Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 2 Carbonyl group properties in triacylglycerols. (A) Increased acidity of the �
carbon to the carbonyl group due to resonance stabilization of the carbanion. (B) The carbonyl
carbon is prone to nucleophilic attack due to the electronegativity of oxygen. (Adapted from
Ref. 30.)
groups permits easy access for nucleophiles to approach and react with the carbonyl
carbon.
The transition state of the reaction is a relatively stable tetrahedral intermediate
with a partial negative charge on the oxygen. As the reaction progresses, a group
leaves and the structure reverts to the planar carbonyl structure. Strong evidence
supports the cleavage of the carbonyl carbon–oxygen bond as the mechanism for
the release of the leaving group.
For acid-catalyzed nucleophilic acyl substitution, a hydrogen easily associates
with the carbonyl oxygen owing to the polarized nature of the carbonyl function and
the presence of free electron pairs on the oxygen, imparting a positive charge to this
atom [50]. The carbonyl carbon is then even more susceptible to nucleophilic attack,
since oxygen can accept � electrons without gaining a negative charge. Acid-catalyzed
interesterification is not discussed further because it is not used for the chemical
interesterification of food lipids.
A. Carbonyl Addition Mechanism
In alkaline conditions encountered during interesterification, the catalyst (which is
nucleophilic) attacks the slightly positive carbonyl carbon at one of three fatty acid–
glycerol ester bonds and forms a tetrahedral intermediate. The fatty acid methyl ester
is then released, leaving behind a glycerylate anion (Fig. 3A). Kinetics of basecatalyzed
hydrolysis of esters shows that the reaction is dependent on both ester and
base concentration (second-order kinetics). This newly formed glycerylate anion is
the nucleophile for subsequent intra- and intermolecular carbonyl carbon attacks,
which continue until a thermodynamic equilibrium has been reached (Fig. 4).
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.