GCSEChemistryWJECRevision Notes2. Chemical Bonding, Application Of Chemical Reactions & Organic Chemistry2.5 Crude Oil, Fuels & Organic ChemistryNaming Alcohols

Naming Alcohols (WJEC GCSE Chemistry)

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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

Alcohol- Functional Group

Diagram of the side chain and -OH group in ethanol which characterises its chemistry

Examiner Tip

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 methanol-
Ethanol C2H5OH screenshot-2024-02-18-191221
Propanol C3H7OH propanol-displayed
Butanol C4H9OH butanol

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

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

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 Tip

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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    Philippa

    Author: Philippa

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.