2009_Book_FoodChemistry

3.2 Fatty Acids 165
3.2.2 Physical Properties
3.2.2.1 Carboxyl Group
Carboxylic acids have a great tendency to form
dimers which are stabilized by hydrogen bonds:
(3.4)
The binding energy of the acid dimer dissolved
in hexane is 38 kJ/mole. Also, the fatty
acid molecules are arranged as dimers in the
crystalline lattice (cf. Fig. 3.2).
The acidic character of the carboxyl group is
based on proton dissociation and on the formation
of the resonance-stabilized carboxylate anion:
(3.5)
The pK s values for the C 2 –C 9 short-chain acid
homologues range from 04.75–4.95. The pK s
of 7.9 for linoleic acid deviates considerably
from this range. This unexpected and anomalous
behavior, which has not yet been clarified, is
clearly illustrated in the titration curves for
propionic, caprylic and linoleic acids recorded
under identical conditions (Fig. 3.1).
3.2.2.2 Crystalline Structure, Melting Points
Melting properties of fats depend on the arrangement
of the acyl residues in the crystal lattice in
addition to other factors attributed solely to the
structure of triglycerides.
Calculations of the energy content of the carbon
chain conformation have revealed that at room
temperature 75% of the C–C bonds of a saturated
fatty acid are present in a fully staggered zigzag
or “trans” conformation and only 25% in the energetically
slightly less favorable skew conformation.
The unsaturated fatty acids, because of their double
bonds, are not free to rotate and hence have
rigid kinks along the carbon chain skeleton. However,
a molecule is less bent by a trans than by
Fig. 3.1. Fatty acid titration curves (according to Bild
et al., 1977). Aqueous solutions (0.1mol/l) of Na-salts
of propionic (1), caprylic (2) and linoleic acids (3) were
titrated with 0.1mol/l HCl
a cis double bond. Thus, this cis-configuration in
oleic acid causes a bending of about 40 ◦ :
(3.6)
The corresponding elaidic acid, with a transconfiguration,
has a slightly shortened C-chain,
but is still similar to the linear form of stearic
acid:
(3.7)
The extent of molecular crumpling is also increased
by an increase in the number of cis double
bonds. Thus, the four cis double bonds in arachidonic
acid increase the deviation from a straight
line to 165 ◦ :
(3.8)
When fatty acids crystallize, the saturated acids
are oriented as depicted by the simplified pattern