Naming Alcohols (WJEC GCSE Chemistry)

• 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 C_nH_2n+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

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

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