Chemistry » Organic Molecules » IUPAC Naming And Formulae

# Naming Alkenes

## Naming Hydrocarbons: Naming Alkenes

The suffix for an alkene is -ene.

## Example:

### Question

Give the IUPAC name for the following compound:

### Identify the functional group

The compound has a double carbon-carbon bond and is an alkene. It will have the suffix -ene.

### Find the longest carbon chain containing the functional group

The functional group is a double bond, so the longest chain must contain the double bond. There are four carbon atoms in the longest chain and so the prefix for this compound will be but-.

### Number the carbon atoms

Remember that the carbon atoms must be numbered so that the functional group is at the lowest numbered carbon atom possible. In this case, it doesn’t matter whether we number the carbons from the left to right, or from the right to left. The double bond will still fall between the second and third carbon atoms.

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

There are no branched groups in this molecule.

### Combine the elements of the name into a single word in the following order: branched groups; prefix; name ending according to the functional group and its position along the longest carbon chain

The name of this compound is but-2-ene or 2-butene.

## Example:

### Question

Draw the structural and molecular formula for the organic compound

3-methylbut-1-ene

### Identify the functional group

The suffix -ene means that this compound is an alkene and there must be a double bond in the molecule. The number 1 immediately before the suffix means that the double bond must be at the first carbon in the chain (but-1-ene).

### Determine the number of carbon atoms in the longest chain containing the functional group

The prefix for the compound is but- so there must be four carbons in the longest chain containing the double bond.

### Look for any branched groups

There is a methyl group at the third carbon atom in the chain. Count from the left so that the double bond carbon is the first carbon atom.

### Reduce the structural formula to the molecular formula

There are 5 carbon atoms and 10 hydrogen atoms so the molecular formula is $$\text{C}_{5}\text{H}_{10}$$.

(Remember that there is no structural information given by the molecular formula)

## Example:

### Question

Give the IUPAC name for the following compound:

### Identify the functional group

The compound is an alkene and will have the suffix -ene. There is a double bond between the first and second carbons and also between the third and fourth carbons. The organic compound therefore contains ‘1,3-diene’.

### Find the longest carbon chain containing the functional group, and number the carbon atoms

Remember that the main carbon chain must contain both the double bonds. There are four carbon atoms in the longest chain containing the double bonds and so the prefix for this compound will be but-. The carbon atoms are already numbered 1 to 4 in the diagram.

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

There is an ethyl group on the second carbon.

Note that if we had numbered from the right to left the suffix would still have been 1,3-diene, however the ethyl group would have been on the third carbon. So we had to number left to right.

### Combine the elements of the name into a single word in the following order: branched groups; prefix; name ending according to the functional group and its position along the longest carbon chain

The name of this compound is 2-ethylbut-1,3-diene.

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