5.6: Redox reactions in organic chemistry and biochemistry (2023)

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

The carbon compounds in organic and biochemical chemistry tend to be covalent molecules. Because they are usually non-ionic, there are usually no charges, making it harder to see if negative electrons have been gained or lost (redox). In these cases, redox can be defined as a change in composition. The original meaning of oxidation was "addition of oxygen". So when oxygen is added to a molecule, the molecule becomes oxidized. In reduction, the opposite is the case: When a molecule loses oxygen atoms, the molecule is reduced. For example, acetaldehyde (CH₃CHO) molecule accepts an oxygen atom and becomes acetic acid (CH).₃COOH).

\[2CH_3CHO + O_2 → 2CH_3COOH\]

This oxidizes acetaldehyde.

Similarly, oxidation and reduction can be defined in terms of the gain or loss of hydrogen atoms. When a molecule adds hydrogen atoms, it is reduced. When a molecule loses hydrogen atoms, the molecule becomes oxidized. For example, in the conversion of acetaldehyde to ethanol (CH₃CH₂OH) hydrogen atoms are added to acetaldehyde so that the acetaldehyde is reduced:

\[CH_3CHO + H_2 → CH_3CH_2OH\]

Figure \(\PageIndex{1}\):Combustion of natural gas. The combustion of natural gas is not only a combustion reaction, but also a redox reaction. Similar reactions include the combustion of gasoline and coal. These are also redox reactions. Image used with permission from Wikipedia.

During respiration, the biochemical process by which the oxygen we breathe in the air oxidizes food to carbon dioxide and water, redox reactions provide living cells with energy. A typical respiratory reaction is the oxidation of glucose (C₆H₁₂Ö₆), the simple sugar that we learned about in the chapter's opening essay, that makes up the yeast food:

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O \label{Eq2}\]

Organic chemists use a variety of redox reactions. For example, potassium dichromate (K₂Cr₂Ö₇) 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 position of the OH functional group in the alcohol molecule, the relative proportions of alcohol and dichromation, and reaction conditions such as temperature. If the OH group is attached to a terminal carbon atom and the product is distilled as it is formed, the product is an aldehyde which has a terminal carbon atomcarbonyl group(C=O) and is often written as RCHO. An example is the breathalyzer reaction to detect ethyl alcohol (C₂H₅OH) in a person's breath:

\[3C_2H_5OH + Cr_2O_7^{2−} + 8H^+ \rightarrow 3CH_3CHO + 2Cr^{3+} + 7H_2O \label{Eq3}\]

If the product acetaldehyde (CH₃CHO) is not removed as it is formed, but is further oxidized to acetic acid (CH).₃COOH). In this case, the overall reaction is as follows:

\[3C_2H_5OH + 2Cr_2O_7^{2−} + 16H^+ \højrepil 3CH_3COOH + 4Cr^{3+} + 11H_2O \label{Eq4}\]

In this reaction, the chromatome is reduced from orange \(Cr_2O_7^{2−}\) to green \(Cr^{3+}\) and the ethanol is oxidized to acetic acid. (A breathalyzer measures the degree of color change in these reactions to determine the amount of alcohol.)

When the alcohol's OH group is attached to an internal carbon atom, the oxidation of an alcohol produces 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 (CH).₃CHOHCH₃). It is the usual solvent acetone [(CH₃)₂CO], which is used in varnishes, lacquers, rubber adhesives and nail polish removers. Acetone can be formed by the following redox reaction:

\[3CH_3CHOHCH_3 + Cr_2O_7^{2−} + 8H^+ \højrepil 3(CH_3)_2CO + 2Cr^{3+} + 7H_2O \label{Eq5}\]

As we have just seen, oxidation of alcohols can produce aldehydes and ketones. Conversely, aldehydes and ketones can be reduced to alcohols. The reduction of the carbonyl group is important in living organisms. For example, in anaerobic metabolism, where biochemical processes take place in the absence of oxygen, pyruvic acid (CH₃COCOOH) is reduced to lactic acid (CH).₃CHOHCOOH) in the muscles.

\[CH_3COCOOH \rightarrow CH_3CHOHCOOH \label{Gl6}\]

(Benzoic acid is both a carboxylic acid and a ketone; only the ketone group is reduced.) The accumulation of lactic acid during intense exercise is largely responsible for the fatigue we experience.

In food chemistry, the so-called antioxidants are among the reducing agents. Ascorbic acid (vitamin C; C₆H₈Ö₆) is designed to delay the potentially harmful oxidation of living cells. It is oxidized to dehydroascorbic acid (C).₆H₆Ö₆). In the stomach, ascorbic acid reduces nitration (NO).₂⁻) to nitric oxide (NO):

\[C_6H_8O_6 + 2H^+ + 2NO_2^− \rightarrow C_6H_6O_6 + 2H_2O + 2NO \label{Gl7}\]

If the reaction in equation \(\ref{Eq7}\) did not occur, nitrates from food would oxidize the iron in hemoglobin and destroy its ability to transport oxygen.

Tocopherol (vitamin E) is also an antioxidant. Vitamin E is believed to work in the body by removing harmful byproducts of metabolism, such as the highly reactive molecular fragments known as free radicals. In food, vitamin E prevents fats from oxidizing and thereby becoming rancid. Vitamin C is also a good antioxidant (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\):citrus fruits. Citrus fruits such as oranges, lemons and limes are good sources of vitamin C, which is an antioxidant. Chunks of pink grapefruit, lime and lemon and half an orange (clockwise from the top). Image used with permission from Wikipedia.

Finally, and most importantly, green plants carry out the redox reaction that makes almost all life on earth possible. They do this through a process called photosynthesis, where carbon dioxide and water are converted into glucose (C).₆H₁₂Ö₆). The synthesis of glucose requires a series of proteins called enzymes and a green pigment called chlorophyll, which converts sunlight into chemical energy (Figure \(\PageIndex{3}\)). The overall change that occurs is as follows:

\[6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 \label{Gl8}\]

In this reaction, carbon dioxide is reduced to glucose and water is oxidized to oxygen gas. Other reactions convert the glucose into more complex carbohydrates, plant proteins and oils.

Figure \(\PageIndex{3}\): life on earth. Photosynthesis is the basic process by which plants use sunlight to convert carbon dioxide and water into glucose and oxygen. Then plants produce more complex carbohydrates. It is the ultimate source of all food on earth and is a redox reaction. Image used with permission (Public Domain; Wikipedia).