Chapter 13 – Unsaturated Hydrocarbons
Physical properties – Similar to saturated hydrocarbons
Chemical properties 1.More reactive than saturated hydrocarbons
2.The carbon-carbon double or triple bonds are the reactive sites (In most cases
we will be working with double bonds)
Functional groups – The part of the molecule where most of its chemical reactions occur
Multiple Bonds
Carbon-carbon multiple bonds
1.There are two types of bonds in carbon-carbon multiple bonds
a. Sigma bonds () – A covalent bond in which atomic orbital
overlap occurs along the axis joining the two bonded carbons
b. Pi bonds () – A covalent bond in which atomic orbital overlap
occurs above and below, but not on, the internuclear axis.


Occurrence of  and  bonds
1.When a single bond is present between two atoms, that bond is always a
-bond.
2.When a double bond is present between two atoms, that bond consists of
one -bond and one -bond.
3.When a triple bond is present between two atoms, that bond always
consists of one -bond and two -bonds.
Importance of -bonds
1.A carbon-carbon -bond is weaker, consequently more reactive
2.The presence of the -bond causes the bond to be structurally rigid.
There is no free rotation.
3.The -bond must be broken for rotation to occur.
Classes of Unsaturated Hydrocarbons
1.
Alkenes – An acyclic hydrocarbon
double bonds
2.
Alkynes – An acyclic hydrocarbon
triple bonds
3.
Aromatic – A cyclic hydrocarbon
three carbon-carbon double bonds.
* known as a benzene ring.
with one or more carbon-carbon
with one or more carbon-carbon
(usually) six*-carbon ring containing
Alkenes: An alkene can be formed by removing a hydrogen atom from two adjacent
carbons in a carbon chain.
Ex:
Hexane
-C—C—C—C—C—C- becomes

Hexene
-C—C—C=C—C—C- (3-Hexene)


Ex:
Ethane
Ethene
-C-C- becomes
-C=C- (also known as ethylene

Ex.:Cycloalkenes
C---C




cyclohexene
C
C
C---C
Nomenclature of Alkenes
1.Select the parent carbon chain with the longest chain of carbon atoms that
contains the double bond.
2.Replace the alkane suffix –ane with –ene to indicate the presence of a double
bond.
3.Number the carbon chain starting with the end of the chain that has the closest
double bond.
4.Indicate location of the double bond using the lowest carbon number of the
carbons associated with the double bond.
5.If more than one double bond is present use the suffixes diene, triene, tetraene,
ect. The associated carbon numbers are used to indicate the position of the double
bonds.
Ex.:
3-Pentene
1,3-Pentadiene
2,4,6-Octatriene
6-Methyl-2,4-octadiene
Nomenclature of Cycloalkenes
1.If there is only one double bond, its position does not need to be indicated. It is
assumed to be located between carbons one and two.
2.If there is more than one double bond in the ring, number the bond locations in a
manner that will give the lowest numbers.
3.In substituted cycloalkenes assign the numbers in a manner that will produce the
lowest combination of numbers.
Ex.:
Cyclopentene
3-Ethylcyclopentene
1,4-Cyclooctadiene
6-propyl-1,4-Cyclooctadiene
Alkenyl Groups: There are THREE important such groups:
Methylene (CH2=) methylidene
Vinyl (CH2=CH-) ethenyl
Ex. Vinyl chloride (CH2=CHCl)
Allyl (CH2=CH-CH2-) 2-propenyl
Structural Isomerism
1.Structural isomer can occur as they do with alkanes
•Positional: 1-butene vs. 2-butene
•Skeletal: 1-butene vs. 2-methylpropene
2.The carbon-carbon double bond allows the formation of two additional types of
isomers, Cis-and Trans- isomers
a)The carbons must have two different types of groups attached to them
A hydrogen functional group
A carbon containing group or a halogen
b)To determine whether cis or trans occurs draw the molecule and examine
the shape.
Ex.: 2-butene
Ex.: Retinal/Opsin
Examples of Structural Isomers
Trans-3-Methyl-3-hexene
Cis-2-Pentene
Trans-2-Pentene
CH3 CH2—CH3
\ /
C=C
/
\
H
H
Cis-1-chloro-1-pentene
Occurrence
Natural
Pheromones
Terpenes (plant odors & fragrances)
Contain 2 or more isoprene units (2-methyl-1,3-butadiene)
Synthetic
Dehydrogenation of Alkanes (at high temperature and in absence of O2)
Ethane ---> Ethene + H2
Physical Properties
Solubility
Insoluble in water
Soluble in nonpolar solvents
Less dense than water
Lower melting point than alkanes
Physical states similar to alkanes
C1 to C5 = gas; C6 to C17 = liquid; > C17 = solid
Chemical Reactions
Addition
Symmetrical: -C=C- + X2 --> X-C-C-X
Hydrogenation - results in formation of alkane
Halogenation*
Unsymmetrical: -C=C- + AB --> A-C-C-B
Hydrohalogenation
Hydration - results in formation of alcohol
Markovnikov’s rule: (“rich get richer”) Hydrogen goes to C with most
hydrogens.
Polymerization: multiple simple molecules (monomers) add together to form a
single, larger molecule (polymer)
These are usually catalyzed reactions!
Addition polymers
C=C + C=C + C=C --> C-C-C-C-C-C (polyethylene)
(C-C)n
Substituted-ethene addition polymers
nC=C-X --> (C-C-X)n
Butadiene-based addition polymers
Ex.: natural rubber
Much more flexible than other polymers
Addition Copolymers (two different monomers)
Ex.: Saran wrap
Alkynes
Formation is similar to that of alkenes (more hydrogens are removed)
Ethyne = Acetylene
Naming: same rules as for alkenes
Isomerism: cis-trans NOT possible
Linear geometry around the triple bond
Properties & Reactions are similar to those of alkenes
Alkenynes
Hydrocarbons with both double & triple bonds.
Naming: Double bond has priority
#ing Carbons: from end closest to a multiple bond.
Aromatics
Unsaturated cyclic hydrocarbons which do not readily undergo addition
reactions.
Benzene: the foundation molecule
Contains both localized and delocalized bonds
Naming Benzene Derivatives
One substituent derivatives: Use IUPAC system
Ex.: methylbenzene
BUT, several of these are considered new Parent molecules:
Toluene
Styrene
Phenol
Two substituent derivatives: Use the following prefixes to indicate substituent
position:
Ortho (1,2)
Meta (1,3)
Para (1,4)
Xylene (dimethylbenzene)
p-dichlorobenzene
Occurances
Coal Tar
Petroleum
Synthetic
Ex.: C6H14 ---> Toluene + 4H2
Physical Properties & Chemical Reactions
Good solvent for non-polar molecules!
Alkylation reactions:
Benzene + R-Cl --->
Halogenation:
Benzene + Cl2 --->
Fused-Ring Aromatics
Naphthalene
Carcinogenic Fused-ring aromatics:
4+ fused rings
Same “angle” in ring series
Form when hydrocarbons are heated to high temperatures
Unsaturated Hydrocarbons: What do you need to know?
Structural characteristics (know the functional group)
Alkene
Alkyne
Aromatic
Nomenclature (the rules for naming the molecules)
Physical and Chemical properties (basic/simple)
Occurrence
and uses (common)
Preparation
(what basic reactions produce the molecules)
Characteristic
reactions of the molecules