1. The Unsaturated Hydrocarbons
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Chapter 11
The Unsaturated Hydrocarbons
Denniston Topping Caret
5th Edition

2. 11.1 Alkenes and Alkynes: Structure and Physical Properties
Both alkenes and alkynes are unsaturated hydrocarbons
The alkene functional group is the carbon-carbon double bond
The alkyne functional group is the carbon-carbon triple bond
Simplest alkene: ethene (ethylene) C2H4
Simplest alkyne: ethyne (acetylene) C2H2

3. Aliphatic Hydrocarbon Structure Comparison
11.1 Structure and Physical
Properties

4. Bonding and Geometry of Two-Carbon Molecules
11.1 Structure and Physical
Properties

5. Structural Comparison of Five Carbon Molecules
11.1 Structure and Physical
Properties
Basic tetrahedral Planar around the Linear at the
zig-zag shape double bond triple bond

6. 11.1 Structure and Physical
Physical Properties
Physical properties of the alkenes and alkynes are quite similar to those of alkanes
Nonpolar
Not soluble in water
Highly soluble in nonpolar solvents
Boiling points rise with molecular weight
11.1 Structure and Physical
Properties

7. 11.2 IUPAC Names
Base name from longest chain containing the multiple bond
Change from -ane to -ene or -yne
Number from the end, that will give the first carbon of the multiple bond the lower number
Prefix the name with the number of the first multiple bond carbon
Prefix branch/substituent names as for alkanes

8. Comparison of Names
11.2 Nomenclature

9. Basic Naming Practice 11.2 Nomenclature 3-ethyl-6-methyl-3-heptene
Name
2-bromo-3-hexyne

10. Molecules With More Than One Double Bond
Alkenes having more than one double bond:
2 double bonds = alkadiene
3 triple bonds = alkatriene
Same rules for alkynes
11.2 Nomenclature

11. Name: Naming Cycloalkenes 11.2 Nomenclature
Cyclic alkenes are named like cyclic alkanes
Prefix name with cyclo
Numbering must start at one end of the double bond and pass through the bond
Substituents must have the lower possible numbers
Either number clockwise or counterclockwise
11.2 Nomenclature
Name:
5-chloro-3-methylcyclohexene

12. Naming Haloalkenes 11.2 Nomenclature
Double or triple bonds take precedence over a halogen or alkyl group
2-Chloro-2-butene
If 2 or more halogens, indicate the position of each
11.2 Nomenclature

13. 11.3 Geometric Isomers: A Consequence of Unsaturation
Carbon-carbon double bonds are rigid
Orbital shape restricts the rotation around the bond
Results in cis-trans isomers
Requires two different groups on each of the carbon atoms attached by the double bond

14. Naming Geometric Isomers
2-butene is the first example of an alkene which can have two different structures based on restricted rotation about the double bond
11.3 Geometric Isomers: Consequence of Unsaturation
trans-2-butene cis-2-butene

15. Identifying cis/trans Isomers
If one end of the C=C has two groups the same, cis-trans isomers are not possible
Both carbons of the C=C must have two different groups attached
Find a group common to both ends of the C=C
If the common group is on the same side of the pi bond, the molecule is cis
If on the opposite side, the molecule is trans
11.3 Geometric Isomers: Consequence of Unsaturation

16. Questions to Identify cis-trans Isomers
Are both groups on a double-bond carbon the same?
A = B? C = D? If no, continue
Is one group on each carbon the same?
A = C or D? B = C or D? If either or both is yes, cis-trans isomer is present
A  B
C  D
So continue
A = C
Isomer!
11.3 Geometric Isomers: Consequence of Unsaturation A C B D

17. Distinguishing cis-trans Isomers
Each carbon has 2 different substituents
One substituent on each carbon is the same (Cl)
The 2 chlorine atoms are attached on opposites of the plane of the double bond = trans
trans-1,2-dichloro-1-butene
11.3 Geometric Isomers: Consequence of Unsaturation

18. 11.3 Geometric Isomers: Consequence of Unsaturation
cis-trans Isomers
Decide whether each compound is
cis
trans
neither
A: methyls are trans
B: no cis-trans. Right C has two isopropyls
C: hydrogens are cis
11.3 Geometric Isomers: Consequence of Unsaturation A B C

19. 11.4 Alkenes in Nature Alkenes are abundant in nature
Ethene is a fruit ripener and promotes plant growth
Polyenes built from the isoprene skeleton are called isoprenoids
Isoprene is the basic 5 carbon unit shown here
The next slide shows some isoprenoids

20. Isoprenoids – Distinctive Aromas
11.4 Alkenes in Nature

21. 11.5 Reactions Involving Alkenes and Alkynes
There are two kinds of reactions typical of alkenes:
Addition: two molecules combine to give one new molecule
Redox: oxidation and reduction
The two classes are not always mutually exclusive

22. Addition: General Reaction
11.5 Reactions Involving Alkenes and Alkynes
A small molecule, AB, reacts with the pi electrons of the double bond
The pi bond breaks and its electrons are used to bond to the A and B pieces
Some additions require a catalyst

23. Types of Addition Reactions
Symmetrical: same atom added to each carbon
Hydrogenation - H2 (Pt, Pd, or Ni as catalyst)
Halogenation - Br2, Cl2
Unsymmetrical: H and another atom are added to the two carbons
Hydrohalogenation - HCl, HBr
Hydration - H2O (requires strong acid catalyst e.g., H3O+, H2SO4, H3PO4)
Self-addition or polymerization
11.5 Reactions Involving Alkenes and Alkynes

24. Hydrogenation: Addition of H2
11.5 Reactions Involving Alkenes and Alkynes
Hydrogenation is the addition of a molecule of hydrogen (H2) to a carbon-carbon double bond to produce an alkane
The double bond is broken
Two new C-H bonds result
Platinum, palladium, or nickel is required as a catalyst
Heat and/or pressure may also be required

25. Halogenation: Addition of X2
11.5 Reactions Involving Alkenes and Alkynes
Halogenation is the addition of a molecule of halogen (X2) to a carbon-carbon double bond to produce an alkane
The double bond is broken
Two new C-X bonds result
Reaction occurs quite readily and does NOT require a catalyst
Chlorine and bromine are most often the halogen added

26. Bromination of an Alkene
11.5 Reactions Involving Alkenes and Alkynes
Left beaker contains bromine, but no unsaturated hydrocarbon
Right beaker contains bromine, but reaction with an unsaturated hydrocarbon results in a colorless solution

27. Unsymmetrical Addition
11.5 Reactions Involving Alkenes and Alkynes
Two products are possible depending how the 2 groups (as H and OH) add to the ends of the pi bond
The hydrogen will add to one carbon atom
The other carbon atom will attach the other piece of the addition reagent
OH (Hydration)
Halogen (Hydrohalogenation)

28. 11.5 Reactions Involving Alkenes and Alkynes
Hydration
11.5 Reactions Involving Alkenes and Alkynes
A water molecule can be added to an alkene
The addition of a water molecule to an alkene is called hydration
Presence of strong acid is required as a catalyst
Product resulting is an alcohol

29. Markovnikov’s Observation
Dimitri Markovnikov (Russian) observed many acid additions to C=C systems
He noticed that the majority of the hydrogen went to a specific end of the double bond
He formulated an explanation
11.5 Reactions Involving Alkenes and Alkynes

30. 11.5 Reactions Involving Alkenes and Alkynes
Markovnikov’s Rule
When an acid adds to a double bond
The H of the acid most often goes to the end of the double bond, which had more hydrogens attached initially
H-OH
H-Cl
H-Br
11.5 Reactions Involving Alkenes and Alkynes

31. 11.5 Reactions Involving Alkenes and Alkynes
Hydration of Alkynes
11.5 Reactions Involving Alkenes and Alkynes
Hydration of an alkyne is a more complex process
The initial product is not stable
Enol produced – both an alkene and an alcohol
Product is rapidly isomerized
Final product is either
Aldehyde
Ketone

32. 11.5 Reactions Involving Alkenes and Alkynes
Hydrohalogenation
An alkene can be combined with a hydrogen halide such as HBr or HCl
The reaction product is an alkyl halide
Markovnikov’s Rule is followed in this reaction
11.5 Reactions Involving Alkenes and Alkynes

33. 11.5 Reactions Involving Alkenes and Alkynes
Alkene Reactions
Predict the major product in each of the following reactions
Name the alkene reactant and the product using IUPAC nomenclature
11.5 Reactions Involving Alkenes and Alkynes

34. Addition Polymers of Alkenes
Polymers are macromolecules composed of repeating units called monomers
Polymers can be made up of thousands of monomers linked together
Many commercially important materials are addition polymers made from alkenes and substituted alkenes
Addition polymers are named for the fact that they are made by the sequential addition of the repeating alkene monomer
11.5 Reactions Involving Alkenes and Alkynes

35. Some Important Addition Polymers of Alkenes
11.5 Reactions Involving Alkenes and Alkynes

36. 11.6 Aromatic Hydrocarbons
Benzene’s structure was first proposed 150 years ago
A cyclic structure for benzene, C6H6
Something special about benzene
Although his structures showed double bonds, the molecule did not react as if it had any unsaturation
Originally named aromatic compounds for the pleasant smell of resins from tropical trees (early source)
Now aromatic hydrocarbons are characterized by a much higher degree of chemical stability than predicted by their chemical composition
Most common group of aromatic compounds is based on the 6-member aromatic ring, benzene

37. 11.6 Aromatic Hydrocarbons
Benzene Structure
The benzene ring consists of:
Six carbon atoms
Joined in a planar hexagonal arrangement
Each carbon is 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 to the true structure
11.6 Aromatic Hydrocarbons

38. Benzene Structure – Modern
Modern concept of benzene structure is based on overlapping orbitals
Each carbon is bonded to two others by sharing a pair of electrons
These same carbon atoms also each share a pair of electrons with a hydrogen atom
Remaining 6 electrons are located in p orbitals that are perpendicular to the plane of the carbon ring
These p orbitals overlap laterally
Form a cloud of electrons above and below the ring
11.6 Aromatic Hydrocarbons

39. Pi Cloud Formation in Benzene
The current model of bonding in benzene
11.6 Aromatic Hydrocarbons

40. 11.6 Aromatic Hydrocarbons
IUPAC Names: Benzenes
Most simple aromatic compounds are named as derivatives of benzene
For monosubstituted benzenes, name the group and add “benzene”
11.6 Aromatic Hydrocarbons
nitrobenzene
chlorobenzene
ethylbenzene

41. 11.6 Aromatic Hydrocarbons
IUPAC Names: Benzenes
For disubstituted benzenes, name the groups in alphabetical order
The first named group is at position 1
If a “special group” is present, it must be number 1 on the ring
An older system of naming indicates groups using
ortho (o) = 1,2 on the ring
meta (m) = 1,3 on the ring
para (p) = 1,4 on the ring
11.6 Aromatic Hydrocarbons

42. IUPAC Names of Substituted Benzenes
11.6 Aromatic Hydrocarbons
1-bromo-2-ethylbenzene
o-bromoethylbenzene
3-nitrotoluene
m-nitrotoluene
1,4-dichlorobenzene
p-dichlorobenzene

43. Historical Nomenclature
Some members of the benzene family have unique names acquired before the IUPAC system was adopted that are still frequently used today
11.6 Aromatic Hydrocarbons

44. Benzene As a Substituent
When the benzene ring is a substituent on a chain (C6H5), it is called a phenyl group
Note the difference between
Phenyl
Phenol (a functional group)
11.6 Aromatic Hydrocarbons
4-phenyl-1-pentene

45. Polynuclear Aromatic Hydrocarbons
Polynuclear aromatic hydrocarbons (PAH) are composed of two or more aromatic rings joined together
Many have been shown to cause cancer

46. 11.6 Aromatic Hydrocarbons
Reactions of Benzene
Benzene does not readily undergo addition reactions
Benzene typically undergoes aromatic substitution reactions:
An atom or group substitutes for an H on the ring
All benzene reactions we consider require a catalyst
The reactions are:
Halogenation
Nitration
Sulfonation
11.6 Aromatic Hydrocarbons

47. 11.6 Aromatic Hydrocarbons
Benzene Halogenation
Halogenation places a Br or Cl on the ring
The reagent used is typically Br2 or Cl2
Fe or FeCl3 are used as catalysts
11.6 Aromatic Hydrocarbons

48. 11.6 Aromatic Hydrocarbons
Benzene Nitration
Nitration places the nitro group on the ring
Sulfuric acid is needed as a catalyst
11.6 Aromatic Hydrocarbons

49. 11.6 Aromatic Hydrocarbons
Benzene Sulfonation
Sulfonation places an SO3H group on the ring
Concentrated sulfuric acid is required as a catalyst
This is also a substitution reaction
11.6 Aromatic Hydrocarbons

50. 11.7 Heterocyclic Aromatic Compounds
Rings with at least one atom other than carbon as part of the structure of the aromatic ring
This hetero atom is typically O, N, S
The ring also has delocalized electrons
The total number of atoms in the ring is typically either:
A six membered ring
Some have a five membered ring

51. Heterocyclic Aromatics
Heterocyclic aromatics are similar to benzene in stability and chemical behavior
Many are significant biologically
Found in
DNA and RNA
11.7 Heterocyclic Aromatic Compounds
Found in hemoglobin
and chlorophyll

52. Reaction Schematic Alkene + HX + H2O + H2 Hydrohalogenation Hydration
acidic
+ H2
Pt, Pd, or Ni
Hydrohalogenation
Hydration
+ X2
adds easily
Hydrogenation
Halogenation

53. Summary of Reactions 1. Addition Reactions of Alkenes
a. Hydrogenation
b. Hydration
c. Halogenation
d. Hydrohalogenation
2. Addition Polymers of Alkenes
3. Reactions of Benzene
a. Halogenation
b. Nitration
c. Sulfonation

54. Diagrammatic Summary of Reactions