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
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:
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
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.
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.
Combine this information and add the hydrogen atoms
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:
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.
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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?
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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?
This lesson is part of:
Organic Molecules