Loudon Ch. 2 Review: Alkanes
Jacquie Richardson, CU Boulder – Last updated 9/2/2020
The simplest organic molecules are hydrocarbons. These contain just carbon and hydrogen.
To be most stable, each C wants to have 4 total bonds and each H wants 1 bond. It’s
possible to link Cs together into long chains. For example, butane has four carbons.
Hydrocarbons can still have multiple bonds between carbons. This gives us a basis to
subdivide them into categories based on what types of bonds they contain: single, double, or
triple. Another category is aromatic hydrocarbons, which react very differently and will be
covered much later. C=C double bonds aren’t considered alkenes if they’re part of an
aromatic ring.
This chapter focuses only on alkanes. If all the carbons in an alkane are in a single,
unbranched line, it’s a normal or n-alkane. The names of these are listed below. These
names are enough to uniquely identify each compound, but if you want to explicitly show
that you’re talking about the normal alkane, you can put “n-” at the start of the name. For
example, five carbons in a row is pentane, but you can name it as n-pentane if you want to
be absolutely clear you’re discussing the linear form.
# of C Name Structure
1 Methane CH4
2 Ethane CH3-CH3
3 Propane CH3-CH2-CH3
4 Butane CH3-CH2-CH2-CH3
5 Pentane CH3-CH2-CH2-CH2-CH3
6 Hexane CH3-CH2-CH2-CH2-CH2-CH3
7 Heptane CH3-CH2-CH2-CH2-CH2-CH2-CH3
8 Octane CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3
9 Nonane CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
10 Decane CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
This is one example of a homologous series – each subsequent item in the list has one
more –CH2– unit (also called a methylene group). Usually all members of a homologous
series will have physical properties – melting point, boiling point, etc. – that vary in
predictable ways.
The more atoms there are in a molecule, the more work it takes to draw out the entire Lewis
dot structure. For this reason, there are several different valid ways to draw the same
compound, each one progressively more compact. We’ll use butane as an example.
• Molecular formula: C4H10 for butane. This one is not actually a unique identifier,
since for an alkane with 4 or more carbons, there can be multiple structures possible
with the same formula. An example is shown below.
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, Loudon Ch. 2 Review: Alkanes
Jacquie Richardson, CU Boulder – Last updated 9/2/2020
These are called structural isomers. The more carbons there are in a molecule, the
more structural isomers are possible.
• Structural formula: This is basically just the Lewis dot structure, with bonds indicated
by lines rather than two shared dots (like the two examples shown above). This gives
us enough information to tell structural isomers apart.
• Condensed structural formula: CH3-CH2-CH2-CH3 for butane. All H atoms are put
either right before or right after the carbon atom to which they’re attached.
• Highly condensed structural formula: CH3-(CH2)2-CH3 for butane. Any similar
groups like the methylenes in the middle of the chain are grouped together with
parentheses.
• Skeletal, line-bond or zigzag formula: for butane. In this system, every bend
or end of a line is a carbon, unless explicitly stated otherwise. Hydrogen atoms are
assumed to be present in the amount needed to fill octets, unless there’s a charge on
the atom that implies it does not have a filled octet. All heteroatoms (atoms other
than C and H) must be shown explicitly, and any C or H can also be shown explicitly
if desired. However, if an atom is shown explicitly, its H atoms must also be shown.
Note that we have a choice in how much to show explicitly, depending on what parts
of the molecule we’re focusing on. In the last example here, the H that’s shown
happens to be important for a particular reaction.
Carbon atoms can also be linked into a ring. These are called cycloalkanes (assuming they’re
all C & H with single bonds only) and are named after the alkane with the same number of
carbons, but with a “cyclo” prefix. (These are not aromatic rings though.)
Degrees of Unsaturation
For noncyclic alkanes, the formula is always CnH2n+2. For example, methane is CH4, ethane is
C2H6, and propane is C3H8. Each pair of missing H atoms from this formula is one
degree of unsaturation. These can be caused by two things: pi bonds or rings. Remember:
in any multiple bond between atoms, the first line of the bond is a sigma bond, and any
additional lines are pi bonds.
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Jacquie Richardson, CU Boulder – Last updated 9/2/2020
The simplest organic molecules are hydrocarbons. These contain just carbon and hydrogen.
To be most stable, each C wants to have 4 total bonds and each H wants 1 bond. It’s
possible to link Cs together into long chains. For example, butane has four carbons.
Hydrocarbons can still have multiple bonds between carbons. This gives us a basis to
subdivide them into categories based on what types of bonds they contain: single, double, or
triple. Another category is aromatic hydrocarbons, which react very differently and will be
covered much later. C=C double bonds aren’t considered alkenes if they’re part of an
aromatic ring.
This chapter focuses only on alkanes. If all the carbons in an alkane are in a single,
unbranched line, it’s a normal or n-alkane. The names of these are listed below. These
names are enough to uniquely identify each compound, but if you want to explicitly show
that you’re talking about the normal alkane, you can put “n-” at the start of the name. For
example, five carbons in a row is pentane, but you can name it as n-pentane if you want to
be absolutely clear you’re discussing the linear form.
# of C Name Structure
1 Methane CH4
2 Ethane CH3-CH3
3 Propane CH3-CH2-CH3
4 Butane CH3-CH2-CH2-CH3
5 Pentane CH3-CH2-CH2-CH2-CH3
6 Hexane CH3-CH2-CH2-CH2-CH2-CH3
7 Heptane CH3-CH2-CH2-CH2-CH2-CH2-CH3
8 Octane CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3
9 Nonane CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
10 Decane CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
This is one example of a homologous series – each subsequent item in the list has one
more –CH2– unit (also called a methylene group). Usually all members of a homologous
series will have physical properties – melting point, boiling point, etc. – that vary in
predictable ways.
The more atoms there are in a molecule, the more work it takes to draw out the entire Lewis
dot structure. For this reason, there are several different valid ways to draw the same
compound, each one progressively more compact. We’ll use butane as an example.
• Molecular formula: C4H10 for butane. This one is not actually a unique identifier,
since for an alkane with 4 or more carbons, there can be multiple structures possible
with the same formula. An example is shown below.
1
, Loudon Ch. 2 Review: Alkanes
Jacquie Richardson, CU Boulder – Last updated 9/2/2020
These are called structural isomers. The more carbons there are in a molecule, the
more structural isomers are possible.
• Structural formula: This is basically just the Lewis dot structure, with bonds indicated
by lines rather than two shared dots (like the two examples shown above). This gives
us enough information to tell structural isomers apart.
• Condensed structural formula: CH3-CH2-CH2-CH3 for butane. All H atoms are put
either right before or right after the carbon atom to which they’re attached.
• Highly condensed structural formula: CH3-(CH2)2-CH3 for butane. Any similar
groups like the methylenes in the middle of the chain are grouped together with
parentheses.
• Skeletal, line-bond or zigzag formula: for butane. In this system, every bend
or end of a line is a carbon, unless explicitly stated otherwise. Hydrogen atoms are
assumed to be present in the amount needed to fill octets, unless there’s a charge on
the atom that implies it does not have a filled octet. All heteroatoms (atoms other
than C and H) must be shown explicitly, and any C or H can also be shown explicitly
if desired. However, if an atom is shown explicitly, its H atoms must also be shown.
Note that we have a choice in how much to show explicitly, depending on what parts
of the molecule we’re focusing on. In the last example here, the H that’s shown
happens to be important for a particular reaction.
Carbon atoms can also be linked into a ring. These are called cycloalkanes (assuming they’re
all C & H with single bonds only) and are named after the alkane with the same number of
carbons, but with a “cyclo” prefix. (These are not aromatic rings though.)
Degrees of Unsaturation
For noncyclic alkanes, the formula is always CnH2n+2. For example, methane is CH4, ethane is
C2H6, and propane is C3H8. Each pair of missing H atoms from this formula is one
degree of unsaturation. These can be caused by two things: pi bonds or rings. Remember:
in any multiple bond between atoms, the first line of the bond is a sigma bond, and any
additional lines are pi bonds.
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