Fatty acids structures, melting points

The influence of fatty acid structure on its melting point has already been mentioned with branch chains and cis double bonds lowering the melting points of equivalent saturated chains. In addition, the melting point of fatty acids depends on whether the chain is even- or odd-numbered (Table 3.5). 

Therefore a special N-containing ether carboxylate was developed [36] with a high melting point ( 90°C) with a good foam and low hard water sensibility. This is obtained by condensation of a fatty acid (e.g., lauric acid) with diglycolamine, followed by carboxymethylation with NaOH and SMCA, washing out of the reaction mixture with a aqueous solution of a strong acid, separation of the oil layer, and neutralization with NaOH or KOH. The result is an ether carboxylate with exactly 2 EO units with the structure ... 

Melting Points of Lipids The melting points of a series of 18-carbon fatty acids are stearic acid, 69.6 °C oleic acid, 13.4 °C linoleic acid, - 5 °C and linolenic acid, - 11 °C. (a) What structural aspect of these 18-carbon fatty acids... 

Structures and Melting Points of Some Common Fatty Acids... 

Question Apart from unsaturation, what other structural feature of a fatty acid could affect its melting point ... 

Body temperatures are approximately 37°C/ 98.6°F for humans 38.5-39.5°C/101-103°F) for domestic animals and 40.5-41.5°C/ 105—107°F) for poultry. As shown in Table 34.1, the longest saturated fatty acid that is fluid at these temperatures is caproic (Cl0 0). All longer saturated fatty acids must be accompanied by lower melting unsaturated fatty acids as in a TAG structure to be fluid. The C18 oleic acid (c-9-octadecenoic acid) has a melting point of 16.3°C, the trans isomer elaidic acid (t-9-octadecenoic acid) melts at 43.7°C, and the biohydrogenated product trans-vaccenic acid (t- 1-octacedenoic acid) melts at 44°C.40... 

In the normal alkanes (Table 31) one finds the alternation of the increase of melting point also well known in the fatty acids. This is a consequence of the difference in crystal structure of even and odd molecules resulting from a difference in symmetry (centre or plane through the middle). 

Table 10.2 lists the structure and melting point of four fatty acids containing 18 carbon atoms. Stearic acid is one of the two most common saturated fatty acids, and oleic and linoleic acids are the most common unsaturated ones. The data show the effect of Z double bonds on the melting point of fatty acids. 

The three-dimensional structures of the fatty acids in Figure 10.6 illustrate how Z double bonds introduce kinks in the long hydrocarbon chain, decreasing the ability of the fatty acid to pack well in a crystalline lattice. The latter the numher of Z double bonds, the more kinks in the hydrocarbon chain, and the lower the melting point. 

The existence of a substance in two or more forms, which are significantly different in physical or chemical properties, is known as polymorphism. The difference between the forms arises firom different modes of molecular packing in the crystal structure of certain triglycerides. Certain pure or mixed fatty acid triglycerides may show as many as five different melting points. Each crystal system has a characteristic melting point, x-ray diffraction pattern, and infrared spectrum. For example, tristearin can exist in three polymorphic forms with melting points of 54.7, 63.2, and 73.5°C. 

A number of different dimers and ohgomers are produced from fatty acids and alcohols. These are branched-chain compounds with significantly lower melting points than straight chain structures of similar molecular weight. Fully saturated dimers... 

It has been reported extensively that fats solidify in more than one crystalline type (2-23). Triglycerides exhibit three main crystal types—ot, p, and p—with increasing degrees of stability and melting point. The molecular conformations and packings in the crystal of each polymorph have been reported. In the ot form, the fatty acid chain axes of the triglyceride are randomly oriented and the ot form reveals a freedom of molecular motion with the most loosely packed hexagonal subcell structure. 

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