9.7.2: Redox Reactions in Organic Chemistry and Biochemistry
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Oxidation-reduction reactions are of central importance in cooking, of course, but also in the production of the foods that we eat, all of which come from photosynthesis (plant foods) and respiration (animal-based foods).
As a reminder: redox processes can be identified by tracking the exchange of electrons, or by following the amount of oxyen or hydrogen bonded to the species in the reaction. To review these tracking methods, return to the prior section.
All combustion reactions are also redox reactions. A typical combustion reaction is the burning of methane, the principal component of natural gas (Figure \(\PageIndex{1}\)).
\[\ce{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O }\label{Eq1} \]
In respiration, the biochemical process by which the oxygen we inhale in air oxidizes foodstuffs to carbon dioxide and water, redox reactions provide energy to living cells. A typical respiratory reaction is the oxidation of glucose (\(\ce{C6H12O6}\)), the simple sugar we encountered in the chapter-opening essay that makes up the diet of yeast:
\[\ce{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O} \label{Eq2} \]
In this reaction, the glucose molecule is oxidized because it gained oxygen atoms from a 1:1 ratio of carbon to oxygen to a 1:2 ratio of carbon to oxgen in CO2. The oxygen in the reaction is reduced - it gains hydrogen to form water, and carbon to form carbon dioxide.
In food chemistry, the substances known as antioxidants are reducing agents. Ascorbic acid (vitamin C; \(\ce{C6H8O6}\)) is thought to retard potentially damaging oxidation of living cells. In the process, it is oxidized to dehydroascorbic acid (\(\ce{C6H6O6}\)). In the stomach, ascorbic acid reduces the nitrite ion (\(\ce{NO_2^{−}}\)) to nitric oxide (\(\ce{NO}\)):
\[\ce{C_6H_8O_6 + 2H^{+} + 2NO_2^{−} \rightarrow C_6H_6O_6 + 2H_2O + 2NO} \label{Eq7} \]
If reaction in Equation \(\ref{Eq7}\) did not occur, nitrite ions from foods would oxidize the iron in hemoglobin, destroying its ability to carry oxygen.
Tocopherol (vitamin E) is also an antioxidant. In the body, vitamin E is thought to act by scavenging harmful by-products of metabolism, such as the highly reactive molecular fragments called free radicals. In foods, vitamin E acts to prevent fats from being oxidized and thus becoming rancid. Vitamin C is also a good antioxidant (Figure \(\PageIndex{2}\)).
Finally, and of greatest importance, green plants carry out the redox reaction that makes possible almost all life on Earth. They do this through a process called photosynthesis, in which carbon dioxide and water are converted to glucose (\(\ce{C6H12O6}\)). The synthesis of glucose requires a variety of proteins called enzymes and a green pigment called chlorophyll that converts sunlight into chemical energy (Figure \(\PageIndex{3}\)). The overall change that occurs is as follows:
\[\ce{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2} \label{Eq8} \]
In this reaction, carbon dioxide is reduced to glucose, and water is oxidized to oxygen gas. Other reactions convert the glucose to more complex carbohydrates, plant proteins, and oils.
Summary
Redox reactions are common in organic and biological chemistry, including the combustion of organic chemicals, respiration, and photosynthesis.