Chapter 11 of General, Organic, and Biochemistry, Denniston, 7th edition

1. Chapter 11
The Unsaturated Hydrocarbons:
Alkenes, Alkynes, and Aromatics

2. 1. Structure
Alkenes are hydrocarbons with a double bond.
CnH2n

3. 1. Structure
Alkynes are hydrocarbons with a triple bond.
CnH2n-2

4. 1. Structure
Alkenes and alkynes are unsaturated (don’t have the
maximum number of hydrogens bonded to each carbon).
Tegrity lecture video

5. 1. Comparison

6. 1. Geometry

7. 1. Geometry [3.4 Lewis structures]
 Four groups of electrons
 Ethane
 tetrahedral
 extend toward the corners of a regular tetrahedron
 bond angle = 109.5o

8. 1. Geometry [3.4 Lewis structures]
8

9. 1. Geometry [3.4 Lewis structures]
 Three groups of electrons
 Ethene
 All in the same plane
 Trigonal planar
 Bond angle = 120o

10. 1. Geometry [3.4 Lewis structures]
10

11. 1. Geometry [3.4 Lewis structures]
 Two groups of electrons
 Ethyne
 Linear
 Bond angle = 180o

12. 1. Geometry [3.4 Lewis structures]
12

13. 1. Physical properties
Name Melting point Boiling point
ethene -160.1oC -103.7oC
propene -185.0oC -47.6oC
1-butene -185.0oC -6.1oC
methylpropene -140.0oC -6.6oC
ethyne -81.8oC -84.0oC
propyne -101.5oC -23.2oC
1-butyne -125.9oC 8.1oC
2-butyne -32.3oC 27.0oC

14. 1. Physical properties
 In each case, the alkyne has a higher boiling point than the
alkene.
 Its structure is more linear.
 The molecules pack together more efficiently.
 Intermolecular forces are stronger.
Tegrity lecture video

15. 2. Nomenclature
The root name is based on the longest chain that
includes both carbons of the multiple bond.
The –ane ending is changed to –ene for double bonds
and –yne for triple bonds.
ethyne
ethene
propyne
propene

16. 2. Nomenclature
The chain is numbered from the end nearest the multiple
bond.
2-pentyne
1-butene [not 3-pentyne]
[not 3-butene]
The position of the multiple bond is indicated with the
lower-numbered carbon in the bond.

17. 2. Nomenclature
Determine the name and number of each substituent
and add in front of the name of the parent compound.
5-chloro-4-methyl-2-hexene
2,6-dimethyl-3-octene
5-bromo-4-ethyl-2-heptene

18. 2. Nomenclature
Alkenes with more than one double bond are called
alkadienes (2 double bonds)
alkatrienes (3 double bonds)
etc…
Each double bond is designated by its lower-numbered
carbon.
2,4-hexadiene

19. 2. Nomenclature
Cycloalkenes must be numbered so the double bond is
between carbons one and two.
3-chloro-cyclopentene
4-ethyl-5-methylcyclooctene

20. 2. Nomenclature
Name the following compounds.
CH3CH=C(CH2CH3)2
H2C=C-CH2-CH=CH2
pencast

21. 2. Nomenclature
Name the following compounds.
pencast

22. 2. Nomenclature
Write a structural formula for each of the following
compounds.
1-hexene
1,3-dicholoro-2-butene
4-methyl-2-hexyne
pencast
1,4-cyclohexadiene

23. 2. Nomenclature
Draw a structural formula for each of the following
compounds:
1-bromo-3-hexyne
2-butyne
pencast
dichloroethyne
9-iodo-1-nonyne

24. 3. Geometric isomers
 Rotation around a double bond is restricted, in much the
same was as rotation is restricted for the cycloalkanes.
 In the alkenes, geometric isomers occur when there are
two different groups on each of the double-bonded carbon
atoms.
1,2-dichloroethene

25. 3. Cis-trans isomers
 If both constituents are on the same side of the double
bond, the isomer is cis-.
cis-1,2-dichloroethene
 If the constituents are on opposite sides of the double bond,
the isomer is trans-.
trans-1,2-dichloroethene

26. 3. Cis-trans isomers
 Alkenes without substituents also may exhibit cis-trans
isomerism.
cis-4-octene
trans-4-octene

27. 3. Cis-trans isomers
 In order for cis and trans isomers to exist, neither double-
bonded carbon may have two identical substituents.
2-methyl-2-butene
no cis/trans isomerism
1-butene
no cis/trans isomerism

28. 3. Cis-trans isomers
28

29. 3. Cis-trans isomers
 Which of the following compounds can exist as geometric
isomers?
 1-bromo-1-chloro-2,2-dimethylpropene
 1,1-dichloroethene
 1,2-dibromoethene
 3-ethyl-2-methyl-2-hexene

30. 5. Reactions of alkenes and alkynes
 The most common reactions of alkenes and alkynes are
addition reactions.
 Hydrogenation: addition of H2
 Halogenation: addition of X2
 Hydration: addition of H2O
 Hydrohalogenation: addition of HX

31. 5. General addition reaction
 A double bond consists of
 a sigma bond: two electrons
concentrated on a line between the two
connected atoms;
 a pi bond: two electrons concentrated in
planes above and below the sigma bond.

32. 5. General addition reaction
32

33. 5. General addition reaction
 In an addition reaction, the pi bond is lost and its electrons
become part of the single bonds to A and B.

34. 5. General addition reaction
 For hydrogenation, halogenation, hydration, and
hydrohalogenation, identify the A and B portions of what is
being added to the double bond.
 hydrogenation, H2
 halogenation, X2 (where X = F, Cl, Br, or I)
 hydration, H2O
 hydrohalogenation, HX (where X = F, Cl, Br, or I)

35. 5. Hydrogenation
 In hydrogenation of an alkene, one molecule of hydrogen
(H2) adds to one mole of double bonds.
 Reaction conditions:
 platinum, palladium, or nickel catalyst
 [sometimes] heat and/or pressure

36. 5. Hydrogenation
 In hydrogenation of an alkyne, two molecules of hydrogen
(H2) add to one mole of triple bonds.
 Reaction conditions: same as for alkenes.

37. 5. Hydrogenation
 Compare the products resulting from the hydrogenation of
trans-2-pentene and cis-2-pentene.
pencast

38. 5. Hydrogenation
 Compare the products resulting from the hydrogenation of
1-butene and cis-2-butene.
pencast

39. 5. Vegetable oil and margarine
 Why does hydrogenation make oils more solid?
MP = 13-14oC
MP = 69.6oC
MP = 62.9oC

40. 5. Halogenation
 In halogenation of an alkene, one mole of a halogen (Cl2,
Br2, I2) adds to one mole of double bonds.
 Since halogens are more reactive than hydrogen, no
catalyst is needed.

41. 5. Halogenation
 In halogenation of an alkyne, two moles of a halogen (Cl2,
Br2, I2) add to one mole of double bonds.

42. 5. Halogenation
 Draw the structure and write a balanced equation for the
halogenation of each of the following compounds.
 3-methyl-1,4-hexadiene
 4-bromo-1,3-pentadiene
 3-chloro-2,4-hexadiene
pencast

43. 5. Halogenation
 A solution of bromine in water
has a reddish-orange color.
 A simple test for the presence
of an alkene or alkane is to
add bromine water.
 If a double or triple bond is
present, the bromine will be
used up in a halogenation
Test of cyclohexane
reaction and the color will
and cyclohexene
disappear.

44. 5. Hydration
 In hydration, one mole of water (H2O) is added to one mole
of double bonds.
 A trace of acid is required as a catalyst.

45. 5. Hydration
 Unlike hydrogenation and halogenation, hydration is not a
symmetric addition to a double bond.
 If the double bond is not symmetrically located in the
molecule, there are two possible hydration products.

46. 5. Hydration
 The predominant product is determined by Markovnikov’s
rule: The rich get richer.
 OR: The carbon that already has more hydrogens will get
the hydrogen from the water.
 Hydration of propene:
+ H 2O 

47. 5. Hydration
 Write a balanced equation for the hydration of each of the
following compounds:
 2-butene
 2-ethyl-3-hexene pencast
 2,3-dimethylcyclohexene
Alkynes undergo a much more complicated hydration that you don’t need
to remember at this time!

48. 5. Hydrohalogenation
 Like hydration, hydrohalogenation is an asymmetric addition
to a double bond.
 Hydrohalogenation also follows Markovnikov’s rule.

49. 5. Hydrohalogenation
2-butene + HBr  ?
3-methyl-2-hexene + HCl  ?
cyclopentene + HI  ?
pencast

50. 6. Aromatic compounds
 Consider the following molecular formulas for unsaturated
hydrocarbons:
 Hexane (all single bonds): C6H14
 Cyclohexane (one ring): C6H12
 Hexene (one double bond): C6H12
 Hexadiene (two double bonds): C6H10
 Cyclohexene (one ring, one double bond): C6H10
 Hexatriene (three double bonds): C6H8
 Cyclohexadiene (one ring, two double bonds): C6H8

51. 6. Aromatic compounds
 The molecular formula for benzene is C6H6.
 The structure must be highly unsaturated.
 One ring, three double bonds?
 Reactions of benzene:
 Benzene does not decolorize bromine solutions.
 Benzene does not undergo typical addition reactions.
 Benzene reacts mainly by substitution.
 The first three items are opposite from what is expected
from unsaturated compounds.
 The last item is identical to what is expected for alkanes.

52. 6. Benzene structure
 The benzene ring consists of:
 six carbon atoms
 joined in a planar hexagonal arrangement
 with each carbon bonded to one hydrogen atom.
 Two equivalent structures proposed by Kekulé are
recognized today as resonance structures.
 The real benzene molecule is a hybrid with each resonance
structure contributing equally to the true structure.

53. 6. Benzene structure
 Sigma and pi bonding in benzene:
 The sharing of six electrons over the entire ring gives the
benzene structure extra stability.
 Removing any one of the six electrons would destroy that
stability.

54. 6. Nomenclature
 Most single-substituent compounds are named as
derivatives of benzene.
 Bromobenzene
 Ethylbenzene

55. 6. Nomenclature
 A few “common” names have been adopted as IUPAC
nomenclature.
 toluene
 phenol
 aniline
 xylene (any benzene ring with two methyl groups)

56. 6. Nomenclature
 There are three ways for the methyl groups on xylene to be
arranged.
 1,2 [ortho-xylene]
 1,3 [meta-xylene]
 1,4 [para-xylene]

57. 6. Nomenclature
 The substituent created by removing one hydrogen from the
benzene ring is called phenyl-.
 2-phenylhexane
 3-phenylcyclopentene

58. 6. Nomenclature
 The substituent consisting of a –CH2 attached to a benzene
ring is called benzyl-.
 Benzyl chloride

59. 6. Polynuclear aromatic hydrocarbons
 These consist of rings joined along one side.
Good news! You don’t have to memorize these names!

60. 6. Reactions of benzene
 Because of the stability of benzene’s ring structure, only
substitution reactions are characteristic.
 Halogenation: substitution of one or more halogen atoms for
hydrogen atoms.
 Cl2 requires FeCl3 catalyst.
 Br2 requires FeBr3 catalyst.
 Nitration: substitution of one or more nitro- (-NO2) groups for
hydrogen atoms.
 Requires nitric acid and concentration sulfuric acid.
 Sulfonation: substitution of one sulfonic acid (-SO3H) group for a
hydrogen atom.
 SO3 reactant and concentration sulfuric acid.

61. 7. Heterocyclic aromatic compounds
 Heterocyclic aromatic compounds have at least one non-
carbon atom incorporated in an aromatic ring or polynuclear
aromatic compound.
 Many of these compounds are biologically important.
 Components of DNA and RNA
 Components of hemoglobin and chlorophyll
 Pharmaceuticals
pyridine