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Chapter 91 Elimination Reactions of Alkyl Halides : Chapter 9 Competition Between Substitution and Elimination.

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Presentation on theme: "Chapter 91 Elimination Reactions of Alkyl Halides : Chapter 9 Competition Between Substitution and Elimination."— Presentation transcript:

1 Chapter 91 Elimination Reactions of Alkyl Halides : Chapter 9 Competition Between Substitution and Elimination

2 Chapter 92 Contents of Chapter 9 The E2 Reaction The E2 Reaction The E1 Reaction The E1 Reaction Competition Between E2 and E1 Reactions Competition Between E2 and E1 Reactions Stereochemistry of Elimination Reactions Stereochemistry of Elimination Reactions Elimination from Cyclic Compounds Elimination from Cyclic Compounds Competition Between Substitution and Elimination Competition Between Substitution and Elimination Substitution and Elimination Reactions in Synthesis Substitution and Elimination Reactions in Synthesis

3 Chapter 93 Elimination Reactions In this chapter we start with elimination rxns then work subst/elim competition A compound with an electronegative atom bonded to an sp 3 carbon, when approached by a nucleophile/base can undergo either a substitution reaction OR an elimination reaction

4 Chapter 94 The E2 Reaction

5 Chapter 95 The E2 Reaction: Regioselectivity 2-bromobutane has two structurally different  -carbons from which to abstract a hydrogen E2 rxns give more stable alkene if possible

6 Chapter 96 The E2 Reaction: Regioselectivity Zaitsev’s rule: The more substituted alkene will be formed in elimination reactions

7 Chapter 97 The E2 Reaction: Regioselectivity Zaitsev’s rule does not apply when the base is bulky E2 Rxn is kinetically-controlled

8 Chapter 98 The E2 Reaction: Regioselectivity Zaitsev’s rule does not apply when the leaving group is poor E2-carbanion mechanism operative

9 Chapter 99 The E2 Reaction: Regioselectivity Zaitsev’s rule may not apply when conjugated dienes might be formed major product

10 Chapter 910 The E2 Reaction: Regioselectivity The major product of an E2 reaction is the more substituted alkene unless: the base is large (i.e. bulky) the leaving group is poor (i.e. F –) the less substituted  –carbon is allylic or benzylic (ie. more stable)

11 Chapter 911 The E1 Reaction “E1” stands for “Elimination unimolecular” The E1 reaction is a two-step reaction The first step is rate-determining

12 Chapter 912 The E1 Reaction Relative reactivities of alkyl halides in an E1 reaction are similar to the relative stabilities of carbocations 3 o benzylic > 3 o allylic > 2 o benzylic > 2 o allylic > 3 o > 1 o benzylic > 1 o allylic  2 o > 1 o > vinyl Increasing reactivity and C+ stability

13 Chapter 913 The E1 Reaction E1 reaction involves a carbocation Therefore rearrangements must be considered

14 Chapter 914 Competition Between E2 and E1 Reactions Summary of the Reactivity of Alkyl Halides in Elimination Reactions primary alkyl halideE2 only unless  -hinderred secondary alkyl halide E1 and E2 tertiary alkyl halideE1 and E2

15 Chapter 915 Competition Between E2 and E1 Reactions E2 reaction is favored by the same factors that favor S N 2 reactions over S N 1 primary alkyl halide electrophiles a high concentration of a strong base (e.g. HO – or – NH 2 ) in 1°, 2°, or 3° electrophiles an aprotic polar solvent in 1°, 2°, or 3° electrophiles An E1 reaction is favored by a weak base (e.g. a neutral solvent) a polar protic solvent (e.g. H 2 O or ROH)

16 Chapter 916 Stereochemistry of Elimination Reactions If the elimination reaction removes two substituents from the same side of the molecule it is syn elimination If the elimination reaction removes two substituents from opposite sides of the molecule it is anti elimination

17 Chapter 917 The E2 Reaction: Stereochemistry  The E2 Reaction is stereoselective, but not stereospecific if 2  H’s are available on carbon bearing eliminated H  The H leading to more stable E isomer is selected to be extracted from  carbon regardless of streochem at  carbon

18 Chapter 918 The E2 Reaction: Stereochemistry In an E2 reaction, the bonds to the eliminated substituents must be in the same plane In this course E2 eliminations will all go via anti-periplanar conformation Product analysis possible by drawing Newman projections if only 1  H is available

19 Chapter 919 The E2 Reaction: Stereochemistry When only one hydrogen is on the  carbon predominantly anti elimination leads to high stereospecificity (2S,3S)-2-bromo-3-phenylbutane (E)-2-phenyl-2-butene (2S,3R)-2-bromo-3-phenylbutane (Z)-2-phenyl-2-butene

20 Chapter 920 The E2 Reaction: Stereochemistry Retro-pro-Fischer analysis can be done to track stereochemistry of reaction For anti elimination put  H on vertical and leaving group on horizontal pos’n Erase LG and  H, draw double bond (2S,3R)-2-bromo-3-phenylbutaneZ isomer

21 Chapter 921 The E1 Reaction: Stereochemistry With C+ both syn and anti elimination can occur, so E1 reaction forms both E and Z products regardless of whether  -carbon is bonded to one or two H’s Product stability leads to stereoselectivity but not stereospecificity

22 Chapter 922 E2 Reactions of Cyclic Compounds E2 reaction of cyclic compounds follows the same stereochemical rules as from open-chain compounds

23 Chapter 923 E2 Reactions of Cyclic Compounds The E2 reaction of menthyl chloride violates Zaitsev’s rule

24 Chapter 924 E1 Reactions of Cyclic Compounds When a cyclohexyl chloride undergoes an E1 reaction, there is no requirement that the two groups to be eliminated be diaxial

25 Chapter 925 E1 Reactions of Cyclic Compounds Carbocation rearrangements must be considered for E1 reactions

26 Chapter 926 Competition Between Substitution and Elimination Conditions that favor E2 also favor S N 2 Conditions that favor E1 also favor S N 1 No need to worry about S N 2/E1 or S N 1/E2 combinations First decide whether the reaction would favor S N 2/E2 or S N 1/E1 reactions If the halide is primary, only S N 2/E2 need be considered If the halide is secondary or tertiary, S N 2/E2 or S N 1/E1 depends on reaction condition

27 Chapter 927 Competition Between Substitution and Elimination S N 2/E2 reactions are favored by high conc of a good nuc/strong base and polar aprotic solvent. S N 1/E1 reactions are favored by poor nuc/weak base and polar protic solvents

28 Chapter 928 Competition Between S N 2 and E2 Primary halides generally undergo substitution, although if the halide or the base is hindered, elimination is possible, favorable if heated Secondary halides are more difficult to predict The stronger and more hindered the base, the more elimination product is produced The higher the temperature, the more elimination product is produced Tertiary halides never undergo S N 2 reaction - elimination is the only possibility

29 Chapter 929 Competition Between S N 1 and E1 Because S N 1 and E1 reactions both proceed through a carbocation, they have the same rate-determining step Primary halides do not undergo either S N 1 or E1 reactions For secondary and tertiary halides, raising the temperature increases the elimination product

30 Chapter 930 Williamson Ether Synthesis If you want to synthesize butyl propyl ether you have a choice of starting materials Other ethers should be made by choosing least-hindered electrophile if possible Ethers usually best made by S N 2 rxn

31 Chapter 931 Williamson Ether Synthesis  If you want to prepare tert-butyl ethyl ether the starting materials must be an ethyl halide and tert-butoxide ion When ethoxide ion and tert-butyl bromide are used, only elimination product is produced ethyl bromide tert-butoxide tert-butyl ethyl ethene ion ether

32 Chapter 932 Substitution and Elimination Reactions in Synthesis S N 1/E1 conditions are rarely useful synthetically

33 Chapter 933 Designing a Synthesis How would you carry out the following?

34 Chapter 934 Designing a Synthesis Under E2 conditions a tertiary halide would yield only the elimination product

35 Chapter 935 Designing a Synthesis We know also that Br 2 addition to an alkene yields only the anti product Overall we might propose:

36 Chapter 936 Designing a Synthesis

37 Chapter 937 Designing a Synthesis Only method we know to prepare a ketone is to add water to an alkyne

38 Chapter 938 Designing a Synthesis The alkyne can be prepared by two successive E2 reactions on a vicinal dihalide

39 Chapter 939 Designing a Synthesis The vicinal dihalide can be prepared via halogenation of an alkene

40 Chapter 940 Designing a Synthesis The alkene can be prepared from the starting material via dehydrohalogenation

41 Chapter 941 Designing a Synthesis II From this analysis we might suggest the following synthesis:

42 Chapter 942 Problem-solving Info  Reaction speed comparisons Increasing speed in E1 reaction Polar protic solvent Relief of steric strain making C+ More stable carbocation formed Anything which destabilizes electrophile Increased leaving group stability (less basic)

43 Chapter 943 Problem-solving Info Increasing speed in E2 reaction Polar aprotic solvent Alkene stability Increased leaving group stability Higher concentration of base used Stronger base used Anything which destabilizes electrophile Cyclohexanes with more or more stable antiperiplanar H’s which lead to more stable products C  stability with bad leaving group (F)

44 Chapter 944 Problem-solving Info E1 vs E2 chemistry Conditions which give E1 Weak base and C+ stability  2 Polar protic solvent and C+ stability  2 No antiperiplanar H’s in cyclohexanes Conditions which give E2 Primary electrophile without  hindrance Polar aprotic solvent Strong/concentrated base

45 Chapter 945 Problem-solving Info Unimolecular vs. bimolecular S N 1/E1 Weak base or bad nucleophile Protic solvent C+ stability  2 S N 2/E2 Concentrated strong base or good nucleophile Polar aprotic solvent Primary electrophile Substitution vs. elimination S N 1 vs. E1 Weaker base and lower temp gives S N 1 Stronger base and higher temp gives E1

46 Chapter 946 Problem-solving Info S N 2 vs. E2 S N 2 Primary unhindered electrophile Larger than second row anion nucleophile Weaker base with other substitution-enhancing cond’s E2  -hindered secondary electrophile Bulky concentrated second-row base S N 1 vs. E1 Weaker base and lower temp gives S N 1 Stronger base and higher temp gives E1

47 Chapter 947 Problem-solving Info Product distribution Most stable alkene formed unless: Bad leaving group (ie. F) gives most stable C-  H in cyclohexane trans to leaving group Only  H in acyclic reactant which can be antiperiplanar to leaving grp leads to Z prod Acyclic electrophile with 1  H & chiral  C Must use Newman projection to predict product Product arises from antiperiplanar elimination Stereospecific – one enantiomer gives E, other Z C+ rearrangements with E1 reactions!!!


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