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

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

16. Nomenclature Hexane hexene cyclopentane cyclopentene 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”
Hexane hexene
cyclopentane cyclopentene

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 a substituted a substituted 2-hexene cyclopentene
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. trans-6-chloro-5-methyl-2-hexene trans-6-chloro-5-methylhex-2-ene
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)
Methylene group
3-methylenecyclohexene
vinyl group
3-vinyl-1,5-hexadiene
3-vinylhexa-1,5-diene
Allyl group
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
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)-1-bromo-1-chloropropene 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. a substituted octatriene
Nomenclature
Naming alkenes with more than one double bond:
Make sure that the longest chain includes as many C=C as possible.
2 C=C diene
3 C=C triene
4 C=C tetraene
a substituted octatriene

28. Nomenclature 3-bromo-2, 4, 6-octatriene 3-bromoocta-2,4,6-triene
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
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. 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 Pt

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.