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The aldehyde group is oxidized and becomes a carboxyl group.
During this process, O is inserted between C and H of the
aldehyde group (-CHO), then becomes the carboxyl group (-COOH).
In my textbook, “Formic acid (HCOOH) is reducing
agent, because it has both an aldehyde group and a carboxyl
group.” So what I expected is that “O is inserted between
C and H of the aldehyde group, i.e. formic acid becomes HOCOOH,
when it acts as a reducing agent.”. However, I realized that
my expectation was not true because my textbook listed the
half equation of formic acid as follows;
HCOOH ↔ CO2
+ 2H+ + 2e-
Why doesn’t the reaction of formic acid follow the
theory above? (Why doesn’t formic acid become HOCOOH?)
Gattu Ippatu (Dec 2001)
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Answer:
About the oxidation of formic acid
To summarize, your expectation is correct, i.e. formic acid becomes HOCOOH when it is oxidized. This seems to be different from what your textbook said, however, please look at the structure carefully. HOCOOH is
H2CO3, which means it is carbonic acid. See the structural formulas below.
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Formic acid |
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Carbonic acid |
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The carbonic acid (H2CO3) is unstable so that it
will easily break down into CO2 and H2O. In
other words, the reaction (H2CO3
↔ CO2
+ H2O) proceeds mainly towards the right side.
Therefore, the formic acid apparently becomes water and carbon
dioxide when it is oxidized.
About the mechanism of the oxidation of the aldehyde group
The mechanism of the reaction “the aldehyde group is oxidized
and becomes the carboxyl group” would be that the aldehyde becomes a
hydrate during the oxidation process. The oxygen atom in an aldehyde
group attracts the electrons of the carbon atom, so the carbon atom
has a slightly positive charge. In addition, the aldehyde group has a
planar structure. Thus, the carbon atom in an aldehyde group is easily
attacked by negatively charged agents (or agents having high electron
density).
Therefore, it is expected that the following reaction would
occur. The marked (^) hydrogens are eliminated.
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Since the definition of oxidation is addition of oxygen or
elimination of hydrogen, this reaction is an oxidation reaction. (Of
course you can use a half-reaction to express this reaction, though it
is not commonly used in organic chemistry.)
In this reaction it seems that a water molecule attacks an
aldehyde group and two hydrogen atoms are eliminated so that one
oxygen atom remains, instead of an oxygen atom being inserted between
carbon and hydrogen in the aldehyde group.
This reaction does not occur naturally. The oxidant which is
present pulls electrons from the hydrate, resulting in the elimination
of the hydrogen atoms.
Acknowledgement
We would like to thank Dr. M.H for his careful teaching and kind
support for this answer.
This article is translated by
Chemistryquestion.com from the original article in
Chemistryquestion.jp. Please let us know
if you find any errors.
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