5.6 Redox Reactions in Organic Chemistry and Biochemistry

Oxidation-reduction reactions are of central importance in organic chemistry and biochemistry. The burning of fuels that provides the energy to maintain our civilization and the metabolism of foods that furnish the energy that keeps us alive both involve redox reactions.

All combustion reactions are also redox reactions. A typical combustion reaction is the burning of methane, the principal component of natural gas (Figure 5.5 "The Burning of Natural Gas").

CH₄ + 2O₂ → CO₂ + 2H₂O

In respirationThe biochemical process by which the oxygen we inhale oxidizes foodstuffs to carbon dioxide and water., 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 (C₆H₁₂O₆), the simple sugar we encountered in the chapter-opening essay that makes up the diet of yeast:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Organic chemists use a variety of redox reactions. For example, potassium dichromate (K₂Cr₂O₇) is a common oxidizing agent that can be used to oxidize alcohols (symbolized by the general formula ROH). The product of the reaction depends on the location of the OH functional group in the alcohol molecule, the relative proportions of alcohol and the dichromate ion, and reaction conditions such as temperature. If the OH group is attached to a terminal carbon atom and the product is distilled off as it forms, the product is an aldehyde, which has a terminal carbonyl group (C=O) and is often written as RCHO. One example is the reaction used by the Breathalyzer to detect ethyl alcohol (C₂H₅OH) in a person’s breath:

3C₂H₅OH + Cr₂O₇²⁻ + 8H⁺ → 3CH₃CHO + 2Cr³⁺ + 7H₂O

If the product acetaldehyde (CH₃CHO) is not removed as it forms, it is further oxidized to acetic acid (CH₃COOH). In this case, the overall reaction is as follows:

3C₂H₅OH + 2Cr₂O₇²⁻ + 16H⁺ → 3CH₃COOH + 4Cr³⁺ + 11H₂O

In this reaction, the chromium atom is reduced from Cr₂O₇²⁻ to Cr³⁺, and the ethanol is oxidized to acetic acid.

When the OH group of the alcohol is bonded to an interior carbon atom, the oxidation of an alcohol will produce a ketone. (The formulas of ketones are often written as RCOR, and the carbon–oxygen bond is a double bond.) The simplest ketone is derived from 2-propanol (CH₃CHOHCH₃). It is the common solvent acetone [(CH₃)₂CO], which is used in varnishes, lacquers, rubber cement, and nail polish remover. Acetone can be formed by the following redox reaction:

3CH₃CHOHCH₃ + Cr₂O₇²⁻ + 8H⁺ → 3(CH₃)₂CO + 2Cr³⁺ + 7H₂O

As we have just seen, aldehydes and ketones can be formed by the oxidation of alcohols. Conversely, aldehydes and ketones can be reduced to alcohols. Reduction of the carbonyl group is important in living organisms. For example, in anaerobic metabolismA biochemical process that takes place in the absence of oxygen., in which biochemical processes take place in the absence of oxygen, pyruvic acid (CH₃COCOOH) is reduced to lactic acid (CH₃CHOHCOOH) in the muscles.

CH₃COCOOH → CH₃CHOHCOOH

(Pyruvic acid is both a carboxylic acid and a ketone; only the ketone group is reduced.) The buildup of lactic acid during vigorous exercise is responsible in large part for the fatigue that we experience.

In food chemistry, the substances known as antioxidantsA substance in foods that acts as a reducing agent. are reducing agents. Ascorbic acid (vitamin C; C₆H₈O₆) is thought to retard potentially damaging oxidation of living cells. In the process, it is oxidized to dehydroascorbic acid (C₆H₆O₆). In the stomach, ascorbic acid reduces the nitrite ion (NO₂⁻) to nitric oxide (NO):

C₆H₈O₆ + 2H⁺ + 2NO₂⁻ → C₆H₆O₆ + 2H₂O + 2NO

If this reaction 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 5.6 "Citrus Fruits").

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 photosynthesisThe process by which plants use solar energy to convert carbon dioxide and water to glucose., in which carbon dioxide and water are converted to glucose (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 5.7 "Life on Earth"). The overall change that occurs is as follows:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

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.