Catalytic hydrogenation

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1. Catalytic hydrogenation: In the presence of catalysts such as Pt, Pd, and Ni, alkenes and alkynes undergo addition reactions with hydrogen to form corresponding alkanes, releasing heat; this heat is known as the heat of hydrogenation (the heat released when 1 mol of an unsaturated hydrocarbon is hydrogenated). Mechanism of catalytic hydrogenation (changing the reaction pathway, reducing the activation energy): Hydrogen molecules adsorbed on the catalyst generate reactive hydrogen atoms, which then add to the double or triple bonds of alkenes and alkynes whose bonds have been weakened by the catalyst. (1) The more alkyl groups attached to the double-bonded carbon atom, the lower the hydrogenation enthalpy and the more stable the alkene: R2C=CR2 > R2C=CHR > R2C=CH2 > RCH=CH2 > CH2=CH2. (2) The trans isomer is more stable than the cis isomer. (3) The hydrogenation enthalpy of acetylene is -313.8 kJ•mol⁻¹, which is greater than twice that of ethylene (-274.4 kJ•mol⁻¹); therefore, acetylene is less stable than ethylene. Control of alkyne hydrogenation: 1. Using a catalyst with lower activity can keep the alkyne hydrogenation at the alkene stage. 2. By using different catalysts and conditions, the configuration of olefins can be controlled: For example, by passivating a palladium/calcium carbonate catalyst with a small amount of lead acetate or quinoline, a Lindlar catalyst is obtained, which catalyzes the hydrogenation of alkynes to cis-olefins ； Alkynes can be reduced to trans alkenes in liquid ammonia using metallic sodium or lithium. Significance of catalytic hydrogenation of alkynes: • ——To selectively produce cis or trans alkenes, thereby achieving directed synthesis ； ——Increase the content and quality of alkanes (from crude gasoline to hydrogenated gasoline) or olefins. Catalytic hydrogenation of cycloalkanes: Catalytic hydrogenation of cycloalkanes results in the formation of alkanes. Comparing the hydrogenation conditions, it can be seen that the difficulty of ring opening increases in the order of cyclopropane, cyclobutane, cyclopentane, and cyclohexane, while the stability of the rings increases in the same order. Second, catalytic oxidation – I guess I won’t go into details on that in inorganic chemistry! In organic chemistry, oxidation reactions carried out in the presence of catalysts such as Pt, Pd, and Ni are considered catalytic oxidations, including both \"oxygenation\" and \"dehydrogenation\". For example, ethanol CH3CH2OH is converted to CH3CHO, which is a dehydrogenation process in which the carbon-oxygen single bond turns into a double bond. As for the conversion of acetaldehyde to acetic acid, CH3CHO – CH3COOH, one additional oxygen atom is present. Oxygenation oxidation. Second, hydrogen peroxide is an important chemical product; the current production methods include electrolysis, oxygen cathode reduction, alcohol oxidation, direct hydroxylation, and the anthraquinone process. (1) Electrolytic method. Electrolysis was the main method for producing hydrogen peroxide in the first half of the 20th century. In this method, platinum was used as the anode and lead or graphite as the cathode; a saturated ammonium bisulfate solution was electrolyzed to produce persulfuric acid, which was then hydrolyzed with dilute sulfuric acid to yield hydrogen peroxide. This method requires high energy consumption and is limited to small-scale production. (2) Oxygen cathode reduction method. The production of hydrogen peroxide using this method involves placing a strongly alkaline electrolyte in an electrolyzer, where the oxygen in the air is reduced to hydroxyl anions at the cathode; these anions are then converted into hydrogen peroxide in a recovery unit. This process relies on the precipitation of calcium salts to form calcium peroxide, which is subsequently filtered and decomposed, with CO2 being used to produce hydrogen peroxide while calcium carbonate is generated for reuse. This method is simple for producing hydrogen peroxide, has low costs, and causes no pollution, but the concentration of hydrogen peroxide in the product is low. (3) Alcohol oxidation method. This method was developed by Shell and DuPont companies. The United States and the former Soviet Union built industrial production facilities using isopropanol as a raw material. In addition to isopropanol, alcohols such as cyclohexanol and 1-phenylethanol can also be used in this method, but isopropanol is the most commonly employed substance, with propylene being produced as a by-product. This method does not use any catalysts; it generates hydrogen peroxide through automatic oxidation using air, but the cost of steam is high, and co-production of propylene is generally not favorable. (4) Direct hydroxylation method. This method uses water as the reaction medium, which contains almost no organic substances; only a small amount of bromides is present as a catalyst. Pt (such as Pt/C) is used as the catalyst, and hydrogen and oxygen (or air) serve as the reactants. Hydrogen peroxide is synthesized through continuous reaction at a reaction temperature of 0–25°C and a pressure of 2.9–19.3 Mpa. The entire process of synthesizing hydrogen peroxide using this method does not produce any organic substances, resulting in virtually no waste. However, the selectivity for hydrogen peroxide in the product is low, and the method is hazardous. To date, there are no reports of its industrial application, but it holds great potential for development. This method will be a focus of research and development worldwide in the coming period. (5) Anthraquinone method. This method uses anthraquinone compounds as hydrogen carriers (or working carriers) to enable the reaction of hydrogen and oxygen to produce hydrogen peroxide. This method is currently the most widely used approach for industrial hydrogen peroxide production, accounting for a dominant share of output – roughly 95% of the world’s total hydrogen peroxide production. This method involves dissolving anthraquinone derivatives (usually 2-alkylanthraquinones) in an organic solvent to form a working solution; in most processes, an appropriate amount of anthraquinone is present in this working solution. The working solution is then hydrogenated in the presence of a catalyst to produce anthrahydroquinones ； Oxidation with air or oxygen in the absence of any catalyst produces hydrogen peroxide and anthraquinone (for reuse) ； Finally, extraction is carried out using pure water, and after purification and concentration, hydrogen peroxide of various concentrations is obtained ； The extract is recycled after regeneration treatment. The industrial production technology for this method is highly mature, featuring advanced technology and a high degree of automation. It has low costs and energy consumption, making it suitable for large-scale industrial production of hydrogen peroxide