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Alkenes C n H 2n “unsaturated” hydrocarbons C 2 H 4 ethylene Functional group = carbon-carbon double bond

Alkenes C n H 2n “unsaturated” hydrocarbons C 2 H 4 ethylene Functional group = carbon-carbon double bond sp 2 hybridization => flat, 120 o bond angles σ bond & π bond => H 2 C=CH 2 No rotation about double bond!. C 3 H 6 propylene CH 3 CH=CH 2

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Alkenes C n H 2n “unsaturated” hydrocarbons C 2 H 4 ethylene Functional group = carbon-carbon double bond

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  1. Alkenes CnH2n “unsaturated” hydrocarbons C2H4 ethylene Functional group = carbon-carbon double bond sp2 hybridization => flat, 120o bond angles σ bond & π bond => H2C=CH2 No rotation about double bond!

  2. C3H6 propylene CH3CH=CH2 C4H8 butylenes CH3CH2CH=CH2 α-butylene 1-butene CH3 CH3CH=CHCH3 CH3C=CH2 β-butylene isobutylene 2-butene 2-methylpropene

  3. there are two 2-butenes: cis-2-butene trans-2-butene “geometric isomers” (diastereomers)

  4. C=C are called “vinyl” carbons If eithervinyl carbon is bonded to two equivalent groups, then no geometric isomerism exists. CH3CH=CHCH3 CH3CH2CH=CH2 yes no CH3 (CH3)2C=CHCH3 CH3CH=CCH2CH3 no yes

  5. Confusion about the use of cis- and trans-. According to IUPAC rules it refers to the parent chain. “cis-” ????????

  6. E/Z system is now recommended by IUPAC for the designation of geometric isomerism. • Use the sequence rules to assign the higher priority * to the two groups attached to each vinyl carbon. • 2. * * * • * • (Z)- “zusammen” (E)- “entgegen” • together opposite

  7. * * (Z)- (E)- * *

  8. Nomenclature, alkenes: • Parent chain = longest continuous carbon chain that contains the C=C. • alkane => change –ane to –ene • prefix a locant for the carbon-carbon double bond using • the principle of lower number. • Etc. • If a geometric isomer, use E/Z (or cis/trans) to indicate which isomer it is.

  9. * * (Z)-3-methyl-2-pentene (3-methyl-cis-2-pentene) * (E)-1-bromo-1-chloropropene *

  10. CH3 CH3CH2 CHCH2CH3 \ / C = C 3-ethyl-5-methyl-3-heptene / \ CH3CH2 H (not a geometric isomer)

  11. -ol takes precedence over –ene CH2=CHCH2-OH 2-propen-1-ol CH3CHCH=CH2 3-buten-2-ol OH

  12. Physical properties: non-polar or weakly polar no hydrogen bonding relatively low mp/bp ~ alkanes water insoluble Importance: common group in biological molecules starting material for synthesis of many plastics

  13. Syntheses, alkenes: • dehydrohalogenation of alkyl halides • 2. dehydration of alcohols • dehalogenation of vicinal dihalide • 4. (later)

  14. dehalogenation of vicinal dihalides • | | | | • — C — C — + Zn  — C = C — + ZnX2 • || • X X • eg. • CH3CH2CHCH2 + Zn  CH3CH2CH=CH2 + ZnBr2 • Br Br • Not generally useful as vicinal dihalides are usually made from alkenes. May be used to “protect” a carbon-carbon double bond.

  15. dehydrohalogenation of alkyl halides • | | | | • — C — C — + KOH(alc.)  — C = C — + KX + H2O • | | • H X • RX: 3o > 2o > 1o • no rearragement  • may yield mixtures  • Saytzeff orientation • element effect • isotope effect • rate = k [RX] [KOH] • Mechanism = E2

  16. rate = k [RX] [KOH] => both RX & KOH in RDS R-I > R-Br > R-Cl “element effect” => C—X broken in RDS R-H > R-D “isotope effect” => C—H broken in RDS Concerted reaction: both the C—X and C—H bonds are broken in the rate determining step.

  17. Mechanism = elimination, bimolecular E2 One step! “Concerted” reaction.

  18. CH3CHCH3 + KOH(alc)  CH3CH=CH2 Br isopropyl bromide propylene CH3CH2CH2CH2-Br + KOH(alc)  CH3CH2CH=CH2 n-butyl bromide 1-butene CH3CH2CHCH3 + KOH(alc)  CH3CH2CH=CH2 Br 1-butene19% sec-butyl bromide + CH3CH=CHCH3 2-butene81%

  19. Problem 8.6. What akyl halide (if any) would yield each of the following purealkenes upon dehydrohalogenation by strong base? CH3 CH3 isobutylene  KOH(alc) + CH3CCH3 or CH3CHCH2-X X 1-pentene  KOH(alc) + CH3CH2CH2CH2CH2-X note: CH3CH2CH2CHCH3 would yield a mixture!  X 2-pentene  KOH(alc) + CH3CH2CHCH2CH3 X 2-methyl-2-butene  KOH(alc) + NONE!

  20. Saytzeff orientation: Ease of formation of alkenes: R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2 Stability of alkenes: R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2 CH3CH2CHCH3 + KOH(alc)  CH3CH2CH=CH2RCH=CH2 Br 1-butene 19% sec-butyl bromide + CH3CH=CHCH3RCH=CHR 2-butene 81%

  21. KOH (alc) CH3CH2CH2CHBrCH3 CH3CH2CH=CHCH3 + CH3CH2CH2CH=CH2 71% 29% CH3 CH3 CH3 CH3CH2CCH3 + KOH(alc)  CH3CH=CCH3 + CH3CH2C=CH2 Br 71% 29% CH3 CH3 CH3 CH3CHCHCH3 + KOH(alc)  CH2=CHCHCH3 + CH3CH=CCH3 Br major product

  22. Order of reactivity in E2: 3o > 2o > 1o CH3CH2-X  CH2=CH2 3 adj. H’s CH3CHCH3  CH3CH=CH2 6 adj. H’s & more stable X alkene CH3 CH3 CH3CCH3  CH=CCH3 9 adj. H’s & most stable X alkene

  23. Elimination unimolecular E1

  24. Elimination, unimolecular E1 • a)RX: 3o > 2o > 1o • b) rearragement possible  • c) may yield mixtures  • d) Saytzeff orientation • e) element effect • f) no isotope effect • g) rate = k [RW]

  25. E1: Rate = k [RW] => only RW involved in RDS R-I > R-Br > R-Cl “element effect” => C—X is broken in RDS R-H  R-D no“isotope effect” => C—H is not broken in the RDS

  26. Elimination, unimolecular E1 • RX: 3o > 2o > 1o carbocation • rearragement possible “ • c) may yield mixtures • d) Saytzeff orientation • e) element effect C—W broken in RDS • f) no isotope effect C—H not broken in RDS • g) rate = k [RW] only R-W in RDS

  27. alkyl halide + base  substitution or elimination?

  28. R-X + base  ???????? • 1) If strong, conc. base: • CH3 > 1o => SN2 R-Z  • 3o > 2o => E2 alkene(s) • If weak, dilute base: • 3o > 2o > 1o => SN1 and E1 R-Z + alkene(s)  • If KOH(alc.) • 3o > 2o > 1o => E2 alkene(s) 

  29. SN2 CH3CH2CH2-Br + NaOCH3 CH3CH2CH2-O-CH3 1o CH3E2 CH3 CH3CCH3 + NaOCH3  CH3C=CH2 + HOCH3 Br 3o E2 CH3CH2CH2-Br + KOH(alc)  CH3CH=CH2

  30. CH3 CH3 CH3CHCHCH3 + dilute OH- CH3CCH2CH3SN1 Br OH  CH3 + CH3C=CHCH2E1 CH3 CH3CHCHCH3 CH3  + CH2=CCH2CH3E1  [1,2-H]  CH3 CH3CCH2CH3 

  31. dehydration of alcohols: • | ||| • — C — C — acid, heat  — C = C — + H2O • | | • H OH • ROH: 3o > 2o > 1o • acid is a catalyst • rearrangements are possible  • mixtures are possible  • Saytzeff • mechanism is E1 • note: reaction #3 for alcohols!

  32. CH3CH2-OH + 95% H2SO4, 170oC  CH2=CH2 CH3 CH3 CH3CCH3 + 20% H2SO4, 85-90oC  CH3C=CH2 OH CH3CH2CHCH3 + 60% H2SO4, 100oC  CH3CH=CHCH3 OH + CH3CH2CH=CH2 CH3CH2CH2CH2-OH + H+, 140oC  CH3CH2CH=CH2 rearrangement!  + CH3CH=CHCH3

  33. Synthesis of 1-butene from 1-butanol: CH3CH2CH2CH2-OH + HBr  CH3CH2CH2CH2-Br SN2 E2  KOH(alc) CH3CH2CH=CH2 only! To avoid the rearrangement in the dehydration of the alcohol the alcohol is first converted into an alkyl halide.

  34. Syntheses, alkenes: • dehydrohalogenation of alkyl halides • E2 • 2. dehydration of alcohols • E1 • dehalogenation of vicinal dihalide • 4. (later)

  35. R-OH H+ R-X KOH Alkene (alc.) Zn vicinal dihalide

  36. Alkyl halides: nomenclature syntheses: 1. from alcohols a) HX b) PX3 2. halogenation of certain alkanes 3. 4. 5. halide exchange for iodide reactions: 1. nucleophilic substitution 2. dehydrohalgenation 3. formation of Grignard reagent 4. reduction

  37. Alcohols: nomenclature syntheses later reactions 1. HX 2. PX3 3. dehydration 4. as acids 5. ester formation 6. oxidation

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