Anwar.Hydrogenation

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Named Reactions_Hydrogenation

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Named Reactions:

Named Reactions Dr. Shaikh Anwar R.

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Hydrogenation, to treat with hydrogen, also a form of chemical reduction , is a chemical reaction between molecular hydrogen (H 2 ) and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds . Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, generally an alkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogen adds to double and triple bonds in hydrocarbons . Hydrogenation

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Because of the importance of hydrogen, many related reactions have been developed for its use. Most hydrogenations use gaseous hydrogen (H 2 ), but some involve the alternative sources of hydrogen, not H 2 : these processes are called transfer hydrogenations . The reverse reaction, removal of hydrogen from a molecule, is called dehydrogenation . A reaction where bonds are broken while hydrogen is added is called hydrogenolysis , a reaction that may occur to carbon-carbon and carbon-heteroatom ( O , N , X ) bonds. Hydrogenation differs from protonation or hydride addition: in hydrogenation, the products have the same charge as the reactants.

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An illustrative example of a hydrogenation reaction is the addition of hydrogen to maleic acid to succinic acid . Numerous important applications are found in the petrochemical , pharmaceutical an d food industries. Hydrogenation of unsaturated fats produces saturated fats and, in some cases, trans fats.

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Process Hydrogenation has three components, the unsaturated substrate, the hydrogen (or hydrogen source) and, invariably, a catalyst. The reaction is carried out at different temperatures and pressures depending upon the substrate and the activity of the catalyst. Substrate The addition of H 2 to an alk e ne affords an alk a ne in the protypical reaction: RCH=CH 2 + H 2 → RCH 2 CH 3 (R = alkyl , aryl ) Hydrogenation is sensitive to steric hindrance explaining the selectivity for reaction with the exocyclic double bond but not the internal double bond. An important characteristic of alkene and alkyne hydrogenations, both the homogeneously and heterogeneously catalyzed versions, is that hydrogen addition occurs with " syn addition ", with hydrogen entering from the least hindered side. Typical substrates are listed in the table

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Substrates for and products of hydrogenation alkene , R 2 C=CR' 2 alkane, R 2 CHCHR' 2 alkyne , RCCR alkene , cis -RHC=CHR' aldehyde , RCHO primary alcohol, RCH 2 OH ketone , R 2 CO secondary alcohol, R 2 CHOH ester , RCO 2 R' two alcohols, RCH 2 OH, R'OH imine , RR'CNR" amine, RR'CHNHR" amide , RC(O)NR' 2 amine, RCH 2 NR' 2 nitrile , RCN imine , RHCNH easily hydrogenated further nitro , RNO 2 amine, RNH 2

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Catalysts With rare exception, no reaction below 480 °F occurs between H 2 and organic compounds in the absence of metal catalysts. The catalyst binds both the H 2 and the unsaturated substrate and facilitates their union. Platinum group metals, particularly platinum , palladium , rhodium , and ruthenium , form highly active catalysts, which operate at lower temperatures and lower pressures of H 2 . Non-precious metal catalysts, especially those based on nickel (such as Raney nickel and Urushibara nickel ) have also been developed as economical alternatives, but they are often slower or require higher temperatures. The trade-off is activity (speed of reaction) vs. cost of the catalyst and cost of the apparatus required for use of high pressures. Notice that the Raney-nickel catalysed hydrogenations require high pressures:

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Two broad families of catalysts are known - homogeneous catalysts and heterogeneous catalysts . Homogeneous catalysts dissolve in the solvent that contains the unsaturated substrate. Heterogeneous catalysts are solids that are suspended in the same solvent with the substrate or are treated with gaseous substrate. Homogeneous catalysts Illustrative homogeneous catalysts include the rhodium -based compound known as Wilkinson's cat alyst and the iridium-based Crabtree's catalyst. An example is the hydrogenation of carvone:

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Heterogeneous catalysts Heterogeneous catalysts for hydrogenation are more common industrially. As in homogeneous catalysts, the activity is adjusted through changes in the environment around the metal, i.e. the coordination sphere . Different faces of a crystalline heterogeneous catalyst display distinct activities, for example. Similarly, heterogeneous catalysts are affected by their supports, i.e. the material upon with the heterogeneous catalyst is bound, a carefully chosen catalyst can be used to hydrogenate some functional groups without affecting others, such as the hydrogenation of alkenes without touching aromatic rings, or the selective hydrogenation of alkynes to alkenes using Lindlar's catalyst . For example, when the catalyst palladium is placed on barium sulfate and then treated with quinoline , the resulting catalyst reduces alkynes only as far as alkenes. The Lindlar catalyst has been applied to the conversion of phenylacetylene to styrene .

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Hydrogen sources For hydrogenation, the obvious source of hydrogen is H 2 gas itself, which is typically available commercially within the storage medium of a pressurized cylinder. The hydrogenation process often uses greater than 1 atmosphere of H 2 , usually conveyed from the cylinders and sometimes augmented by "booster pumps". Gaseous hydrogen is produced industrially from hydrocarbons by the process known as steam reforming . Hydrogen may, in specialised applications, also be extracted ("transferred") from "hydrogen-donors" in place of H 2 gas. Hydrogen donors, which often serve as solvents include hydrazine , dihydronaphthalene, dihydroanthracene, isopropanol , and formic acid . [10] In organic synthesis , transfer hydrogenation is useful for the reduction of polar unsaturated substrates, such as ketones , aldehydes , and imines .

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Thermodynamics and mechanism Hydrogenation is a strongly exothermic reaction. In the hydrogenation of vegetable oils and fatty acids, for example, the heat released is about 25 kcal per mole (105 kJ/mol), sufficient to raise the temperature of the oil by 1.6-1.7 °C per iodine number drop. The mechanism of metal-catalyzed hydrogenation of alkenes and alkynes has been extensively studied. First of all isotope labeling using deuterium confirms the regiochemistry of the addition: RCH=CH 2 + D 2 → RCHDCH 2 D

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Heterogeneous catalysis On solids, the accepted mechanism today is called the Horiuti-Polanyi mechanism. Binding of the unsaturated bond, and hydrogen dissociation into atomic hydrogen onto the catalyst Addition of one atom of hydrogen; this step is reversible Addition of the second atom; effectively irreversible under hydrogenating conditions. Homogeneous catalysis In many homogeneous hydrogenation processes, the metal binds to both components to give an intermediate alkene-metal(H) 2 complex. The general sequence of reactions is assumed to be as follows or a related sequence of steps: binding of the hydrogen to give a dihydride complex ("oxidative addition"): L n M + H 2 → L n MH 2 binding of alkene: L n M(η 2 H 2 ) + CH 2 =CHR → L n-1 MH 2 (CH 2 =CHR) + L transfer of one hydrogen atom from the metal to carbon (migratory insertion) L n-1 MH 2 (CH 2 =CHR) → L n-1 M(H)(CH 2 -C H 2 R) transfer of the second hydrogen atom from the metal to the alkyl group with simultaneous dissociation of the alkane ("reductive elimination") L n-1 M(H) (CH 2 -C H 2 R) → L n-1 M + C H 3 -C H 2 R

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Industrial applications Catalytic hydrogenation has diverse industrial uses. In petrochemical processes, hydrogenation is used to convert alkenes and aromatics into saturated alkanes (paraffins) and cycloalkanes (napthenes). Hydrocracking of heavy residues into diesel is another application. In isomerization and catalytic reforming processes, some hydrogen pressure is maintained to hydrogenolyze coke and prevent its accumulation. Xylitol , a polyol, is produced by hydrogenation of the sugar xylose , an aldehyde.

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In the food industry Hydrogenation is widely applied to the processing of vegetable oils and fats . Complete hydrogenation converts unsaturated fatty acids to saturated ones. In practice the process is not usually carried to completion. Since the original oils usually contain more than one double bond per molecule (that is, they are polyunsaturated), the result is usually described as partially hydrogenated vegetable oil; that is some, but usually not all, of the double bonds in each molecule have been reduced. This is done by restricting the amount of hydrogen (or reducing agent) allowed to react with the fat.

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Hydrogenation results in the conversion of liquid vegetable oils to solid or semi-solid fats, such as those present in margarine . Changing the degree of saturation of the fat changes some important physical properties such as the melting point, which is why liquid oils become semi-solid. Semi-solid fats are preferred for baking because the way the fat mixes with flour produces a more desirable texture in the baked product. Since partially hydrogenated vegetable oils are cheaper than animal source fats, they are available in a wide range of consistencies, and have other desirable characteristics (e.g., increased oxidative stability (longer shelf life)), they are the predominant fats used in most commercial baked goods. Fat blends formulated for this purpose are called shortenings .

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