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overlap of one sp3 orbital on each carbon. The two orbitals, one from each carbon, point right at each other. Rotating one of the carbons does not change the overlap of the orbitals. There are favored configurations because the hydrogens on the two carbons want to avoid each other as much as possible, but the molecule can readily rotate from one favored configuration to another without changing the carbon-carbon sp3 orbital overlap. This is in contrast to the situation for ethylene, which has a carbon-carbon double bond. Figure 14.15 shows the orbitals used to form the double bond in ethylene. Each carbon uses three sp2 hybrids to make σ bonds to two hydrogens and the other carbon as shown in the top portion of Figure 14.15. The three sp2 orbitals on each carbon are formed by superpositions of the 2s, 2px, and 2py orbitals. These orbitals and the σ bonds are in the plane of the page, which is taken to be the xy plane. That leaves one 2pz orbital on each carbon atom, which will point perpendicular to the plane of the page. As shown in the bottom portion of Figure 14.15, the two 2pz orbitals overlap side to side to formaπ bond. If you could grab one of the carbons and rotate it, you would be rotating that 2pz orbital away from the z direction toward the xy plane. Such a rotation will decrease the overlap between the two 2pz orbitals, breaking the π bond. As shown in the table that follows the discussion of Figure 13.9, a double bond is much stronger than a single bond. Therefore, it will take a great deal of energy to rotate around a carbon-carbon double bond because it is necessary to break the π bond for the rotation to occur. The large energy penalty that would be paid prevents rotation.

NATURE PRODUCES CIS FATS BUT CHEMICAL PROCESSING PRODUCES TRANS FATS

Unsaturated fats, both monounsaturated and polyunsaturated, are produced in nature virtually exclusively in cis conformations. Small amounts of trans fats are found in the meat and milk of cattle, sheep, goats, and other ruminants. However, large amounts of trans fats are present in partially hydrogenated oils, and trans fats are also found in hydrogenated oils because the chemical processing does not result in an oil that is 100% saturated. Unprocessed monounsaturated and polyunsaturated vegetable oils have only cis conformations about their double bonds. Partial hydrogenation of the naturally occurring oils generates large quantities of trans fats. This transformation from cis to trans occurs during the hydrogenation process. As discussed above, one of the carbons of a carbon-carbon double bond binds to a metal catalyst in the reaction vessel, which is at very high temperature. When bound to the catalyst, the carbon-carbon bond is effectively a single bond, and rotation from cis to trans can occur. The oil can break its bond to the catalyst before hydrogenation occurs, so the double bond is not hydrogenated, and it reforms. So, rotation from cis to trans can take place before the oil is released from the catalyst. If this occurs, the result is the conversion of a cis conformation to a trans conformation without hydrogenation of the double bond. The processing is intended to reduce the number of double bonds, not eliminate all of them. But a substantial number of double bonds are converted from cis to trans. The result is that partially hydrogenated oils can contain substantial fractions of trans double bonds.

TRANS FATS CAN BE DELETERIOUS

Trans fats have been shown to have a number of deleterious health effects. The basic reason for the harmful effects of trans fats arises from the fact that biological systems have developed to deal with cis fats, and shape matters. Enzymes are proteins (large biological molecules) that act as very specific chemical factories. They can convert fats into other useful molecules, as well as break down fats for elimination. However, an enzyme that will work on a cis fat will, in general, not produce the same chemical reactions or any reaction at all for the trans fat, although it has the identical chemical formula. So two