Unsaturated Hydrocarbons
Chapter 20
Hein * Best * Pattison * Arena
Version 1.0
Colleen Kelley
Chemistry Department
Pima Community College
1
© John Wiley and Sons, Inc.
Chapter Outline
20.1 Bonding in Unsaturated
Hydrocarbons
20.2 Nomenclature of
Alkenes
20.3 Geometric Isomerism in
Alkenes
20.4 Cycloalkenes
20.8 Physical and Chemical Properties
of Alkynes
20. 9 Aromatic Hydrocarbons:
Structure
20.10 Naming Aromatic Hydrocarbon
20.11 Polycyclic Aromatic
Hydrocarbons
20.5 Preparation and Physical
20.12 Sources and Physical
Properties of Alkenes
Properties of Aromatic
20.6 Chemical Properties of
Hydrocarbons
Alkenes
20.13 Chemical Properties of
20.7 Alkynes: Nomenclature and
Aromatic Hydrocarbons
Preparation
2
• Unsaturated hydrocarbons enhance our
lives in many ways:
1.
2.
3.
4.
Polyethylene plastic bags and bottles
Polystyrene Styrofoam cups
Plastic wraps
Essential oils in plants contain multiple bonds between
carbon atoms.
– Cosmetics, medicines, flavorings, perfumes
5. Hydrocarbons also form rings of carbon atoms
(aromatics)
– Detergents, insecticides, and dyes
3
Bonding in Unsaturated
Hydrocarbons
4
• The unsaturated hydrocarbons consist of
three families of homologous compounds
that contain multiple bonds between carbon
atoms.
• Alkenes contain carbon-carbon double bonds.
• Alkynes contain carbon-carbon triple bonds.
• Aromatic compounds contain benzene rings.
5
The four orbitals available for bonding in
alkenes are three sp2 orbitals and one p orbital.
Figure 20.1 Schematic hybridization of 2s22px12py1
orbitals of carbon to form three sp2 electron orbitals
and one p electron orbital
6
Figure 20.2 (a) A single sp2 electron orbital and (b) a
side view of three sp2 orbitals all lying in the same
plane with a p orbital perpendicular to the three sp2
7
orbitals.
• The carbon-carbon pi () bond is much weaker and,
as a consequence, much more reactive than the
carbon-carbon sigma () bond.
8
• The formation of a triple bond between carbon
atoms, as in acetylene, CHCH, may be visualized as
shown below.
• These pi bond electrons are not as tightly held by
the carbon nuclei as the sigma bond electrons.
Acetylene, consequently, is a very reactive
substance.
9
Nomenclature of Alkenes
The general formula for alkenes is:
CnH2n
10
IUPAC Rules for Naming Alkenes
1. Select the longest continuous carbon-carbon
chain that contains the double bond.
2. Name this parent compound as you would an
alkane, but change the –ane ending to –ene.
CH3CH2CH3 propane
CH3CH=CH2 propene
11
IUPAC Rules for Naming Alkenes
3. Number the carbon chain of the parent
compound starting with the end nearer to the
double bond. Use the smaller of the two
numbers on the double-bonded carbon atoms to
indicate the position of the double bond. Place
this number in front of the alkene name.
CH3CH=CHCH3 2- butene
CH3CH2CH2CH=CH2 1-pentene
12
IUPAC Rules for Naming Alkenes
4. Branch chains and other groups are treated as
in naming alkanes, by numbering and assigning
them to the carbon atom to which they are
bonded.
H2C
CH3
H3C
5
CH
4
H2
C
3
H
C
2
4-methyl-1-pentene
CH2
1
H3C
6
H2 H2
C
C CH
5 4
3
H2
C
CH3
H
C
2
CH2
1
3-propyl-1-hexene
13
How would we write the structural formula
for 4-methyl-2-pentene?
• The name indicates:
– Five carbons in the longest chain
– A double bond between carbons 2 and 3
– A methyl group on carbon 4
CH 3
H
C
H3C
CH
C
H
CH 3
14
Write a structural formula for:
7-methyl-2-octene
• Octene indicates an 8-carbon chain
• The chain contains a C=C between carbons
2 and 3.
• There is a –CH3 group on carbon 7
H2
C
H
C
H3C
C
H
H2
C
C
H2
CH 3
CH
CH 3
15
Name this compound:
H2
C
H3C
H2
C
C
CH 3
C
H2
CH 2
• Longest chain containing C=C is 5 carbons
• 2-ethyl-1-pentene
16
Geometric Isomerism
in Alkenes
17
Geometric Isomerism in Alkenes
• Compounds containing a carbon-carbon
double bond (pi bond) have restricted
rotation about that double bond.
• This restricted rotation in a molecule gives
rise to a type of isomerism known as
geometric isomerism.
• Isomers that differ from each other only in
the geometry of their molecules and not in
the order of their atoms are known as
geometric isomers.
– They are also called cis-trans isomers.
18
Geometric Isomers in Alkenes
Cl
Cl
C
H
Cl
H
C
C
H
cis-1,2-dichloroethene
(bp = 60.1 C)
Cl
C
H
trans-1,2-dichloroethene
(bp = 48.4 C)
19
An alkene shows cis-trans isomerism when each
carbon atom of the double bond has two
different kinds of groups attached to it.
20
An alkene does not show cis-trans isomerism
if one carbon of the double bond has two
identical groups attached to it.
21
Draw a structure for
cis-5-chloro-2-hexene
22
Is the compound below the cis or
trans isomer?
H3C
CH 3
C
H
C
H2C
CH 3
trans-3-methyl-2-pentene
23
Dienes
• Many compounds have more than one C=C.
• Compounds with two C=C are called dienes.
H
C
H2C
CH 2
C
H
• Compounds with
three C=C are called
1,3-butadiene
trienes.
24
Cycloalkenes
25
Cycloalkenes
• As the name implies, cycloalkenes are cyclic
compounds that contain a C=C in the ring.
• The carbons of the double bond are assigned
numbers 1 and 2.
cyclohexene
1
2
2
cyclopentene
1
26
Cycloalkenes
CH 3
CH 3
5
4
4
1
2
3
1-methylcyclopentene
3
2
5
6
1
CH 3
1,3-dimethylcyclohexene
27
Preparation and
Physical Properties of
Alkenes
28
Preparation of Alkenes
(ie, an alkene is a product of each)
•
•
•
•
•
Cracking
Dehydration of Alcohol
Dehydrohalogenation of Alkyl
Halides
Hydrogenation of alkynes
Dehalogenation of vicinal dihalides
29
Cracking
• Cracking, or pyrolysis, is the process in which saturated
hydrocarbons are heated to very high temperatures in the
presence of a catalyst (usually silica-alumina):
Heat
Alkane (CnH2n+2) 
Mixture of alkenes + Alkanes + H2 (g)
catalyst
~500°C
2CH3CH2CH3  CH3CH=CH2 + CH2=CH2 + CH4 + H2
30
Dehydration of Alcohols
Dehydration involves the elimination of a
molecule of water from a reactant molecule.
Ex:
31
Dehydrohalogenation of alkyl
halides
involves eliminating a hydrogen and a halogen
Ex
32
Hydrogenation of alkynes
involves adding hydrogen (H2) to an alkyne
Ex:
33
Dehalogenation of vicinal
dihalide
Ex:
involves removing a halogen molecule (X2)
34
Physical Properties of Alkenes
• Alkenes have physical properties very
similar to the corresponding alkanes.
35
Chemical Properties
of Alkenes
36
Addition Reactions of Alkenes
• Addition at the C=C bond is the most common
reaction of alkenes.
– H2
– Br2 and Cl2
– HBr, HCl
– H2SO4
– H20
37
Addition of H2
• Hydrogenation
H
H3C
C
H
C
CH3 + H
2
Pt, 25 C
1 atm
H3C
H
H
C
C
H
H
CH3
38
Addition of X2
• Bromination (Br2) or Chlorination (Cl2)
H3C
H
H
C
C
CH 3 + Br
2
H3C
H
H
C
C
Br
Br
CH 3
39
Addition of HX
• Hydrobromination (HBr) or Hydrochlorination (HCl)
H3C
H
H
C
C
CH3 + HCl
H3C
H
H
C
C
H
Cl
CH3
40
Addition of H2SO4
H3C
H
H
C
C
CH 3 +
HOSO3H
H3C
H
H
C
C
H
OSO3H
CH 3
41
Addition of H2O
H
H
H
H
C
C
H
OH
+
H3C
C
C
CH3 +
H2O
H
H3C
CH 3
42