Difference Between Hydrogenation and Reduction

Main Difference – Hydrogenation vs Reduction

Hydrogenation and reduction reactions are related chemical processes. Hydrogenation is a type of reduction. Reduction has three different definitions: decrease of the oxidation number, loss of oxygen, and gaining of hydrogen. The most acceptable definition is the decrease of the oxidation number. The third definition (gaining hydrogen) fits the definition of hydrogenation. However, hydrogenation essentially requires a catalyst whereas reduction does not require a catalyst, unless it is a hydrogenation. This is the main difference between hydrogenation and reduction.

Key Areas Covered

1. What is Hydrogenation
      – Definition, Mechanism
2. What is Reduction
     – Definition, Explanation, Examples
3. What is the Relationship Between Hydrogenation and Reduction
4. What is the Difference Between Hydrogenation and Reduction
     – Comparison of Key Differences

Key Terms: Catalyst, Hydrogenation, Nickel, Oxidation, Oxidation Number, Palladium, Platinum, Redox Reactions, Reduction, Saturation, Unsaturated

What is Hydrogenation

Hydrogenation is a chemical reaction between molecular hydrogen (H₂) and another chemical species. A hydrogenation usually occurs in the presence of a catalyst such as nickel, palladium or platinum and their oxides. Hydrogenation is used for the reduction and saturation of a chemical compound. There can be two types of hydrogenation:

1. Addition of hydrogen to a double bond or triple bond in a compound
2. Addition of hydrogen that causes dissociation of the molecule

Nearly all organic compounds composed of double bonds or triple bonds can react with molecular hydrogen in the presence of a catalyst. The hydrogenation reaction is of great importance to industrial synthesis processes; for example, in the petroleum industry, hydrogenation is used to manufacture gasoline and various petrochemicals.

Mechanism of Hydrogenation

When considering the mechanism of hydrogenation, there are three main components that need to be considered. They are the unsaturated substrate, hydrogen source, and a catalyst. The temperature and pressure of the system may vary depending on the substrate and the catalyst used.

The hydrogen source is often hydrogen gas itself. It is commercially available in pressurized cylinders. The substrates for hydrogenation can be alkenes, alkynes, aldehydes, alcohols, ketones, esters, carboxylic acids and nitro compounds. Catalysts are found in two types as homogeneous catalysts and heterogeneous catalysts. The difference between homogeneous and heterogeneous catalysts is that homogeneous catalysts dissolve in the solution containing the substrate whereas heterogeneous catalysts are suspended in that solution.

Figure 1: Mechanism of Hydrogenation

Hydrogenation that occurs in solids (heterogeneous catalyst) is explained by the Horiuti-Polanyi mechanism. It states the following steps.

1. Binding of the unsaturated bond and atomic hydrogen (from dissociation of molecular hydrogen) to the surface of the catalyst.
2. Addition of one hydrogen atom to the unsaturated bond. This step is reversible.
3. Addition of second hydrogen atom; this is irreversible.

But in hydrogenation processes that involve homogeneous catalysis, the metal binds to both components to give an intermediate alkene-metal(H)₂ complex.

What is Reduction

Reduction is decreasing the oxidation number of a chemical species. It is usually done by addition of electrons. A reduction reaction is a half reaction that occurs parallel to an oxidation reaction in redox reactions. Oxidation reaction increases the oxidation number. Therefore, reduction is the opposite of oxidation.

But in some reactions, oxidation and reduction refer to the addition or removal of oxygen respectively. Here, oxidation is the gain of oxygen whereas reduction is the loss of oxygen. Another older definition for oxidation and reduction involves the transfer of hydrogen. Here, oxidation is the loss of hydrogen whereas reduction is the gain of hydrogen. However, according to the generally accepted definitions, oxidation is known to be the increase of oxidation state whereas reduction is known to be the decrease of the oxidation state.

Figure 2: Redox Reactions

The reduction can be one of the three following phenomena.

1. Decreasing the oxidation number from a positive value to a negative value
2. Decreasing the oxidation number from a positive value to zero
3. Decreasing the oxidation number from zero to a negative value.

Examples

Some common examples of reduction reactions:

• Reduction of iron (III) to iron (II):
• Reduction of H⁺ to H₂
• Reduction of Cu⁺² to Cu

Relationship Between Hydrogenation and Reduction

• Hydrogenation is a type of reduction reaction. One of the three definitions of reduction indicates that gain of hydrogen is a reduction.

Difference Between Hydrogenation and Reduction

Definition

Hydrogenation: Hydrogenation is a chemical reaction between molecular hydrogen (H₂) and another chemical species.

Reduction: Reduction is the decrease of the oxidation number of a chemical species.

Mechanism

Hydrogenation: Hydrogenation is essentially the addition of hydrogen.

Reduction: Reduction is the decrease of the oxidation number; loss of oxygen; gaining of hydrogen.

Components

Hydrogenation: Compounds containing double bonds or triple bonds will undergo hydrogenation.

Reduction: Chemical species having a higher oxidation number can undergo reduction.

Catalyst

Hydrogenation: Hydrogenation requires a catalyst as an essential component.

Reduction: Reduction reaction does not require a catalyst unless it is a hydrogenation.

Conclusion

Hydrogenation is a type of reduction reaction. It is used to convert unsaturated compounds into saturated compounds. Reduction refers to decreasing the oxidation number of a chemical species. It always occurs parallel to an oxidation reaction. The main difference between hydrogenation and reduction is that hydrogenation essentially requires a catalyst whereas reduction does not require a catalyst unless it is a hydrogenation.

Reference:

1. Helmenstine, Anne Marie. “Reduction Definition in Chemistry.” ThoughtCo, Sep. 3, 2017, Available here.
2. Libretexts. “Definitions of Oxidation and Reduction.” Chemistry LibreTexts, Libretexts, 2 May 2016, Available here.
3. “Hydrogenation.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 1 May 2017, Available here.

Image Courtesy:

1. “Mechanism Involving Palladium Hydrogenation” By Wesleye1000 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Redox Halves” By Cameron Garnham – Own work (CC BY-SA 3.0) via Commons Wikimedia