Are All Combustion Reactions Redox

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5.vi Redox Reactions in Organic Chemical science and Biochemistry

Learning Objective

Place oxidation-reduction reactions with organic compounds.

Oxidation-reduction reactions are of central importance in organic chemical science 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 united states live both involve redox reactions.

Figure 5.5 The Called-for 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.

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

CH_four + 2O₂ → CO₂ + 2H₂O

In respirationThe biochemical procedure by which the oxygen nosotros 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 h2o, redox reactions provide free energy to living cells. A typical respiratory reaction is the oxidation of glucose (C₆H₁₂O_{half dozen}), the uncomplicated sugar we encountered in the chapter-opening essay that makes up the diet of yeast:

C_{half dozen}H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Organic chemists use a variety of redox reactions. For example, potassium dichromate (K₂Cr₂O_vii) 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 grouping in the booze 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 final carbonyl group (C=O) and is frequently written as RCHO. One instance is the reaction used by the Breathalyzer to notice ethyl alcohol (C₂H₅OH) in a person'due south jiff:

3C_twoH_fiveOH + Cr₂O₇ ⁱⁱ⁻ + 8H⁺ → 3CH₃CHO + 2Crⁱⁱⁱ⁺ + 7H₂O

If the product acetaldehyde (CH₃CHO) is non removed as it forms, it is further oxidized to acetic acrid (CH_threeCOOH). In this example, the overall reaction is as follows:

3C₂H_vOH + 2Cr₂O₇ ²⁻ + 16H⁺ → 3CH_threeCOOH + 4Cr³⁺ + 11H₂O

In this reaction, the chromium atom is reduced from Cr₂O₇ ²⁻ to Cr^three+, and the ethanol is oxidized to acetic acid.

When the OH grouping 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 equally RCOR, and the carbon–oxygen bond is a double bail.) The simplest ketone is derived from two-propanol (CH₃CHOHCH_iii). It is the common solvent acetone [(CH₃)₂CO], which is used in varnishes, lacquers, safety cement, and smash polish remover. Acetone can be formed by the following redox reaction:

3CH₃CHOHCH₃ + Cr₂O₇ ²⁻ + 8H⁺ → 3(CH₃)₂CO + 2Cr³⁺ + 7H_twoO

As we accept only 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 of import in living organisms. For example, in anaerobic metabolismA biochemical process that takes identify in the absence of oxygen. , in which biochemical processes take place in the absenteeism of oxygen, pyruvic acrid (CH₃COCOOH) is reduced to lactic acid (CH_iiiCHOHCOOH) in the muscles.

CH_threeCOCOOH → 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 big 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; C₆H₈O_{half dozen}) is thought to retard potentially damaging oxidation of living cells. In the procedure, it is oxidized to dehydroascorbic acrid (C_half-dozenH₆O₆). In the stomach, ascorbic acid reduces the nitrite ion (NO_two ⁻) to nitric oxide (NO):

C₆H_eightO₆ + 2H⁺ + 2NO₂ ⁻ → C₆H₆O₆ + 2H_iiO + 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 Due east) is also an antioxidant. In the body, vitamin E is idea to human activity past scavenging harmful by-products of metabolism, such every bit the highly reactive molecular fragments called costless radicals. In foods, vitamin E acts to prevent fats from beingness oxidized and thus becoming rancid. Vitamin C is besides a good antioxidant (Figure 5.6 "Citrus Fruits").

Figure v.6 Citrus Fruits

Citrus fruits, such as oranges, lemons, and limes, are skilful sources of vitamin C, which is an antioxidant.

Finally, and of greatest importance, green plants behave out the redox reaction that makes possible near all life on Earth. They practise this through a procedure called photosynthesisThe process by which plants use solar free energy to catechumen carbon dioxide and h2o to glucose. , in which carbon dioxide and water are converted to glucose (C_half-dozenH₁₂O_vi). The synthesis of glucose requires a diverseness of proteins chosen enzymes and a light-green pigment chosen chlorophyll that converts sunlight into chemical energy (Figure five.7 "Life on Globe"). The overall change that occurs is as follows:

6CO_ii + 6H_twoO → C₆H₁₂O₆ + 6O₂

Figure v.7 Life on Earth

Photosynthesis is the cardinal procedure by which plants apply sunlight to convert carbon dioxide and h2o 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.

In this reaction, carbon dioxide is reduced to glucose, and h2o is oxidized to oxygen gas. Other reactions convert the glucose to more circuitous carbohydrates, plant proteins, and oils.

Concept Review Exercise

Requite some biochemical examples of oxidation and reduction reactions.

Answer

photosynthesis and antioxidants in foods (answers volition vary)

Primal Takeaway

Redox reactions are common in organic and biological chemical science, including the combustion of organic chemicals, respiration, and photosynthesis.

Exercises

A typical respiratory reaction discussed in the text is the oxidation of glucose (C_sixH₁₂O₆):

C_half-dozenH₁₂O₆ + 6O₂ → 6CO_ii + 6H₂O

Is this a redox reaction? If and then, what are the oxidizing and reducing agents?
The major cyberspace reaction in photosynthesis is as follows:

6CO_ii + 6H₂O → C₆H₁₂O₆ + 6O₂

Is this a redox reaction? If so, what are the oxidizing and reducing agents?
What would be the ultimate organic product if CH₃CH_twoCH₂OH were to react with a solution of K₂Cr₂O_vii?
What would exist the ultimate organic product if CH₃CH_iiCH_twoOH were to react with a solution of 1000₂Cr₂O₇?
What would exist the terminal organic product if CH₃CH_iiCHOHCH_three were to react with a solution of K₂Cr_twoO_vii?
What would exist the major organic product if CH₃CH₂CHOHCH_twoCH₃ were to react with a solution of Yard_twoCr_twoO₇?
What alcohol is produced in the reduction of acetone [(CH_three)₂CO]?
What alcohol is produced in the reduction of propanal (CH₃CH_twoCHO)?

Answers

yes; oxidizing amanuensis: O₂; reducing amanuensis: C_{half dozen}H₁₂O_half-dozen
CH₃CH₂COOH
CH_iiiCH_twoC(O)CH_three, where the carbon is double bonded to the oxygen
CH₃CHOHCH_three, or isopropyl booze