What is Hydrogenation?
A chemical reaction between molecular hydrogen and an element or compound, generally in the presence of a catalyst, is known as hydrogenation. The process could be one in which hydrogen merely adds to a double or triple bond joining two atoms in the molecule's structure, or one in which hydrogen causes the molecule to dissociate (break up) (known as destructive hydrogenation or hydrogenolysis).
Margarine, mineral turpentine, and aniline are hydrogenation examples. Let us know what hydrogenation is, and more details associated with it from this article.
Catalyst Used in Hydrogenation
Let us look at the catalyst used in hydrogenation here.
The metals nickel, platinum, and palladium, as well as their oxides, are the most often used catalysts for hydrogenation reactions and are the best hydrogenation examples. In high-pressure hydrogenations, copper chromite and nickel supported on kieselguhr (loose or porous diatomite) are frequently used.
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Hydrogenation Reactions
The treatment of substances with molecular hydrogen (H2), which includes the addition of pairs of hydrogen atoms to compounds, is known as hydrogenation (generally unsaturated compounds). For the reaction to take place under normal temperature and pressure, a catalyst is usually required. Although gaseous hydrogen is used in most hydrogenation reactions, other hydrogen sources have been developed. Dehydrogenation is the reverse of hydrogenation, in which hydrogen is removed from the compounds. Because the products of hydrogenation have the same charge as the reactants, it differs from protonation or hydride addition.
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Hydrogenation A catalyst, such as palladium, can be used to add hydrogen across a double bond, such as the olefin in maleic acid.
In most hydrogenation reactions, three components are required: the substrate, the hydrogen source, and the catalyst. Depending on the catalyst and substrate used, the reaction is carried out at temperatures and pressures. An alkene is transformed to an alkane by hydrogenation. Hydrogen is added to compounds in a syn addition manner, with hydrogen being added to the same face of the compound and entering from the least hindered side. Alkenes become alkanes, alkynes become alkenes, aldehydes and ketones become alcohols, esters become secondary alcohols, and amides become amines through the process of hydrogenation.
Catalysts of Hydrogenation
Hydrogenation reactions between hydrogen and organic compounds will not occur below 480 degrees Celsius without the use of metal catalysts. The hydrogen-to-substrate conversion is facilitated by catalysts, which bind the H2 molecule. Platinum, palladium, rhodium, and ruthenium are active catalysts that can operate at low temperatures and pressures. Non-precious metal catalysts with similar activity at lower temperatures and pressures are now being researched. Raney nickel and other nickel-based catalysts have been created, although they still require high temperatures and pressures.
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Heterogeneous Catalysis - Heterogeneous catalysis is shown via the hydrogenation of ethylene (C2H4) on a solid support.
Catalysts are divided into two categories: homogeneous and heterogeneous catalysts. In the solvent containing the unsaturated substrate, homogeneous catalysts are soluble. In industry, heterogeneous catalysts are more common, and they are not soluble in the fluid containing the substrate. Metal-based heterogeneous catalysts are frequently attached to carbon or oxide-based substrates. The choice of support for these materials is critical, as the supports have the potential to affect the catalysts' activity. The most frequent source of hydrogen is hydrogen gas, which is commercially available.
Hydrogenation of vegetable oils and fatty acids (hydrogenation of oil catalyst) is an exothermic process that produces about 25 kcal/mol. The Horiuti-Polanyi process explains how hydrogenation happens on heterogeneous catalysts. The unsaturated bond connects to the catalyst first, then H2 dissociation as atomic hydrogen binds to the catalyst. The hydrogenation process is then irreversibly completed by attaching one hydrogen atom to the substrate in a reversible step, followed by the addition of a second atom.
By oxidative addition, the metal binds to hydrogen to form a dihydride complex for homogeneous catalysis. The metal is used to bind to the substrate, and then one of the hydrogen atoms is moved from the metal to the substrate by migratory insertion. As the newly produced alkane is dissociated by reductive elimination, the metal's second hydrogen atom is transferred to the substrate.
Industrial Uses of Hydrogenation Reactions
In industrial processes, heterogeneous catalytic hydrogenation is important. Hydrogenation is used in petrochemical processes to saturate alkenes and aromatics, making them less toxic and reactive. Because most vegetable oils are made up of polyunsaturated fatty acids, hydrogenation is also vital in their processing. Most, but not all, carbon-carbon double bonds are decreased during partial hydrogenation, making them more suitable for sale and consumption. The melting range of oils is influenced by the degree of saturation of fats; for example, liquid vegetable oils turn semi-solid at different temperatures.
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In margarine, partial hydrogenation is used. Margarine is a semi-solid butter substitute produced from unsaturated vegetable oil, which is liquid at room temperature. At room temperature, partial hydrogenation adds hydrogen atoms to fatty acids while decreasing double bonds, producing a semi-solid vegetable oil.
Incomplete hydrogenation of double bonds has health implications because some double bonds might isomerize from the cis to the trans form. Because the trans configuration has less energy than the cis form, this isomerization happens. The trans isomers have been linked to pathological disorders of the blood circulatory system (i.e.,atherosclerosis and heart disease).
Hydrogenation Products
In the industry, hydrogenation is widely used. Many products, raw materials, or ingredients are frozen, stored, or purified by hydrogenation. Ammonia, fuels (hydrocarbons), alcohols, pharmaceuticals, margarine, polyols, various polymers, and chemicals are all treated by hydrogenation (hydrogen chloride and hydrogen peroxide).
Vegetable oil is the most commonly hydrogenated product. Vegetable oil is converted from a liquid to a solid or semi-solid fat through hydrogenation. D-sorbitol syrup is created by hydrolyzing starches to produce dextrose, which is then hydrogenated to create sorbitol, or sugar alcohol. Hydrocracking, a process that breaks heavy crude's long hydrogen carbon chains into lighter petroleum products like diesel, gasoline, and jet fuel, uses hydrogenation in the petroleum industry.
The hydrogenation of oil catalyst is represented below.
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Key Points
Hydrogenation reactions usually include three components: hydrogen, the substrate, and catalysts, which help speed up the reaction at lower temperatures and pressures.
Heterogeneous and homogeneous catalysts are two types of catalysts with different hydrogenation mechanisms.
Hydrogenation reactions aren't just for converting alkenes to alkanes; they cover a wide range of reactions in which substrates can be effectively reduced.
Incomplete hydrogenation processes have been linked to circulatory disorders and have serious effects.
FAQs on Hydrogenation
1. What is hydrogenation and what happens during this chemical reaction?
Hydrogenation is a chemical reaction that involves the addition of hydrogen (H₂) across a double or triple bond in a molecule, typically an unsaturated organic compound. This process effectively saturates the compound by converting double bonds to single bonds or triple bonds to double or single bonds. For this reaction to occur at practical rates and moderate temperatures, a catalyst is almost always required.
2. What is a classic example of a hydrogenation reaction, as per the NCERT syllabus?
A classic and fundamental example of hydrogenation is the conversion of an alkene to an alkane. For instance, the hydrogenation of ethene to form ethane. The reaction is carried out by passing ethene gas and hydrogen gas over a finely divided catalyst like nickel, platinum, or palladium.
The chemical equation is:
CH₂=CH₂ (Ethene) + H₂ (Hydrogen) → (in the presence of Ni/Pt/Pd catalyst) → CH₃-CH₃ (Ethane)
3. How is hydrogenation used in the food industry to make products like margarine?
In the food industry, hydrogenation is used to convert liquid vegetable oils into solid or semi-solid fats. Vegetable oils are rich in polyunsaturated fatty acids, which makes them liquid at room temperature. Through partial hydrogenation, hydrogen atoms are added to some of these double bonds, increasing their saturation level and melting point. This process turns the liquid oil into a semi-solid fat, which is the basis for products like margarine and vanaspati ghee, giving them a butter-like consistency.
4. What is the key difference between homogeneous and heterogeneous catalysis in hydrogenation?
The key difference lies in the physical state (phase) of the catalyst relative to the reactants.
- In heterogeneous catalysis, the catalyst is in a different phase from the reactants. For example, a solid catalyst like Nickel (Ni) is used to hydrogenate a gaseous alkene. This is the most common type used in industrial processes because the catalyst can be easily separated from the product.
- In homogeneous catalysis, the catalyst exists in the same phase as the reactants. For example, a catalyst like Wilkinson's catalyst, which is soluble in the organic solvent containing the unsaturated compound.
5. Why is a catalyst like Nickel, Platinum, or Palladium essential for hydrogenation reactions?
A catalyst is essential because the direct reaction between molecular hydrogen (H₂) and an unsaturated compound is extremely slow at room temperature and pressure. The hydrogen molecule has a very strong H-H bond that requires a large amount of energy to break. A metal catalyst provides an alternative reaction pathway with a lower activation energy. It does this by adsorbing the hydrogen molecule onto its surface, weakening the H-H bond, and facilitating its addition across the double or triple bond of the other reactant.
6. What are trans fats, and how are they an unintended consequence of the partial hydrogenation of oils?
Trans fats are a type of unsaturated fatty acid with a specific chemical structure (trans configuration) that is rarely found in nature. They are an unintended byproduct of partial hydrogenation. During this process, the goal is to saturate some, but not all, double bonds. However, the reaction conditions can cause some of the remaining natural cis-configured double bonds to isomerise, or change their shape, into the more stable trans-configured form. Consumption of these trans fats has been linked to negative health effects, including an increased risk of heart disease.
7. What is the primary purpose of hydrogenation in organic chemistry?
The primary purpose of hydrogenation in organic chemistry is the reduction of unsaturated compounds. This process is strategically used to achieve several outcomes:
- Saturation: To convert unstable or more reactive alkenes and alkynes into stable, saturated alkanes.
- Functional Group Conversion: To transform various functional groups. For example, reducing aldehydes and ketones to alcohols, or nitro compounds to amines.
- Changing Physical Properties: To alter the physical state of a substance, such as increasing the melting point of oils to make them solid fats.
