高等有機(jī)化學(xué)chapter v

1、Chapter VAddition & EliminationA. Polar Addition : distinct from radical addition1. MechanismH-X Additions a. Markovnikovs RuleRule: X goes to more subsituted carbonb. Unconjugated alkenes: plex, anti-addition, termolecularprimarily associationlewis acid-base complex plex, -bondWhy the major product

2、 is trans?+a: Methyl is electron-donating group ,Thats why bromide attacks on the more substituted carbon. If EWG , the attack goes the other wayb: plex decide anti additionMechanism:c. Conjugated alkenes: bimolecular, syn addition majorly , nice carbocationAdE23. Hydration4. Halogenationa. stereoch

3、emistrySyn V.S. AntiConclusion:1) For brominations, anti addition is preferred for alkenes that do not contain substituent groups that would strongly stablize a C+ intermidiate. 2) When the alkene is conjugated with an aryl group ,the extent of syn addition e much larger.Examples:b. kineticRate = k1

4、ethyleneBr2 + k2ethyleneBr22 + k3ethyleneBr2Br-First term:k1ethyleneBr2 this term implies AdE2 mechanism complexSecond term:k2ethyleneBr22 this term implies AdE3 mechanismSummary5. Addition of carbocationWhat is the driving force?In this case, a double bond breaks while a single bond forms. loss of

5、C=C bond: 64 Kcal/molgain of C-C bond: 81 Kcal/molFrom the data above, we know the reaction is driven by the energy gained in the stronger bond formation.This can be a standard polymerization reaction, so it is a problem to use this energy to generate single addition molecule.To make the reaction be

6、tter: a: add L- so as to trap the C+ easier b: in terms of forming the rings, this reaction is applied generallyExample:Which is the main product?T.S. state is chair-like The p orbital of carbocation is flat, orbital of C=C should be flat too, so 6-member-ring dominates in C+ addtion reaction. norma

7、lly,in the case of 6-member ring or 5-member ring , it always forms 6-member ring.solvolysisExample:B . Elimination1. types of Elimination-eliminationCarbene a lone pair of electrons an empty orbital neutral-eliminationOnly get 1,1-E when 1,2-E is impossible , Typically, 1,2-E is common -elimination

8、Rare happens when there s ring somewhere to fixA. 1,1-EliminationB. 1,2- EliminationC. 1,3- Eliminaton2 .Mechanisms of 1,2-Eliminationmore substituted alkenea. E1 : Classic carbocation intermediateb. E2 : c. E1cb (E1 conjugated base)Proton abstraction from less hindered position. It typically happen

9、s with acid proton and poor leaving group. The leaving group is to a carbonyl , -CN, sulfonyl or other carbanion stablizing group. Carbanion intermediate (less hindered alkene)If k2 k-1 , E1cb(irr) , very hard to distinguish from E2Often k-1 k2 ,E1(rev), distinguish from E2 by look at H/D into SMFou

10、nd D incorporation into SM3. E1 analogous to SN1 : carbcation intermediate Favored over E2, E1cb by weak bases and good leaving group. Base is too weak to get involved in the reaction.Why get this major alkene ?the more alkyl group substituents,the more stable the product . By hyperconjunction, weak

11、 C-H bond to get more stable resonance structure .C+Actually two RXN with similar E#, two products should be very similar. So why ?4. E2- favored by strong base and poorer leaving group Mechanisms T.S.E1-like. Not completely concerted .X almost leaves , H still there. Formation of the more highly su

12、bstituted alkene .E2 full . Concerted. Mainly the more substituted alkene.E1cb-like . The direction of the elimination is governed by the ease of proton removed, less substituted alkene products dominatesExample :Y-I2.070 means a partial negative charge building up-Cl2.61 is bigger:a-Cl is bad LG,ne

13、eds more push from C-;b-Cl is more electronegative,stablize C-NMe33.77 is even higher:a-NMe3 cation stablizes C-; more hard to break C-N bond.b. RegioselectivitySaytzeff products more stable products because of C = C bond character in T.S. E1 & E2 involving relatively good leaving group.2) Hofmann p

14、roducts the acidity of the hydrogen (Ha, Hb) controls the reaction. Hb is more acidic because of sterics and electron-donating R groups.Which one? Basically both are right at different time and different places Once building up negative charge , the acidity of H makes a big deal.Strong base pull off

15、 the Hb , makes it different . 69% 78%X=I19%63%18%Cl33%33%17%F69%69%9%X=I(BuO-/BuOH)78%15%9%poor LGExample :Building up lots of -, acidity is the point.E1cb-Like: Cl makes Ha more acidicAnti eliminationAnti elimination is favored because of the anti conformation, syn is ellipsedc. syn V.S. anti elim

16、inationE2 Stereochemistry anti between L & H syn orbital alignExample :2) Syn elimination Typically happens when anti is not achievable at all or syn-positive hydrogen is more reactivea) AcidityPh group makes syn-H more acidic.Strong bases are more likely to provide a high proportion of syn eliminat

17、ion product.c) Ion pairing: the idea is solvent effect.an ion pair promotes syn-elimination of anionic leaving groups.-less polar solvent favor ion pairing.-bulky bases and not well solvated system favor ion pairing.b) Conformationrelative rate1386steric repulsionfaster,none of them can go anti elim

18、ination, so syn eliminationTypically:Thus ,if one elimination needs in TIP,using tBuOK/tBuOH as the reaction conditions.concerted,syn eliminationK+ really needs to be associated with Br before it leaves. good leaving group=0.15.0 should build up a negative change1 means that k2 is not RDS,k1 is RDS.

19、=1,if k2 is slow.which means k1 is RDS.This anion is far away from01if its E2,it will be close to 1, a full negative charge is formed.6. non-catalyzed eliminationCope Elimination:syn-eliminationIf -H changes to -D,since its a fast step.The primary isotope effect is not obvious.KH/KD=1KH/KD will be v

20、ery obvious.-OMe is a poor leaving group,the intermadiate exists.But if :7. Grob fragmentationan easy way to make a 9-member ringnot very kinetically acidic.much easier to be pulled off.C.Carbenes1.Generation2.Structure:a.singletb. triplet3. Reactiontripletreacts like radicalssingletreacts like elec

21、tronphilesC+ dominates reactivitya. Migration1) Singlet carbene:Typically the fastest reaction of carbenes.After generation of the carbene, the migration can happen.as the leaving of Br-,the migration of PhBr occurs simutaneouslybutits really carbene-like reaction.2) Triplet carbenes:dont migrate.However ,there is only a small energy gap between triplet to singlet. triplet -singlet -migratefairly stable carbene: adjacent to a carbonyl groupb. Additions:1