Reactions of carbon compounds
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.
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:
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.
The above example shows how the oxygen-to-hydrogen ratio decreases during reduction.
It is important to remember the colour changes which occur in the reactions, from starting to final colours.
Oxidising agents must always be reduced.
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).
|Oxidising agent||Change in appearance||Reason|
|Copper(II) oxide||black solid
|Cu2+ + 2e-|
|Acidified potassiumdichromate solution||orange solution
|Cr2O72-(aq) reduced to Cr3+(aq)|
|Benedict's orFehling's solution||blue solution
|Cu2+ + e-|
|Tollen's Reagent||colourless solution
|Ag+(aq) + e-|
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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