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)
So, common reactive sites are:
Multiple bond sites
Functional group sites
Multiple Bonds

Carbon-carbon multiple bonds (ex.: C2H4)
1. There are two types of bonds in carbon-carbon multiple
bonds



Occurrence of  and  bonds
1.
2.
3.

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.
When a single bond is present between two atoms, that bond is always
a -bond.
When a double bond is present between two atoms, that bond consists
of one -bond and one -bond.
When a triple bond is present between two atoms, that bond always
consists of one -bond and two -bonds.
Importance of -bonds
1.
2.
3.
A carbon-carbon -bond is weaker, consequently more reactive
The presence of the -bond causes the bond to be structurally rigid.
There is no free rotation.
The -bond must be broken for rotation to occur.
Classes of Unsaturated
Hydrocarbons
 1.
Alkenes – An acyclic hydrocarbon
with one or more carbon-carbon
double bonds (with one double bond : C H )
Alkynes – An acyclic hydrocarbon
with one or more carbon-carbon
triple bonds (with one triple bond : C H )
Aromatic – A cyclic hydrocarbon
six*-carbon (usually) ring containing
three carbon-carbon double bonds.
n
 2.
n
 3.
* known as a benzene ring (C6H6).
2n-2
2n
Alkenes
 An alkene can be formed by removing a hydrogen atom
from two adjacent carbons in a carbon chain.
 Ex:

Hexane
Hexene
-C—C—C—C—C—C-C—C—C=C—C—C-
 Ex:

Ethane
Ethene
-C-C- becomes
-C=C-
(also known as ethylene)
 Ex.:Cycloalkenes




C---C
cyclohexene
C
C
C---C
becomes
(3-Hexene)
In ethene, the atoms are
in a flat (planar) rather
than a tetrahedral
arrangement.
QuickTi me™ a nd a
Graphi cs decompre ssor
are need ed to se e th is p icture.
Ethene is the
compound that
causes tomatoes to
ripen.
 Bonding in Ethylene
 Bonding in Ethene
p
2 sp2
each carbon has an unhybridized 2 p orbital
axis of orbital is perpendicular to the plane of the  bonds
 Bonding in Ethylene
 Bonding in Ethene
p
2 sp2
side-by-side overlap of half-filled
p orbitals gives a bond
double bond in ethylene has a
 component and a  component
Top View C2H4
H
H
C
H
C
H
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.
2.
3.





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.
If there is more than one double bond in the ring, number the bond
locations in a manner that will give the lowest numbers.
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
(these are also known as stereoisomers)
a)
b)
The double bond restricts rotation around the C atoms.
The carbons must have two different types of groups
attached to them
*
*
c)
A hydrogen functional group
A carbon containing group or a halogen
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*
 Asymmetrical: -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.
A bromine in water
solution is reddish brown. When a
small amount of such a solution is
added to an unsaturated hydrocarbon,
the added solution is decolorized
.
Chemical Reactions
 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 (ex.: PVC)
 Butadiene-based addition polymers
 Ex.: natural rubber (2-methyl-1,3-butadiene; isoprene)
 Much more flexible than other polymers
 Addition Copolymers (two different monomers)
 Ex.: Saran wrap (1953) - polyvinylidene chloride
(2004) - polyethylene
Alkynes
 Formation is similar to that of alkenes
(more hydrogens are removed; higher
temp.)
 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
 Bonding in Acetylene
 Bonding in Acetylene
2p
2 sp
one  bond involves one of the p orbitals on each carbon
there is a second  bond perpendicular to this one
 Bonding in Acetylene
 Bonding in Acetylene
2p
2 sp
 Bonding in Acetylene
 Bonding in Acetylene
2p
2 sp
C2H2
H
C
C
H
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; bromobenzene
 BUT, several of these are considered
new Parent molecules:
 Toluene
 Styrene
 Phenol
Naming Benzene Derivatives
 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.: C7H16 ---> Toluene + 4H2
Physical Properties & Chemical
Reactions
 Good solvent for non-polar molecules!
 Alkylation reactions:
 Benzene + R-Cl --->
 Halogenation:
 Benzene + Cl2 --->
 Polymerization
 Styrene --> Polystyrene
 Largest Synthetic Molecule
Fused-Ring Aromatics
 Naphthalene
 Carcinogenic Fused-ring aromatics:
 4+ fused rings
 Same “angle” in ring series
 Form when hydrocarbons are heated to
high temperatures
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