Organic Chemistry
Zumdahl Chapter 22
Alkanes: Saturated Hydrocarbons
• Hydrocarbons are molecules composed of carbon & hydrogen
– Each carbon atom forms 4 chemical bonds
– A saturated hydrocarbon is one where all C - C bonds are “single”
bonds & the molecule contains the maximum number of H-atoms
– Saturated hydrocarbons are called ALKANES
Normal vs Branched Alkanes
• NORMAL alkanes consist of
continuous chains of carbon
CH2
CH2
CH3
CH2
CH3
atoms
• Alkanes that are NOT
continuous chains of carbon
atoms contain branches
CH3
CH2
CH
CH3
• The longest continuous chain
of carbons is called the
CH3
parent chain
Structural Isomerism
• Structural isomers are
molecules with the same
chemical formulas but different
molecular structures - different
“connectivity”.
• They arise because of the many
ways to create branched
hydrocarbons.
• a.k.a. “Constitutional Isomers”
CH2
CH2
CH3
CH2
CH3
n-pentane, C5H12
CH3
CH2
CH3
CH
CH3
2-methlbutane, C5H12
The First 10 “Normal” Alkanes
•
•
•
•
•
•
•
•
•
•
Name
Formula
M.P.
B.P.
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
-183
-172
-187
-138
-130
-95
-91
-57
-54
-30
-162
-89
-42
0
36
68
98
126
151
174
# Structural Isomers
1
1
1
2
3
5
9
18
35
75
C1 - C4 are Gases
at Room Temperature
C5 - C16 are Liquids
at Room Temperature
IUPAC Rules for
Naming Branched Alkanes
– Find and name the parent chain in the hydrocarbon - this
forms the root of the hydrocarbon name
– Number the carbon atoms in the parent chain starting at
the end closest to the branching
– Name alkane branches by dropping the “ane” from the
names and adding “yl”. A one-carbon branch is called
“methyl”, a two-carbon branch is “ethyl”, etc…
– When there are more than one type of branch (ethyl and
methyl, for example), they are named alphabetically
– Finally, use prefixes to indicate multiple branches
Example 1: 2,2-dimethylpentane
2
4
• The parent chain is indicated by
CH2 5
CH2 3
1
the ROOT of the name CH3
CH2
“pentane”. This means there are CH3
5 carbons in the parent chain.
• “dimethyl” tells us that there are TWO
CH3
methyl branches on the parent chain.
A methyl branch is made of a single
carbon atom.
CH2
C
CH3
CH2
CH3
• “2,2-” tell us that BOTH methyl
CH3
branches are on the second carbon
atom in the parent chain.
4
1
3
5
Example 2: 3-ethyl-2,4-dimethylheptane
2
4
• The parent chain is indicated by
CH2 5
CH2 3
1
the ROOT of the name CH2
CH2
CH3
“heptane”. This means there are
7 carbons in the parent chain.
H2C 6
CH3
CH3
CH
CH
CH3
CH3
7
CH3
• “2,4-dimethyl” tells us there are
TWO methyl branches on the
parent chain, at carbons #2 and #4.
CH
CH2
CH2
• “3-ethyl-” tell us there is an ethyl
CH2
branch (2-carbon branch) on
CH3 carbon #3 of the parent chain.
Example 3: 2,3,3-trimethyl-4-propyloctane
• The parent chain is indicated by
the ROOT of the name - “octane”.
This means there are 8 carbons
in the parent chain.
3
5
4
2
6
7
1
8
CH3
2
1
3
5
4
6
7
• “2,3,3-trimethyl” tells us there are
CH3THREECmethyl branches
CH2 - one on
CH #2 andCH
CH2#3.
carbon
two on carbon
8
CH3
CH3
• “4-propyl-”
tellCH
us there
is CH
a propyl
2
2
branch (3-carbon branch)
CH2 on CH3
carbon #4 of the parent chain.
CH3
Example 4: Name the molecules shown!
CH3
• parent chain has 5 carbons “pentane”
CH
CH2
• two methyl branches - start
CH3
CH
CH3
counting from the right - #2 and #3
• 2,3-dimethylpentane
3
4
5
CH3
• parent chain has 8 carbons - “octane”
• two methyl branches - start counting
from the left - #3 and #4
• one ethyl branch - #5
• name branches alphabetically
5-ethyl- 3,4-dimethyl octane
HOMEWORK ASSIGNMENT
• Read Zumdahl section 22-1
– make notes on REACTIONS OF ALKANES and
on CYCLIC ALKANES
– Copy table 22.2 on page 1040
– Pay attention to sample exercises!
• Answer end-of-chapter problems:
• 15, 16, 17, 18
Alkanes Review - Cycloalkanes
• A cycloalkane is made of a hydrocarbon chain
that has been joined to make a “ring”.
H2
C
109.5° bond angle
CH2
CH3
CH3
n-propane
C3H8
H2C
60° bond angle
unstable!!
CH2
cyclopropane
C3H6
•Note that two hydrogen atoms were lost in forming the ring!
•What is the general formula for a cycloalkane?
Other Cycloalkanes
Angle
(ring) Strain - results from compression of
cyclobutane
the
bondbond
angles.angles
Cyclopropane has the
C4internal
H8 - ~90°
greatest angle strain (60° bond angles) while the
strain is eliminated in cyclohexane.
cylcopentane
C5H10 ~109.5° bond angles
Torsional Strain - a barrier to free rotation around
single bonds, due to the eclipsing of atoms in a
molecule. This results when atoms are brought
cyclohexane
too
close together in a particular conformation of
6H12 = 109.5° bond angles
aCmolecule.
Cycloalkanes: Cis-Trans Isomerism
• Consider
The Trans-isomer
1,2-dimethylcycloalkane
is the molecule with
- abranches
moleculeon
thatOPPOSITE
illustrates
GEOMETRIC
sides
of the ring
ISOMERISM - compounds with the same molecular
and connectivity
but differ
their geometries.
• formula
The Cis-isomer
is the molecule
within branches
on the SAME SIDE
• of
The
themolecule
ring. on the left shows the two methyl branches on
SIDES ofisthe
ring.
molecule on
the right shows
• OPPOSITE
Cis-Trans isomerism
one
typeThe
of geometric
isomerism
the two methyl branches on the SAME SIDE of the ring.
Trans-1,2-dimethylcyclopentane
Cis-1,2-dimethylcyclopentane
Cis-Trans Isomers - Examples
cis-1,3-dimethylcyclobutane
Cl
1-chloro-1-methylcyclohexane
cis-1,2-dichlorocyclohexane
2
CH2
6
H3C
CH
3
CH2
5
Cl
Cl
CH
1
CH3
4
trans-1-ethyl-2-methylcyclopropane
Cyclohexane - Boat & Chair Conformations
• Cyclohexane is NOT a planar molecule. To achieve its
109.5° bond angles and reduce angle strain, it adopts
several different conformations.
• The BOAT and CHAIR (99%) are two conformations
Alkenes & Alkynes
• Alkenes are
hydrocarbons that
contain at least one
carbon-carbon double
bond
• Alkynes are
hydrocarbons that
contain at least one
carbon-carbon triple
bond
• The suffix for the parent
chains are changed from
“ane” to “ene” and “yne”
– e.g. ethene, propyne
• Where it is ambiguous,
the BONDS are
numbered like branches
so that the location of
the multiple bond may
be indicated
Alkenes & Alkynes: Examples
H2C
CH
CH2
H2C
H
C
HC
ethene
CH3
ethyne
propyne
1-butene
H2
C
CH
C
CH3 H3C
propene
2-pentyne
C
C
CH2
16
H3C
H2C
C
H
CH3
Cis-Trans Isomerism…Again!
• Like rings, alkenes and alkynes show
restricted rotation - this time about the
multiple bonds
• Because of the 120° bond angles in
alkenes, cis-trans isomerism is possible
– If one of the carbons in the double bond
have two of the same substituents, there is
NO cis-trans isomerism!
– Remember that “trans” means opposite
sides of the double bond and “cis” means
the same side of the bond!
– Use molar mass to prioritize the substituents
to decide cis-trans isomerism
cis-2-butene
H
C
H
C
H3C
CH3
trans-2-butene
H
C
H3C
CH3
C
H
Name those alkenes...
H
C
H
C
cis-4-methyl-2-pentene
HC
CH CH
3
2-methyl-2-hexene
3
H3C
trans-2-bromo-3-methylBr 2-pentene
H2
C
H2cyclopentene
C
CH2
C
H
C
H
cis-3-heptene
Homework Assignment
• READ sections 22.2 (Alkenes & Alkynes), 22.3
(Aromatic Hydrocarbons) and 22.4 (The
Petrochemical industry)
– Don’t get hung up on “s and p bonds”
– Copy figure 22.11 and 22.12
– Summarize the use of ortho, meta, para prefixes when
naming benzene derivatives
– Make notes on section 22.4 - the Petrochemical Industry
• Complete Questions 19 - 25, 27, 29, 31, 32, 54 - 56