Fats Organic Chemistry

Chapter 33

Fats 

The organic chemistry of biomolecules

• The study of biology at the molecular level is called biochemistry. It is a branch of biology, but it is equally a branch of organic chemistry. Most of the molecules involved, the biomolecules, are bigger and more complicated than the ones we have so far studied, and their environment a living organism is a far cry from the stark simplicity of the reaction mixture of the organic chemist. But the physical and chemical properties of these compounds depend on molecular structure in exactly the same way as do the properties of other organic compounds.

• The detailed chemistry of biological processes is vast and complicated and is beyond the scope of this book; indeed, the study of biochemistry must be built upon a study of the fundamentals of organic chemistry. We can, however, attempt to close the gap between the subject "organic chemistry" and the subject "biochemistry.

• In the remaining chapters of this book, we shall take up the principal classes of biomolecules: fats, carbohydrates, proteins, and nucleic acids. Our chief concern will be with their structures since structure is fundamental to everything else and with the methods used to determine these structures. Because biomolecules are big ones, we shall encounter structure on several levels: first, of course, the sequence of functional groups and the configuration at any chiral centers or double bonds; then, conformation, with loops, coils, and zigzags on a grander scale than anything we have seen yet; finally, the arrangement of collections of molecules, and even of collections of these collections. We shall see remarkable effects due to our familiar intermolecular forces: operating between biomolecules; between biomolecules or parts of them and the solvent; between different parts of the same biomolecule.

Occurrence and composition of fat

• Biochemists have found it convenient to define one set of biomolecules, the lipids, as substances, insoluble in water, that can be extracted from cells by organic solvents of low polarity like ether or chloroform. This is a catch-all sort of definition, and lipids include compounds of many different kinds: steroids (Sec. 15.16), for example, and terpenes (Sec. 8.26). Of the lipids, we shall take up only iChat and certain closely related compounds. These are not the only important lipids indeed, every compound in an organism seems to play an important role, if only as an unavoidable waste product of metabolism but they are the most abundant. Fats are the main constituents of the storage fat cells in animals and plants and are one of the important food reserves of the organism. We can extract these animal and vegetable fats liquid fats are often referred to as oils and obtain such substances as corn oil, coconut oil, cottonseed oil, palm oil, tallow, bacon grease, and butter.

• Chemically, fats are carboxylic esters derived from the single alcohol, glycerol, HOCH2CHOHCH2OH, and are known as glycerides. More specifically, they are triacylglycerols. As Table 33.1 shows, each fat is made up of glycerides derived from many different carboxylic acids. The proportions of the various acids vary from fat to fat; each fat has its characteristic composition, which does not differ very much from sample to sample.

 Hydrolysis of fats. Soap. Micelles

• The making of soap is one of the oldest of chemical syntheses. (It is not nearly so old, of course, as the production of ethyl alcohol; man's desire for cleanliness is much newer than his desire for intoxication.) When the German tribesmen of Caesar's time boiled goat tallow with potash leached from the ashes of wood fires, they were carrying out the same chemical reaction as the one carried out on a tremendous scale by modern soap manufacturers: hydrolysis fog/cyprides. Hydrolysis yields salts of the carboxylic acids, and glycerol, CH2OHCHOHCH2OH.

• Ordinary soap today is simply a mixture of sodium salts of long-chain fatty acids. It is a mixture because the fat from which it is made is a mixture, and for washing our hands or our clothes a mixture is just as good as a single pure salt. Soap may vary in composition and method of processing: if made from olive oil, it is Castile soap; alcohol can be added to make it transparent; air can be beaten in to make it float; perfumes, dyes, and germicides can be added; if a potassium salt (instead of a sodium salt), it is soft soap. Chemically, however, soap remains pretty much the same, and does its job in the same way.

Fats as sources of pure acids and alcohols

• Treatment of the sodium soaps with mineral acid (or hydrolysis of fats under acidic conditions) liberates a mixture of the free carboxylic acids. In recent fractional distillation of these mixtures has been developed on a commercial scale to furnish individual carboxylic acids of over 90" purity.

• Fats are sometimes converted by transesterification into the methyl esters of carboxylic acids; the glycerides are allowed to react with methanol in the presence of a basic or acidic catalyst. The mixture of methyl esters can be separated by fractional distillation into individual esters, which* can then be hydrolyzed to individual carboxylic acids of high purity. Fats are thus the source of straight-chain acids of even carbon number ranging from six to eighteen carbons.

Detergents

• Of the straight-chain primary alcohols obtained from fats or in other ways (Sec. 32.6) the C8 and C, members are used in the production of high-boiling esters used as plasticizers (e.g., octyl phthalate). The Ci 2 to C| S alcohols are used in enormous quantities in the manufacture of detergents (cleansing agents). Although the synthetic detergents vary considerably in their chemical structure, the molecules of all of them have one common feature, a feature they share with ordinary soap: they are amphipathic and have a large non-polar hydrocarbon end that is oil-soluble, and a polar end that is water-soluble. The C J2 to C ls alcohols are converted into the salts of alkyl hydrogen sulfates. For example:

• molecule water-soluble. Alternatively, the ethoxylates can be converted into sulfates and used in the form of the sodium salts.

• Formerly, polypropylene was commonly used in the synthesis of these alkylbenzene sulfonates: but the highly branched side chain it yields blocks the rapid biological degradation of the detergent residues in sewage discharge and septic tanks. Since about 1965 in this country, such "hard" detergents have been replaced by "soft" (biodegradable) detergents: alkyl sulfates, ethoxylates and their sulfates; and alkylbenzene sulfonates in which the phenyl group is randomly attached to the various secondary positions of a long straight chain (Ci 2-C 18 range). (See Problem 17, p. 403.) The side chains of these "linear" alkyl benzenesulfonates are derived from straight-chain 1-alkenes (Sec. 32.6) or chlorinated straight-chain alkanes separated (by use of molecular sieves) from kerosene.

Unsaturated fats. Hardening of oils. Drying oils

• We have seen that fats contain, in varying proportions, glycerides of unsaturated carboxylic acids. We have also seen that, other things being equal, unsaturation in a fat tends to lower its melting point and thus tends to make it a liquid at room temperature. In the United States the long-established use of lard and butter for cooking purposes has led to a prejudice against the use of the cheaper, equally nutritious oils. Hydrogenation of some of the double bonds in such cheap fats as cottonseed oil, corn oil, and soybean oil converts these liquids into solids having a consistency comparable to that of lard or butter. This hardening of oils is the basis of an important industry that produces cooking fats (for example, Crisco, Spry) and. oleomargarine. Hydrogenation of the carbon-carbon double bonds takes place under such mild conditions (Ni catalyst, 175-190, 20-40 lb./in. 2) that hydrogenolysis of the ester linkage does not occur.

Phosphoglycerates'. Phosphate esters

• So far, we have talked only about glycerides in which all three ester linkages are to acyl groups, that is, triacylglycerols. There also occur lipids of another kind, phosphoglycerates, which contain only two acyl groups and, in place of the third, a phosphate group. The parent structure is diacylglycerol phosphate, or phosphatidic acid.

•  Phosphoglycerates are, then, not only carboxylate esters but phosphate esters as well. Just what are phosphate esters like? It will be well for us to learn something about them since we shall be encountering them again and again: phospholipids make up the membranes of cells (Sec. 33.8); adenosine triphosphate lies at the heart of the energy system of organisms, and it does its job by converting hosts of other compounds into phosphate esters (Sec. 37.3); nucleic acids, which control heredity, are polyesters of phosphoric acid.

• In acidic solution, phosphate esters are readily cleaved to phosphoric acid. In alkaline solution, however, only trialkyl phosphates, (RO)3 PO, are hydrolyzed, and only one alkoxy group is removed. Mono alkyl and diallyls esters, ROPO(OH)2 and (RO)2PO(OH), are inert to alkali, even on long treatment. This may seem unusual behavior, but it has a perfectly rational explanation. The Mono alkyl and diallyls esters contain acidic OH groups on phosphorus, and in alkaline solution exist as anions; repulsion between like charges prevents attack on these anions by hydroxide ion.

Phospholipids and cell membranes

• The fats are found, we said, in storage fat cells of plants and animals. Their function rests on their chemical properties: through oxidation, they are consumed to help provide energy for the life processes.

• The phospholipids. on the other hand, are found in the membranes of cells all cells and are a basic structural element of living organisms. This vital function depends, in a fascinating \Vay, on their physical properties.

• Phosphoglycerates molecules are amphipathic, and in this respect differ from fats -but resemble soaps and detergents. The hydrophobic part is, again, the long fatty acid chains. The hydrophilic part is the dipolar ionic end: the substituted phosphate group with its positive and negative charges. In aqueous solution, as we would expect, phosphoglycerate's form micelles. In certain situations, however at an aperture between two aqueous solutions, for example- -they tend to form bilayers: two rows of molecules are lined up, back-to-back, with their polar ends projecting into water on the two surfaces of the bilayer (Fig. 33.3). Although the polar groups are needed to hold molecules in position, the bulk of the bilayer is made up of the fatty acid chains. Non-polar molecules can therefore dissolve in this mostly hydrocarbon wall and pass through it, but it is an effective barrier to polar molecules and ions.

• It is in the form of bilayers that phosphoglycerates are believed to exist in cell membranes. They constitute walls that not only enclose the cell but also very selectively control the passage, in and out, of the various substance's nutrients, waste products, hormones, etc. even from a solution of low concentration to a solution of high concentration. Now, many of these substances that enter and leave the cells are highly polar molecules like carbohydrates and amino acids, or ions like sodium and potassium. How can these molecules pass through cell membranes when they cannot pass through simple bilayers? And how can permeability be so highly selective?

• The answer to both these questions seems to involve the proteins that are also found in cell membrane: embedded in the bilayer, and even extending clear through it. Proteins, as we shall see in Chap. 36, are very long chain amides, polymers of twenty-odd different amino acids. Protein chains can be looped and coiled in a variety of ways; the conformation that is favored for a particular protein molecule depends on the exact sequence of amino acids along its chain.

• It has been suggested that transport through membranes happens in the following way. A protein molecule, coiled up to turn its hydrophobic parts outward, is dissolved in the bilayer, forming a part of the cell wall. A molecule approaches: a potassium ion, say. If the particular protein is the one designed to handle potassium ion, it receives the ion into its polar interior. Hidden in this hydrophobic wrapping, the ion is smuggled through the bilayer and released on the other side.