Many recipes call for lemon juice to be squirted on fish to mitigate its unpleasant odor. In this example, the citric acid of lemon juice reacts with the amines in fish – often trimethylamine – to create salt complexes. Draw the arrow pushing mechanism and product for this reaction. Why does this trick eliminate the classic “fishy” smell? (Hint: Consider physical properties.) HO OH OH
The Correct Answer and Explanation is:
Arrow-Pushing Mechanism and Product
The reaction between citric acid and trimethylamine follows a straightforward acid-base mechanism. Citric acid, a tricarboxylic acid, donates protons to trimethylamine, a basic compound with a lone pair on its nitrogen atom. This leads to the formation of trimethylammonium citrate.
Mechanism Steps:
- Citric acid has three carboxyl groups (-COOH), each capable of donating protons (H⁺).
- The lone pair on the nitrogen in trimethylamine (N(CH₃)₃) attracts and accepts a proton, forming its conjugate acid, trimethylammonium (N(CH₃)₃H⁺).
- The resulting trimethylammonium ion then associates with the citrate anion, forming an ionic salt.
Why This Trick Works
The strong fishy smell primarily comes from trimethylamine, a volatile amine. Volatility is determined by a molecule’s ability to evaporate and enter the air, where it can be detected by the nose. Trimethylamine, being small and non-ionic, readily evaporates, producing its characteristic odor.
When citric acid protonates trimethylamine to form trimethylammonium citrate, the resulting product is an ionic salt. Ionic compounds are significantly less volatile than neutral molecules, meaning the salt complex remains in the fish rather than evaporating into the air. This greatly reduces the fishy smell.
Additionally, the salt formed is water-soluble, allowing it to be rinsed away more easily. The reduction in volatility and the increased solubility contribute to the effectiveness of lemon juice in neutralizing unpleasant fish odors.
