Chemistry » Organic Molecules » IUPAC Naming And Formulae

Naming Alcohols

Naming alcohols

The rules used to name the alcohols are similar to those already discussed for the hydrocarbons. The suffix of an alcohol is –ol (see the table in this lesson).

Example:

Question

Give the IUPAC name for the following organic compound

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Identify the functional group

The compound has an \(-\text{OH}\) (hydroxyl) functional group and is therefore an alcohol. The compound will have the suffix -ol.

Find the longest carbon chain containing the functional group

There are three carbon atoms in the longest chain that contains the functional group. The prefix for this compound will be prop-. As there are only single bonds between the carbon atoms, the prefix includes an to become propan-.

Number the carbons in the carbon chain

In this case, it doesn’t matter whether you start numbering from the left or right. The hydroxyl group will still be attached to second carbon atom (-2-ol).

Look for branched groups

There are no branched groups in this compound.

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 propan-2-ol or 2-propanol.

Example:

Question

Give the IUPAC name for the following compound:

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Identify the functional group

The compound has an \(-\text{OH}\) (hydroxyl) functional group and is therefore an alcohol. There are two hydroxyl groups in the compound, so the suffix will be -diol.

Find the longest carbon chain that contains the functional group

There are four carbon atoms in the longest chain that contains the functional group (but-) and only single bonds (an-). The prefix for this compound will be butan-.

Number the carbons in the carbon chain

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There are two hydroxyl groups attached to the main chain. If we number as shown in red (on the left) they are attached to the first and second carbon atoms. If we number as shown in blue (on the right) they are attached to the third and fourth carbon atoms.

The functional groups should have the lowest numbers possible. Therefore the red numbering is correct. The hydroxyl groups are attached to the first and second carbon atoms (1,2-diol).

Look for branched groups

There are no branched groups in this compound.

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 butan-1,2-diol.

Example:

Question

Draw the structural and condensed structural representations for the organic compound 4-ethyloctan-2,5-diol

Identify the functional group

The compound has the suffix -ol. It is therefore an alcohol.

Find the longest carbon chain that contains the functional group

The prefix is oct- therefore there are 8 carbons in the longest chain containing the functional group.

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Place the functional group as well as any branched groups

There is one \(-\text{OH}\) attached to carbon 2 and one attached to carbon 5. There is also an ethyl (\(-\text{CH}_{2}\text{CH}_{3}\)) branched group attached to carbon 4.

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Combine this information and add the hydrogen atoms

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Condense the structural formula

First condense the main chain: \(\text{CH}_{3}\text{CHCH}_{2}\text{CHCHCH}_{2}\text{CH}_{2}\text{CH}_{3}\)

Then add the side chains and alcohol functional groups (in brackets) on the relevant carbon atoms: \(\text{CH}_{3}\text{CH}(\text{OH})\text{CH}_{2}\text{CH}(\text{CH}_{2}\text{CH}_{3})\text{CH}(\text{OH})\text{CH}_{2}\text{CH}_{2}\text{CH}_{3}\)

Optional Activity: Building Alcohols

The structural representation of butan-1-ol built using an atomic model kit is given below:

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Using atomic model kits, build the molecules of methanol, ethanol, propan-1-ol, pentan-1-ol and octan-1-ol. If you don’t have an atomic model kit remember that you can use jelly tots (or playdough) and toothpicks. Use different colour jelly tots to represent the different atoms.

  • You should see that all these compounds have a similar formula. Remember, they belong to the homologous series of the alcohols. What is the general formula for this series?

  • Try placing the hydroxyl group at different positions within the molecule. Does this make any difference to the total number of carbon, hydrogen and oxygen atoms in the molecule?

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