9. Thermodynamics of Organic Transformations in Hydrothermal Fluids

Hydrothermal Photochemistry as a Mechanistic Tool in Organic Geochemistry: The Chemistry of Dibenzyl Ketone

“…The increased thermal energy and increased K w facilitate rapid Brønsted acid- and base-catalyzed reactions that are not observed under ambient conditions, specifically dehydration of the ketones 3a and 4a . Although alcohol dehydration has been observed many times in hydrothermal organic reactions, this work points to dehydration as a generally favorable process for different kinds of structures . The follow-up reactions in Scheme are mainly Brønsted acid-catalyzed, and it is well-known that natural geologic systems are characterized by widely varying pH and distributions of organic materials and functional groups .…”

“…Reactions of organic material in the Earth’s crust represent an important component of the deep (or geochemical) carbon cycle, which is an important factor in controlling atmospheric carbon dioxide levels . Subsurface organic reactions are critical to petroleum generation and maturation processes, can provide energy sources for deep microbial communities, and have been implicated in the formation of ore deposits …”

“…Examples include the endothermic elimination of hydrogen from alcohols and alkanes to form carbonyls and alkenes and the dehydration of alcohols to form alkenes. − Alcohol dehydration is particularly surprising since water is the solvent. The equilibrium between alcohol and alkene + water is strongly temperature-dependent, and for butanol the equilibrium is on the side of butene + water at temperatures above 200 °C at the vapor–liquid saturation pressure of water . At higher temperatures the entropic contribution to the free energy increases, so eventually water elimination, which increases the entropy, becomes favorable despite being endothermic.…”

Organic Oxidations Using Geomimicry

“…In conventional aqueous oxidations of aldehydes to carboxylic acids, water first adds to the carbonyl to form the hydrate, which is subsequently oxidized (Scheme 4, pathway A). 48 Under hydrothermal conditions, however, dehydration reactions (loss of H 2 O) tend to dominate over hydration (addition of H 2 O), 49 which suggests that an alternate mechanism should be considered, i.e., oxidation followed by H 2 O addition (Scheme 4, pathway B). Reaction for 6 h of 0.20 molal Cu(II) with benzaldehydes substituted in the paraposition with −CH 3 and −CF 3 substituents resulted in conversions of 45% and 31%, respectively, compared to 39% for the unsubstituted aldehyde.…”