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Example of Structural Isomers of Alkanes: Butane
Butane is a saturated hydrocarbon, an alkane, with the molecular formula C4H10.
We can draw a structural formula for butane using the information in its name:
- "but" tells us there are 4 carbon atoms in a chain
- "ane" tells us there are only single bonds between the carbon atoms in the chain
Structural formula of butane is shown below:
|
H | |
|
H | |
|
H | |
|
H | |
|
H- |
C |
- |
C |
- |
C |
- |
C |
-H |
|
| H |
|
| H |
|
| H |
|
| H |
|
Count the number of carbon atoms in total (4), and the number of hydrogen atoms (10), so the molecular formula of butane is C4H10.
But this is NOT the only structure we could draw for a molecule with the molecular formula C4H10.
We could draw an alkane chain of 3 carbon atoms (propane), with a CH3 (methyl) side-chain (or branch) attached to the second carbon atom in the chain (C(2)), as shown below:
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|
|
H | |
|
|
|
|
H | |
|
H-C-H | |
|
H | |
|
H- |
C(1) |
- |
C(2) |
- |
C(3) |
-H |
|
| H |
|
| H |
|
| H |
|
The name of this branched-chain alkane is produced by adding the prefix 2-methyl to the name of the parent alkane (propane), that is, this molecule is 2-methylpropane.
Count the number of carbon atoms in total in the molecule (4), and the total number of hydrogen atoms (10), so the molecular formula of 2-methylpropane is C4H10.
Because butane and 2-methylpropane have the same molecular formula (C4H10) but different structural formula they are known as structural isomers.
Butane and 2-methylpropane are structural isomers.
Name |
Structural Formula |
Molecular Formula |
butane |
|
H | |
|
H | |
|
H | |
|
H | |
|
H- |
C |
- |
C |
- |
C |
- |
C |
-H |
|
| H |
|
| H |
|
| H |
|
| H |
|
|
C4H10 |
2-methylpropane |
|
|
|
H | |
|
|
|
|
H | |
|
H-C-H | |
|
H | |
|
H- |
C |
- |
C |
- |
C |
-H |
|
| H |
|
| H |
|
| H |
|
|
C4H10 |
Structural isomers of alkanes have:
Example of Structural Isomers of Alkanes: Pentane
Pentane is a saturated hydrocarbon with the molecular formula C5H12.
How many different structural formula can you draw for molecules with the molecular formula C5H12 ?
First, we could draw the structural formula of pentane which has 5 carbon atoms in a straight chain ("pent" = 5) and only single bonds between all the carbon atoms in the chain ("ane"):
|
H | |
|
H | |
|
H | |
|
H | |
|
H | |
|
H- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
-H |
|
| H |
|
| H |
|
| H |
|
| H |
|
| H |
|
Count the number of carbon atoms in total (5), and the total number of hydrogen atoms (12), so the molecular formula for pentane is C5H12.
What if we drew a chain of 4 carbon atoms and added a CH3 (methyl group) to the second carbon atom (C(2))in the chain ?
The structure of this molecule is shown below:
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|
|
H | |
|
|
|
|
|
H | |
|
H-C-H | |
|
H | |
|
H | |
|
H- |
C(1) |
- |
C(2) |
- |
C(3) |
- |
C(4) |
-H |
|
| H |
|
| H |
|
| H |
|
| H |
|
|
OR |
|
|
|
|
|
H | |
|
|
|
H | |
|
H | |
|
H-C-H | |
|
H | |
|
H- |
C(4) |
- |
C(3) |
- |
C(2) |
- |
C(1) |
-H |
|
| H |
|
| H |
|
| H |
|
| H |
|
|
The name of the parent alkane is butane because there are 4 carbon atoms in an alkane chain.
The methyl group (CH3) is located on the second carbon atom in the chain, so we use the prefix 2-methyl
The name of this branched-chain alkane is 2-methylbutane
Count the number of carbon atoms in total in a molecule of 2-methylbutane (5), and the total number of hydrogen atoms (12), so the molecular formula of 2-methylbutane is the same as that for pentane, that is, C5H12, therefore pentane and 2-methylbutane are structural isomers.
Can you draw another structural isomer of pentane, that is, a molecule with the molecular formula C5H12 but with a different structural formula from either pentane or 2-methylbutane?
Let's draw an alkane with 3 carbon atoms in the chain (propane), then add 2 methyl groups (CH3) to the second carbon atom in the propane chain (C(2))as shown below:
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|
|
H | |
|
|
|
|
H | |
|
H-C-H | |
|
H | |
|
H- |
C(1) |
- |
C(2) |
- |
C(3) |
-H |
|
| H |
|
| H-C-H |
|
| H |
|
|
|
|
| H |
|
|
|
The name of this branched-chain alkane is made up of the name of the parent alkane, propane, and a prefix showing the number (di = 2) and location (2,2-) of the both methyl groups on the chain.
The name of this molecule is 2,2-dimethylpropane
Count the total number of carbon atoms in a molecule of 2,2-dimethylpropane (5), and count the total number of hydrogen atoms in the molecule (12), so 2,2-dimethylpropane has the same molecular formula as pentane and 2-methylbutane, that is, C5H12.
The three structural isomers with the molecular formula C5H12 we have drawn are summarised in the table below:
name |
structural formula |
molecular formula |
pentane |
|
H | |
|
H | |
|
H | |
|
H | |
|
H | |
|
H- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
-H |
|
| H |
|
| H |
|
| H |
|
| H |
|
| H |
|
|
C5H12 |
2-methylbutane |
|
|
|
H | |
|
|
|
|
|
H | |
|
H-C-H | |
|
H | |
|
H | |
|
H- |
C |
- |
C |
- |
C |
- |
C |
-H |
|
| H |
|
| H |
|
| H |
|
| H |
|
|
C5H12 |
2,2-dimethylpropane |
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|
|
H | |
|
|
|
|
H | |
|
H-C-H | |
|
H | |
|
H- |
C |
- |
C |
- |
C |
-H |
|
| H |
|
| H-C-H |
|
| H |
|
|
|
|
| H |
|
|
|
|
C5H12 |
Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers because they have: