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Dehydrohalogenation of Alkyl Halides E2 and E1 Reactions in Detail.

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Presentation on theme: "Dehydrohalogenation of Alkyl Halides E2 and E1 Reactions in Detail."— Presentation transcript:

1 Dehydrohalogenation of Alkyl Halides E2 and E1 Reactions in Detail

2 X Y dehydration of alcohols: X = H; Y = OH dehydrohalogenation of alkyl halides: X = H; Y = Br, etc.   C C C C +XY  -Elimination Reactions Overview

3 X Y dehydration of alcohols: acid-catalyzed dehydrohalogenation of alkyl halides: consumes base   C C C C +XY  -Elimination Reactions Overview

4 is a useful method for the preparation of alkenes (100 %) likewise, NaOCH 3 in methanol, or KOH in ethanol NaOCH 2 CH 3 ethanol, 55°C Dehydrohalogenation Cl

5 CH 3 (CH 2 ) 15 CH 2 CH 2 Cl When the alkyl halide is primary, potassium tert-butoxide in dimethyl sulfoxide is the base/solvent system that is normally used. KOC(CH 3 ) 3 dimethyl sulfoxide (86%) CH 2 CH 3 (CH 2 ) 15 CH Dehydrohalogenation

6 Br 29 % 71 % + Regioselectivity follows Zaitsev's rule More highly substituted double bond predominates = More Stable KOCH 2 CH 3 ethanol, 70°C

7 Zaitsev’s Rule The more substituted alkene is obtained when a proton is removed from the  -carbon that is bonded to the fewest hydrogens

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9 Conjugated alkenes are preferred !

10 Steric hindrance effects the product distribution

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12 more stable configuration of double bond predominates Stereoselectivity KOCH 2 CH 3 ethanol Br + (23%)(77%)

13 more stable configuration of double bond predominates Stereoselectivity KOCH 2 CH 3 ethanol + (85%)(15%)Br

14 Mechanism of the Dehydrohalogenation of Alkyl Halides: The E2 Mechanism

15 Facts Dehydrohalogenation of alkyl halides exhibits second-order kinetics first order in alkyl halide first order in base rate = k[alkyl halide][base] implies that rate-determining step involves both base and alkyl halide; i.e., it is bimolecular

16 Facts Rate of elimination depends on halogen weaker C—X bond; faster rate rate: RI > RBr > RCl > RF implies that carbon-halogen bond breaks in the rate-determining step

17 concerted (one-step) bimolecular process single transition state C—H bond breaks  component of double bond forms C—X bond breaks The E2 Mechanism

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19 – O R.... : CCCCCCCCHX.. :: Reactants The E2 Mechanism

20 – O R.... : CCCCCCCCHX.. :: Reactants The E2 Mechanism

21 CCCCCCCC –––– OR.... H X..:: –––– Transition state The E2 Mechanism

22 OR.... H CCCCCCCC–X.. ::.. Products

23 Stereoelectronic Effects Anti Elimination in E2 Reactions

24 Stereochemistry of the E2 Reaction Remember: The bonds to the eliminated groups (H and X) must be in the same plane and anti to each other H X More stable conformation than syn-eclipsed

25 The best orbital overlap of the interacting orbitals is achieved through back side attack of the leaving group X as in an S N 2 displacement.

26 Regioselectivity

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29 Configuration of the Reactant

30 Elimination from Cyclic Compounds Configuration must be trans, which is (anti).

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35 (CH 3 ) 3 C Br KOC(CH 3 ) 3 (CH 3 ) 3 COH cis-1-Bromo-4-tert- butylcyclohexane Stereoelectronic effect

36 (CH 3 ) 3 C Br KOC(CH 3 ) 3 (CH 3 ) 3 COH trans-1-Bromo-4-tert- butylcyclohexane Stereoelectronic effect

37 (CH 3 ) 3 C Br Br KOC(CH 3 ) 3 (CH 3 ) 3 COH cis trans Rate constant for dehydrohalogenation of cis is 500 times greater than that of trans Stereoelectronic effect

38 (CH 3 ) 3 C Br KOC(CH 3 ) 3 (CH 3 ) 3 COH cis H that is removed by base must be anti periplanar to Br Two anti periplanar H atoms in cis stereoisomer H H Stereoelectronic effect

39 (CH 3 ) 3 C KOC(CH 3 ) 3 (CH 3 ) 3 COH trans H that is removed by base must be anti periplanar to Br No anti periplanar H atoms in trans stereoisomer; all vicinal H atoms are gauche to Br H H (CH 3 ) 3 C BrHH Stereoelectronic effect

40 cis more reactive trans less reactive Stereoelectronic effect

41 An effect on reactivity that has its origin in the spatial arrangement of orbitals or bonds is called a stereoelectronic effect. The preference for an anti periplanar arrangement of H and Br in the transition state for E2 dehydrohalogenation is an example of a stereoelectronic effect.

42 E2 in a cyclohexane ring

43 Can you predict the products? Cis or trans? Axial or equatorial? a,e  e,a e,e  a,a Can you explain the products?

44 Cyclohexane Stereochemistry Revisited l-menthol How many stereoisomers are possible for menthol?

45 A Different Mechanism for Alkyl Halide Elimination: The E1 Mechanism

46 CH 3 CH 2 CH 3 Br CH 3 Ethanol, heat + (25%) (75%) C H3CH3CH3CH3C CH 3 C C H3CH3CH3CH3C H CH 2 CH 3 CH 3 C H2CH2CH2CH2C Example

47 1. Alkyl halides can undergo elimination in absence of base. 2. Carbocation is intermediate 3. Rate-determining step is unimolecular ionization of alkyl halide. The E1 Mechanism

48 slow, unimolecular C CH 2 CH 3 CH 3 + CH 2 CH 3 Br CH 3 C :.. : :.. : Br..– Step 1

49 C CH 2 CH 3 CH 3 + C CH 2 CH 3 CH 3 CH 2 + C CHCH 3 CH 3 – H + Step 2 Which alkene is more stable and why?

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53 Reaction coordinate diagram for the E1 reaction of 2-chloro-2-methylbutane

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56 Must consider possible carbocation rearrangement

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58 Stereochemistry of the E1 Reaction

59 E1 Elimination from Cyclic Compounds E1 mechanism involves both syn and anti elimination

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61 Summary & Applications (Synthesis) S N 1 / E1 vs. S N 2 / E2

62 E2 and E1 Reactions

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64 Substitution vs. Elimination Alkyl halides can undergo S N 2, S N 1, E2 and E1 Reactions 1) Which reaction conditions favor S N 2/E2 or S N 1/E1? S N 2/E2 reactions are favored by a high concentration of nucleophile/strong base S N 1/E1 reactions are favored by a poor nucleophile/weak base 2) What will be the relative distribution of substitution product vs. elimination product?

65 Consider S N 1/E1 vs. S N 2/E2 Consider the Substrate

66 NOTE: a bulky base encourages elimination over substitution

67 Returning to Sn2 and E2: Considering the differences Can you predict the products? Can you explain the products?

68 Substitution and Elimination Reactions in Synthesis

69 A hindered alkyl halide should be used if you want to synthesize an alkene

70 Which reaction produces an ether?

71 Consecutive E2 Elimination Reactions: Alkynes

72 Intermolecular vs. Intramolecular Reactions A low concentration of reactant favors an intramolecular reaction The intramolecular reaction is also favored when a five- or six-membered ring is formed

73 Three- and four-membered rings are less easily formed Three-membered ring compounds are formed more easily than four-membered ring compounds The likelihood of the reacting groups finding each other decreases sharply when the groups are in compounds that would form seven-membered and larger rings.

74 Designing a synthesis …

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