有機化學(xué)：第七章-2 Chemical Properties of Halogen-Substituted Hydrocarbons

1、Elimination reaction副反應(yīng)副反應(yīng)親核試劑的影響親核試劑的影響溶劑的影響溶劑的影響CH3 1oRX 2oRX 3oRXR-X的反應(yīng)活性的反應(yīng)活性Walden Inversion立體化學(xué)立體化學(xué)= kRXNu- Second order reaction動力學(xué)動力學(xué)反應(yīng)機理反應(yīng)機理SN2R X+Nu-NuCX-Nu R+X-(one step) PhCH2X，CH2=CHCH2X離去基團離去基團I- Br- Cl- F-polar aprotic solventsHigh nucleophilicity is favorable(2) The SN1 Reaction-單分子反

2、應(yīng)單分子反應(yīng) (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 構(gòu)型翻轉(zhuǎn)構(gòu)型翻轉(zhuǎn)retention構(gòu)型保持構(gòu)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 (外消旋體外消旋體). 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(溶劑化溶劑化), 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消除反應(yīng)消除反應(yīng)消除反應(yīng)，消除反應(yīng)，重排重排副反應(yīng)副反應(yīng)親核性強有利于反應(yīng)親核性強有利于反應(yīng)影響不明顯影響不明顯親核試劑的影響親核試劑的影響非質(zhì)子性溶劑非質(zhì)子性溶劑對反應(yīng)有利對反應(yīng)有利極性溶劑極性溶劑對反應(yīng)有利對反應(yīng)有利溶劑的影響溶劑的影響R-X的反應(yīng)活性的反應(yīng)活性構(gòu)型翻轉(zhuǎn)構(gòu)型翻轉(zhuǎn)外消旋化外消旋化立體化學(xué)立體化學(xué) = kRXNu-= kRX動力學(xué)動力學(xué)反應(yīng)機理反應(yīng)機理SN2SN1R XslowRXR+X-R+Nu-fastR Nu(two steps)R X+Nu-NuCX-Nu R+X-(one step)離去基團離去基團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環(huán)上亞甲基的遷移環(huán)上亞甲基的遷移Chemical Properties of Halogen-Substituted HydrocarbonsChemical Prope

15、rties of Halogen-Substituted Hydrocarbons3. Elimination Reactions(消除反應(yīng)消除反應(yīng)) 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( (區(qū)域選擇性區(qū)域選擇性) )優(yōu)先消除含氫少的優(yōu)先消除含氫少的- C上的氫上的氫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