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Chapter 11 Alkenes and IR

Chapter 11 Alkenes and IR. Alkene Nomenclature Unsaturation Alkanes: C n H 2n+2 Alkenes: C n H 2n Degree of Unsaturation Tells us how many rings and double bonds in molecule H sat = 2C + 2 – X + N (Ignore O, S) Degree of Unsaturation = (H sat – H act )/2 Example: C 5 H 8 NOCl

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Chapter 11 Alkenes and IR

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  1. Chapter 11 Alkenes and IR • Alkene Nomenclature • Unsaturation • Alkanes: CnH2n+2 • Alkenes: CnH2n • Degree of Unsaturation • Tells us how many rings and double bonds in molecule • Hsat = 2C + 2 – X + N (Ignore O, S) • Degree of Unsaturation = (Hsat – Hact)/2 • Example: C5H8NOCl • Hsat = 2(5) + 2 – 1 + 1 = 12 • (Hsat – Hact)/2 = (12 – 8)/2 = 2 degrees of unsaturation

  2. Nomenclature • Common Names end with –ylene • Ethylene • Propylene • IUPAC: Replace –ane of an alkane with –ene of an alkene • Ethene • Propene • Alkenes follow alkane nomenclature, with double bond location numbered closest to end • 1-butene • 2-butene • Cylclohexene • Substituents named as prefixes with lowest numbers • 3-methyl-1-pentene • 3-methylcyclohexene

  3. Disubstituted Alkenes can be cis or trans streoisomers • cis-2-butene • trans-2-butene • Cycloalkenes cis unless large • Tri- or Tetra-substituted Alkenes can be E or Z stereoisomers • Use priorities from R/S nomenclature • Assign 1-2 on each carbon • Move from 1-2-1-2 to trace out an E or Z • Alcohols have priority over alkene in numbering: Alkenol • Alkene substituents are named alkenyl E-1-chloro-3-ethyl-4-methyl-3-heptene Z-1-bromo-1,2-difluoroethene Z-5-chloro-3-ethyl-4-hexen-2-ol 2-propen-1-ol trans-1-propenyl- ethenylcyclohexane 2-propenyl-

  4. Pi-bonding in Alkenes • The p-bond • sp2 hybridization results in 120o angles • H1s-Csp2 overlap gives the CH s-bonds • Csp2—Csp2 overlap gives the C—C s-bond • Cp—Cp overlap gives the C—C p-bond • Bond Strength • Bond strength is proportional to orbital overlap • The s-bond in ethene is very strong because of good overlap • The p-bond in ethene is fairly weak because of poor overlap • Overall, the double bond is stronger than a C—C single bond • The weak p-bond will be the reactive part of the molecule

  5. 6) Orbital and Energy level diagrams for ethene

  6. Thermal Isomerization tells us the p-bond energy • cis/trans interconversion must go through broken p-bond T.S. • Ea = 65 kcal/mol should be about the same as the p-bond strength • The s-bond is slightly stronger than alkane due to better sp2 overlap • C—H bonds are also stronger than in alkanes (110 kcal/mol) • Radical H-atom abstraction doesn’t occur in alkenes because of the strong C-H bonds. The chemistry is dominated by the weak p-bond.

  7. Physical properties of Alkenes • Boiling points are about like alkanes • Melting points depend on the isomer • cis-alkenes have a U-shape that disrupts packing in the solid, giving lower temperatures (Vegetable oils have cis-alkenes) 2) trans-alkenes have melting points close to the alkanes • Polarization • Alkenes are more polar than alkanes due to more e-withdrawing sp2 hybrid orbitals (more s-character draws e- closer to nucleus) • cis-alkenes are more polar than trans-alkenes due to their shape • Acidity of alkenes > alkanes, again because of the greater s-character of sp2 hybrid orbitals. • Ethane pKa = 50 • Ethene pKa = 44

  8. NMR of Alkene • p-electrons deshield hydrogens • Alkane H 1.0 ppm • Alkene H 5-6 ppm • Spectrum of an alkene

  9. Coupling in Alkenes depends on the isomer • 13C NMR of alkenes gives peaks at 100-150 ppm due to deshielding

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