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The review is mainly focused on developments of hydrogenation and dehydrogenation reaction on organic compounds using a homogeneous catalyst. In most of the cases, it has been found that hydrogenation and dehydrogenation reactions are carried out by complexes of transition elements or sometimes alone elemental transition elements. Organic chemists are also inclined towards the development of metal catalysts having a high degree of selectivity on specific functional groups.
In an alkene, the main functionality is at “C=C” where we have C-C σ bond and a π bond. Now we know that π- bond is weaker than σ bond – so the reaction is expected to proceed at π bond initially. When an alkene is treated with hydrogen in presence of a catalyst such as Ni, we get an alkane. Raney Ni is prepared by dissolving Ni-Al alloy (in the form of dust) in NaOH. Aluminum dissolves via the formation of sodium aluminate and H2. Ni remains in particle form. It absorbs the liberated H2 partially. The powdered Ni is called Raney Ni which is washed with distilled water several times that makes it free from alkali. Raney Ni under methanol is a primitive reducing agent. But the question is how reaction proceeds. We have given a simple schematic representation of mechanism of catalytic reduction of alkene which is done by Raney Ni.
Through this way, catalyst decreases the energy of activation of the hydrogenation and it is a syn addition. As reaction proceeds reacting species converted to products, ΔS = -ve. Here 1 σ bond in H2 and 1 π bond in alkene are broken in reactants. Besides two σ bonds are formed in the product. As a result, enthalpy of reaction (ΔH) is negative. Activation of dihydrogen is a key step in catalytic hydrogenation as bond dissociation energy of molecular hydrogen is very high (~105kcal/mol). One possible way of activation of molecular hydrogen is to transfer the bonding electron of hydrogen to a vacant orbital of metal M-H2+ 1. The second possible way is to the allocation of a metal orbital electron to antibonding σ* of molecular hydrogen i.e. M-H2-. The transition state would be understood by three centers two electrons system for both possibilities.
Metals ion like Cu (II) having 3d9, Cu(I) having 3d10 etc. are capable of forming M-H2- complex as metal ions have almost or filled ‘d’ orbital i.e. they are in low oxidation state. Significant research has been done on the development of the hydrogenation catalyst using noble elements like Ru, Ir, Pd etc2. But those metals are highly expensive so it is better to replace those elements by first-row transition elements like Fe and Mn3. Manganese complexes are being used in oxidation purpose for several years. Nowadays research is going on to explore the catalytic reduction of organic compounds using Mn complexes....