有机化学：第七章-2 Chemical Properties of Halogen-Substituted Hydrocarbons

1、Elimination reaction副反应副反应亲核试剂的影响亲核试剂的影响溶剂的影响溶剂的影响CH3 1oRX 2oRX 3oRXR-X的反应活性的反应活性Walden Inversion立体化学立体化学= kRXNu- Second order reaction动力学动力学反应机理反应机理SN2R X+Nu-NuCX-Nu R+X-(one step) PhCH2X，CH2=CHCH2X离去基团离去基团I- Br- Cl- F-polar aprotic solventsHigh nucleophilicity is favorable(2) The SN1 Reaction-单分子反

2、应单分子反应 (unimolecular)Chemical Properties of Halogen-Substituted Hydrocarbons(CH3)3CBr + H2O(CH3)3COH + HBr= k(CH3)3BrFor SN1 reaction: the reaction rate is dependent only on the alkyl halide concentration and is independent of the H2O concentration. In other words, the reaction is a first-order proc

3、ess.The mechanism of SN1:Chemical Properties of Halogen-Substituted Hydrocarbons+H2OCCH3CH3CH3+ Fast step+ CCH3CH3CH3+OH2 CCH3CH3CH3+OH2H2O+CCH3CH3CH3OHH3O+CCH3CH3CH3BrRate-limiting stepCCH3CH3CH3+ +Br-Carbocation-H+2-Bromo-2-methylpropaneStereochemistry of the SN1 Reaction:Chemical Properties of Ha

4、logen-Substituted HydrocarbonsRSInversion 构型翻转构型翻转retention构型保持构型保持 Since an SN1 reaction occurs through a carbocation intermediate, carbocations are planar and are achiral. The carbocation can be attacked by a nucleophile equally well from either side, leading to a 50:50 mixture of enantiomers a ra

5、cemic mixture (外消旋体外消旋体). Racemization 外消旋化外消旋化 The characteristics of the SN1 reaction:Chemical Properties of Halogen-Substituted Hydrocarbons(a) The substrates : The more stable the carbocation intermediate, the faster the SN1 reaction.3RX 2 CRX 1RX CH3X PhCH2XCH2=CHCH2XXH2CCHXNo reactionThe relat

6、ive reactivity of substrate in SN1:(b) The nucleophiles:Chemical Properties of Halogen-Substituted Hydrocarbons The SN1 reaction occurs through a rate-limiting step in which the added nucleophile has no kinetic role ,Thus, nucleophles can not affect the reaction rate. The nucleophiles must be no bas

7、ic to prevent a competitive elimination (消除消除) of HXGood leaving group increase the reaction rate.(c) The leaving group:(d) The solvent:Chemical Properties of Halogen-Substituted HydrocarbonsSol-OH: C2H5OH 80%C2H5OH 50%C2H5OH20%H2O 50%H2OH2ORelative reactivity：(CH3)3CBr + Sol-OH(CH3)3C OSol + HBrSN1

8、1 10 2 9 1450 Polar solvent stabilize the carbocation intermediate by solvation(溶剂化溶剂化), thereby increasing the reaction rate.Chemical Properties of Halogen-Substituted HydrocarbonsTypical SN1 reaction:The relative reactivity of the reaction is:RX + AgNO3RONO2 + AgXEtOH PhCH2XCH2=CHCH2X 3RX 2 CRX 1R

9、X CH3X消除反应消除反应消除反应，消除反应，重排重排副反应副反应亲核性强有利于反应亲核性强有利于反应影响不明显影响不明显亲核试剂的影响亲核试剂的影响非质子性溶剂非质子性溶剂对反应有利对反应有利极性溶剂极性溶剂对反应有利对反应有利溶剂的影响溶剂的影响R-X的反应活性的反应活性构型翻转构型翻转外消旋化外消旋化立体化学立体化学 = kRXNu-= kRX动力学动力学反应机理反应机理SN2SN1R XslowRXR+X-R+Nu-fastR Nu(two steps)R X+Nu-NuCX-Nu R+X-(one step)离去基团离去基团I- Br- Cl- F-I- Br- Cl- F-3RX

10、 2 RX 1RX CH3X CH3X 1RX 2 RX 3RXChemical Properties of Halogen-Substituted HydrocarbonsExercise：1. Predict the relatively reaction rate of the following pairs of reaction:(CH3)2CHCH2Cl+SH(CH3)2CHCH2SH+Cl(CH3)2CHCH2I+SH(CH3)2CHCH2SH+IfasterCH3CH2CHCH2BrCH3+CNCH3CH2CHCH2CNCH3+BrCH3CH2CH2CH2CH2Br+CNCH3

11、CH2CH2CH2CH2CN+Br(1)(2)SN2fasterCH3CH2BrAgNO3-EtOHCH3CH2ONO2AgBr+BrAgNO3-EtOHONO2AgBr+BrAgNO3-EtOHONO2AgBr+Chemical Properties of Halogen-Substituted Hydrocarbons(3)fastestSN1CH3CH2ClCH3CH2I +NaCl(CH3)2CHClNaI-CH3COCH3(CH3)2CHI+NaClNaI-CH3COCH3(4)SN2fasterChemical Properties of Halogen-Substituted H

12、ydrocarbons2. Complete the following reactions:1)ICH3BrCl2hvHCCNaICH2CBrCHHgSO4H2SO4 /H2OICH2CCH3BrOICH2ClBrCH3Cn-C3H7C2H5BrH2OCH3Cn-C3H7C2H5OH+CHOCH3n-C3H7C2H5Chemical Properties of Halogen-Substituted HydrocarbonsSN1， Racemization HCH3BrHNaI丙丙酮酮HCH3HISN2， Walden Inversion2)3)Chemical Properties of

13、 Halogen-Substituted Hydrocarbons3. Order each of the following sets of compounds according to the different reaction conditions.(1) Reacted with 2% AgNO3 in ethanol solution： (A) 1- Bromobutane (B) 1- Chlorobutane (C) 1- Iodobutane C A B(2) Reacted with NaI in acetone: (A) 3- Bromopropylene (B) Vin

14、yl bromide (C) 1- Bromobutane (D) 2- Bromobutane A C D B(3) Nucleophilicity towards SN2 Reaction: OO2NOCH3CH2OABCC B AH2O-H+H2O-H+Predicate the mechanism of the following reactionCH3+CHCH3+CHCH3Ag+H2OCHCH3+CH3OHClOH环上亚甲基的迁移环上亚甲基的迁移Chemical Properties of Halogen-Substituted HydrocarbonsChemical Prope

15、rties of Halogen-Substituted Hydrocarbons3. Elimination Reactions(消除反应消除反应) of Alkyl Halides When a nucleophile/Lewis base reacts with an alkyl halide, the nucleophile can attack at a neighboring hydrogen and cause elimination of HX to form an alkene.CXCH 12NaOHH2OsubstitutionEliminationCOHCHCC+ X+

16、X+ H2O-H(1) Regiochemistry of Elimination of Alkyl Halides- Zaitsevs RuleChemical Properties of Halogen-Substituted Hydrocarbons 1875, the Russian chemist Alexander Zaitsev indicated that base-induced elimination reactions generally give the more highlysubstituted (more stable) alkene product.RCH2CH

17、CH3BrKOH乙乙醇醇RCHCHCH3RCH2CHCH2+81%19%CH2CHCH3BrC2H5OHNaOHCH=CHCH3( (区域选择性区域选择性) )优先消除含氢少的优先消除含氢少的- C上的氢上的氢Chemical Properties of Halogen-Substituted HydrocarbonsCH2=CHCH2CHCH(CH3)2BrC2H5OHNaOHCH2=CHCH=CHCH(CH3)2CH2=CHCH2CH=C(CH3)2CH3ClRO-HHCH3+CH3O-(CH3)3CO-33 %67 %91 %9 %(2) The E2 ReactionChemical

18、Properties of Halogen-Substituted HydrocarbonsCRRCRRHXB:CRRCRRB.HX. +_ Transition stateCRRCRR:X_+B H when an alkyl halide is treated with a strong base, such as hydroxide ion (HO-) or alkoxide ion (RO-). The E2 reaction takes place in one step without intermediate. As the attacking base begins to ab

19、stract H+ from a carbon next the leaving group, the C-H bond begins to break, a C=C bond begins to form, and the leaving group begins to depart taking with it the electron pair from the C-X bond.Chemical Properties of Halogen-Substituted HydrocarbonsThe relative activity of E2:3RX 2RX 1RXRI RBr RCl

20、RFH3CCCH3CH3XH3CCHCH3XH3CCHHXWhy?The stereochemistry of E2 reactionChemical Properties of Halogen-Substituted HydrocarbonsE2 reactions always occur with a anti- periplanar geometry,periplanar geometry: meaning that all four reacting atoms-the hydrogen, the two carbons, and the leaving group lie in t

21、he same plane.anti periplanar geometry: in which the H and the X are on opposite sides of the molecule. CHCXXHCHCXXHanti periplanar geometry:syn periplanar geometry:反式共平面反式共平面Chemical Properties of Halogen-Substituted HydrocarbonsCHBrPhCHPhBrKOHEthanolCBrPhCHPhMeso-1,2-Dibromo-1,2-diphenylethaneE-CH

22、3HHClHCH(CH3)2OH-HCl(CH3)2CHCH3E2MechanismAlkyl 3RX 2RX 1RXLeaving Group RI RBr RCl RF BaseReaction rate is related to basicity of baseSolventHave some effectsstereochemistryanti- periplanar geometryThe factors that affect the E2 reaction:One step reaction with a transition stateChemical Properties

23、of Halogen-Substituted Hydrocarbons1. Draw structures corresponding to the following IUPAC names:(a) 2,3-Dichloro-4-methylhexane(b) 4-Bromo-4-ethyl-2-methylhexane(c) 3-Iodo-2,2,4,4-tetramethylpentane(d) cis-1-Bromo-2-ethylcyclopentane2. Predict the products of the following reactions:H3C OHHBrEther(a) Homework for this chapter:Chemical Properties of Halogen-Substituted HydrocarbonsNBSCCl4(c) OHPBr3Ether(d) 3. Order each of the following sets of compounds with respect to SN1 reactivity:H3CCClCH3CH3ClH3CCH3CH3C