Alcohols, carboxylic acids and esters

Alcohols, carboxylic acids and esters are three homologous series that contain oxygen. Alcohols have the –OH group, carboxylic acids the –COOH group and esters the –COO– group. Ethanol is made by fermenting sugar or from ethene, and can be oxidised to ethanoic acid (vinegar). An alcohol and a carboxylic acid react to make a sweet-smelling ester and water.

Key facts

  • Alcohols contain –OH: methanol CH3OH, ethanol C2H5OH, propanol C3H7OH, butanol C4H9OH.
  • Alcohols burn to give carbon dioxide and water, react with sodium to give hydrogen, and are oxidised to carboxylic acids.
  • Fermentation: yeast turns sugar into ethanol and carbon dioxide, warm (about 30 °C) and without air: C6H12O6 → 2C2H5OH + 2CO2.
  • Carboxylic acids contain –COOH: methanoic, ethanoic, propanoic and butanoic acid. They are weak acids and react with carbonates to give carbon dioxide.
  • Alcohol + carboxylic acid ⇌ ester + water, with concentrated sulfuric acid as a catalyst. Ethanol + ethanoic acid → ethyl ethanoate.
  • Esters are volatile with fruity smells: they are used in perfumes and flavourings.

Worked examples

Example 1

Draw ethanol and ethanoic acid and name their functional groups.

  1. Ethanol has two carbons and an –OH group on the end: it is an alcohol. C2H5OH
  2. Ethanoic acid has two carbons; the second carbon has a double-bonded O and an –OH. Together they are –COOH. CH3COOH
  3. Oxidising ethanol turns its –CH2OH end into –COOH, which is how wine turns to vinegar.

Answer: ethanol: –OH (hydroxyl); ethanoic acid: –COOH (carboxyl)

Example 2

Name the ester made from methanol and propanoic acid.

  1. The alcohol gives the first word, ending in "-yl": methanol → methyl.
  2. The acid gives the second word, ending in "-anoate": propanoic acid → propanoate.
  3. So the ester is methyl propanoate. Water is the other product.

Answer: methyl propanoate

Example 3

A student adds sodium carbonate to ethanoic acid and to ethanol. What would be seen?

  1. Ethanoic acid is a carboxylic acid, a weak acid.
  2. Acids react with carbonates: fizzing as carbon dioxide is given off. acid + carbonate → salt + water + CO2
  3. Ethanol is not an acid, so nothing is seen.

Answer: fizzing (carbon dioxide) only with ethanoic acid