for Reaction B can be calculated at 95.7 kJ mol-1 in p-cymene
but only 56.8 kJ mol-1 in DMSO. This observation, in general
terms, states that DH‡ is reduced with increasing b of the solvent.
Such a relationship would be expected to produce enhanced
rates of reaction in hydrogen bond accepting solvents. In Fig.
2 the opposite is demonstrated to occur because polar solvents
introduce a large entropic penalty during the formation of the
activated complex. So although hydrogen bonding stabilises the
activated complex of the reaction, the molecular organisation
within the reaction medium required to do so is ultimately a
hindrance at the high temperatures required for the reaction
to proceed in the absence of coupling agents or a catalyst
(Scheme 4).
Scheme 4 Proposed transition state stabilisation for a nucleophilic
attack on a carboxylic acid where [Sol] indicates a solvent molecule
capable of accepting hydrogen bonds.
The solvent influence controlling the rates of Reaction A
and Reaction B mirrors a previously reported acid catalysed
Fischer esterification and the condensation of carboxylic acids
with dicyclohexylcarbodiimide.17,32 All of these reactions are tied
by the common theme of a nucleophilic attack upon a carbonyl
moiety. Although varied reasons have been proposed for the
kinetic dependence of these reactions on the b solvent polarity
parameter, ultimately entropic control is not incompatible with
any of these chemistries. In conclusion, the low hydrogen bond
basicity of limonene and p-cymene has been shown to provide
an effective medium for the synthesis of esters and amides.
Readily obtainable from abundant citrus waste feedstocks,
these low polarity solvents complement the existing array of
hydrogen bonding bio-derived solvents. Because it is common
for the solvent to have a profound effect on the kinetics
and thermodynamics of a reaction,31 developing a range of
sustainable solvents with varied attributes is of some merit.
Given its resemblance to toluene, p-cymene would seem to
be an ideal drop-in replacement with the added benefit of a
straightforward synthesis from waste biomass.
Fig. 1 (i) LSER describing Reaction A without accounting for the
cohesive energy of the solvent (see Table 2, entry 1). (ii) Comparison
between predicted and experimental ln(k2) values for Reaction A when
both b and dH are used in the construction of the LSER (see Table
2, entry 2). Key: squares indicate citrus waste derived solvents; circles
2
indicate conventional solvents.
for the observed reaction rates of the amidation (Fig. 2).
This similarity is not unexpected given that the two reactions
are mechanistically related, both proceeding as a result of a
nucleophilic attack on a carbonyl group. Investigating further,
Reaction B was performed at different temperatures as a means
to separate the enthalpy and entropy of activation from the
observed rate constants by means of manipulating the Eyring
equation.31 Again only b is required to predict the enthalpy
(DH‡) and entropy of activation (DS‡) for Reaction B in a given
solvent. For example, using the coefficients listed in Table 2, DH‡
Notes and references
1 M. Pourbafrani, G. Forga´cs, I. S. Horva´th, C. Niklasson and M. J.
Taherzadeh, Bioresour. Technol., 2010, 101, 4246.
2 F. R. Mar´ın, C. Soler-Rivas, O. Benavente-Garc´ıa, J. Castillo and J.
A. Pe´rez-Alvarez, Food Chem., 2007, 100, 736.
3 M. Pourbafrani, F. Talebnia, C. Niklasson and M. J. Taherzadeh,
Int. J. Mol. Sci., 2007, 8, 777.
4 C. G. Garzo´n and R. A. Hours, Bioresour. Technol., 1992, 39, 93.
5 M. Lohrasbi, M. Pourbafrani, C. Niklasson and M. J. Taherzadeh,
Bioresour. Technol., 2010, 101, 7382.
6 M. Pourbafrani, in Citrus Waste Biorefinery: Process Development,
Simulation and Economic Analysis, PhD thesis, Go¨teborg, 2010.
7 A. Farhat, A.-S. Fabiano-Tixier, M. El Maataoui, J.-F. Maingonnat,
M. Romdhane and F. Chemat, Food Chem., 2011, 125, 255.
8 B. Mira, M. Blasco, A. Berna and S. Subirats, J. Supercrit. Fluids,
1999, 14, 95.
Fig. 2 LSER describing Reaction B at 100 ◦C (see Table 2, entry 3).
Key: see Fig. 1.
92 | Green Chem., 2012, 14, 90–93
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The Royal Society of Chemistry 2012
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