Naming Alcohols (WJEC GCSE Chemistry): Revision Note
Names & Formulae of Alcohols
All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions
Alcohols are a homologous series of compounds that have the general formula CnH2n+1OH
They differ by one -CH2 in the molecular formulae from one member to the next
The -OH group in alcohols
Diagram of the side chain and -OH group in ethanol which characterises its chemistry
Examiner Tips and Tricks
Don’t confuse the -OH functional group with the hydroxide ion, OH–, they are not the same thing!
The first four alcohols
The names and structures of the first four alcohols are shown below
In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘ol’
Table to show the name, formula and displayed formula of the first four alcohols
Name | Formula | Displayed formula |
|---|---|---|
Methanol | CH3OH |  |
Ethanol | C2H5OH |  |
Propanol | C3H7OH |  |
Butanol | C4H9OH |  |
Positional Isomers of Alcohols
Higher Tier
You will have already looked at isomers in hydrocarbons and this also occurs in alcohols
Positional Isomers
Positional isomers arise from differences in the position of a functional group in each isomer
The functional group of an alcohol, –OH, can be located on different carbon atoms
For example, butan-1-ol and butan-2-ol
Butan-1-ol and butan-2-ol
In butan-1-ol the –OH functional group is on carbon number 1. In Butan-2-ol the –OH group is on carbon number 2
Chain isomerism
Chain isomerism is when compounds have the same molecular formula, but their longest hydrocarbon chain is not the same
This is caused by branching
For example butan-1-ol and 2-methylpropan-1-ol
One –CH3 group has moved positions to the second carbon atom
The longest chain now has 3 carbon atoms so the final part of the name will be propan-2-ol
As the –CH3 is on carbon 2, the name will start with 2-methyl
It is still an alcohol, as there is an –OH group on carbon 1, so the name is 2-methylpropan-1-ol
Butan-1-ol and 2-methylpropan-1-ol
A –CH3 group has been moved to the 2nd carbon in 2-methylpropan-1-ol. The number of carbon, hydrogen and oxygen atoms has not changed, C4H9OH
Examiner Tips and Tricks
Butan-1-ol and butan-4-ol are the same molecule, just flipped around, but we use the lowest numbers possible to name the molecule, therefore we use butan-1-ol. The same occurs with butan-2-ol and butan-3-ol.
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