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5.6: Redox Reactions in Organic Chemistry and Biochemistry

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    Skills to Develop

    • To identify oxidation-reduction reactions with organic compounds.

    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.

    The carbon compounds in organic and biochemistry tend to be covalent molecules. Because they are not usually ionic, there are not usually charges, which makes it more difficult to see if negative electrons have been gained or lost (redox). In these cases, redox can be defined in terms of changes in composition. The original meaning of oxidation was “adding oxygen,” so when oxygen is added to a molecule, the molecule is being oxidized. The reverse is true for reduction: if a molecule loses oxygen atoms, the molecule is being reduced. For example, the acetaldehyde (CH3CHO) molecule takes on an oxygen atom to become acetic acid (CH3COOH).

    \[2CH_3CHO + O_2 → 2CH_3COOH\]

    Thus, acetaldehyde is being oxidized.

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

    \[CH_3CHO + H_2 → CH_3CH_2OH\]

    Example \(\PageIndex{1}\)

    In each conversion, indicate whether oxidation or reduction is occurring. 

    1. N2 → NH3
    2. CH3CH2OHCH3 → CH3COCH3
    3. HCHO → HCOOH


    1. Hydrogen is being added to the original reactant molecule, so reduction is occurring.
    2. Hydrogen is being removed from the original reactant molecule, so oxidation is occurring.
    3. Oxygen is being added to the original reactant molecule, so oxidation is occurring.

    Example \(\PageIndex{2}\)

    Rank these three one-carbon compounds in order from least to most oxidized. Methanol (CH3OH), or wood alcohol is much more toxic than beverage alcohol and used for many purposes, included antifreeze in windshield wiper fluid. Formaldehyde (H2CO) is often used as an enbalming agent. Formic acid (HCO2H) is what gives red ant bites their sting.


    The order from least to most oxidized would be CH3OH < H2CO < HCO2H.

    CH3OH is the most reduced (least oxidized) form of the four because it has only one oxygen and the most hydrogens in the formula. H2CO also has one oxygen, but only two hydrogens instead of four. HCO2H is the most oxidized of the three, as it also has only two hydrogens, but has two oxygens instead of one.

    Combustion Reactions

    A combustion reaction is a type of redox reaction that occurs when a substance combines with molecular oxygen to make oxygen-containing compounds of other elements in the reaction. One example is the burning of methane, the principal component of natural gas (Figure \(\PageIndex{1}\)).

    \[CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O \label{Eq1}\]

    Oxygen (in its elemental form) is a crucial reactant in combustion reactions, and it is also present in the products. In combustion, the oxygen (O2) is reduced and the other elements are oxidized. Because of its abundance in the air, O2 is the most ubiquitous oxidizing agent on Earth.

    Energy in the form of heat is usually given off as a product in a combustion reaction as well.  


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

    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 (C6H12O6), the simple sugar we encountered in the chapter-opening essay that makes up the diet of yeast:

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

    Organic chemists use a variety of redox reactions. For example, potassium dichromate (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:

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

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

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

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

    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:

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

    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 metabolism, in which biochemical processes take place in the absence of oxygen, pyruvic acid (CH3COCOOH) is reduced to lactic acid (CH3CHOHCOOH) in the muscles.

    \[CH_3COCOOH \rightarrow CH_3CHOHCOOH \label{Eq6}\]

    (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 antioxidants 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):

    \[C_6H_8O_6 + 2H^+ + 2NO_2^− \rightarrow C_6H_6O_6 + 2H_2O + 2NO \label{Eq7}\]

    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 \(\PageIndex{2}\)).


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

    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 (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:

    \[6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 \label{Eq8}\]

    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.


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


    Redox reactions are common in organic and biological chemistry, including the combustion of organic chemicals, respiration, and photosynthesis. When an organic compound is oxidized, the result will usually be more oxygen atoms and/or fewer hydrogen atoms in the formula.

    Concept Review Exercise

    1. What are two ways to determine if an organic compound has been oxidized?
    2. Give some biochemical examples of oxidation and reduction reactions.


    1. If the compound is more oxidized it will have more oxygen and/or less hydrogen in the formula.
    2. respiration, photosynthesis, antioxidants in foods,... (answers will vary)


    1. In the combustion of methane, is the carbon reduced or oxidized? Since these are not ionic compounds and there are no charges, how can you tell? Does that make the carbon in methane the oxidizing agent or reducing agent?

      CH4 + 2O2 → CO2 + 2H2O

    2. List these four three-carbon organic molecules from least to most oxidized: propanal (CH3CH2CHO), propane (CH3CH2CH3), propanoic acid (CH3CH2CO2H), and propene (CH3CH=CH2)?


    1. The carbon in methane is oxidized. Carbon gains more oxygens (more oxidized) and loses hydrogens (less reduced) when CH4 is converted to CO2. This means the carbon in methane is the reducing agent. (Note: O2 is the oxidizing agent here, as is usually the case.)

     2.  (least oxidized) propane (CH3CH2CH3) < propene (CH3CH=CH2) < propanal (CH3CH2CHO) < propanoic acid (CH3CH2CO2H) (most oxidized)


    5.6: Redox Reactions in Organic Chemistry and Biochemistry is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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