Structure–Property Effects in the Generation of Transient Aqueous Benzoic Acid Anhydrides by Carbodiimide Fuels

The design of dissipative systems, which operate out-of-equilibrium
by consuming chemical fuels, is challenging. As yet, there are a few
examples of privileged fuel chemistries that can be broadly applied
in abiotic systems in the same way that ATP hydrolysis is exploited
throughout biochemistry. The key issue is that designing nonequilibrium
systems is inherently about balancing the relative rates of coupled
reactions. The use of carbodiimides as fuels to generate transient
aqueous carboxylic anhydrides has recently been used in examples of
new nonequilibrium materials and supramolecular assemblies. Here,
we explore the kinetics of formation and decomposition of a series
of benzoic anhydrides generated from the corresponding acids and EDC
under typical conditions (EDC = N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride). The reactions
can be described by a simple mechanism that merges known behavior
for the two processes independently. Structure–property effects
in these systems are dominated by differences in the anhydride decomposition
rate. The kinetic parameters allow trends in concentration-dependent
properties to be simulated, such as reaction lifetimes, peak anhydride
concentrations, and yields. For key properties, there are diminishing
returns with the addition of increasing amounts of fuel. These results
should provide useful guidelines for the design of functional systems
making use of this chemistry.