Organic molecules

Methane (CH₄), as shown earlier, is the simplest alkane. The bonding in propane, the alkane containing three carbon atoms, is shown below using a dot diagram.

Propane (C₃H₈)

We can also represent the molecules with diagrams. Select the link below for an example as a line diagram.

Propane (C₃H₈)

These are two-dimensional diagrams, which show the structure and bonding clearly. However, in reality the molecules are three-dimensional, so they can be represented with models or three-dimensional representations. Select the link below for an example of the propane 3-D molecule.

Propane (C₃H₈)

You can use your molecular models to make this molecule. You can see from the model that single covalent bonds in the carbon chain are free to rotate. This means that the chain can form into a variety of shapes. This is important, especially when the carbon chain is longer and hydrocarbon molecules can tangle with each other.

The names of all alkanes end in 'ane'. The number of carbon atoms in the molecule is shown by the prefixes in the following table.

Branches on alkanes have names based on the number of carbon atoms in the branch, as shown by the table below.

If there is more than one of the above groups present in the molecule, this is shown by using the prefixes listed below.

The positions of the groups are identified by using numbers along the carbon chain in the molecule. To decide which end of the carbon chain to start numbering from you must number from the end that will ensure that the lowest possible numbers are required in the final name. This means that normally you number from the end closest to the groups or branches.

Once you have more than three carbon atoms in an alkane, there will be more than one way for the atoms to be arranged. Molecules that have the same formula, but a different structure are called structural isomers. Both of the molecules shown below have the formula of C₆H₁₄, but are clearly different structures.

Use your molecular models to make two versions of an alkane with the formula C₄H₁₀. Draw the structure of the two isomers into your notes and name them.

Click here to compare your notes

Properties of isomers

The physical properties of isomers will normally vary, often due to differences between the intermolecular forces that will occur between the molecules.

For example, the boiling point of hexane is 69° C compared with the boiling point of 3-methylpentane, which is 64° C.

1

Which of the following is the best explanation for the difference in boiling points of hexane and 3-methylpentane? a) The vapour pressure of the hexane is higher due to more carbon atoms in the longest chain of the molecule. b) The dispersion forces between molecules of hexane are stronger because there will be a greater available surface. c) There are fewer covalent bonds to break in 3-methylpentane than in hexane, so this compound evaporates more easily. d) The mass of the hexane is greater than the mass of the 3-methylpentane molecule, so more energy is required. Please select an answerIncorrect. A high vapour pressure will cause a lower boiling point.Well done! Can you draw a diagram to show this difference?Incorrect. Covalent bonds are not broken when a substance boils. Only the forces between the molecules are affected.Incorrect. The molecular masses of the two isomers must be the same because they have the same molecular formula. (Otherwise they would not be isomers.)

Ethene is a hydrocarbon that contains a double bond between carbon atoms; it is the simplest example of an alkene.

The bonding in the next alkene (propene) is shown below.

Propene (C₃H₈)

Just like with the alkanes, we can represent the molecules with diagrams. Select the link below for an example as a line diagram.

Ethene (C₂H₄)

Propene (C₃H₆)

Use your molecular models to make the alkenes shown above.

The names of all alkenes end in 'ene', and the number of carbon atoms in the molecule is shown by the same prefixes as used for the alkanes.

Alkenes follow the same naming rules as the alkanes.

Once you have more than three carbon atoms in the chain of an alkene, there will be more than one place where the double bond can be. An example of this is shown below, where there are two versions (isomers) of butane (C₄H₁₀) represented.

1-butene

But-2-ene

The position of the double bond is indicated by the number at the beginning of the name.

Geometric isomerism (cis/trans isomerism)

Look at the two molecules shown below.

The structure as defined by the bonds is the same, but the arrangement of the atoms on either side of the double bond is different. This type of isomerism is called geometric isomerism. The two forms are labelled as cis (meaning the same side) or trans (meaning across), and often referred to as cis/trans isomerism. The double bond between the two carbons cannot rotate, which means that two versions are different molecules.

To predict whether geometric isomerism is possible in a molecule you need to ask the following questions.

1. Is there a double bond between two carbon atoms?
2. Are there two different groups or atoms attached to the carbon at one end of the double bond?
3. Are there two different groups or atoms attached to the carbon at the other end of the double bond?

Only a 'yes' to all three questions will indicate that the molecule can exist as a cis and trans form.