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Home > Chemistry > The world of carbon > Reactions of carbon compounds

Chemistry

Reactions of carbon compounds

Oxidation reactions

The most important oxidation reactions are oxidation of alcohols (alkanols) and aldehydes (alkanals), using a variety of oxidising agents.

Oxidation just means joining with oxygen.

Complete combustion is an extreme oxidation reaction.

Example: complete combustion of methanol

2 CH3OH + 3 O2 → 2 CO2 + 4 H2O

Alcohols burn in oxygen to produce carbon dioxide and water.

In organic chemistry, oxidation can mean either adding oxygen or removing hydrogen.

The oxidations to remember are:

  1. primary alcohol →aldehyde →carboxylic acid;
  2. secondary alcohol→ketone →no further oxidation;
  3. tertiary alcohol →not oxidised.

 

Example: oxidation of ethanol (primary alcohol).

Oxidation of ethanol to produce ethanal (when hyrogen is removed from ethanol) and then ethanoic acid (when oxygen is added to ethanal).
Ethanol is oxidised to produce ethanal by removing two hydrogen atoms. Ethanal is then oxidised to produce ethanoic acid [acid: A corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions., by adding an oxygen atom. The O:H ratio changes from 1:6 (ethanol) to 1:4 (ethanal) to 1:2 (ethanoic acid).

Example: oxidation of propan-2-ol (secondary alcohol).

Oxidation of propan-2-ol to produce propanone (hydrogen is removed)
Propan-2-ol is oxidised to produce propanone by removing two hydrogen atoms. The O:H ratio changes from 1:8 (propan-2-ol) to 1:6 (propanone).

Both examples show that oxidation leads to an increase in the oxygen to hydrogen ratio.

Tertiary alcohols are not easily oxidised because, unlike primary and secondary alcohols, they do not have a hydrogen atom attached to the same carbon atom as the hydroxyl group.

Of course, the opposite of oxidation is reduction, and the previous two examples can also go in reverse.

Example: reduction of ethanoic acid.

Reduction of ethanoic acid to produce first ethanal (when oxygen is removed) and then ethanol (when hydrogen is added)
Ethanoic acid is reduced to produce ethanal, by removing an oxygen atom. Ethanal is then reduced to produce ethanol, by adding two hydrogen atoms. The O:H ratio changes from 1:2 (ethanoic acid) to 1:4 (ethanal) to 1:6 (ethanol).

The above example shows how the oxygen-to-hydrogen ratio decreases during reduction.

The oxidising agents

It is important to remember the colour changes which occur in the reactions, from starting to final colours.

Oxidising agents must always be reduced.

Document

Ion-electron equations can be found on page 11 of the SQA Higher Chemistry data book. This will open as a pdf file (192 kb).

Summary of oxidising agents:

Oxidising agentChange in appearanceReason
Copper(II) oxideblack solid

brown solid
Cu2+ + 2e-

Cu
Acidified potassiumdichromate solutionorange solution

(blue)-green solution
Cr2O72-(aq) reduced to Cr3+(aq)
Benedict's orFehling's solutionblue solution

orange-red precipitate
Cu2+ + e-

Cu+
Tollen's Reagentcolourless solution

silver mirror
Ag+(aq) + e-

Ag(s)

Prescribed practical activity

Oxidising agents can be used to distinguish between aldehydes and ketones. Because aldehydes can be oxidised, they will produce the colour changes in oxidising agents shown above. Ketones can't be easily oxidised, and so do not produce these colour changes.

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