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Alkanoic acids

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Introduction to Chemical Formulae
Organic Chemistry | Alkanes | Alkenes | Alkynes | Alkanols
Alkanoic acids | Esters | Alkanals | Alkanones | Further information

Introduction to Alkanoic Acids

Alkanoic acids are also known as carboxylic acids and in biology as fatty acids. One alkanoic acid (ethanoic acid) is found in vinegar.

Alkanoic acids can be produced by the oxidisation of an alkanol1. For example, propan-1-ol can be oxidised to propanoic acid (an alkanoic acid) when heated with acidified sodium dichromate(VI) solution. Water is also produced in this reaction.

Structure of Alkanoic Acids

Alkanoic acids have, like alkanes a backbone of carbon atoms all bonded to each other by single, covalent bonds. The functional group, the arrangement of atoms that all alkanoic acids share, is a carboxyl group (COOH).

Any 'unused' bonds are filled with hydrogen atoms. For example, shown below is the alkanoic acid that contains three carbon atoms in its longest carbon chain.

H H O | | // H - C - C - C | | \ H H O - H

When writing the functional group as part of a chemical equation, it should be written as COOH (not CO2H).

Naming Alkanoic Acids

Alkanoic acids are named as follows:

  • The number of carbon atoms in the longest carbon atom chain is noted
  • The corresponding alkane is then named
  • Finally, the name of this corresponding alkane is modified by removing the ane and replacing it with anoic acid

Therefore, the alkanoic acid shown above has three carbon atoms in its longest carbon atom chain. The corresponding alkane is propane. The ane is replaced with anoic acid to give the alkanoic acid its name: propanoic acid.

Reactions of Alkanoic Acids

Despite being weak acids,2 alkanoic acids can react with many compounds in a similar way to dilute hydrochloric acid.

For example, when hydrochloric acid reacts with sodium hydroxide, sodium chloride (table salt) and water are formed:

HCl (aq) + NaOH (aq)→ NaCl (aq) + H2O (l)

In a similar way, when ethanoic acid reacts with sodium hydroxide, sodium ethanoate and water are formed:

CH3COOH (l) + NaOH (aq)→ NaCH3COO (aq) + H2O (l)

1The alkanol must have the hydroxyl group attached to carbon-12This is because an acid is only formed in the presence of water and, the more hydrogen ions in solution, the stronger the acid. Hydrogen chloride completely dissolves in water to form hydrochloric acid (a strong acid). However, only small amounts of the alkanoic acids dissolve to form hydrogen ions and so they are weak acids.

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