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Alcohols

Organic Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable
Low
Stable6%

Fermentation vs hydration routes and uses of ethanol are commonly tested.

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:

CnH2n+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)
Ethanol molecule labelled to show the −OH hydroxyl functional group and the carbon side chain (R)
Source: Alcohols by Save My Exams

The first four alcohols

The members of the series most commonly tested:

nNameMolecular formulaState at room temperature
1MethanolCH3OHLiquid
2EthanolC2H5OHLiquid
3Propan-1-olC3H7OHLiquid
4Butan-1-olC4H9OHLiquid

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

Displayed formula of ethanol, C₂H₅OH, showing the hydroxyl (−OH) functional group on the end carbon
Source: Ethanol (displayed formula) by Save My Exams

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