Homogeneous Catalysts

A homogeneous catalyst is any catalyst that is present in the same phase as the reactant molecules. There are numerous examples of homogeneous catalysts, and we will examine one that is commonly used in textbooks.

The alkene but-2-ene can exist as one of two isomers: cis where the methyl groups are located on the same side of the double bond, and trans where the methyl groups are located on opposite sides of the double bond (Figure 1.1 “Isomerization of But-2-ene”). To convert (isomerize) between the two structures, the carbon-carbon double bond must be broken and the molecule must rotate. This process has a relatively high activation energy of approximately 264 kJ/mol and is therefore fairly slow to occur at room temperature.

Figure1.1. Isomerization of But-2-ene

But-2-ene can exist as one of two isomers. This diagram shows the isomerization of but-2-ene.

A catalyst like iodine can be used to provide an alternate pathway for the reaction with a much lower activation energy of approximately 118 kJ/mol (Figures 1.2 “Catalyzed and Uncatalyzed Reaction Pathways”).

Figure 1.2. Catalyzed and Uncatalyzed Reaction Pathways

Potential energy diagrams of catalyzed and uncatalyzed reaction pathways.

In the catalyzed pathway, an iodine atom is generated that reacts with cis-but-2-ene to produce a reaction intermediate that has broken its carbon-carbon double bond and formed a new C-I bond and a radical (Figure 1.3 “But-2-ene Catalyzed Isomerization Steps”). The molecule can rotate more easily and the C-I breaks to reform the double bond.

Figure 1.3. But-2-ene Catalyzed Isomerization Steps

But-2-ene catalyzed isomerization steps using iodine as the catalyst.