Alcohols

What an alcohol is

• Alcohols are a homologous series of organic compounds that contain the hydroxyl functional group, −OH
• The hydroxyl group is what fixes the chemistry of every alcohol — it can be oxidised, react with carboxylic acids to form esters, and is responsible for the alcohol being a polar molecule that dissolves in water
• The short alcohols are colourless liquids at room temperature; they mix fully with water and the resulting solution is neutral on the pH scale
• Methanol, ethanol, propanol and butanol — the first four members of the series — are all used either as fuels or as solvents
• General formula of the alcohol homologous series:

C_nH_2n+1OH

• The naming rule is the standard stem-plus-ending convention from topic 21: replace the final "-e" of the parent alkane with "-ol"
  • meth-an-e + -ol → methanol
  • eth-an-e + -ol → ethanol
  • prop-an-e + -ol → propanol (numbered as propan-1-ol or propan-2-ol when the position of −OH matters)

The first four alcohols

The members of the series most commonly tested:

All four are liquids at room temperature, unlike the smaller alkanes which are gases. The extra −OH group lets each molecule form hydrogen bonds with its neighbours, which raises the boiling point well above the equivalent-length alkane.

Position of the −OH on longer alcohols

• For propanol and onwards, the hydroxyl group can sit on different carbons in the chain; a number between the stem and the "-ol" shows which carbon
• Propan-1-ol has the −OH on carbon 1 (the end of the chain): CH₃CH₂CH₂OH
• Propan-2-ol has the −OH on carbon 2 (the middle): CH₃CH(OH)CH₃
• In exam answers, "propanol" and "butanol" without a number always mean the propan-1-ol and butan-1-ol isomers