Chapter11 140330084827-phpapp01
- 1. Chapter 11: AlkenesChapter 11: Alkenes CC CC DoubleDouble bondbond Names:Names: EndingEnding aneane eneene 1. Find longest chain wit1. Find longest chain wit bothboth CCspsp22 s in it.s in it. Rules:Rules: AnAn octocteneene 11 22 33 4455 66 77 88 44 For example: Ethene, propene, butene, etc.For example: Ethene, propene, butene, etc.
- 2. Lingo: Double bondLingo: Double bond positionposition RR RR InternalInternalTerminalTerminal RR RR CHCH22 3. Name and # substituents, in alphabetical order3. Name and # substituents, in alphabetical order 4-Ethyl-3-methyl-4-Ethyl-3-methyl-3-3-octocteneene 2. # Chain with2. # Chain with C CC C close to terminusclose to terminus 11 22 33 4455 66 77 88 AA 3-3-octocteneene (only the(only the first of the two Cfirst of the two Cspsp2 s2 s is named by a #)is named by a #) 44
- 3. 4. Cycloalkenes4. Cycloalkenes 11 22 33 CHCH33 CC CC 11 22 5. Stereoisomers:5. Stereoisomers: RR RRRR RR ciscis transtrans Cis/trans used for 1,2-disubstituted ethenes.Cis/trans used for 1,2-disubstituted ethenes. 6. For tri- and tetrasubstituted alkenes:6. For tri- and tetrasubstituted alkenes: E, ZE, Z naming.naming. Use R, S priority rules at eachUse R, S priority rules at each spsp22 -carbon-carbon separatelyseparately, to, to find higher priority groups at each end.find higher priority groups at each end. By definitionBy definition EE--4-ethyl-3methyl-4-ethyl-3methyl-3-3-octocteneene 11 22 33 4455 66 77 88 44 Opposite sides:Opposite sides: EE Same side:Same side: ZZ 3-Methyl3-Methylcyclohexcyclohexeneene
- 4. 8. Substituents: Alkenyl8. Substituents: Alkenyl CHCH22 CHCH Ethenyl (vinyl)Ethenyl (vinyl) CHCH22 CHCH CHCH22 2-Propenyl (allyl)2-Propenyl (allyl) 9. Exocyclic alkenes:9. Exocyclic alkenes: AlkylidenecycloalkanesAlkylidenecycloalkanes MethylidenecyclohexaneMethylidenecyclohexane (Methylenecyclohexane)(Methylenecyclohexane) OH ( SH)OH ( SH) >> eneene 11 22 33 OHOH 2-Propen-1-ol2-Propen-1-ol7.7.
- 5. Structure of DoubleStructure of Double BondBond ““Electron rich”Electron rich”
- 7. The Pi BondThe Pi Bond EtheneEthene
- 8. ππ Bond isBond is relatively weakrelatively weak OrbitalOrbital EnergiesEnergies
- 9. How Weak is theHow Weak is the ΠΠ Bond?Bond? EEaa = 65 kcal/mol= 65 kcal/mol
- 10. Bond StrengthsBond Strengths (kcal/mol)(kcal/mol) Unusually strong because C usesUnusually strong because C uses spsp22 hybridshybrids
- 11. Acidity: CC CC HH Alkenyl hydrogensAlkenyl hydrogens are relatively “acidic”are relatively “acidic” ppKKaa ~ 44~ 44 Cf.Cf. CHCH33CHCH33 HH ++ CHCH33LiLi RCHRCH CC HH LiLi CHCH44++ ~ 50.~ 50. CC CC HH HH HH RR Therefore, in principle:Therefore, in principle: Problems:Problems: Regio-, stereoselectivity. Better:Regio-, stereoselectivity. Better: CHCH22 CC HH BrBr ++ LiLi CHCH22 CC HH LiLi CC CC BrBr HH HH RR MgMg++ CC CC MgBrMgBr HH HH RR Useful:Useful: ReactReact withwith carbonylscarbonyls
- 12. Why are alkenyl hydrogens acidic?Why are alkenyl hydrogens acidic? CC 33%33% ss character.character. In contrast:In contrast: spsp33 has 25%has 25% ss charactercharacter Net effect: relativelyNet effect: relatively e-withdrawinge-withdrawing HH :: CC spsp22 HasHas
- 13. 11 H NMRH NMR CC CC HH δδ ~ 4.5-6 ppm: deshielded!~ 4.5-6 ppm: deshielded! Why?Why? 1.1. spsp22 2.2. e-Flow ofe-Flow of ππ cloudcloud
- 15. Coupling ConstantsCoupling Constants CC CC HH HH HH RR JJHHHH transtrans= 11-18 Hz;= 11-18 Hz; JJHHHH ciscis= 6-14 Hz= 6-14 Hz JJHHHH geminalgeminal ~ 0-3 Hz~ 0-3 Hz For cis/trans isomers:For cis/trans isomers: JJtranstrans alwaysalways JJciscis.. Double bond “transmits”Double bond “transmits” long rangelong range (over 3-4 C)(over 3-4 C) couplingcoupling (1-3 Hz).(1-3 Hz). >> Depend onDepend on stereochemistry.stereochemistry. ““Vicinal” coupling:Vicinal” coupling: ““Geminal” coupling:Geminal” coupling:
- 21. 1313 CC NMRNMR CCspsp22 deshielded (reasons are complex)deshielded (reasons are complex) δδ = 110 – 150 ppm “left half” of total spectral window= 110 – 150 ppm “left half” of total spectral window CC CC CCHH33 CCHH33 HH33CC HH33CC CC CC HH CHCH22CHCH33 HH HH33CC 132.7132.7 122.8122.8 18.918.9 12.312.3 123.7123.7 20.520.5 14.014.0 AlkenesAlkenes HHCC CCHH CCHH33 CCHH33 HH33CC HH33CC 34.034.0 19.219.2 CCHH33CCHH22CCHH22CHCH22CHCH33 13.513.5 34.134.1 22.222.2 AlkanesAlkanes
- 23. Vibrations inVibrations in Molecules: Infrared (IR)Molecules: Infrared (IR) SpectroscopySpectroscopy
- 24. Compounds resemble a mechanical frame:Compounds resemble a mechanical frame: theythey “rattle”.“rattle”. Rattling is quantized.Rattling is quantized. ΔΔEE == hhνν ~ 1-10 kcal mol~ 1-10 kcal mol-1-1 inin λλ or 1/or 1/λλ == υυ “wave numbers”“wave numbers”~~ Range:Range: 600-4000 cm600-4000 cm-1-1 EE Excited stateExcited state Ground stateGround state AA BB stretchingstretching υυ : Determined by Hooke’s Law: Determined by Hooke’s Law~~ υυ == kk~~ √√ ff mmAA+m+mBB mmAAmmBB f = force constant m =f = force constant m = mass (reflects bond strength)mass (reflects bond strength) υυ goes up with larger f, smaller mgoes up with larger f, smaller m~~ Not only stretching: also bending and coupled modesNot only stretching: also bending and coupled modes Complex patterns 600-1500 cmComplex patterns 600-1500 cm-1-1 : The: The fingerprintfingerprint regionregion
- 32. Most useful:Most useful: υυC HC H ~~ --1.1. AlkylAlkyl = 2900cm= 2900cm-1-1 υυCCspsp 22 HH ~~ -- = 3080 cm= 3080 cm-1-1 ,, υυC CC C ~~ ---- = 1640 cm= 1640 cm-1-1 ,, CC CC HH RR RR HH transtrans υυ = 970 cm= 970 cm-1-1~~ R O HR O H 2.2. AlkenesAlkenes 3.3. HH HH TransTrans-2-hexene-2-hexene 4.4. OO CC 1740 cm1740 cm-1-1 33503350 cmcm-1-1 (broad)(broad)
- 33. Degrees of UnsaturationDegrees of Unsaturation Molecular formulaMolecular formula tells us how many rings and/ortells us how many rings and/or ππ bonds are present in a molecule. Reference is abonds are present in a molecule. Reference is a saturated acyclicsaturated acyclic hydrocarbon: Chydrocarbon: CnnHH22nn+2+2.. Simple examples:Simple examples: We need to determine the deviation of theWe need to determine the deviation of the molecular formula from Cmolecular formula from CnnHH22nn+2+2 (in increments of(in increments of 2H). Every ring or double bond takes away 2H,2H). Every ring or double bond takes away 2H, triple bond 4H from Ctriple bond 4H from CnnHH22nn+2+2.. CC66HH1212,, not Cnot C66HH14.14. CC66HH1010,, not Cnot C66HH14.14.
- 35. Halogen:Halogen: -1-1;; CC HH CC XX Nitrogen:Nitrogen: +1+1;; CC HH CC NN RR HH Effect of Presence ofEffect of Presence of HeteroatomsHeteroatoms on Con CnnHH22nn+2+2 CC HH CC OO HH S, OS, O no effect on count (still Cno effect on count (still CnnHH22nn+2+2 + S+ Sxx or Oor Oyy)) Depends on valency of element:Depends on valency of element:
- 36. Steps:Steps: 1.1. Calculate HCalculate Hsatsat = 2= 2nn C + 2 –C + 2 – nnXX ++ nnNN nn = “number of”= “number of” 2.2. Count HCount Hactualactual in givenin given molecular formulamolecular formula.. 3.3. Degree of UnsaturationDegree of Unsaturation: (H: (Hsatsat – H– Hactualactual)/2)/2
- 37. CC55HH55NN ExamplesExamples:: CC1010HH1616 1. H1. Hsatsat = (2x10) + 2 = 22= (2x10) + 2 = 22 2. Degree of unsaturation: (22-16)/2 = 32. Degree of unsaturation: (22-16)/2 = 3 etc.etc.oror 1.1. HHsatsat= 10 + 2 + 1 = 13= 10 + 2 + 1 = 13 2. (13 - 5)/2 = 4 degrees of unsaturation:2. (13 - 5)/2 = 4 degrees of unsaturation: NN PyridinePyridine CC NN Or?Or?OrOr
- 38. ProblemProblem CC33HN: How many degrees of unsaturation?HN: How many degrees of unsaturation? HHsatsat = 2= 2nn C + 2 –C + 2 – nnXX ++ nnNN Degree of Unsaturation: (HDegree of Unsaturation: (Hsatsat – H– Hactualactual)/2)/2 A. 2A. 2 B. 3B. 3 C. 4C. 4
- 39. Relative Stability ofRelative Stability of AlkenesAlkenes Measure heat of hydrogenationMeasure heat of hydrogenation ΔΔHHH2H2 of isomers,of isomers, e.g., butenee.g., butene ++ HH22 ++ HH22 ++ HH22 ΔΔHHH2H2 (kcal mol(kcal mol-1-1 )) -28.6-28.6 -27.6-27.6 -30.3-30.3 InternalInternal terminalterminal transtrans ciscisStability:Stability: >> >>,, cat.cat. cat.cat. cat.cat.
- 41. Why?Why? 1.1. Hyperconjugation:Hyperconjugation: 2. Steric hindrance (strain)2. Steric hindrance (strain) CC HH CC CC :: CisCis isis less stableless stable thanthan transtrans because ofbecause of steric hindrancesteric hindrance GeneralGeneral order of stability:order of stability: CHCH22 CHCH22 RCH CHRCH CH22 RCH CHRRCH CHR ciscis<< << RCH CHRRCH CHR transtrans tritri tetrasubstitutedtetrasubstituted<< <<<<
- 42. Synthesis of AlkenesSynthesis of Alkenes E revisitedE revisited. Best: E. Best: E22 on RX.on RX. Regioselectivity?Regioselectivity? Saytzev-RuleSaytzev-Rule Non-bulky base:Non-bulky base: MoreMore stable product.stable product. CHCH33 CHCH22 C CHC CH33 CHCH33 XX basebase more stablemore stable less stableless stable CC CC HH CHCH33 CHCH33 HH33CC CC CHCH22 HH33CC HH22CCHCCH33 ++
- 44. Hofmann RuleHofmann Rule Bulky base: Less stable, terminal product is majorBulky base: Less stable, terminal product is major
- 45. 51%51% 18%18% 31%31% ++ ++ BrBr Na OCHNa OCH33 CHCH33OHOH --++ Trans predominatesTrans predominates (not much)(not much) Stereospecificity?Stereospecificity? YesYes.. E or Z from respectiveE or Z from respective diastereomeric haloalkanes:diastereomeric haloalkanes: C CC C HH XX **** IsIs elimination stereoselectiveelimination stereoselective?? I.e., will it make preferentiallyI.e., will it make preferentially ciscis oror transtrans product? Yes, but not completely.product? Yes, but not completely.
- 46. Stereospecificity:Stereospecificity: Good! EachGood! Each diastereomerdiastereomer givesgives only oneonly one stereoisomer ofstereoisomer of alkene productalkene product
- 47. Alkenes from ROH byAlkenes from ROH by Dehydration:Dehydration: OftenOften MessyMessy RRprimprim OHOH :::: CHCH22 CHCH22 OO + H+ H22SOSO44 conc., goes by Econc., goes by E2,2, requires heat:requires heat: ++ HH HH :: HH ++ HSOHSO44 CHCH22 CHCH22 HH22SOSO44 HH22OO :::: ++ ++ --++ RRsecsec, R, Rterttert OH : EOH : E11 + rearrangements+ rearrangements CHCH33CHCH22OOH +H + HH
- 48. Acid-Mediated Dehydration of AlcoholsAcid-Mediated Dehydration of Alcohols CC CC HH OHOH Acid,Acid, ΔΔ CC CC ++ HHOHOH Relative ReactivityRelative Reactivity of Alcohols (ROH) inof Alcohols (ROH) in Dehydration ReactionsDehydration Reactions R = primaryR = primary << secondarysecondary << tertiarytertiary CHCH33CHCH22OHOH CHCH22 CHCH22 CHCH33CC CCHCCH33 HH HH HOHO HH conc. Hconc. H22SOSO44,, 170°C170°C HOHHOH 50% H50% H22SOSO44,, 100°C100°C HOHHOH CHCH33CHCH CHCHCHCH33 CHCH22 CHCHCHCH22CHCH33 80%80% TraceTrace ++
- 49. HH22CC CC(CH(CH33))33COHCOH CHCH33 CHCH33 100%100% Dilute HDilute H22SOSO44,, 50°C50°C HOHHOH CHCH33CC CCHCCH33 HH HH OHOHCHCH33 CHCH22 HH22SOSO44,, ΔΔ HOHHOH HH33CC CC CC HH HH33CC CHCH22CHCH33 54%54% 8%8% CHCHCHCH33CHCH33CCHCCH HH CHCH33 other minor isomersother minor isomers++ ++ Dehydration with RearrangementDehydration with Rearrangement
- 50. OHOH αα-Terpineol-Terpineol ++ TerpinoleneTerpinolene 15%15% LimoneneLimonene 9%9% 33% H33% H22SOSO44, 1 h, 100°C, 1 h, 100°C HH22OO ++ ++ 28.5%28.5% IsoterpinoleneIsoterpinolene 18.5%18.5% γγ-Terpinene-Terpinene 15%15% ++ αα-Terpinene-Terpinene Acid-catalyzedAcid-catalyzed dehydrations givedehydrations give mixturesmixtures Terpenes: The scentTerpenes: The scent of soapof soap