9/15/2009
Alkanes: Saturated Hydrocarbons
• Hydrocarbons are molecules composed of carbon & hydrogen
Organic Chemistry
– 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
SCH 3U Review
Normal vs Branched Alkanes
• NORMAL alkanes consist of
continuous chains of carbon
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
CH2
CH2
The First 10 “Normal” Alkanes
Name
•
•
•
•
•
•
•
•
•
•
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
Formula
M.P.
B.P.
# Structural Isomers
CH4
-183 -162
1
-130 36
-95
68
-91
98
-57 126
-54 151
-30 174
3
5
9
18
35
75
C H- C4 are
-172 Gases
-89
1
C1
CH
-187 -42
1
at Room
Temperature
CH
-138 0
2
2 6
3 8
4 10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
C5 - C16 are Liquids
at Room Temperature
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-methylbutane, C5H12
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
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Example 2: 3-ethyl-2,4-dimethylheptane
Example 1: 2,2-dimethylpentane
2
2
• 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
4
CH
CH
CH3
CH
CH2
CH2
CH2
5
CH3
7
CH3
• “2,4-dimethyl” tells us there are
TWO methyl branches on the
parent chain, at carbons #2 and #4.
CH3
CH3
1
3
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
• “3-ethyl-” tell us there is an ethyl
branch (2-carbon branch) on
CH3 carbon #3 of the parent chain.
Example 3: 2,3,3-trimethyl-4-propyloctane
Example 4: Name the molecules shown!
• The parent chain is indicated by
the ROOT of the name - “octane”.
This means there are 8 carbons
in the parent chain.
• parent chain has 5 carbons “pentane”
CH
CH2
• two methyl branches - start
CH3
CH
CH3
counting from the right - #2 and #3
3
5
4
2
6
7
1
8
CH3
2
3
5
4
6
7
1
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
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
n-propane
C3H8
• 2,3-dimethylpentane
• “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
H2C
60° bond angle
unstable!!
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
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
formula
and connectivity
but differ
their geometries.
• 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.
CH2
cyclopropane
C3H6
•Note that two hydrogen atoms were lost in forming the ring!
•What is the general formula for a cycloalkane?
Trans-1,2-dimethylcyclopentane
Cis-1,2-dimethylcyclopentane
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Cis-Trans Isomers - Examples
Alkenes & Alkynes
cis-1,3-dimethylcyclobutane
Cl
1-chloro-1-methylcyclohexane
cis-1,2-dichlorocyclohexane
2
CH2
6
H3C
CH
CH
3
1
CH2
CH3
5
Cl
Cl
4
trans-1-ethyl-2-methylcyclopropane
H2C
H2C
CH2
ethene
CH3
ethyne
propyne
propene
1-butene
H2
C
CH
C
2-pentyne
CH3 H3C
C
C
CH2
16
H3C
H2C
CH3
C
H
– 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
• 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
H
C
HC
• The suffix for the parent
chains are changed from
“ane” to “ene” and “yne”
Cis-Trans Isomerism…Again!
Alkenes & Alkynes: Examples
CH
• Alkenes are
hydrocarbons that
contain at least one
carbon-carbon double
bond
• Alkynes are
hydrocarbons that
contain at least one
carbon-carbon triple
bond
– 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
CH3
C
H
H3C
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
3