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The enzyme HMG–CoA reductase, shown here as a so-called ribbon model, catalyzes a crucial step in the body’s synthesis of cholesterol. Understanding how this enzyme functions has led to the development of drugs credited with saving millions of lives.

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FIGURE 1-1 The position of carbon in the periodic table. Other elements commonly found in organic compounds are shown in the colors typically used to represent them.

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FIGURE 1-2 A schematic view of an atom. The dense, positively charged nucleus contains most of the atom’s mass and is surrounded by negatively charged electrons. The three-dimensional view on the right shows calculated electron-density surfaces. Electron density increases steadily toward the nucleus and is 40 times greater at the blue solid surface than at the gray mesh surface.

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FIGURE 1-3 Representations of s, p, and d orbitals. An s orbital is spherical, a p orbital is dumbbell-shaped, and four of the five d orbitals are cloverleaf-shaped. Different lobes of p and d orbitals are often drawn for convenience as teardrops, but their actual shape is more like that of a doorknob, as indicated.

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FIGURE 1-4 The energy levels of electrons in an atom. The first shell holds a maximum of 2 electrons in one 1s orbital; the second shell holds a maximum of 8 electrons in one 2s and three 2p orbitals; the third shell holds a maximum of 18 electrons in one 3s, three 3p, and five 3d orbitals; and so on. The two electrons in each orbital are represented by up and down arrows. Although not shown, the energy level of the 4s orbital falls between 3p and 3d.

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FIGURE 1-5 Shapes of the 2p orbitals. Each of the three mutually perpendicular, dumbbell-shaped orbitals has two lobes separated by a node. The two lobes have different algebraic signs in the corresponding wave function, as indicated by the different colors.

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FIGURE 1-6 A representation of a tetrahedral carbon atom. The solid lines represent bonds in the plane of the paper, the heavy wedged line represents a bond coming out of the plane of the page, and the dashed line represents a bond going back behind the plane of the page.

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FIGURE 1-7 The cylindrical symmetry of the H ] H s bond in an H2 molecule. The intersection of a plane cutting through the s bond is a circle.

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FIGURE 1-8 Relative energy levels of two H atoms and the H2 molecule. The H2 molecule has 436 kJ/mol (104 kcal/mol) less energy than the two H atoms, so 436 kJ/mol of energy is released when the H ] H bond forms. Conversely, 436 kJ/mol is absorbed when the H ] H bond breaks.

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FIGURE 1-9 A plot of energy versus internuclear distance for two H atoms. The distance between nuclei at the minimum energy point is the bond length.

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FIGURE 1-10 Four sp3 hybrid orbitals, oriented to the corners of a regular tetrahedron, are formed by the combination of an s orbital and three p orbitals (red/blue). The sp3 hybrids have two lobes and are unsymmetrical about the nucleus, giving them a directionality and allowing them to form strong bonds when they overlap an orbital from another atom.

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FIGURE 1-11 The structure of methane, showing its 109.5° bond angles.

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FIGURE 1-12 The structure of ethane. The carbon–carbon bond is formed by s overlap of sp3 hybrid orbitals. For clarity, the smaller lobes of the sp3 hybrid orbitals are not shown.

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FIGURE 1-13 sp2 Hybridization. The three equivalent sp2 hybrid orbitals lie in a plane at angles of 120° to one another, and a single unhybridized p orbital (red/blue) is perpendicular to the sp2 plane.

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FIGURE 1-14 The structure of ethylene. One part of the double bond in ethylene results from s (head-on) overlap of sp2 orbitals, and the other part results from p (sideways) overlap of unhybridized p orbitals (red/blue). The p bond has regions of electron density above and below a line drawn between nuclei.

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FIGURE 1-15 sp Hybridization. The two sp hybrid orbitals are oriented 180° away from each other, perpendicular to the two remaining p orbitals (red/blue).

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FIGURE 1-16 The structure of acetylene. The two carbon atoms are joined by one sp–sp s bond and two p–p (pi) bonds.

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FIGURE 1-17 Molecular orbitals of H2. Combination of two hydrogen 1s atomic orbitals leads to two H2 molecular orbitals. The lower-energy, bonding MO is filled, and the higher-energy, antibonding MO is unfilled.

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FIGURE 1-18 A molecular orbital description of the C ] C p bond in ethylene. The lower-energy, p bonding MO results from an additive combination of p orbital lobes with the same algebraic sign and is filled. The higher-energy, p antibonding MO results from a subtractive combination of p orbital lobes with opposite algebraic signs and is unfilled.

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How dangerous is the pesticide being sprayed on this crop?

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