Scheme 3 Possible mechanistic scheme to explain the product selectivity in the acetalisation of glycerol with formaldehyde and acetone.
The distribution of the two acetals formed in the reactions of
glycerol with aqueous formaldehyde did not change very much
with the catalyst used, being around 70% for the six-membered
ring and 30% for the five-membered ring acetal. We did not
find in the literature a more systematic study to account for the
difference in selectivity between acetone and formaldehyde in
the acetalisation of glycerol. A possible explanation is shown in
Scheme 3. Upon the formation of the hemiacetal or hemiketal,
there could be two different pathways for the two systems.
For acetone/glycerol hemiketal, dehydration yields a tertiary
carbenium ion, also stabilized by resonance with the non-
bonded electron pairs of the adjacent oxygen atom. Then, there
occurs a rapid nucleophilic attack of the secondary hydroxyl
group to form the five-membered ring ketal. As the lifetime
of the carbenium ion in the reaction medium is supposed
to be short compared with the lifetime of the hemiketal, the
product distribution is governed by kinetics, which favors the
formation of the less thermodynamically stable five-membered
ring transition state, as already observed in other cyclisation
The results of glycerol acetalisation with acetone and
formaldehyde solution shows that the choice of the proper
heterogeneous catalyst might prevent the use of hazardous
solvents, such as benzene and chloroform, normally used to distil
off the water formed and shift the equilibrium. This procedure
might be employed in other reactions, where water is present in
the reaction medium or is formed during the reaction and affects
the catalyst activity.
Acknowledgements
Authors thank Repsol/YPF, FINEP, CNPq, FAPERJ and
PRH/ANP for financial support.
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0 | Green Chem., 2009, 11, 38–41
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