Comparison of Unimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels

Quantum
mechanical molecular modeling is used [M06-2X/6-311++G­(2df,p)] to
compare activation energies and rate constants for unimolecular decomposition
pathways of saturated and unsaturated carboxylic acids that are important
in the production of biofuels and that are models for plant and algae-derived
intermediates. Dehydration and decarboxylation reactions are considered.
The barrier heights to decarboxylation and dehydration are similar
in magnitude for saturated acids (∼71 kcal mol–1), with an approximate 1:1 [H2O]/[CO2] branching
ratio over the temperature range studied (500–2000 K). α,β-Unsaturation
lowers the barrier to decarboxylation between 2.2 and 12.2 kcal mol–1 while increasing the barriers to dehydration by ∼3
kcal mol–1. The branching ratio, as a result, is
an order of magnitude smaller, [H2O]/[CO2] =
0.07. For some α,β-unsaturated acids, six-center transition
states are available for dehydration, with barrier heights of ∼35.0
kcal mol–1. The branching ratio for these acids
can be as high as 370:1. β,γ-Unsaturation results in a
small lowering in the barrier height to decarboxylation (∼70.0
kcal mol–1). β,γ-Unsaturation also leads
to a lowering in the dehydration pathway from 1.7 to 5.1 kcal mol–1. These results are discussed with respect to predicted
kinetic values for acids of importance in biofuels production.