Alkenes and Alkynes

1. Alkenes and Alkynes • Structure and Properties • Nomenclature • Synthesis of Alkenes • Reactions of Alkenes Alkenes - Synthesis and Reactions

2. Structure and Properties • Alkene: • hydrocarbon with one or more C-C double bond • also called olefin • C=C consists of 1 s bond and 1 p bond Ethylene ethene

3. Structure and Properties • C=C is a functional group • BDE (s bond) = ~83 kcal/mol • BDE (p bond) = ~ 63 kcal/mol • p bond is weaker than s bond • reactions take place at the p bond • sp2 hybridization • trigonal planar

4. Structure and Properties • Trigonal planar geometry • approximately 120o bond angle for alkenes • vs. ~109.5o bond angle for alkanes • Double bonds are shorter than single bonds.

5. Structure and Properties • Alkanes • saturated hydrocarbons • each C has the maximum # of H’s possible • Alkenes • unsaturated hydrocarbons • fewer H atoms per C than an alkane • capable of adding hydrogen

6. Structure and Properties • Element of unsaturation • a structural feature that reduces the number of hydrogen atoms by 2 relative to the corresponding alkane • ring • p bond • used to help determine possible structures

7. Structure and Properties • Elements of unsaturation = 1/2 (2C + 2 - H) • C6H12 • EU = ½ (2x6 +2 – 12) = 1

8. Structure and Properties Example: Calculate the elements of unsaturation for C4H8. Draw 5 structural isomers with this formula.

9. Structure and Properties 5 structural isomers of C4H8

10. Structure and Properties • To determine the elements of unsaturation for compounds with heteroatoms (atoms other than C and H): • use same formula as given previously BUT • Each halogen counts as a hydrogen atom • Ignore any oxygen atoms • Each nitrogen counts as 1/2 C

11. Structure and Properties Example: Calculate the elements of unsaturation for C6H9ClO. Draw at least 4 structural isomers.

12. Structure and Properties • 4 possible structural isomers The structures you draw should contain reasonable functional groups….i.e. don’t make up strange functional groups!

13. IR • Alkenes have two characteristic peaks in the IR: • sp2 C-H at >3000 cm-1 • C=C at ~1620 – 1680 cm-1 • Conjugated alkene C=C is at lower frequency • Isolated alkene C=C is at higher frequency • C=C peak has variable intensity but is typically weak to moderate.

14. sp2 C-H Alkene C=C sp3 C-H

15. Nomenclature • Alkenes can be named using either IUPAC names or common names. ethene ethylene propene propylene 2-methylpropene isobutylene Blue = IUPAC Red = common

16. Hexane hexene cyclopentane cyclopentene Nomenclature • To name alkenes: • Find the longest continuous chain (or ring) that contains the double bond. • Base name = name of corresponding alkane or cycloalkane with ending changed to “ene”

17. Nomenclature • To name alkenes: • Number from the end of the chain closest to the double bond • the double bond is given the lower number of the two double-bonded carbons • Cycloalkenes: double bond is always between carbons 1 and 2 5 1 2 6 4 4 2 1 5 3 3

18. Nomenclature • Place the number of the double bond in front of the base name of the alkene (omit the number for cycloalkenes unless > 2 double bonds) a substituted 2-hexene a substituted cyclopentene a substituted hex-2-ene Newer IUPAC system places the position number just before the “ene” ending

19. Nomenclature • Name substitutent groups in the same manner as in alkanes. trans-6-chloro-5-methyl-2-hexene or trans-6-chloro-5-methylhex-2-ene 3-bromo-4-methylcyclopentene

20. Nomenclature • Alkenes as substitutents (often named using common names) Methylenegroup 3-methylenecyclohexene vinyl group 3-vinyl-1,5-hexadiene 3-vinylhexa-1,5-diene Allylgroup Allyl chloride

21. Nomenclature • For compounds that show geometric isomerism, add the appropriate prefix: • cis • trans OR • E • Z • NOTE: Cycloalkenes are assumed to be cis unless otherwise indicated.

22. Nomenclature • Cis/trans isomers trans cis Cis: 2 identical groups located on the same side of the double bond Trans: 2 identical groups located on opposite sides of the double bond

23. Nomenclature Example: Name the following compounds.

24. Nomenclature • Some compounds form geometric isomers that cannot be named using the cis/trans nomenclature • Cis/trans nomenclature can’t be used: • two identical groups are not attached to adjacent carbons in the C=C

25. Nomenclature • The E-Z system of nomenclature for geometric isomers: • Break the double bond into two halves • Separately, assign priorities to the groups on each carbon in the double bond using the Cahn-Ingold-Prelog rules (R & S configuration rules) 1 1 2 2

26. Nomenclature • Z (Zusammen) isomer • both high priority groups are on the same side of the double bond • similar to cis • E (Entgegen) isomer • high priority groups are on the opposite side of the double bond • similar to trans (Z)-1-bromo-1-chloropropene

27. Nomenclature • Naming alkenes with more than one double bond: • Make sure that the longest chain includes as many C=C as possible. • 2 C=Cdiene • 3 C=Ctriene • 4 C=Ctetraene a substituted octatriene

28. Nomenclature • Show the location of each double bond • Designate the isomer present for each double bond (use location and E or Z) 3-bromo-2, 4, 6-octatriene 3-bromoocta-2,4,6-triene (2Z,4E,6E)-3-bromo-2,4,6-octatriene (2Z,4E,6E)-3-bromoocta-2,4,6-triene

29. Nomenclature Example: Name the following compounds.

30. Nomenclature Example: Draw the following compounds. cis-3-methyl-2-pentene 1-ethylcyclohexene (2E, 4Z)-2,4-hexadiene Remember:You must show the trigonal planar geometry around the C=C.

31. Uses and Physical Properties • Alkenes are important intermediates in the synthesis of polymers, drugs, pesticides, and other chemicals. • Ethylene is used as a feedstock for: • ethanol • ethylene glycol (antifreeze) • acetic acid • Propylene is used as a feedstock for: • isopropyl alcohol • acetone

32. Uses and Physical Properties • Alkenes are important “monomers” for the production of polymers like poly(vinyl chloride), and Teflon.

33. Uses and Physical Properties • Physical Properties • Similar to alkanes • Density • ~0.6 g/mL to ~ 0.7 g/mL • Boiling Point • increases with increasing MW • decreases with branching • Polarity • relatively non-polar • insoluble in water

34. Pt Stability of Alkenes • The heat of hydrogenation is used to compare the relative stabilities of alkenes. • Heat of hydrogenation: • The heat released (DH) during a catalytic hydrogenation • Catalytic hydrogenation:the addition of H2 to a double (or triple) bond in the presence of a catalyst

35. Stability of Alkenes • As the heat of hydrogenation becomes more negative, the stability of the alkene decreases.

36. Stability of Alkenes • More highly substituted double bonds are more stable • larger angular separation between the bulky alkyl groups

37. Stability of Alkenes • For acyclic alkenes, trans isomers are more stable than cis isomers. • Trans isomers of cycloalkenes with fewer than 8 carbons are unstable. • Large amount of ring strain • Because of ring strain, cycloalkenes with less than 5 carbons in the ring are less stable than those with 5 or more carbons.

38. Stability of Alkenes Example: Which of the following alkenes is more stable.