Structural Isomers of Alkenes Chemistry Tutorial

Key Concepts

• Structural isomers are molecules with:

  ⚛ same molecular formula

  ⚛ different structural formula

• Structural isomers of alkenes have:

  ⚛ same number of atoms of carbon and hydrogen

  ⚛ different arrangement of carbon and hydrogen atoms

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

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Drawing the Structure of Isomers of Alkenes

If you were told that a particular hydrocarbon had the molecular formula C₄H₈, how many different 2-dimensional structural formula do you think you could draw?

First, you might recognise that the molecular formula C₄H₈ has the general formula of an alkene, C_nH_2n where n = 4.
So we could start by drawing a chain of 4 carbon atoms and placing a double bond at the beginning of the carbon chain as shown below:

The name of an alkene is made up of three parts:

• prefix: determined by number of carbon atoms in the chain

  (4 = but)

• infix: a number giving the location (locant) of the double bond

  Start numbering from the red C to give the double bond the lowest number (-1-)

• suffix: derived from the parent hydrocarbon

  The parent hydrocarbon is an alkene so the suffix is ene

Putting it all together, we can name this hydrocarbon but-1-ene (or 1-butene):

But this is NOT the only 2-dimensional structural formula we could have for the molecular formula C₄H₈.

We could move the double bond to the second carbon atom in the chain as shown below:

The number of carbon atoms in the chain is the same so the prefix remains unchanged (4 = but), there is still one double bond so the suffix is unchanged (ene), but now the infix (for the location of the double bond) is 2 instead of 1 (-2-).
This molecule is but-2-ene.

But-1-ene and but-2-ene are called structural isomers because they have:

• same molecular formula (C₄H₈)
• different structural formula (position of double bond is differrent)

There are other structural formula we could draw for the molecular formula C₄H₈.
For instance, what if the parent hydrocarbon was a 3 carbon chain with a double bond (propene), AND, it had a methyl group as a side-chain (or branch)?
Let's draw the 2-dimensional structural formula for this hydrocarbon:

Count the number of carbon atoms (4), and, the number of hydrogen atoms (8)
So, the molecular formula for this molecule is C₄H₈ and it will be a structural isomer of but-1-ene (1-butene) and but-2-ene (2-butene).

To name this branched-chain alkene:

1. Name the longest carbon chain containing the double bond to give the name of the parent alkene:3 = prop, parent akene is propene
2. Determine the infix (the locant or position of the double bond in the parent alkene): -1-
3. Name the parent alkene: prop-1-ene (or 1-propene)
4. Name the prefix for the CH₃ group (1 carbon in chain = methyl) including the locant (position) of this methyl group: prefix is 2-methyl
5. Construct the name of this alkene by attaching the prefix to the name of the parent alkene: 2-methylprop-1-ene (or 2-methyl-1-propene)

So far we have drawn 3 structural isomers for C₄H₈. There is another one. Consider the molecule drawn below:

Count the number of carbon atoms (4), and, the number of hydrogen atoms (8).
This molecule also has the molecular formula C₄H₈ but it has a very different structure to but-1-ene, but-2-ene, and 2-methylprop-1-ene.
This also a structural isomer of but-1-ene.

This is cyclic compound, the carbon atoms are joined to make a ring.
There are 4 carbon atoms in the ring and only single bonds between them, so the parent alkane is butane.
In order to differentiate this ring compound from the straight chain alkane butane, we write "cyclo" as a prefix in front of "butane", that is, the name of this molecule is cyclobutane.

Now that we have introduced one cyclic alkane, cyclobutane, you might see that there is another cyclic alkane you could draw with the same molecular formula, C₄H₈:

Methylcyclopropane also has the the molecular formula C₄H₈ but a different structure to but-1-ene, but-2-ene, 2-methylprop-1-ene and cyclcobutane, so it is also a structural isomer.

We have drawn 5 possible structural isomers with the molecular formula C₄H₈, as shown in the summary below:

But-1-ene (1-butene), but-2-ene (2-butene), 2-methylprop-1-ene (2-methylpropene), cyclobutane and methylcyclopropane are structural isomers.
They have:

• same molecular formula, C₄H₈
• different structural formulae

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