Absolutely Small - Michael D. Fayer [107]
IN THIS CHAPTER, we will use some of the ideas developed previously to talk about some of the large molecules commonly discussed in everyday life. We hear about such things as saturated fats, unsaturated fats, transfats, and cholesterol. What are these things, and how do they differ? What are the relationships between molecular structure and health effects?
WHAT IS A FAT MOLECULE?
When we say fat, you may think of butter, lard, olive oil, or cottonseed oil. Each of these is actually a mixture of different fats. Figure 16.1 shows one particular molecule that is a fat. It is stearic acid. It is a long hydrocarbon chain with an organic acid group on the end. Stearic acid has 18 carbon atoms. The end carbon on the right is the organic acid group. Figure 15.4, bottom, shows acetic acid, which is an acid group with one methyl attached. Figure 15.5 shows tetradecane, a 14-carbon hydrocarbon, which is a component of heating oil. Stearic acid is like acetic acid but with a 17-carbon chain attached to the acid group instead of a single methyl group, or stearic acid is a somewhat longer hydrocarbon than tetradecane with an acid group on the end. Basically, a fat is a long carbon chain with an organic acid group on one end. The acid group forms hydrogen bonds to water. As discussed in Chapter 15, acetic acid is soluble in water because of the strong hydrogen bonding interactions between the acid group and water. Stearic acid, and fats in general, are not soluble in water because of the long hydrocarbon chain. While the acid group is strongly attracted to water (hydrophilic), the long hydrocarbon chain, like hydrocarbons discussed in Chapter 15, does not like to interact with water (hydrophobic). For fats, the long hydrocarbon portion of the molecules wins, and in general fats are not water soluble.
FIGURE 16.1. Ball-and-stick (top) and space-filling models (bottom) of stearic acid. Stearic acid has 18 carbon atoms, 36 hydrogen atoms, and two oxygen atoms. It is a 17-carbon hydrocarbon with an acid group, -COOH, on the end (right side).
SATURATED AND UNSATURATED FATS
Stearic acid is a saturated fat. Each carbon is bonded to one or two other carbons only by single bonds. There are no double bonds connecting carbon atoms. A saturated fat is a fat with only single bonds between carbon atoms.
Figure 16.2 shows a ball-and-stick model of oleic acid. Like stearic acid, oleic acid has 18 carbon atoms with an acid group on one end. However, it has one double bond between carbons 9 and 10, where the carbon atoms are numbered beginning with the carboxylic acid carbon. Oleic acid is a monounsaturated fat. It is unsaturated because it has a double bond. It is monounsaturated because it has only one double bond. Saturated fats have no double bonds between carbon atoms. In stearic acid, except for the carbon in the acid group, all of the carbons use four sp3 hybrid orbitals to form bonds. Carbons not at the ends of the stearic acid molecule use two of the four sp3 hybrid orbitals to form single bonds to the adjacent carbons and two to bond two hydrogens. Each carbon, except for the acid carbon, has a tetrahedral arrangement of bonds to other carbons and hydrogens. Oleic acid’s carbons 9 and 10 use three sp2 hybrids to form three σ bonds, one to a hydrogen and two to the adjacent carbons. Carbons 9 and 10 each use their remaining 2p orbitals, one on each carbon, to form a π bond between them. So carbons 9 and 10 have a double bond, and these carbons have a trigonal, rather than a tetrahedral, bonding geometry. The difference in the geometry can be seen clearly in Figure 14.14 by comparing the model of ethane (carbon-carbon single bond) with ethylene (carbon-carbon double bond). In ethane, the carbon centers are tetrahedral. In ethylene, the carbons are trigonal. Oleic acid has 34 hydrogen atoms in contrast to stearic acid, which has 36 hydrogen atoms. Oleic acid uses two orbitals to form the double bond that is used in stearic acid to bond hydrogens. A saturated fat has as many hydrogen atoms as possible,