Difference Between Hydrogenation and Hydrogenolysis

Main Difference – Hydrogenation vs Hydrogenolysis

Hydrogenation and hydrogenolysis are important chemical reactions that occur in the presence of hydrogen gas and catalysts. Although the names seem similar, the two processes are different from each other. Basically, the main difference between hydrogenation and hydrogenolysis is that hydrogenation includes the formation of a saturated compound from an unsaturated compound whereas hydrogenolysis includes the formation of two small compounds from a large molecule.

Key Areas Covered

1. What is Hydrogenation
     – Definition, Process
2. What is Hydrogenolysis
     – Definition, Process, Uses
3. What are the Similarities Between Hydrogenation and Hydrogenolysis
     – Outline of Common Features
4. What is the Difference Between Hydrogenation and Hydrogenolysis
     – Comparison of Key Differences

Key Terms: Catalyst, Hydrogenation, Hydrogen Gas, Hydrogenolysis, Saturated, Unsaturated

What is Hydrogenation

Hydrogenation is a chemical reaction that occurs between molecular hydrogen (H₂) and another chemical species. This reaction usually takes place in the presence of a catalyst such as nickel, palladium or platinum (or 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

Figure 1: Hydrogenation of Alkene

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

What is Hydrogenolysis

Hydrogenolysis is a cleavage reaction where the hydrogen molecule (H₂) reacts with an organic compound, resulting in two small compounds. The bond that is broken down can be a C-C single bond or C-X heteroatom single bond. X (heteroatom) is usually oxygen, nitrogen or sulfur. This process is done using hydrogen gas in the presence of a catalyst.

Figure 2: Hydrogenolysis of 4-(dimethylamino) benzyl acetate to N,N,4-trimethylaniline by Nickel boride

Hydrogenolysis reaction has laboratory scale applications as well as industrial scale applications. For example, in laboratories, hydrogenolysis is used in organic synthesis reactions. ex: debenzylation – cleavage of benzyl ethers. However, the operation of hydrogenolysis in the laboratory is somewhat similar to hydrogenation. This reaction is widely used in petrochemical industry. It is used to remove sulfur from feedstock in petroleum refineries. The sulfur is removed in the form of hydrogen sulfide (H₂S).  

Similarities Between Hydrogenation and Hydrogenolysis

• Both reactions occur in the presence of molecular hydrogen.
• Both processes occur in the presence of a catalyst.

Difference Between Hydrogenation and Hydrogenolysis

Definition

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

Hydrogenolysis: Hydrogenolysis is a cleavage reaction in which the hydrogen molecule (H₂) reacts with an organic compound resulting in two small compounds.

Process

Hydrogenation: Hydrogenation involves the reduction and saturation of chemical compounds.

Hydrogenolysis: Hydrogenolysis involves bond cleavage.

Chemical Bonding

Hydrogenation: Hydrogenation occurs in compounds containing double bonds or triple bonds.

Hydrogenolysis: Hydrogenolysis occurs in compounds containing C-C or C-X (heteroatom) single bonds.

End Product

Hydrogenation: Hydrogenation produces a saturated compound.

Hydrogenolysis: Hydrogenolysis produces two small molecules.

Conclusion

Hydrogenation and Hydrogenolysis are important chemical reactions used in laboratories as well as in industries. The main difference between hydrogenation and hydrogenolysis is that hydrogenation includes the formation of a saturated compound from an unsaturated compound whereas hydrogenolysis includes the formation of two small compounds from a large molecule.

Reference:

1. “Hydrogenolysis.” Wikipedia, Wikimedia Foundation, 31 Aug. 2017, Available here.
2. “HYDROGENOLYSIS TECHNOLOGY.” Johnson Matthey Process Technologies, Available here.
3. “Hydrogenation.” Wikipedia, Wikimedia Foundation, 24 Jan. 2018, Available here.

Image Courtesy:

1. “Alkene to alkane” By The original uploader was Robert at English Wikibooks – Transferred from en.wikibooks to Commons by Adrignola using CommonsHelper (Public Domain) via Commons Wikimedia
2. “Hydrogenolysis of a benzylic ester by Nickel boride” By LHcheM – Own work (CC BY-SA 3.0) via Commons Wikimedia