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Elimination Reactions of Alkyl Halides Chapter 9 “A pessimist finds difficulty in every opportunity. An optimist finds opportunity in every difficulty.”

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Presentation on theme: "Elimination Reactions of Alkyl Halides Chapter 9 “A pessimist finds difficulty in every opportunity. An optimist finds opportunity in every difficulty.”"— Presentation transcript:

1 Elimination Reactions of Alkyl Halides Chapter 9 “A pessimist finds difficulty in every opportunity. An optimist finds opportunity in every difficulty.” A. Lincoln

2 Elimination Reactions Elimination Rx can be of two types E1 mechanism (unimolecular) E2 mechanism (bimolecular)

3 E2 Eliminations Concerted one step reaction

4 E2 Eliminations Called “dehydrohalogenations” Called  Eliminiation Called 1-2 Elimination

5 E2 Eliminations Relative reactivities of RX in an E2 reaction: (most reactive) RI > RBr > RCl > RF (least reactive)) F - is a better base than Br -

6 E2 Eliminations Sometimes only one elimination product is possible Sometimes More than one product may result

7 E2 Eliminations Kinetic and Thermodynamic Product (A regioselective reaction !)

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

9 Reactivity of Alkyl Halides *

10 Reactivity Considerations The more substituted product is not always the most rapidly formed… That Conjugated Diene thing just keeps showing up…….

11 Reactivity Considerations The more substituted product is not always the most rapidly formed… That Pesky Bulky Base – Changes things……

12 Reactivity Considerations Steric hindrance may dictate removal of a more accessible H (The most substituted is not formed here.)

13 * (What does this mean?)

14 Alkyl fluorides Dehydrohalogenation of R-I, R-Br, R-Cl yield more substituted alkene Dehydrohalogenation of RF yields the less substituted alkene

15 Elimination of Alkyl fluorides

16 Fluoride is a strong base Fluoride is a poor leaving group A partial negative charge develops on the carbon that lost the proton (a carbanion)

17 Carbo - Ions Carbocations are stabilized by alkyl groups Carbanions are destabilized by alkyl groups C R’ R” R : - 3 O < 2 O < 1 O Increasing stability of Carbanions

18 Elimination Considerations The major product is the more substituted alkene unless… à the alkyl halide contains conjugation à the base is large à the alkyl halide is an alkyl fluoride

19

20 E1 Mechanism

21 E1 Mechanism Major product will generally be the more substituted - Zatsev

22 E1 Mechanism Product depends on stability of the carbocation

23 E1 Mechanism Because a carbocation is formed – rearrangement is possible

24 E1 Mechanism Because a carbocation is formed – rearrangement is possible

25 Competition E1 vs. E2 RXElimination comments 1 o alkyl halides E2 only Difficulty in forming 1 o carbocations 2 o alkyl halides E1 and E2 Favored by same as S N 2 conditions 3 o alkyl halides E1 and E2 Favored by same as S N 2 conditions

26 Review of S N Reactions S N 1 is Favored By More Substituted Carbocation Weak Nucleophile Polar Protic Solvent S N 2 is Favored By Less Substituted Carbocation Strong Nucleophile Polar AProtic Solvent

27 Stereochemistry of E2 In an E2 reaction, 2 groups are eliminated in one step The 2 groups MUST be located in the same plane (periplanar) (syn-periplanar) slower (anti-periplanar) faster

28 E2 is regioselective

29 E2 Is Stereoselective The conformers that have the bulkiest groups on opposite sides will be the major product.

30 Stereochemistry of E2 Note: E isomer is major product because bulky groups, methyl and t-butyl, are anti.

31 Stereochemistry of E2 Important note: use models! E2 for 2-bromo-3-phenylbutane only E isomer only Z isomer (2S, 3S) (2S, 3R)

32 E1 Eliminations –planar, carbocation intermediate –syn and anti elimination can occur (Major product is the one with bulkiest groups on opposite sides.) bulkiest groups on opposite sides

33 Elimination from Cyclics - E2 –In an E2 reaction, both groups must be periplanar and anti to each other. –This is NOT the most stable conformer.

34 Elimination from Cyclics - E2

35 Occurs 200 x faster! More stable conformer has groups equatorial.

36 Elimination from Cyclics - E2 In an E2 reaction, the H is removed from the most substituted  -carbon unless that hydrogen is not in the axial

37 Elimination from Cyclics - E1 In an E1 reaction, the elimination is not concerted. Intermediate is a carbocation Groups do not need to be in the axial position

38 Elimination from Cyclics - E1 Carbocation intermediates may rearrange. (Zaitsev’s rule is followed, but first watch out for a rearrangement.)

39 Stereochemistry Review mechanismproducts comments SN1SN1 Both enantiomers formed (more inverted than retained) Racemization w/ some inversion E1 Both E and Z formed (more of the stereoisomer with bulkiest groups trans) Also C+ intermediate S N 2 Only the inverted product is formed Inversion E2 Both enantiomers formed (more of the stereoisomer with bulkiest groups trans)

40 Using Isotopes In Chemistry Look at mechanism Look at research handout Look at medical questions handout

41 Substitution vs. Elimination

42 Steps To Deciding Decide if it favors Sn2/E2 or Sn1/E1 conditions Decide if it favors Substitution or Elemination …….. That’s all there is to it From Yahoo Images

43 Deciding Between First or Second order Substrate –1 st order – primary and secondary 2 nd order – secondary and tertiary Nucleophile1 st order – weaker nuclephile 2 nd order – strong nucleophile Solvent1 st order – polar protic solvent 2 nd order – polar aprotic or neutral 2 out of three ain’t bad………… or is that just a long lost song

44 Deciding Between Sn2 and E2 Substrate Base Temperature From Yahoo Images

45 Deciding Between Sn2 and E2 Substrate Sn2 – primary alkyl halide E2 – secondary alkyl halide

46 Deciding Between Sn2 and E2 Nuclophile Sn2 – Weaker and Smaller E2 – stronger and Bulkier

47 Deciding Between Sn2 and E2 Temperature Sn2 – Lower Temperature E2 – Higher Temperatue

48 S N 1 / E1 Conditions All alkyl halides have same order of reactivity for S N 1 or E1 Substitution is favored at lower temperatures

49 Practice Activity Each person create a reaction and conditions Have your neighbor decide which mechanism it will undergo Switch and do it again From Yahoo Images

50 Williamson Ether Synthesis Williamson ether synthesis –Alkyl halide with alkoxide (S N 2 reaction) –Alexander Williamson (1850!) –React alcohol with sodium metal or sodium hydride CH 3 OH + Na CH 3 O - + Na + + 1/2 H 2 NaH

51 Williamson Ether Synthesis If you want to prepare n-propyl n-butyl ether, there are two methods.

52 Williamson Ether Synthesis If you want to prepare ethyl t-butyl ether, there are two methods… HOWEVER, if you use t-butyl halide, you will obtain primarily the elimination product.

53 Alkyne Syntheses Elimination Reactions involving geminal dihalides (note: use a very strong base NH 2, not OH -, or it stops at vicinyl dihalide!)

54 Synthesis Problems Synthesize 1,3-cylcohexadiene from cyclohexane.

55 Synthesis Problems Synthesis Pathway

56 Synthesis Problems Synthesize trans- 1,2-dibromo-1-methylcylcohexane from methylcyclohexane.

57 Synthesis Problems Synthesis Pathway

58 Synthesis Problems Synthesize 2-butanone from 1-bromobutane.

59 Synthesis Problems Retrosynthetic Analysis

60 Synthesis Problems Synthetic pathway

61 END “A pessimist finds difficulty in every opportunity. An optimist finds opportunity in every difficulty.” A. Lincoln


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