Linolenic acid melting point

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... 

The reaction mechanism for the selective hydrogenation of edible oils is very complex. Figure 14.1 illustrates a reaction scheme for linoleic acid. In this scheme, (n m) is used to represent an oil with n carbon atoms and m double bonds. There are several parallel, consecutive, and side reactions. Oleic acid (cis 18 1) is the desired product when the reaction starts with linolenic (all-cis 18 3) or linoleic acid (cis, cis 18 2). In the hydrogenation of linolenic and linoleic acid, elaidic acid (trans 18 1) is formed in a cisjtrans isomerization reaction. From the viewpoint of dietics, elaidic acid is an undesirable product however, its presence increases the melting point of the product in a desirable way. Stearic acid (18 0) is formed in a consecutive reaction, but direct formation from linoleic acid is also possible. 

A second double bond lowers the melting point further (linoleic acid, mp — 5 °C), and a third double bond lowers it still further (linolenic acid, mp —11 °C). The trans double bonds in eleostearic acid (mp 49 °C) have a smaller effect on the melting point than the cis double bonds of linolenic acid. The geometry of a trans double bond is similar to the zigzag conformation of a saturated acid, so it does not kink the chain as much as a cis double bond. 

D) Linolenic Acid.—The ester is dissolved in 400 cc. of a 5 per cent alcoholic solution of sodium hydroxide in a 1-1. beaker, and the solution is allowed to stand overnight at room temperature. The resulting jelly is dissolved in 400 cc. of warm water, and a slow stream of carbon dioxide is introduced beneath the surface of the liquid while it is acidified with 50 cc. of dilute sulfuric acid (1 1 by volume). The stream of carbon dioxide is maintained throughout the subsequent operations. The linolenic acid rises to the surface as a clear layer, which is washed once vith hot water. The acid is dried over anhydrous sodium sulfate and is preserved under carbon dioxide. The yield of acid is 22-24 g., and the product has a melting point of —17° to —16° (Note 11). 

Explain why arachidonic acid has a lower melting point (-50 °C) than linolenic acid, even though it contains two more carbon atoms. 

Clarity at or below ambient room temperature is the primary characteristic of a liquid oil. Natural vegetable oils that are liquid at room temperatures in temperate climates, 75 5°F (23.4 3°C), contain high levels of unsaturated fatty acids with low melting points. Fatty acids with one or more double bonds and 18 carbon atoms are the most important unsaturated fatty acids for liquid oils. Oleic (18 1), a monounsaturated fatty acid, is the most widely distributed and most stable Ci8 unsaturated fatty acid. Linoleic (18 2) and linolenic (18 3) are the most widely distributed di- and triunsaturated fatty acids. Both of these polyunsaturated fatty acids are termed essential because they cannot be synthesized by animals, including man, and must be supplied in the diet. Complete exclusion of the essential fatty acids from the diet results in scaly skin, loss of weight, kidney lesions and eventually death. 

The melting points of unsaturated fatty acids are lower than those of saturated fatty acids with the same number of carbons (compare stearic, oleic, linoleic, and linolenic acids in Table 19-1). The more double bonds present, the lower the melting point of the fatty acid. The effect of a double bond in lowering the melting point is a consequence of its presence in nature in the cis geometry instead of trans. Unsaturated fatty acids are liquids at temperatures where saturated fatty acids are solids. 

Saturated FA, such as palmitic and stearic acids, are stable toward oxidation and polymerization. Because they have a high melting point, they add structure to certain products. However, due to this characteristic the appearance of a fried food may be adversely affected (e.g., waxy mouth-feel, dry surface of stored fried food). Monoenoic FA, primarily oleic acid, are considered to be beneficial from a health standpoint. Frying oils rich in such FA do not add to the structure they are stable against oxidation and provide a light taste. Polyenoic FA (PEFA) deteriorate more rapidly that monoenoic FA and the shelf life of products fried in oils rich in these acids is shorter. Oxidation products formed from PEFA vary widely, depending on the structure of the FA and the relative concentration of linoleic and linolenic acids. The percentage of linolenic acid in heated oils should be very low. 

The naturally occurring triglycerols are mixtures. They are solid at room temperature if their content of unsaturated fatty acids with a low melting point, e.g., oleic aicd, linoleic acid and linolenic acid (Table 26) is low, and they are liquid if it is high. 

Linolenic acid (Table 10.2) and stearidonic acid are omega-3 fatty acids, unsaturated fatty acids that contain the first double bond located at C3, when numbering begins at the methyl end of the chain. Predict how the melting point of stearidonic acid compares with the melting points of linolenic and... 

Rapeseed Canola) oil is used as an edible oil. It is susceptible to autoxidation because of its relatively high content of linolenic acid. It is saturated by hyrogenation to a melting point of 32-34 °C and, with its stability and melting properties, resembles coconut oil. 

A figure of value for the detection of fish oils in linseed oil is the determination of ETHER-INSOLUBLE BROMIDES. The test depends upon the separation of the linolenic and clupanodonic acids as insoluble bromides, and the presence of the bromide of clupanodonic acid (derived from fish oils) is detected by its melting-point. The insoluble bromides of vegetable oils melt between 170° and 180°, but those from fish oils do not melt under 200°, although they darken above 180°. The following quantitative test is briefly that due to Gemmell ... 

The fatty acids that concern us from a dietary point of view range in length from Cjj to Cjg. The acids may contain one or more double bonds the presence of the double bonds lowers the melting point of the fat, because the chains pack less well. In the Cjg series (Figure 11.96), we can identify stearic acid (saturated, mainly found in animal fats but also in cocoa butter), oleic acid (one double bond, the triglyceride is the main constituent of olive oil) linoleic acid (two double bonds, one of the essential fatty acids that we must consume for good health, found in safflower, sunflower, and corn oil), and linolenic acid (three double bonds, another essential fatty acid, found in canola, soya, and walnuts). In this series (and related ones), the saturated fats are solids, whereas the unsaturated fats are liquids at room temperature. So in order to... 

---