Chemistry » Organic Molecules » IUPAC Naming And Formulae

# Naming Hydrocarbons

## Naming Hydrocarbons: Naming Alkanes

The suffix for an alkane is -ane.

## Example:

### Question

Give the IUPAC name for the following compound:

Note: The numbers attached to the carbon atoms would not normally be shown. The carbon atoms have been numbered to help you to name the compound.

### Identify the functional group

The compound is a hydrocarbon with single bonds between the carbon atoms. It is an alkane and will have a suffix of -ane.

### Find the longest carbon chain

There are four carbon atoms in the longest chain. The prefix of the compound will be but-.

### Number the carbon atoms in the longest chain

The numbering has been done for you here.

### Look for any branched group, name them and give their position on the carbon chain

There are no branched groups in this compound.

### Combine the elements of the name into a single word

The name of the compound is butane.

## Example:

### Question

Give the IUPAC name for the following compound:

### Identify the functional group

The compound is a hydrocarbon with single bonds between the carbon atoms. It is an alkane and will have the suffix -ane.

### Find the longest carbon chain

There are three carbon atoms in the longest chain. The prefix for this compound is prop-.

### Number the carbons in the carbon chain

If we start at the carbon on the left, we can number the atoms as shown in red (left). If we start at the carbon on the right, we can number the atoms as shown in blue (right).

### Look for any branched groups, name them and give their position on the carbon chain

There is a branched group attached to the second carbon atom. In this case the methyl group is on carbon 2 regardless of which side you number the longest chain from.

This group has the formula $$\text{CH}_{3}$$, which is methane without a hydrogen atom. However, because it is not part of the main chain, it is given the suffix -yl (i.e. methyl). The position of the methyl group comes just before its name (see the next step).

### Combine the elements of the compound’s name into a single word in the order of branched group; prefix; name ending according to the functional group

The compound’s name is 2-methylpropane.

## Example:

### Question

Give the IUPAC name for the following compound:

$$\text{CH}_{3}\text{CH}(\text{CH}_{3})\text{CH}(\text{CH}_{3})\text{CH}_{3}$$

(Remember that the side groups are shown in brackets after the carbon atom to which they are attached).

### Draw the structural formula from its condensed structural formula

The structural formula of the compound is:

### Identify the functional group

The compound is a hydrocarbon with single bonds between the carbon atoms. It is an alkane and will have the suffix -ane.

### Find the longest carbon chain

There are four carbon atoms in the longest chain. The prefix for this compound is but-.

### Number the carbons in the carbon chain

If we start at the carbon on the left, we can number the atoms as shown in red (left). If we start at the carbon on the right, we can number the atoms as shown in blue (right).

### Look for any branched groups, name them and give their position on the carbon chain

There are two methyl groups attached to the main chain. The first one is attached to the second carbon atom and the second methyl group is attached to the third carbon atom. Notice that in this example it does not matter how you have chosen to number the carbons in the main chain; the methyl groups are still attached to the second and third carbon atoms and so the naming of the compound is not affected.

This group will be 2,3-dimethyl-

### Combine the elements of the compound’s name into a single word in the order of branched groups; prefix; name ending according to the functional group

The compound’s name is 2,3-dimethylbutane.

## Example:

### Question

Give the IUPAC name for the following compound:

### Identify the functional group

The compound is a hydrocarbon with single bonds between the carbon atoms. It is an alkane and will have the suffix -ane.

### Find the longest carbon chain and number the carbons in the longest chain

There are six carbons in the longest chain if they are numbered as shown in red (on the left). There are only five carbon atoms if they are numbered as shown in blue (right). Therefore, the red numbering (on the left) is correct and the prefix for the compound is hex-.

### Look for any branched groups, name them and give their position on the carbon chain

There is one methyl group attached to the main chain. If we number as shown in red (on the left) the methyl is attached to the fourth carbon atom. If we number as shown in blue (on the right) the methyl is attached to the third carbon atom.

After functional groups, the branched groups should have the lowest numbers possible. Therefore the blue numbering (on the right) is correct. The methyl is attached to the third carbon atom (3-methyl).

### Combine the elements of the compound’s name into a single word in the order of branched groups; prefix; name ending according to the functional group

The compound’s name is 3-methylhexane.

## Example:

### Question

Draw the semi-structural structural and condensed structural formula for the organic compound 2,2,4-trimethylhexane

### Identify the functional group

The name ends in -ane therefore the compound is an alkane.

### Determine the number of carbon atoms in the longest chain

The longest chain has the prefix hex-. There are therefore 6 carbon atoms in the longest chain.

### Look for any branched groups and place them on the structure

The compound is 2,2,4-trimethylhexane. Therefore there are three branched groups. Two on carbon 2 and one on carbon 4.

### Combine this information and add the hydrogen atoms

Carbon atoms can have four single bonds. Therefore wherever a carbon atom has less than four bonds draw in hydrogen atoms until there are four bonds.

### Condense the structural formula

First condense the main chain: $$\text{CH}_{3}\text{CCH}_{2}\text{CHCH}_{2}\text{CH}_{3}$$

Then add the side chains (in brackets) on the relevant carbon atoms:

$$\text{CH}_{3}\text{C}(\text{CH}_{3})_{2}\text{CH}_{2}\text{CH}(\text{CH}_{3})\text{CH}_{2}\text{CH}_{3}$$

## Optional Activity: Building hydrocarbons

An example of ethane, ethene and ethyne built with atomic model kits is given in the picture below:

1. Using atomic model kits, build the molecules of methane, propane, butane, pentane and octane. If you don’t have atomic model kits, jelly tots (or playdough) and toothpicks will work just as well. Use one colour jelly tot for the carbon atoms and one for the hydrogen atoms.

• Remember that carbon atoms should have four bonds and hydrogen atoms can have only one. You should see that all these compounds have a similar formula, remember they all have the general formula $$\text{C}_{\text{n}}\text{H}_{2\text{n}+2}$$.
• What is the name of the homologous series that all these molecules belong to?
2. Build the molecules of prop-1-ene, but-1-ene, pent-1-ene and oct-1-ene. Use two toothpicks to represent a double bond. You should see that all these compounds have a similar formula. Remember, they all have the general formula $$\text{C}_{\text{n}}\text{H}_{2\text{n}}$$.

• Try placing the double bond at different positions within the molecule. Does this make any difference to the total number of carbon and hydrogen atoms in the molecule?
• What is the name of the homologous series that all these molecules belong to?
3. Build the molecules of prop-1-yne, but-1-yne, pent-1-yne and oct-1-yne. Use three toothpicks to represent a triple bond. You should see that all these compounds have a similar formula. Remember, they are all alkynes.

• Try placing the triple bond at different positions within the molecule. Does this make any difference to the total number of carbon and hydrogen atoms in the molecule?
• What is the general formula for the alkynes?

[Attributions and Licenses]

This is a lesson from the tutorial, Organic Molecules and you are encouraged to log in or register, so that you can track your progress.