CATALYTIC HYDROGENATION: CHARACTERISTICS, TYPES AND MECHANISM - CHEMISTRY - 2025

The catalytic hydrogenation is the reaction by which molecular hydrogen is added to a compound at higher speeds. Not only must the H ₂ molecule 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. Examples of catalytic hydrogenation are most often found in organic compounds; in particular, those that present pharmacological activity, or that have metals incorporated in their structures (organometallic compounds).

Source: Gabriel Bolívar

What happens when H ₂ is added to a carbon-packed structure? Its unsaturation decreases, that is, carbon reaches the maximum degree of simple bonds that it can form.

Therefore, H ₂ is added to 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 H ₂ just by detecting double and triple bonds.

Characteristics of catalytic hydrogenation

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.

Hydrogen bond breaks

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 CH bonds formed, which require more energy for their subsequent breaking than the HH bond of molecular hydrogen requires.

On the other hand, hydrogenation always involves breaking the HH bond first. This rupture can be homolytic, as it happens in many cases:

HH => H ∙ + ∙ H

Or heterolytic, which can occur, for example, when zinc oxide, ZnO, is hydrogenated:

HH => 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; while if the distribution is ionic, one ends up without electrons, H ⁺, and the other gains them completely, H ^-.

Both breaks are possible in catalytic hydrogenation, although the homolytic one allows to give way to the development of a logical mechanism for this.

Experimental

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 the hydrogenation would proceed very slowly.

Once the compound to be hydrogenated is dissolved, 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 phase with the compound and hydrogen dissolved, and a solid phase, that of the catalyst.

These metals provide their surface for the hydrogen and the compound to 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.

Types

Homogeneous

We speak of homogeneous catalytic hydrogenation when the reaction medium consists of a single phase. The use of metals in their pure states does not fit here, 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, ₃ RhCl. These compounds form a complex with H ₂, activating it for its subsequent addition reaction to alkene or alkyne.

Homogeneous hydrogenation presents 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

Heterogeneous catalytic hydrogenation, as just mentioned, has two phases: one liquid and one 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 peculiarity 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 + H ₂ => RHC = CHR

Mechanism

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 H ₂ molecule (purple color) approaches the metal surface as does the tetra substituted alkene, R ₂ C = CR ₂.

The H ₂ interacts with the electrons that run through the atoms of the metal, and a rupture and formation of a temporary bond HM 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 HH bond has already dissociated and each hydrogen atom remains bonded to the metal; it does the same with the metal centers in organometallic catalysts, forming an intermediate HMH complex.

There then occurs 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 ₂ HC-CHR ₂, is finally released.

This mechanism will be repeated as many times as necessary, until all the H ₂ has fully reacted.

References

1. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10 ^th edition.). Wiley Plus.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
4. Lew J. (nd). Catalytic Hydrogenation of Alkenes. Chemistry LibreTexts. Recovered from: chem.libretexts.org
5. Jones D. (2018). What is Catalytic Hydrogenation? - Mechanism & Reaction. Study. Recovered from: study.com