化学专业英语《Chemistry English》课程教学资源（PPT课件）Lecture 11

Heterolytic nucleophilic substitution:Sl and S2: halogenoalkanes with nucleophilesHeterolytic nucleophilicsubstitution:Sμ1 and Sn2:halogenoalkanes withnucleophiles

1 Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles

Heterolytic nucleophilic substitution:Snl and S2: halogenoalkanes with nucleophilesIf the bond breaksWhen carbon is bondedThis can result inheterolytically, twoto a morecompleteions. a halide and aelectronegative atomeitherheterolyticcarbocation,arebreakage of the3.0￥2.80made.HalCbond.XCHalHalThis atom applies aninductive effect on theCharge separationelectrons in the bond,creates an electronThis can result inpulling the electronsdeficient carbonchargetowards it.oratom which is aseparation alongreactive site with athe bond.XHalcenter of positiveo&18+charge.C-Hal18+C-Hal1

2 Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles When carbon is bonded to a more electronegative atom This atom applies an inductive effect on the electrons in the bond, pulling the electrons towards it. This can result in complete heterolytic breakage of the bond. If the bond breaks heterolytically, two ions, a halide and a carbocation, are made. This can result in charge separation along the bond. Charge separation creates an electron deficient carbon atom which is a reactive site with a center of positive charge. either or

Heterolytic nucleophilic substitution:Sl and Sn2: halogenoalkanes with nucleophilesThese two possibilities lead to two different reaction mechanisms:SUBSTITUTION BY A NUCLEOPHILE: FIRST ORDERKINETICS: S~1NoHalCoreither+CNuNuOO10HalSopofeCStage 1Stage 2The carbon-halogenThe carbocation is attacked by a nucleophilebond breaksThis step is very fast, so the rate determiningheterolyticallystep is stage 1. The rate of stage 1 dependsonly on the concentration of halogencompound, so the kinetics are first order.3

3 Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles SUBSTITUTION BY A NUCLEOPHILE; FIRST ORDER KINETICS: SN1 These two possibilities lead to two different reaction mechanisms: Stage 1 The carbon-halogen bond breaks heterolytically. Stage 2 The carbocation is attacked by a nucleophile. This step is very fast, so the rate determining step is stage 1. The rate of stage 1 depends only on the concentration of halogen compound, so the kinetics are first order

Heterolytic nucleophilic substitution:Sl and S2: halogenoalkanes with nucleophilesThese two possibilities lead to two different reaction mechanisms:SUBSTITUTION BY A NUCLEOPHILE: FIRST ORDERKINETICS: SN121o4NulHaloreither+CNUNuO?！1NuHalCSSOfStage3In the product, the halogen atom has been substituted by thenucleophile so the result is a substitution reaction. Because thecarbocation is planar, it can be attacked equally well form either sideIf the product has an asymmetric carbon atom, equal amounts of eachoptical isomer will be made. The product, although it containsoptically active particles, contains equal amounts of each kind and sowill not affect plane polarized light. It is called a racemic mixture. 4

4 Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles SUBSTITUTION BY A NUCLEOPHILE; FIRST ORDER KINETICS: SN1 These two possibilities lead to two different reaction mechanisms: Stage 3 In the product, the halogen atom has been substituted by the nucleophile so the result is a substitution reaction. Because the carbocation is planar, it can be attacked equally well form either side. If the product has an asymmetric carbon atom, equal amounts of each optical isomer will be made. The product, although it contains optically active particles, contains equal amounts of each kind and so will not affect plane polarized light. It is called a racemic mixture

Heterolytic nucleophilic substitution:Sl and Sn2: halogenoalkanes with nucleophilesSUBSTITUTION BY A NUCLEOPHILE: SECOND ORDERKINETICS: S~20ICDO&84HalNHalNuHalNuCStage 1Initial attack is by nucleophile on the electron deficient carbonatom.This initial step is the rate determining step and involves acollision between two particles, the halogen compound and thenucleophile, so the kinetics are second order.5

5 Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles SUBSTITUTION BY A NUCLEOPHILE; SECOND ORDER KINETICS: SN2 Stage 1 Initial attack is by nucleophile on the electron deficient carbon atom. This initial step is the rate determining step and involves a collision between two particles, the halogen compound and the nucleophile, so the kinetics are second order

Heterolytic nucleophilic substitution:Sl and Sn2: halogenoalkanes with nucleophilesSUBSTITUTION BY A NUCLEOPHILE: SECOND ORDERKINETICS: S~20I0DO&84HalNUHalNu..HalNuCStage2The intermediate has a high energy, which means that the overallrate of reaction is slowThe carbon being attacked is saturated and has a full outer shellbecause carbon cannot expand its outer shell. As the electron pairfrom the nucleophile moves in, the pair forming the sigma bondto the halogen moves away. This bond breaks heterolytically6

6 Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles SUBSTITUTION BY A NUCLEOPHILE; SECOND ORDER KINETICS: SN2 Stage 2 The intermediate has a high energy, which means that the overall rate of reaction is slow. The carbon being attacked is saturated and has a full outer shell because carbon cannot expand its outer shell. As the electron pair from the nucleophile moves in, the pair forming the sigma bond to the halogen moves away. This bond breaks heterolytically

Heterolytic nucleophilic substitution:Sl and Sn2: halogenoalkanes with nucleophilesSUBSTITUTION BY A NUCLEOPHILE: SECOND ORDERKINETICS: S~20I0DO&84HalNUHalNu..HalNuCStage 3In the product the halogen has been substituted by the nucleophile sothe reaction is a substitutionThe halide ion is a leaving group. The strength of the bond to theleaving group and the stability of the leaving group will affect theamount of product make.If the product has an asymmetric carbon atom, only one optical7isomer will be made

7 Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles SUBSTITUTION BY A NUCLEOPHILE; SECOND ORDER KINETICS: SN2 Stage 3 In the product the halogen has been substituted by the nucleophile so the reaction is a substitution. The halide ion is a leaving group. The strength of the bond to the leaving group and the stability of the leaving group will affect the amount of product make. If the product has an asymmetric carbon atom, only one optical isomer will be made

Heterolytic nucleophilic substitution:S1 and Sn2: halogenoalkanes with nucleophilesWordsWords and Expressionsheterolytic nucleophilic substitutionhalogenoalkaneinductive effectcarbocationsubstitute: substitutionintermediateasymmetricoptical isomerracemic8

8 Heterolytic nucleophilic substitution:SN1 and SN2: halogenoalkanes with nucleophiles Words heterolytic nucleophilic substitution halogenoalkane inductive effect carbocation substitute; substitution intermediate asymmetric optical isomer racemic Words and Expressions

Heterolytic nucleophilic addition and addition/elimination:carnonyl compounds withhydrogencyanideHeterolytic nucleophilic addition andaddition/elimination: carbonylcompounds with hydrogen cyanide9

9 Heterolytic nucleophilic addition and addition/elimination:carnonyl compounds with hydrogen cyanide Heterolytic nucleophilic addition and addition/elimination: carbonyl compounds with hydrogen cyanide

Heterolytic nucleophilic addition and addition/elimination:carnonyl compounds withhydrogencyanideHydrogen cyanide adds across the double bond in acarbonyl compound, if there is a trace of the catalystpotassium cyanide present.HC0+H-CNONC10

10 Heterolytic nucleophilic addition and addition/elimination:carnonyl compounds with hydrogen cyanide Hydrogen cyanide adds across the double bond in a carbonyl compound, if there is a trace of the catalyst potassium cyanide present