Hydrogenation
Hydrogenation
Hydrogenation in the research laboratory
Hydrogenation is a chemical reaction in which hydrogen atoms add to carbon-carbon multiple bonds. In order for the reaction to proceed at a practical rate, a catalyst is usually needed. Hydrogenation reactions are used in many industrial processes as well as in the research laboratory and occur in living systems.
The hydrogenation reaction
Hydrogen gas, H2, can react with a molecule containing carbon-carbon double or triple bonds. In its simplest form, a molecule with one double bond would react with one molecule of hydrogen gas. An example is shown below.
Many carbon compounds have triple bonds, and in a case such as that, two molecules of hydrogen are necessary to completely saturate the carbon compound with hydrogen.
Hydrogenation of a double or triple carbon-carbon bond will not occur unless the catalyst is present. Scientists have developed many catalysts for this kind of reaction. Most of them include a heavy metal, such as platinum or palladium, in finely divided form. The catalyst adsorbs both the carbon compound and the hydrogen gas on its surface, in such a way that the molecules are arranged in just the right position for addition to occur. This allows the reaction to proceed at a fast enough rate to be useful.
Because at least one of the reagents (hydrogen) is a gas, often the reaction will occur at an even faster rate if it occurs in a pressurized container, at a pressure several times higher than atmospheric pressure.
Hydrogenation in the research laboratory
The hydrogenation reaction is a useful tool for a scientist trying to determine the structure of a new molecule. The molecular formula, showing the exact number of each kind of atom, can be determined in several ways, but discovering the arrangement of these atoms requires a large amount of detective work.
Sometimes, for example, a new substance is isolated from a plant, and a chemist needs to determine what the structure of this substance is. One method of attack is to find out how many molecules of hydrogen gas will react with one molecule of the unknown substance. If the ratio is, for example, two molecules of hydrogen to one of the unknown, the scientist can deduce that there are two carbon-carbon double bonds, or else one carbon-carbon triple bond in each molecule. Other kinds of chemical clues lead to the rest of the structure, and help the scientist to decide where in the unknown molecule the multiple bonds are.
One of the simplest uses of hydrogenation in the research laboratory is to make new compounds. Almost any organic molecule that contains multiple bonds can undergo hydrogenation, and this sometimes leads to compounds that were unknown before. In this way, scientists have synthesized and examined many molecules not found in nature, or not found in sufficient quantity. These newly synthesized molecules are of use to humanity in a variety of ways.
Hydrogenation in industry
Many of the carbon compounds found in crude petroleum are of little use. These compounds may contain multiple bonds, but can be converted to saturated compounds by catalytic hydrogenation. This process is one source for gasoline that humans use today. Other chemicals besides gasoline are made from petroleum, and for these, too, the first step from crude oil may be hydrogenation.
Another commercial use of the hydrogenation reaction is the production of fats and oils in more useful forms. Fats and oils are not hydrocarbons, like the simple molecules that have earlier been looked at, since they contain oxygen atoms, too. However, they do contain long chains of carbon and hydrogen, joined together in part by carbon-carbon double bonds. Partial hydrogenation of these molecules, so that some, but not all of the double bonds react, gives compounds with different cooking characteristics, more satisfactory for consumers in some situations than the original oils. This is the source of the partially hydrogenated vegetable oil on the grocery shelf.
Hydrogenation with respect to such fats and oils has been implicated in health problems such as heart disease in humans. Hydrogenation converts fat and oil molecules to trans-unsaturated fatty acids, usually shortened to trans fats). Medical studies have shown that trans fats increase bad cholesterol (low-density lipoprotein, LDL) while decreasing good cholesterol
KEY TERMS
Addition —A type of chemical reaction in which two molecules combine to form a single new molecule.
Adsorb —To attach to the surface of a solid. The more finely divided the solid is, the more molecules can absorb on its surface.
Catalyst —Any agent that accelerates a chemical reaction without entering the reaction or being changed by it.
Fat —A solid ester of glycerol and long-chain carboxylic acids.
Le Châtelier’s principle —A statement describing the behavior of mixtures undergoing a chemical reaction. This principle states that in a chemical reaction at its steady state, addition of more of a reactant or product will cause the readjustment of concentrations to maintain the steady state.
Oil —A liquid ester of glycerol and long-chain carboxylic acids.
Organic —A term used to describe molecules containing carbon atoms.
Saturation —A molecule is said to be saturated if it contains only single bonds, no double or triple bonds.
(high-density lipoprotein, HDL), numbers that show increased risk to heart disease and heart attacks
Biological hydrogenation
Many chemical reactions within the body require the addition of two atoms of hydrogen to a molecule in order to maintain life. These reactions are much more complex than the ones described above, because hydrogen gas is not found in the body. These kinds of reactions require carrier molecules, which give up hydrogen atoms to the one undergoing hydrogenation. The catalyst in biological hydrogenation is an enzyme, a complex protein that allows the reaction to take place in the blood, at a moderate temperature, and at a rate fast enough for metabolism to continue.
Hydrogenation reactions can happen to many other types of molecules as well. However, the general features for all of the reactions are the same. Hydrogen atoms add to multiple bonds in the presence of a catalyst, to product a new compound, with new characteristics. This new compound has different properties than the original molecule had.
Resources
BOOKS
Carey, Francis A. Organic Chemistry. New York: McGraw-Hill, 2002.
Nishimura, Shigeo. Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis. New York: J. Wiley, 2001.
Zweifel, George S. Modern Organic Synthesis: An Introduction. New York: W.H. Freeman, 2007.
OTHER
Chemicals from Petroleum. London: Audio Learning, 1982. 35mm Film strip.
G. Lynn Carlson
Hydrogenation
Hydrogenation
Hydrogenation is a chemical reaction in which hydrogen atoms add to carbon-carbon multiple bonds. In order for the reaction to proceed at a practical rate , a catalyst is almost always needed. Hydrogenation reactions are used in many industrial processes as well as in the research laboratory, and occur also in living systems. We will look at a few examples in each category in this article.
The hydrogenation reaction
Hydrogen gas, H2, can react with a molecule containing carbon-carbon double or triple bonds. In its simplest form, a molecule with one double bond would react with one molecule of hydrogen gas. An example is shown below.
Many carbon compounds have triple bonds, and in a case such as that, two molecules of hydrogen are necessary to completely saturate the carbon compound with hydrogen.
Hydrogenation of a double or triple carbon-carbon bond will not occur unless the catalyst is present. Scientists have developed many catalysts for this kind of reaction. Most of them include a heavy metal , such as platinum or palladium, in finely divided form. The catalyst adsorbs both the carbon compound and the hydrogen gas on its surface, in such a way that the molecules are arranged in just the right position for addition to occur. This allows the reaction to proceed at a fast enough rate to be useful.
Because at least one of the reagents (hydrogen) is a gas, often the reaction will occur at an even faster rate if it occurs in a pressurized container, at a pressure several times higher than atmospheric pressure .
Hydrogenation in the research laboratory
The hydrogenation reaction is a useful tool for a scientist trying to determine the structure of a new molecule. The molecular formula , showing the exact number of each kind of atom, can be determined in several ways, but discovering the arrangement of these atoms requires a large amount of detective work.
Sometimes, for example, a new substance is isolated from a plant , and a chemist needs to determine what the structure of this substance is. One method of attack is to find out how many molecules of hydrogen gas will react with one molecule of the unknown substance. If the ratio is, for example, two molecules of hydrogen to one of the unknown, the scientist can deduce that there are two carbon-carbon double bonds, or else one carbon-carbon triple bond in each molecule. Other kinds of chemical clues lead to the rest of the structure, and help the scientist to decide where in the unknown molecule the multiple bonds are.
One of the simplest uses of hydrogenation in the research laboratory is to make new compounds. Almost any organic molecule that contains multiple bonds can undergo hydrogenation, and this sometimes leads to compounds that were unknown before. In this way scientists have synthesized and examined many molecules not found in nature, or not found in sufficient quantity. These newly synthesized molecules are of use to humanity in a variety of ways.
Hydrogenation in industry
Many of the carbon compounds found in crude petroleum are of little use. These compounds may contain multiple bonds, but can be converted to saturated compounds by catalytic hydrogenation. This is one source for much of the gasoline that we use today. Other chemicals besides gasoline are made from petroleum, and for these, too, the first step from crude oil may be hydrogenation.
Another commercial use of the hydrogenation reaction is the production of fats and oils in more useful forms. Fats and oils are not hydrocarbons, like the simple molecules we have been looking at, since they contain oxygen atoms, too. But they do contain long chains of carbon and hydrogen, joined together in part by carbon-carbon double bonds. Partial hydrogenation of these molecules, so that some, but not all of the double bonds react, gives compounds with different cooking characteristics, more satisfactory for consumers in some situations than the original oils. This is the source of the "partially hydrogenated vegetable oil" on the grocery shelf.
Biological hydrogenation
Many chemical reactions within the body require the addition of two atoms of hydrogen to a molecule in order to maintain life. These reactions are much more complex than the ones described above, because hydrogen gas is not found in the body. These kinds of reactions require "carrier" molecules, which give up hydrogen atoms to the one undergoing hydrogenation. The catalyst in biological hydrogenation is an enzyme , a complex protein that allows the reaction to take place in the blood , at a moderate temperature , and at a rate fast enough for metabolism to continue.
Hydrogenation reactions can happen to many other types of molecules as well. However, the general features for all of the reactions are the same. Hydrogen atoms add to multiple bonds in the presence of a catalyst, to product a new compound, with new characteristics. This new compound has different properties than the original molecule had.
Resources
books
Bettelheim, Frederick A., and Jerry March. Introduction to General, Organic, and Biological Chemistry. 3rd ed. Fort Worth: Saunders College Publishing, 1991.
Carey, Francis A. Organic Chemistry. New York: McGraw-Hill, 2002.
Cross, Wilbur. Petroleum. Chicago: Children's Press, 1983.
other
Chemicals from Petroleum. London: Audio Learning, 1982. 35mm Film strip.
G. Lynn Carlson
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Addition
—A type of chemical reaction in which two molecules combine to form a single new molecule.
- Adsorb
—To attach to the surface of a solid. The more finely divided the solid is, the more molecules can absorb on its surface.
- Catalyst
—Any agent that accelerates a chemical reaction without entering the reaction or being changed by it.
- Fat
—A solid ester of glycerol and long-chain carboxylic acids.
- Le Châtelier's principle
—A statement describing the behavior of mixtures undergoing a chemical reaction. This principle states that in a chemical reaction at its steady state, addition of more of a reactant or product will cause the readjustment of concentrations to maintain the steady state.
- Oil
—A liquid ester of glycerol and long-chain carboxylic acids.
- Organic
—A term used to describe molecules containing carbon atoms.
- Saturation
—A molecule is said to be saturated if it contains only single bonds, no double or triple bonds.