHCH2CH32-甲基-3-己烯2-己烯4CH2CHCH3CH CH2CH2CHCH2乙烯基丙烯基烯丙基Vinyl groupPropanyl groupAllyl groupCHCH3H2C(CH3)2C亚亚乙乙基基亚亚甲甲基基亚亚异异丙丙基基亚亚- -3 3- -环环己己烯烯基基乙乙酸酸2 2- -( (或或 - -) )亚亚环环戊戊基基环环戊戊酮酮CHCO2HO M

2、ethylene Ethylene Isopropylene5*3.2 cis-trans isomers of Alkenes烯烃的顺烯烃的顺反异构体反异构体 Rotation of Bond Is Prohibitive6The presence of a carbon-carbon double can create two possible structures cis isomer - two similar groups on same side of the double bond trans isomer similar groups on opposite sides Eac

3、h carbon must have two different groups for these(cis/trans) isomers to occur7Cis-Trans Isomers Require That End Groups Must Differ in Pairs Bottom pair cannot be superposed (重叠) by 180rotation without breaking C=CX89101112131415Sequence Rules: The E,Z Designation E, Z 标示的次序规那么 Neither compound is c

4、learly “cis or “trans Substituents on C1 are different than those on C2 We need to define “similarity in a precise way to distinguish the two stereoisomers Cis/trans nomenclature only works for disubstituted double bonds 161. Considering each of the double-bond carbons separately, identify the two a

5、toms directly attached and rank them according to atomic number 分别考虑与双键碳原子直接相连的原子，根据原子序数排序 2. If a decision cant be reached by ranking the first atoms in the substituent, look at the second, third, or forth atoms away from the double-bond carbons until the first difference is found 如果第一个原子难以判别，再看第二、

6、三、四个原子直到能够作出区分 3. Multiple-bonded atoms are equivalent to the same number of single-bonded atoms 多键原子相等于同样数量的单键原子 17Ranking Priorities: Cahn-Ingold-Prelog Rules Must rank atoms that are connected at comparison point Higher atomic number gets higher priority Br Cl O N C H In this case,The higher prio

7、rity groups are opposite: (E )-2-bromo-2-chloro-propene18E,Z Stereochemical NomenclaturePriority rules of Cahn, Ingold, and Prelog Compare where higher priority group is with respect to bond and designate as prefix E -entgegen, opposite sides Z - zusammen, together on the same side19If atomic number

8、s are the same, compare at next connection point at same distance Compare until something has higher atomic numberDo not combine always compare Extended Comparison2021222324 *3.3 Kinds of Organic Reactions In general, we look at what occurs and try to learn how it happens Common patterns describe th

9、e changes Addition reactions two molecules combine Elimination reactions one molecule splits into two Substitution reaction parts from two molecules exchange Rearrangement reactions a molecule undergoes changes in the way its atoms are connected 25ABC+ABDCABABCABDDCD_C_ABCDABDC26How Organic Reaction

10、s Occur: Mechanisms In a clock the hands move but the mechanism behind the face is what causes the movement In an organic reaction, we see the transformation that has occurred. The mechanism describes the steps behind the changes that we can observe Reactions occur in defined steps that lead from re

11、actant to product 27CO2C2H5CO2C2H5CO2C2H5CO2C2H5+HBrOORBrHRHBr+RRHBrRLONu-+RLO-Nunucleophileelectrophile+-28+RNu ( E, R )RL+Nu- ( E+, R . )L- ( L+, L . )+A+B-A B. Organic Reactions: A B.B.A.+Radical:Polar:29Homogenic Formation of a BondOne electron comes from each fragmentNo electronic charges are i

12、nvolved and Not common in organic chemistry 30Heterogenic Formation of a Bond One fragment supplies two electrons One fragment supplies no electrons Combination can involve electronic charges and Common in organic chemistry31*An Example of a Polar Reaction: Addition of HCl to EthylenenucleophileElec

13、trophile323334Describing a Reaction: Rates and Equiilibria 35 The energy needed to go from reactant to transition state is the activation energy (DG). The highest energy point in a reaction step is called the transition stateDescribing a Reaction: Reaction Energy Diagrams and Transition States36Desc

14、ribing a Reaction: Intermediate HBr, a Lewis acid, adds to the bond This produces an intermediate with a positive charge on carbon - a carbocation(碳正离子) This is ready to react with bromide37 If a reaction occurs in more than one step, it must involve species that are neither the reactant nor the fin

15、al product. They are called reaction intermediates or simply “intermediates . Each step has its own free energy of activation. The complete diagram for the reaction shows the free energy changes associated with an intermediate 38Energy Diagram for Electrophilic Addition Rate determining (slowest) st

16、ep has highest energy transition state Independent of direction In this case it is the first step in forward direction “rate is not the same as “rate constant 39Carbocation Intermediate Reacts with Anion 40Reaction Diagram for Addition of HBr to Ethylene 41Mechanistic Source of Regiospecificityin Ad

17、dition Reactions If addition involves a carbocation intermediate and there are two possible ways to add the route producing the more alkyl substituted cationic center is lower in energy alkyl groups stabilize carbocation423.4 Addition of HX to Alkenes: Hydrohalogenation General reaction mechanism: e

18、lectrophilic addition Attack of electrophile (such as HBr) on bond of alkene Produces carbocation and bromide ion Carbocation is an electrophile, reacting with nucleophilic bromide ion43Two step process First transition state is high energy pointElectrophilic Addition Energy Path44Example of Electro

19、philic AdditionAddition of hydrogen bromide to 2-Methyl-propene H-Br transfers proton to C=C Forms carbocation intermediate More stable cation forms Bromide adds to carbocation45Orientation of Electrophilic Addition: Markovnikovs Rule In an unsymmetrical alkene, HX reagents can add in two different

20、ways, but one way may be preferred over the other If one orientation predominates, the reaction is regiospecific(位置选择性) Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most Hs and X attaches to the other end (to the one with the mo

21、st alkyl substituents)This is Markovnikovs rule(马氏规那么)46Addition of HCl to 2-methylpropeneRegiospecific one product forms where two are possible Example of Markovnikovs Rule47Energy of Carbocations and Markovnikovs RuleMore stable carbocation forms faster Tertiary cations and associated transition s

22、tates are more stable than primary cations48Carbocation Structure and Stability Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2 orbitals The fourth orbital on carbon is a vacant p-orbital The stability of the carbocation is increased by the presence of a

23、lkyl substituentsTherefore stability of carbocations: 3 2 1 +CH34950513.5 Addition of H2O to Alkenes: Hydration52533.6 Addition of Water to Alkenes: Oxymercuration(羟汞化反响羟汞化反响) For laboratory-scale hydration of an alkene Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientati

24、on via mercurinium ion543.7 Addition of Water toAlkenes: Hydroboration(硼氢化反响) Borane (BH3) is electron deficient is a Lewis acid Borane adds to an alkene to give an organoboraneH. C. Brown55Addition of Water toAlkenes: Hydroboration(硼氢化反响) Borane (BH3) is electron deficient is a Lewis acid Borane ad

25、ds to an alkene to give an organoboraneH. C. Brown56BH3 is a Lewis AcidSix electrons in outer shell Coordinates to oxygen electron pairs in ethers57 Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane Oxidation with alkaline hydrogen peroxide(H2O2) in water produces the

26、alcohol derived from the alkeneHydroboration-Oxidation Forms an Alcohol from an Alkene 58Orientation in Hydration via HydroborationRegiochemistry is opposite to Markovnikov orientation OH is added to carbon with most Hs H and OH add with syn stereochemistry, to the same face of the alkene (opposite

27、of anti addition)59Hydroboration, Electronic Effects Give Non-Markovnikov More stable carbocation is also consistent with steric preferences60Hydroboration - Oxygen Insertion StepH2O2, OH- inserts OH in place of B Retains syn orientation61Oxymercuration(羟汞化反响羟汞化反响) 62633.8 Addition of X2 to Alkenes:

28、 HalogenationBromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process F2 is too reactive and I2 does not add Cl2 reacts as Cl+ Cl- , Br2 is similar64Addition of Br2 to Cyclopentene+65Mechanism of Bromine AdditionBr+ adds to an alkene producing a cyclic ion Bromoniu

29、m ion(溴鎓离子), bromine shares charge with carbon Gives trans addition Product66Bromonium Ion MechanismElectrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile67The Reality o

30、f Bromonium Ions Bromonium were postulated more than 60 years ago to expain the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkene Olah showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly683.9 Addition of H2 to Alkenes: Hydro

31、genationAddition of H-H across C=C Reduction in general is addition of H2 or its equivalent Requires Pt or Pd as powders on carbon and H2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution) 69Hydrogen Addition- SelectivitySelective for C=C. No reaction with

32、C=O, C=N Polyunsaturated liquid oils become solids If one side is blocked, hydrogen adds to other70Mechanism of Catalytic HydrogenationHeterogeneous reaction between phases Addition of H-H is syn713.10 Oxidation of Alkenes Hydroxylation adds OH to each end of C=C Catalyzed by KMnO4 / OH- Stereochemi

33、stry of addition is syn Product is a 1,2-dialcohol or diol (also called a glycol)OHOHHHKMnO4+NaOHH2O72Permangante Oxidation of Alkenes in acid medium cleave alkenes Potassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if Hs are present on C=C 73Reaction with osmium tetraoxi

34、de7475Alkenes Cleavage: OzoneOzone, O3, adds to alkenes to form molozonide Reduce molozonide to obtain ketones and/or aldehydes76Examples of Ozonolysis of Alkenes Used in determination of structure of an unknown alkene 77Structure Elucidation With Ozone Cleavage products reveal an alkenes structure7

35、8Epoxidation ReactionCH3CO3HOHH3CCH3HOH3CHCH3HCH3CO3HOOOHMCPBAButOOH / Ti(OPri)4*或CCRRROHO*CCRRROHO(D-)或(L-)DETCCRRROH793.11 Halohydrin formation(-卤代醇的产生卤代醇的产生)803.12 Addition of carbene(卡宾卡宾):cyclopropane synthesis 81Dichlorocarbene:8283Simmons-Smith Reaction843.13 Allylic bromination (烯丙基溴代反响烯丙基溴代

36、反响)8586878889CH2=CHCH2CH3+HBrPeroxideBrCH2CH2CH2CH3*Peroxide effectCH2=CHCH3+Cl25000CCH2=CHCH2Cl903.14 Alkene Polymers A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization Alkenes react with radical catalysts to undergo radical polymerizatio

37、n Ethylene is polymerized to polyethylene, for example 91Free Radical Polymerization of Alkenes Alkenes combine many times to give polymer Reactivity induced by formation of free radicals92Free Radical Polymerization: InitiationInitiation(引发) - a few radicals are generated by the reaction of a molec

38、ule that readily forms radicals from a nonradical molecule A bond is broken homolytically93Polymerization: PropagationRadical from intiation adds to alkene to generate alkene derived radical This radical adds to another alkene, and so on many times 94Polymerization: TerminationChain propagation ends

39、 when two radical chains combine Not controlled specifically but affected by reactivity and concentration 95Other Polymers Other alkenes give other common polymers96Unsymmetrical MonomersIf alkene is unsymmetrical, reaction is via more highly substituted radical 97Cationic Polymerization Vinyl monom

40、ers react with Brnsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations983.15 Preparation of Alkenes: Elimination reactionsCCCCXY加成消除additionelimination991. Dehydrohalogenation100101102 Zaizsevs rule: in the elimination of HX from an al

41、kyl halide, the more highly substituted alkene product predominates.103Z-14%E-41%25%20%BrCH3CH2CHCH3C2H5OKC2H5OH+OC2H5CH3CH2CHCH3CH3CHCHCH3CH3CH2C(CH3)CH22. DehydrationH2OOHHCH3CH3OHHHCCHCCHHmajorminorH2O+104CCXCCZnX-+CCXXZnCCBrBrCC+IBr+ZnX2Br2Br-. .I-3. Dehalogenation1053.16 Conjugated Dienes Compo

42、unds can have more than one double or triple bond If they are separated by only one single bond they are conjugated(共共轭轭) and their orbitals interact The conjugated diene 1,3-butadiene has properties that are very different from those of the nonconjugated diene, 1,5-pentadiene106*Measuring Stability

43、 Conjugated dienes are more stable than nonconjugated based on heats of hydrogenation Hydrogenating 1,3-butadiene takes up 16 kJ/mol more heat than 1,4-pentadiene107Conjugation effect is a series of overlapping p orbitals, usually in alternating single and multiple bonds.CH2CHCHCHCHCH2O- conjugation

44、: A serios of overlapping p orbitals, usually in alternating single and multiple bonds. For example, 1,3-butadiene is a conjugated diene, 3-buten-2-one is a conjugated enone, and benzene is a cyclic conjugated triene108CCXCCX+-.CCCHHH(a)(b)p- conjugation: The donation or withdrawal of electrons thro

45、ugh orbital overlap with neighboring bonds (a).Hyperconjugation(超共轭): An interaction that results from overlap of a vacant p orbital on one atom with a neithboring CH bond(b). 109Molecular Orbital Description of 1,3-Butadiene The single bond between the conujgated double bonds is shorter and stronge

46、r than sp3 The bonding -orbitals are made from 4 p orbitals that provide greater delocalization and lower energy than in isolated C=C The 4 molecular orbitals include fewer total nodes than in the isolated case In addition, the single bond between the two double bonds is strengthened by overlap of p

47、 orbitals In summary, we say electrons in 1,3-butadiene are delocalized over the bond system and Delocalization leads to stabilization110HHHHHHHCCCCHHHHH111Carbocations from Conjugated Dienes Addition of H+ leads to delocalized secondary allylic carbocation112 Addition to a conjugated diene at or be

48、low room temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct At higher temperature, product ratio changes and 1,4 adduct predominates 113Products of Addition to Delocalized Carbocation Nucleophile can add to either cationic site The transition

49、 states for the two possible products are not equal in energy114*Kinetic vs. Thermodynamic Control of ReactionsAt completion, all reactions are at equilibrium and the relative concentrations are controlled by the differences in free energies of reactants and products (Thermodynamic Control) If a rea

50、ction is irreversible or if a reaction is far from equilibrium, then the relative concentrations of products depends on how fast each forms, which is controlled by the relative free energies of the transition states leading to each (Kinetic Control) 115ABC116117118*The Diels-Alder Cycloaddition Reaction Conjugate dienes can combine with alkenes to form six-membered cyclic compounds T