Structural Isomers of Alkanes Chemistry Tutorial

Key Concepts

• Structural isomers are molecules with:

  ⚛ same molecular formula

  ⚛ different structural formula

• Structural isomers of alkanes have:

  ⚛ same number of atoms of carbon and hydrogen

  ⚛ different arrangement of carbon and hydrogen atoms

• The structural isomers of a particular alkane are different compounds so they have different chemical and physical properties.
• Structural isomers are also known as constitutional isomers.

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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:

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:

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.

Structural isomers of alkanes have:

• same number of atoms of carbon and hydrogen

  (same molecular formula)

• different arrangement of carbon and hydrogen atoms

  (different structural formula)

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"):

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:

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:

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:

Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers because they have:

• same number of atoms of carbon and hydrogen

  (same molecular formula)

• different arrangement of carbon and hydrogen atoms

  (different structural formula)

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