Transfer hydrogenation is a chemical reaction that adds pairs of hydrogen atoms to a substance using donor solvents as the source of hydrogen. It is used in organic synthesis and industrial treatment of carbon-based compounds. Metal and non-metallic catalysts are used, with research focusing on making the process more cost-effective. Non-gaseous donors have advantages over hydrogen gas, including easier handling and lower costs.
Transfer hydrogenation refers to the treatment of an element or compound with hydrogen from a source other than hydrogen gas. A chemical reaction occurs between the substance to be modified and molecular hydrogen in the presence of catalysts, which facilitate the reaction. This process is often used in the industrial treatment of carbon-based organic compounds. For example, coal liquefaction involves the large-scale use of transfer hydrogenation to produce synthetic fuels from coal.
The chemical reaction essentially consists in adding pairs of hydrogen atoms to the material to be treated. In transfer hydrogenation, this is achieved by using donor solvents as the source of hydrogen. Common donor solvents include formic acid and isopropyl alcohol, although some are synthesized for use in a particular transfer process. The reaction usually takes place in the presence of a metal catalyst, which reduces the minimum energy required to start the reaction.
Transfer hydrogenation is particularly useful in organic synthesis, the production of carbon-based compounds by organic reactions. Organometallic catalysts, based on the platinum group of metals, have been developed for use in this process. Isopropyl alcohol is often the solvent donor and becomes acetone after adding its hydrogen. The catalysts themselves are unchanged by the reaction.
Organocatalytic transfer hydrogenation makes use of non-metallic catalysts. These are formed from elements common to organic compounds, such as carbon, sulfur and hydrogen. The development of these catalysts allows the transfer process to be applied to a wider range of chemicals. The most commonly used metal catalysts are ineffective for the hydrogenation of organic groups such as the benzene series. This chemical class plays an important role in the production of pharmaceuticals, plastics and dyes.
Hydrogenation using non-gaseous donors has long been a standard laboratory procedure. Research into the transfer hydrogenation process itself has been motivated by its importance to the pharmaceutical and petrochemical industries. The development of hydrogen donors and catalysts for use with substances unsuitable for the traditional transfer process is an area of interest. Research into catalysts based on common metals, such as nickel, instead of platinum and other rare metals, seeks to make the industrial process more cost-effective.
The use of a non-gaseous hydrogen donor has several advantages when implemented on a large scale. Typically, standard industrial equipment can be used in the transfer process, rather than the pressurized equipment needed when using a gas. Hydrogen is also extremely flammable and requires great care in storage and handling. These considerations make the use of hydrogen gas a much more costly undertaking than using non-gaseous hydrogen donors.
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