E2 reaction

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E2 reaction

Transcript of E2 reaction

  • 1.Elimination ReactionsOutline1. Introduction2. E2, E1, and E1 cb mechanisms3. Regiochemistry of Elimination reactions4. Stereochemistry of Elimination reactions6. Dehydration of Alcohols7. Competition Between E2 and E1 Reactions8. Summary9. Reference

2. IntroductionGroup 42 3. Organic compounds with anelectronegative atom or an -electron-withdrawing groupbonded to a sp3 carbonundergo substitution orelimination reactionsntios tituSub Elimination Halide ions are good leavinggroups. Substitution reactionon these compounds are easy alkyl fluoride alkyl chloride alkyl bromide alkyl iodideand are used to get a wide variety of compounds 4. Elimination Reactions1-bromo-1,1-dimethylethane2-methylpropene Rate law: rate = k [1-bromo-1,1-dimethylethane][OH-]this reaction is an example of a E2 reaction. E stands for elimination 2 stands for bimolecular 5. Zaytzeffs ruleThe more substituted alkene will be formedin elimination reactions Chapter 9 5 6. Zaytzeffs rule6 7. Zaytzeffs rule In the transition state for elimination, theincreased stability of the most substituteddouble bond is already felt so that there is alower energy barrier for elimination of Ha 8. Zaytzeffs rule Zaytzeffs rule dose not apply when the base isbulky 9. Zaytzeffs rule Zaytzeffs rule does not apply when theleaving group is poor E2-carbanion mechanism operative 10. Zaytzeffs rule Zaytzeffs rule may not apply when conjugated dienes might be formed CH3 CH3 HOCH2 CHCH2CHCHCH3CH2 CHCH CHCHCH3 major productCl+ CH3 CH2 CHCH2CH CCH3 11. Hofmans rule Hofman: the alkene will predominate whichhas least alkyl substituents on the doublebond carbon (1851; working on RN+Mecompounds, i.e. Y=+NMe) 12. Hofmans rule+ NMe will exert a powerful, electron-withdrawing, inductive/field affect on both -carbon atoms 13. E2, E1, and E1 cb mechanisms Mechanism of E1 elimination reaction 14. E1-elimination = k [substrate] The mechanism is similar to that of the SN1reaction: the first step is formation of thecarbocation via heterolytic cleavage 14 15. E1 Orbital Picture 16. Bases used to Promote Elimination Reaction The most common bases used in elimination reactions arenegatively charged oxygen compounds, such as HO and itsalkyl derivatives, RO, called alkoxides. 17. Products of Elimination To draw any product of dehydrohalogenationFind the carbon. Identify all carbons with H atoms. Remove theelements of H and X form the and carbons and form a bond. 18. Order of Alkene Stability The stability of an alkene increases as the number of R groupsbonded to the double bond carbons increases. The higher the percent s-character, the more readily an atomaccepts electron density. Thus, sp2 carbons are more able toaccept electron density and sp3 carbons are more able to donateelectron density. Consequently, increasing the number of electron donating groupson a carbon atom able to accept electron density makes thealkene more stable. 19. Stability of Trans Substituted Alkenes trans-2-Butene (a disubstituted alkene) is more stable thancis-2-butene (another disubstituted alkene), but both aremore stable than 1-butene (a monosubstituted alkene). 20. The E1 Pathway Competes with SN1 21. First Step: Same as SN1 22. The E2 elimination = k [substrate] [base] As in the SN2 reaction, the rate is dependent onthe concentration of both reaction partners 24 23. Energy Diagram for the E2 Mechanism 24. Effect of the Substrate on E2 Reactivity 25. E2 vs SN2 There is a strong similarity between the E2 and SN2reactions: strong nucleophiles favor substitution, whilestrong bases favor elimination28 26. Steric bulk favors elimination We already saw that SN2 substitution on a tertiarycarbon is not possible, therefore E2 eliminationwill prevail (beside E1 elimination) 29 27. Other examples Note that at a primary carbon atom, only SN2and no SN1 substitution is possible30 28. Effect of the leaving group Especially quarternary ammonium leavinggroups favor the Hofmann product31 29. Effect of the LG on E2 Reactivity 30. The E1cB elimination The E1cB reaction resembles the SN2 reaction,with the difference that there is an anion formedprior to the loss of the leaving group33 31. Example of an E1cB reaction This reaction is possible if there is a group presentthat can stabilize the negative charge 34 32. Stability of anions The more substituted the carbanion, the lessstable it is; this is a result of the inductivelyelectron donating alkyl groups35 33. Stereochemistry of Elimination Reactions If the elimination reaction removes twosubstituents from the same side of themolecule it is syn elimination If the elimination reaction removes twosubstituents from opposite sides of themolecule it is anti elimination 34. The E2 Reaction: Stereochemistry The E2 Reaction is stereoselective, but notstereospecific if 2 Hs are available on carbonbearing eliminated H The H leading to more stable E isomer is selected tobe extracted from carbon regardless of streochemat carbon 35. The E2 Reaction: Stereochemistry In an E2 reaction, the bonds to the eliminatedsubstituents must be in the same plane In this course E2 eliminations will all go via anti-periplanar conformation Product analysis possible by drawing Newmanprojections if only 1 H is available 36. The E2 Reaction: StereochemistryWhen only one hydrogen is on the carbonpredominantly anti elimination leads to highstereospecificity (2S,3S)-2-bromo-3-phenylbutane (E)-2-phenyl-2-butene (2S,3R)-2-bromo-3-phenylbutane (Z)-2-phenyl-2-butene 37. 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 posn Erase LG and H, draw double bond(2S,3R)-2-bromo-3-phenylbutane Z isomer 38. 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 Hs Product stability leads to stereoselectivity but not stereospecificity 39. E2 Reactions of Cyclic CompoundsE2 reaction of cyclic compounds follows thesame stereochemical rules as from open-chain compounds 40. E2 Reactions of Cyclic CompoundsThe E2 reaction of menthyl chlorideviolates Zaitsevs rule 41. E1 Reactions of Cyclic CompoundsWhen a cyclohexyl chloride undergoes an E1reaction, there is no requirement that thetwo groups to be eliminated be diaxial 42. E1 Reactions of Cyclic CompoundsCarbocation rearrangements must beconsidered for E1 reactions 43. Competition Between Substitutionand Elimination Conditions that favor E2 also favor SN2 Conditions that favor E1 also favor SN1 No need to worry about SN2/E1 or SN1/E2 combinations First decide whether the reaction would favor SN2/E2 or SN1/E1 reactions If the halide is primary, only SN2/E2 need beconsidered If the halide is secondary or tertiary, SN2/E2 or SN1/E1depends on reaction condition 44. Competition Between Substitutionand Elimination SN2/E2 reactions are favored by high concof a good nuc/strong base and polaraprotic solvent. SN1/E1 reactions are favored by poor nuc/weak base and polar protic solvents 45. Competition Between SN2 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 SN2 reaction -elimination is the only possibility 46. Competition Between SN1 and E1 Because SN1 and E1 reactions both proceedthrough a carbocation, they have the samerate-determining step Primary halides do not undergo either SN1 orE1 reactions For secondary and tertiary halides, raising thetemperature increases the eliminationproduct 47. Substitution and Elimination Reactionsin Synthesis SN1/E1 conditions are rarely useful synthetically 48. Dehydration of alcoholUnder the acidic reagent and involves an E1 mechanism 49. Dehydration of alcohol Rearrangement of products 50. Other 1,2-Elimination 51. 1,1-()-EliminationA very strong basePowerfully electron-withdrawinggroupAbsence of -H atom 52. Summary E1cB: the proton is removed first, an anion is formed E1: the leaving group departs first, a cation is formed E2: all processes occur at the same time 56 53. Summary57