Deamination of alicyclic and aromatic amines under geologically relevant hydrothermal conditions

Amines are a particular group of organic compounds of interest to deep-sea biology, organic geochemistry, and astrobiology research, in large part due to their involvement in biological metabolism, such as in the form of amino acids and proteins, participation in the subsurface carbon (C) and nitrogen (N) cycles, as well as their relevance to biomolecular precursors on early Earth and potential biosignatures beyond Earth. Although there have been many studies on the deamination of amines under hydrothermal conditions, few have determined the relative deamination rates and degradation mechanisms among different amine structures. In this study, we investigate the reaction kinetics and pathways of a group of aromatic and alicyclic amines, including aniline, benzylamine, cyclohexylamine, and cyclohexylmethylamine, under geologically relevant temperatures (200–275 °C) and pH ranges (5–9) for up to 120 h. Among the studied amines, the amine reactivity generally follows a trend of aniline < cyclohexylamine < cyclohexylmethylamine < benzylamine. Alcohols and secondary amines/imines are observed as the major products of amines, whose formation could follow a nucleophilic substitution (S_N1 or S_N2) mechanism. Hydrothermal experiments at different pH also show that deamination occurs more readily under acidic than alkaline hydrothermal conditions, indicating that the aminium form (R-NH₃⁺) accelerates deamination. These results suggest that the kinetics and pathways of hydrothermal amine transformations are controlled by both the amine structure and solution pH, which have implications for predicting the deamination processes of organic N, release of inorganic N (e.g., ammonia), and N cycling in geologically relevant hydrothermal systems.