The conversion of triglycerides into biodiesel can be achieved in the presence of either catalytic acid or catalytic base. We have seen a mechanism for transesterification with catalytic acid. In contrast, the mechanism for base-catalyzed transesterification has fewer steps. The base, such as hydroxide, functions as a catalyst by establishing an equilibrium in which some alkoxide ions are present.

This equilibrium favors the hydroxide ions. Nevertheless, some ethoxide ions are present at equilibrium. These ethoxide ions are strong nucleophiles that can attack each ester moiety of the triglyceride according to the following mechanism.

In the final step, water is deprotonated, regenerating the catalyst.

(a) Draw a mechanism for the following process.

(b) When sodium hydroxide is used as a catalyst for transesterification, it is essential that only a small amount of the catalyst is present. Explain what would happen in the presence of too much sodium hydroxide.

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но: Н-о: HOEt :OEt Ethoxide OEt :ö: :ÖEt Et Et -ӧн Alcohol Ester