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Oxidation Reactions
Ck12 Science
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Printed: January 5, 2016
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Oxidation Reactions
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Oxidation Reactions
• Define organic oxidation reaction.
• Give examples of organic oxidation reactions.
Keeping Food Safe
Benzoic acid crystals in polarized light are pictured above. Benzoic acid is widely used as a food preservative, either
as the carboxylic acid or as the sodium benzoate salt. This compound is most effective when added to acidic foods
such as fruit juices and soft drinks. The major industrial source of benzoic acid is the partial oxidation of toluene
with oxygen. The process is inexpensive and environmentally benign.
Oxidation Reactions
Oxidation can be defined as the addition of oxygen to a molecule or the removal of hydrogen from a molecule. When
an alkane is heated in the presence of an appropriate catalyst, it can be oxidized to the corresponding alkene in a
reaction called a dehydrogenation reaction. Two hydrogen atoms are removed in the process. The alkene can be
further oxidized to an alkyne by the removal of two more hydrogen atoms.
oxidation
−H2
−H2
CH3 CH3 −−→
CH2 =CH2 −−→
CH≡CH
The reactions are reversible, and so an alkyne can be reduced first to an alkene and then to an alkane.
reduction
+H2
+H2
CH≡CH −−→
CH2 =CH2 −−→
CH3 CH3
The alkane is the most reduced form of a hydrocarbon, while the alkyne is the most oxidized form.
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Oxidation reactions in organic chemistry often involve the addition of oxygen to a compound, which changes the
particular functional group of that compound. The following sequence shows how methane can be oxidized first to
methanol, then to methanal, then to methanoic acid, and finally to carbon dioxide.
gain of oxygen
loss of hydrogen
gain of oxygen
loss of hydrogen
CH4 −−−−−−−−−−→ CH3 OH −−−−−−−−−−−→ CH2 O −−−−−−−−−−→ HCOOH −−−−−−−−−−−→ CO2
methane
methanol
methanal
methanoic acid
Each step in the process is either a gain of oxygen or a loss of hydrogen. Each step also releases energy, which
explains why the complete combustion of alkanes to carbon dioxide is an extremely exothermic reaction.
The oxidation of an alcohol can produce either an aldehyde or a ketone. Ethanol can be oxidized in the laboratory
through a heating process combined with the addition of an oxidizing agent such as the dichromate ion, which
catalyzes the reaction in an acidic solution. The reaction produces the aldehyde ethanal (acetaldehyde).
Cr2 O2−
7
→
CH3 CHO
CH3 CH2 OH −−−−
+
H
When the alcohol to be oxidized is a secondary alcohol, the oxidation product is a ketone rather than an aldehyde.
The oxidation of the simplest secondary alcohol, 2-propanol, yields propanone.
Cr2 O2−
7
CH3 CHOHCH3 −−−−
→
CH3 COCH3
+
H
Tertiary alcohols cannot be oxidized in this way because the carbon to which the hydroxyl group is attached does
not have another hydrogen atom attached to it.
When a primary alcohol is oxidized to an aldehyde, the reaction is difficult to stop because the aldehyde is easily
oxidized further to the corresponding carboxylic acid. The oxidation of ethanal produces ethanoic (acetic) acid.
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Chapter 1. Oxidation Reactions
Cr2 O2−
7
CH3 CHO −−−−
→
CH3 COOH
+
H
Ethanol-containing beverages such as wine are susceptible to such oxidation if kept for long periods of time after
having been opened and exposed to the air. Wine that has become oxidized will have an unpleasant vinegary taste
due to the production of acetic acid.
Unlike aldehydes, ketones are resistant to further oxidation because the carbonyl group is in the middle of the carbon
chain and so the ketone cannot be converted to a carboxylic acid.
Summary
• Oxidation reactions are described.
• Examples of oxidation reactions are given.
Practice
Questions
Read the material at the link below and answer the following questions:
http://www.chemguide.co.uk/organicprops/alcohols/oxidation.html
1. What happens to a primary alcohol that is completely oxidized?
2. How can you get an aldehyde from a primary alcohol?
3. What would you use to convert a secondary alcohol to a ketone?
Review
Questions
1. What is the product of the oxidation of a primary alcohol?
2. What type of compound gives a ketone when oxidized?
3. What will produce acetic acid when completely oxidized?
References
1. Image copyright Pi-Lens, 2014. http://www.shutterstock.com .
2. Ben Mills (Wikimedia: Benjah-bmm27). http://commons.wikimedia.org/wiki/File:Acetaldehyde-2D.png .
3. Propanol: User:A.Nath/Wikipedia; Propanone: Ben Mills (Wikimedia: Benjah-bmm27). Propanol: http:/
/commons.wikimedia.org/wiki/File:1-Propanol.png; Propanone: http://commons.wikimedia.org/wiki/File:A
cetone-structural.png .
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