Abstract:
A detailed systematic theoretical study of the mechanism of the homogeneous Brønsted -acid catalysis of D-glucose in
aqueous solution phase ("acid hydrolysis") is reported. G4MP2 with the SMD solvation model at B3LYP/6-31G(2df,p) are
employed to compute the free energies of the molecular and ionic species pertaining to the isomerization, protonation,
hydrogen cation transfer and decomposition processes of D-glucopyranose in aqueous solution phase. This information is
used to hypothesise a reaction mechanism that is of improved accuracy and completeness from the existing art. It is found
that rotation of the D-glucose alkyl carbon-carbon bond is a facile process and is very important to the subsequent catalytic
mechanism. This rotation produces two rotameric isomers which are of notably different thermodynamic stability and
reactivity, even with regard to the products of this acid catalysis. As a low energy process =E2 = ~3.8 6.7 kcal/mol), the
alkyl carbon-carbon bond may rotate toward the hydroxyl group at the adjacent "4" position reducing the energy required
to protonate that position by 3.0 7.2 kcal/mol (or 15 30 %). The combination of two rotomeric isomers with the six
structural isomers owing to the oxygen atoms, means that protonated D-glucose cations embark on a complex competition
of interconversion and decomposition that is both thermodynamically and kinetically influenced. The calculations support
the hypothesis that the acid-catalysed hydrolysis of D-glucose may yield a number of platform chemicals that have not
previously been suggested. These include the prospect of three isomers of 5-hydroxymethylfurfural (HMF); 5-
(hydroxymethyl)furan-2-carbaldehyde, 5-(hydroxymethyl)furan-3-carbaldehyde and 5-(hydroxymethyl)furan-4-
carbaldehyde. Vibrational spectra of these HMF isomers are also computed and compared to experimentally determined
infrared spectra of "humins". On this basis, it is cautiously speculated that the alternative HMF isomers, may be monomeric
constituent of the polymeric "humins".