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

Introduction to Alkanals

Alkanals are also commonly referred to as aldehydes.

Alkanals can be produced by the partial oxidation of a primary alkanol. This process is very similar to how alkanoic acids are produced (the full oxidation of a primary alkanol.

For example, ethanol can be oxidised to ethanal (an alkanal) by heating it with a few drops of acidified soldium dichromate(VI) solution until the colour of the solution changes from orange to green.1

Structure of Alkanals

Alkanals are very similar in structure to alkanoic acids, however, instead of a hydroxyl group, they have merely a hydrogen.

For example, the alkanal containing two carbon atoms is shown below.

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

Naming Alkanals

Alkanals are named as follows:

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

Therefore, the alkanal shown above contains two carbon atoms in its longest carbon atom chain. The corresponding alkane is ethane. The ane is replaced with anal to give the alkanal its name: ethanal.

Reactions of Alkanals

Oxidation reactions

Alkanals can be oxidised to form alkanoic acids in the presence of acidified sodium dichromate(VI) solution, an oxidising agent.

For example, ethanal can be oxidised to ethanoic acid.

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

The extra oxygen comes from the oxidising agent used.

Reaction with Benedict's Solution

Alkanals reduce2 the Cu(II) ions3 in Benedict's solution to Cu(I) ions4. This results in the precipitation of brick red copper(I) oxide from the blue Benedict's Solution.

1If this solution was further heated, the solution would change colour from green to blue. At this point, ethanoic acid (an alkanoic acid) would have been produced.2Reduction occurs when an atom gains electrons.3Cu(II) is the notation meaning that copper (Cu) has lost two electrons (represented by the Roman number II) to form an ion. An ion is an atom that has either gained or lost electrons.4This indicates that copper has formed an ion by losing one electron. In other words, the Cu(II) ion has gained one electron to form the Cu(I) ion.

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