Absolutely Small - Michael D. Fayer [110]
READ THE LABEL
As discussed above, polyunsaturated fats can be beneficial. As they occur naturally, both sunflower oil and safflower oil have very large percentages of polyunsaturated fats. However, many brands of sunflower oil and safflower oil have been partially hydrogenated, so they are more usable for high-temperature cooking. It is possible to tell by reading the nutrition label if sunflower oil and safflower oil have been partially hydrogenated. If the label shows that the amount per serving of monounsaturated fat is greater than the amount of polyunsaturated fat, then the oil has been partially hydrogenated. If these oils have not been partially hydrogenated, then the amount of polyunsaturated fat will greatly exceed the amount of monounsaturated fat. So to get the benefits of significant quantities of polyunsaturated fat from sunflower or safflower oils, the oils should not be partially hydrogenated. Reading the label will tell.
TRANS FATS
All of this hydrogenation of oils sounds fine except for one thing; the big elephant in the room is trans fats. What is a trans fat? Figure 16.4 shows oleic acid in both its cis and trans geometries. Both molecules have 18 carbons, 34 hydrogens, and one carbon-carbon double bond. The atoms are connected one to another in the same order. The difference is the geometry around the double bond.
In the cis conformation, the two hydrogens bonded to carbons 9 and 10 are on the same side of the molecule. They point at an angle toward the top of the page. The molecule is drawn with the double bond horizontal. The angle between the double bond and one of the H atoms is 120° because trigonal sp2 hybrids are used to make the σ bonds. So the angles made by the bonds from carbons 9 and 10 to the hydrogens are 30° from the vertical direction. For the cis molecule, the two chains of atoms connected to carbons 9 and 10 on either side of the double bond angle downward, again making angles that are 30° from the vertical line that is perpendicular to the double bond.
FIGURE 16.4. Ball-and-stick models of cis-oleic acid and trans-oleic acid. Both have 18 carbons and one double bond. However, their geometries differ.
In the trans conformation, the two hydrogens bonded to carbons 9 and 10 are on opposite sides of the molecule. One points almost straight up toward the top of the page and the other points almost straight down toward the bottom of the page. The two chains of carbon atoms that are bound to carbons 9 and 10 come out in opposite directions relative to the double bond. The net result is that the cis molecule is “bent” around the double bond, while the trans molecule is “straight” relative to the double bond.
Rotation around a carbon-carbon double bond cannot occur under normal conditions. The inability of rotation to occur around double bonds is fundamentally important. Figure 14.13 shows the gauche and trans conformations of n-butane, which has only single bonds. Rotation around single bonds can occur readily at room temperature. Therefore, in the example of n-butane, the gauche and trans conformations are not fixed. In fact, in liquid solution at room temperature, the n-butane gauche and trans conformations will interconvert by rotation around the middle carbon-carbon single bond in approximately 50 ps (50 trillionth of a second), which is a very short time. In contrast, the cis and trans conformations of oleic acid shown in Figure 16.4 are locked in. They will not interconvert without very high temperature and a catalyst.
To understand why rotation around a carbon-carbon single bond occurs readily while rotation about a double bond does not, we need to look again at the hybrid orbitals used by carbon to make single and double carbon-carbon bonds. Figure 14.9 illustrates the hybrid orbitals used in ethane to form the carbon-carbon single bond. Each carbon bonds to the other atoms using four sp3 hybrid orbitals. The middle part of Figure 14.9 shows schematically the formation of the carbon-carbon single bond by the