The catalytic hydrogenation It is the reaction by which molecular hydrogen is added to a compound at higher rates. The H moleculetwo Not only must it first break its covalent bond, but also, being so small, efficient collisions between it and the compound to which it will be added are less likely..
The hydrogen receptor compound can either be organic or inorganic. In organic compounds are where most examples of catalytic hydrogenation are found; in particular, those that present pharmacological activity, or that have incorporated metals in their structures (organometallic compounds).
What happens when H is addedtwo to a structure full of carbon? Its unsaturation decreases, that is, carbon reaches the maximum degree of simple bonds that it can form.
Therefore, the Htwo it is added to the double (C = C) and triple (C≡C) bonds; although it can also be added to carbonyl groups (C = O).
Thus, the alkenes and alkynes added react by catalytic hydrogenation. By superficially analyzing any structure, it can be predicted whether or not it will add Htwo just by detecting double and triple bonds.
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The image shows the mechanism of this reaction. However, it is necessary to address some theoretical aspects before describing it.
The surfaces of the greyish spheres represent the metallic atoms that, as will be seen, are the catalysts of hydrogenation par excellence..
To begin with, hydrogenation is an exothermic reaction, that is, it releases heat as a result of the formation of compounds with lower energy..
This is explained by the stability of the C-H bonds formed, which require more energy for their subsequent breaking than the H-H bond of molecular hydrogen requires..
On the other hand, hydrogenation always involves breaking the H-H bond first. This rupture can be homolytic, as it happens in many cases:
H-H => H ∙ + ∙ H
Or heterolytic, which can occur, for example, when zinc oxide, ZnO, is hydrogenated:
H-H => H+ + H-
Note that the difference between the two breaks lies in how the electrons in the bond are distributed. If they are distributed evenly (covalently), each H ends up conserving one electron; whereas if the partition is ionic, one ends up without electrons, H+, and the other one wins them completely, H-.
Both ruptures are possible in catalytic hydrogenation, although homolytic allows to give way to the development of a logical mechanism for this..
Hydrogen is a gas, and therefore, it must be bubbled and it must be ensured that only it predominates on the surface of the liquid.
On the other hand, the compound to be hydrogenated has to be solubilized in a medium, be it water, alcohol, ether, esters or a liquid amine; otherwise hydrogenation would proceed very slowly.
Dissolved the compound to be hydrogenated, there must also be a catalyst in the reaction medium. This will be responsible for accelerating the speed of the reaction.
In catalytic hydrogenation, finely divided metals of nickel, palladium, platinum or rhodium are often used, which are insoluble in almost all organic solvents. Therefore there will be two phases: a liquid with the compound and hydrogen dissolved, and a solid, that of the catalyst..
These metals provide their surface so that the hydrogen and the compound react, in such a way that the breaking of bonds is accelerated.
Likewise, they decrease the diffusion space of the species, increasing the number of effective molecular collisions. Not only that, but even the reaction takes place inside the pores of the metal..
We speak of homogeneous catalytic hydrogenation when the reaction medium consists of a single phase. Here there is no place for the use of metals in their pure states, since they are insoluble.
Instead, organometallic compounds of these metals are used, which are soluble, and have been shown to have high yields..
One of these organometallic compounds is Wilkinson's catalyst: tris (triphenylphosphine) rhodium chloride, [(C6H5)3P]3RhCl. These compounds form a complex with Htwo, activating it for its subsequent addition reaction to alkene or alkyne.
Homogeneous hydrogenation has many more alternatives than heterogeneous. Why? Because chemistry is the organometallic compounds is abundant: it is enough to change the metal (Pt, Pd, Rh, Ni) and the ligands (the organic or inorganic molecules linked to the metal center), to obtain a new catalyst.
Heterogeneous catalytic hydrogenation, as just mentioned, has two phases: one liquid and the other solid..
In addition to metallic catalysts, there are others that consist of a solid mixture; for example, Lindlar's catalyst, which is made up of platinum, calcium carbonate, lead acetate, and quinoline.
The Lindlar catalyst has the particularity that it is deficient for the hydrogenation of alkenes; However, it is very useful for partial hydrogenations, that is, it works excellently on alkynes:
RC≡CR + Htwo => RHC = CHR
The image shows the mechanism of catalytic hydrogenation using powdered metal as a catalyst..
The greyish spheres correspond to the metallic surface of, say, platinum. The molecule Htwo (purple color) approximates the metal surface as does tetra substituted alkene, RtwoC = CRtwo.
The Htwo it interacts with the electrons that travel through the metal atoms, and a rupture and formation of a temporary H-M bond occurs, where M is the metal. This process is known as chemisorption; that is, an adsorption by chemical forces.
Alkene interacts in a similar way, but the bond is formed by its double bond (dotted line). The H-H bond has already dissociated and each hydrogen atom remains bonded to the metal; in the same way it does it with the metallic centers in the organometallic catalysts, forming an intermediate complex H-M-H.
Then there is a migration of an H towards the double bond, and this opens forming a bond with the metal. The remaining H then binds to the other carbon of the original double bond, and the produced alkane, R, is finally released.twoHC-CHRtwo.
This mechanism will be repeated as many times as necessary, until all the Htwo have fully reacted.
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