CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 25. Organic Chemistry

C 6 H 6 +3H 2 −→Pt C 6 H 12

Oxidation Reactions

Oxidation can be defined as the addition of oxygen to a molecule or the removal of hydrogen. Reduction is
therefore the addition of hydrogen or the removal of oxygen. Several classes of organic compounds are related
to one another by oxidation and reduction reactions. Alkanes, alkenes, and alkynes represent different levels of
oxidation of a hydrocarbon. 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. This is
also considered an oxidation according to the modern definition, because the oxidation number of each carbon atom
goes from -3 to -2 to -1.

oxidation: CH 3 CH 3

−H 2

−→CH 2 =CH 2

−H 2

−→CH≡CH

The reactions are reversible, so an alkyne can be reduced first to an alkene and then to an alkane. These are addition
reactions, as seen in the previous section.

reduction: CH≡CH

+H 2

−→CH 2 CH 2

+H 2

−→CH 3 CH 3

The alkane is the most reduced form of a hydrocarbon, while the alkyne is the most oxidized form.

Oxidation reactions in organic chemistry often involve the addition of oxygen to a compound, which changes the
functional group that is present. The following sequence shows how methane can be oxidized first to methanol, then
to methanal, then to methanoic acid, and finally to carbon dioxide.

CH 4
methane

gain of oxygen

−→ CH 3 OH

methanol

loss of hydrogen

−→ CH 2 O

methanal

gain of oxygen

−→ HCOOH

methanoic acid

loss of hydrogen

−→ CO 2

carbon dioxide

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
by slight warming combined with the addition of an oxidizing agent such as the dichromate ion, which catalyzes this
reaction in acidic solution. The reaction produces the aldehyde ethanal.

CH 3 CH 2 OH

Cr 2 O^27 −

?−→

H+

CH 3 CHO

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.