Reduction
Reduction reactions are essential to life. In photosynthesis, carbon dioxide and water are reduced to form the carbohydrates required as a source of energy for plant and animal life. Photosynthesis ultimately provides all energy, with the exception of nuclear power and the relatively small contributions from solar, wind, water, and geothermal power, for human and animal life and most of the fuel used by society and industry. If research into artificial means of using sunlight to convert water and carbon dioxide into carbohydrates without the intervention of green plants succeeds, the impact on our global energy problems will be enormous.
Reduction reactions are also used by chemists to synthesize pharmaceuticals, textiles, dyes, paints, and a multitude of other important products. A major industrial use of hydrogen is the reduction of unsaturated liquid vegetable oils to make edible solid fats like margarine.
The original definition of reduction reactions centered on the removal of oxygen from a species, most typically by the action of hydrogen. When oxygen is removed from a metal oxide to derive the metal itself, the mass of the metal oxide is reduced. Metal-containing ores have been reduced with carbon-containing materials to obtain metals for thousands of years. However, it was not until the eighteenth century that experimenters such as French chemist Antoine Lavoisier (1743-1794) carefully weighed metal ores and metals produced from them to quantitatively establish that weight reduction occurs in such reactions. Species that cause molecules to lose oxygen are called reductants or reducing agents.Oxidation always occurs in tandem with reduction: if a species in a reaction is oxidized, then something else must be reduced.
Industrial processes to recover a metal from its oxidized forms are examples of reduction reactions. Many metal oxides are powdery, non-malleable, non-ductile substances. Only as the free metal do they have significant tensile strength and can they be shaped and formed into useful tools or other objects. Carbon is a reducing agent in metallurgical processes. In the case of making iron in a blast furnace, the carbon is added as coke, which is converted into carbon monoxide--the actual reducing agent.
The current approach to oxidation-reduction reactions has been developed from the idea that electrons are crucial to chemical bonding. When a metal oxide or salt is reduced to form the metal, a cation is converted to an uncharged atom. In reduction, electrons are gained by the cation. Reduction is most generally defined as a gain of electrons.
Most compounds are covalent, not ionic. To encompass all oxidation-reduction reactions, it is necessary to determine the gain and loss of electrons for covalent as well as ionic and elemental species. Electrons are assigned to atoms in covalent and ionic compounds and the oxidation number of each atom is determined. Reduction and oxidation involve a change in the number of electrons assigned to atoms that undergo the reaction. To determine if a change has occurred, the starting and ending states of atoms in terms of the number of electrons must be assigned. This requires determining the oxidation states of atoms, which are specified by oxidation numbers. A species is reduced in a reaction if it undergoes a decrease in oxidation number; oxidation occurs when the oxidation number of a species increases.
Oxidation-reduction (redox) reactions are classified into categories to facilitate our ability to understand trends and predict the course of possible reactions. The first redox reactions to be classified involved addition and removal of oxygen atoms from chemical species. The next definition developed focused on electron transfer. It is consistent with the earlier definition but broadens it to include reactions not involving oxygen atoms. We now recognize that redox reactions cover a wide range of chemical changes, including reactions of atoms of the same element in different oxidation states. Three categories that encompass all these different redox reactions are atom transfer redox reactions, electron transfer reactions, and disproportionation reactions.
Although the most common atom transfer oxidation-reduction reactions involve oxygen, several critical biochemical reactions as well as reactions used to synthesize organic compounds involve hydrogen atoms. Hydrogen atoms have three possible oxidation states: +1 when they are bound to more electronegative elements like other non-metals, 0 for elemental hydrogen, and -1 when they are bound to less electronegative elements like the metals. In acid-base reactions, the hydrogen atoms remain in the +1 oxidation state. In oxidation-reduction reactions of hydrogen, its oxidation state changes.
In biochemical systems, the hydrogen atoms are typically produced and used one at a time and are not present as diatomic hydrogen gas. For example, the enzyme nitrogenase converts atmospheric nitrogen into ammonia in a series of steps involving hydrogen atoms.
One example of the use of hydrogen atom transfer in organic synthesis is hydrogenation, i.e. the addition of hydrogen to a carbon-carbon double bond. Hydrogenation converts an unsaturated oil to a saturated oil, which is an essential process in making solid margarine from liquid vegetable oil.
Electron transfer reactions are essential steps in many biological processes, including photosynthesis, in which light energy induces the transfer of electrons from chlorophyll molecules to chemical intermediates in a very complex reaction sequence. The energetic electrons released from chlorophyll eventually cascade through a series of redox reactions, resulting in the production of organic compounds such as carbohydrates and the liberation of oxygen. The carbon in the carbohydrates is reduced relative to the starting material, carbon dioxide.
We can use the Periodic Table to predict the reactivity of atoms in each of category of redox reaction. The most straightforward reactions to predict are those that occur between elements themselves. Metals act as reductants, providing electrons to nonmetallic elements. The relative reactivity of metals in electron transfer reactions in aqueous medium is commonly displayed as an activity series. The activity series is a list of elements in order of the ease with which they lose electrons in redox reactions. Metals high on the activity series react more vigorously with oxidants including the halogens and the hydronium ion than those lower on the scale. Metals higher on the series are also capable of reducing ions of metals lower on the series. For example, zinc is higher on the activity series than copper and a solution of a copper salt will corrode metallic zinc because zinc metal reduces copper ion. Copper metal does not react with a solution of a zinc salt to produce copper ion and metallic zinc.
The reaction of metals with acid to form hydrogen gas was one of the earliest oxidation-reduction reactions of the metals to be characterized. The conversion of hydronium ion to hydrogen or the reverse reaction is, in fact, used as the reference reaction for oxidation-reduction reactions. The relative activity of metals to act as reductants is judged by their ability to reduce hydronium ion to hydrogen, and the relative reactivity of oxidants is judged by their ability to oxidize hydrogen to hydronium ion.
Hydrogen transfer reactions play an important role in transformations in organic synthesis. The borohydride ion (BH4-) in salts like sodium borohydride and the aluminum hydride ion (AlH4-) in lithium aluminum hydride are often used as sources of hydride ions that react with and reduce many organic compounds. Aluminum hydrides are powerful reducing agents, while borohydrides are more selectively reactive. However, both BH4-) and AlH4-) react with ketones to reduce them efficiently to alcohols. Literally dozens of hydride sources have been developed for use in organic synthesis, and many hydride transfer agents are highly specific for one type of substrate or for one particular reduction. Some agents may reduce an acid to an aldehyde and stop there, while others may reduce an acid all the way to an alcohol.
Reduction reactions (e.g. photosynthesis) are essential to life and are important in the chemical manufacturing industry. Reductions, especially those carried out in the metal processing industry, often require the use of carbon or carbon monoxide to remove oxygen from ores. In other types of reductions, especially those involving organic compounds, hydrogen is added to molecules. Other species can serve as sources of hydrogen or electrons and likewise participate in redox reactions as reductants.
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