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6.9 Attractive Forces in Compounds The protein shape is stabilized by attractive forces between functional groups of side chains (R groups) on the amino acids, causing it to twist and bend into a specific three-dimensional shape. Learning Goal Describe the attractive forces between ions, polar covalent molecules, and nonpolar covalent molecules. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Dipole-Dipole, Hydrogen Bonds In covalent compounds, polar molecules • exert attractive forces called dipole-dipole attractions. • form strong dipole attractions called hydrogen bonds between hydrogen atoms bonded to F, O, or N, and a lone pair on F, O, or N. Hydrogen bonds are the strongest force between molecules and play a major role in the shape of DNA. Core Chemistry Skill Identifying Attractive Forces General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Dispersion Forces Dispersion forces are • weak attractions between nonpolar molecules. • caused by temporary dipoles that develop when molecules bump into each other. • weak but make it possible for nonpolar molecules to form liquids and solids. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Melting Points and Attractive Forces Melting points of compounds • are related to the strength of attractive forces between molecules or compounds. • are lower due to weak forces such as dispersion forces. • are higher due to stronger attractive forces such as hydrogen bonding. • are highest in ionic compounds due to the strong attractive forces between ions in the compound. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Melting Points and Attractive Forces General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Comparison of Bonding and Attractive Forces General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Check Identify the main type of attractive forces that are present in liquids of the following compounds: ionic bonds, dipole-dipole, hydrogen bonds, or dispersion forces. A. NCl3 B. H2O C. Br-Br D. KCl E. NH3 General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution Identify the main type of attractive forces that are present in liquids of the following compounds: ionic bonds, dipole-dipole, hydrogen bonds, or dispersion forces. A. NCl3 dipole-dipole forces B. H2O hydrogen bonds C. Br-Br dispersion forces D. KCl ionic bonds E. NH3 hydrogen bonds General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Attractive Forces, Proteins Proteins are biological molecules with many different functions. They are needed for • structural components such as cartilage, muscles, hair. • the formation of enzymes that regulate biological reactions. • myoglobin and hemoglobin, which transport oxygen in blood and muscle. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Attractive Forces, Proteins Proteins are composed of building blocks called amino acids. Every amino acid has a central carbon atom bonded to • an –NH3+, from an amine. • a –COO−, from a carboxylic acid. • an H atom. • an R group, which is unique for each amino acid. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Attractive Forces, Proteins Several amino acids have R groups or side chains that contain • an amide group, –CONH2. • a hydroxyl, – OH, group. • a carboxyl, –COOH, group ionized as carboxylate, –COO−. • an amine, –NH2, group ionized as ammonium, –NH3+. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Attractive Forces, Proteins The primary structure of a protein is its sequence of amino acids. It is the sequence of amino acids that determines the protein’s function. Amino acids in a protein are linked by a peptide bond between the COO− of one amino acid and the –NH3+ of the next amino acid. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Attractive Forces, Proteins Proteins have a higher level of structure that is determined by the attractive forces between the amino acids. When hydrogen bonds form between a hydrogen atom in an N—H group and the oxygen of the C═O group, the protein forms an alpha helix. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake H-bonds can form between the –OH of serine and the –NH2 of asparagine. © 2016 Pearson Education, Inc. Chemistry Link to Health: Attractive Forces, Proteins Hydrogen bonds can also form between the polar side chains of the amino acids on the outside of the protein and the –OH and –H of polar water molecules in the external aqueous environment. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake Attractive forces hold the protein in a specific shape. © 2016 Pearson Education, Inc. Concept Map General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.
