Covalent Bonding & Shapes of Molecules PDF

WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER ONE Covalent Bonding and Shapes of Molecules Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-1 Organic Chemistry Organic chemistry: The study of the compounds of carbon. Over 10 million organic compound...

WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER ONE Covalent Bonding and Shapes of Molecules Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-1 Organic Chemistry Organic chemistry: The study of the compounds of carbon. Over 10 million organic compounds have been identified. – About 1000 new ones are discovered or synthesized and identified each day! C is a small atom – It forms single, double, and triple bonds. – It is intermediate in electronegativity (2.5). – It forms strong covalent bonds with C, H, O, N, S, the halogens, and some metals. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-2 Electronic Structure of Atoms Figure 1.1 Schematic of an Atom – A small dense nucleus, diameter 10-14 – 10-15 m, which contains positively charged protons, neutrons, and most of the mass of the atom. – Extranuclear space, diameter 10-10 m, which contains negatively charged electrons. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-3 Electronic Structure of Atoms Electrons are confined to regions of space called principle energy levels (shells). – Each shell can hold 2n2 electrons (n = 1, 2, 3, 4......). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-4 Electronic Structure of Atoms Shells are divided into subshells called orbitals, which are designated by the letters s, p, d,… – s (one per shell) – p (set of three per shell 2 and higher) – d (set of five per shell 3 and higher) … Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-5  Electronic Structure of Atoms ure 1.2 Relative energies and order of filling o als through the 3d orbitals. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-6 Electronic Structure of Atoms Figure 1.3 The pairing of electron spins. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-7 Electronic Structure of Atoms Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-8 Electronic Structure of Atoms Problem: Write the ground-state electron configuration of each element, given its atomic number, and describe the relationship between an atom’s ground- state electron configuration and its position in the Periodic Table. (a) Mg (12) and Ar(18) (b) P(15) and Cl (17) Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-9 Lewis Structures Gilbert N. Lewis Valence shell: The outermost electron shell of an atom. Valence electrons: Electrons in the valence shell of an atom. These electrons are used in forming chemical bonds. Lewis structure of an atom – The symbol of the atom represents the nucleus and all inner shell electrons. – Dots represent electrons in the valence shell of the atom. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-10 Lewis Structures Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-11 Lewis Model of Bonding Atoms bond together so that each atom in the bond acquires the electron configuration of the noble gas nearest it in atomic number. – An atom that gains electrons becomes an anion. – An atom that loses electrons becomes a cation. – Ionic bond: A chemical bond resulting from the electrostatic attraction of an anion and a cation. – Covalent bond: A chemical bond resulting from two atoms sharing one or more pairs of electrons. We classify chemical bonds as ionic, polar covalent, and nonpolar covalent based on the difference in electronegativity between the bonded atoms. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-12 Electronegativity Electronegativity: A measure of the force of an atom's attraction for the electrons it shares in a chemical bond with another atom. Pauling scale – Increases from left to right within a period. – Increases from bottom to top in a group. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-13 Electronegativity Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-14 Electronegativity Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-15 Ionic Bonds An ionic bond forms by the transfer of electrons from the valence shell of an atom of lower electronegativity to the valence shell of an atom of higher electronegativity. – We show the transfer of a single electron by a single-headed (barbed) curved arrow. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-16 Covalent Bonds A covalent bond forms when electron pairs are shared between two atoms whose difference in electronegativity is 1.9 or less. – An example is the formation of a covalent bond between two hydrogen atoms. – The shared pair of electrons completes the valence shell of each hydrogen. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-17 Polar Covalent Bonds In a polar covalent bond: – The more electronegative atom has a partial negative charge, indicated by the symbol d–. – The less electronegative atom has a partial positive charge, indicated by the symbol d+. In an electron density model: – Red indicates a region of high electron density. – Blue indicates a region of low electron density. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-18 Drawing Lewis Structures To draw a Lewis structure: – Determine the number of valence electrons in the molecule or ion. – Determine the connectivity (arrangement) of atoms. – Connect the atoms by single line between atoms. – Arrange the remaining electrons so that each atom has a complete valence shell. – Show bonding electrons as single lines. – Show nonbonding electrons as pairs of dots. – Atoms share 1 pair of electrons in a single bond, 2 pairs in a double bond, and 3 pairs in a triple bond. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-19 Lewis Structures-Table 1.6 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-20 Lewis Structures Table 1.6 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-21 Lewis Structures In neutral molecules containing C, H, N, O, and halogen (X) – Hydrogen has one bond. – Carbon has 4 bonds and no unshared electrons. – Nitrogen has 3 bonds and 1 unshared pair of electrons. – Oxygen has 2 bonds and 2 unshared pairs of electrons. – Halogen has 1 bond and 3 unshared pairs of electrons. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-22 Formal Charge Formal charge: the charge on an atom in a molecule or polyatomic ion. – Write a Lewis structure for the molecule or ion. – Assign each atom all its unshared (nonbonding) electrons and one-half its shared (bonding) electrons. – Compare this number with the number of valence electrons in the neutral, unbonded atom. – If the number is less than that assigned to the unbonded atom, the atom has a positive formal charge. – If the number is greater, the atom has a negative formal charge. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-23 Lewis Structures Problem: Draw a Lewis structure for each molecule or ion and show all formal charges. (a) NH4+ (b) CO (c) NO2+ (d) CH3+ (e) N3– (f) CH3NH3+ (g) BF4– (h) CH3– (i) CH3OH2+ Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-24 Lewis Structures Problem: Which is an acceptable Lewis structure (formal charges are not shown) for carbon monoxide, CO? For an acceptable structure, assign formal charges as appropriate. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-25 Valence-shell Electron-Pair Repulsion VSEPR is based on two concepts. – Atoms are surrounded by regions of electron density. – Regions of electron density repel each other. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-26 Shapes of Molecules Methane, Ammonia, and Water molecules. For each, VSEPR predicts tetrahedral distribution of electron density and bond angles of 109.5° Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-27 Shapes of Molecules Figure 1.9 Shapes of Formaldehyde and Ethylene. VESPR predicts trigonal planar geometry Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-28 Shapes of Molecules Figure 1.10 Shapes of carbon dioxide and acetylene. Both are planar molecules. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-29 VSEPR Problem: Draw a Lewis structure and predict all bond angles for these molecules and ions. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-30 Shapes of Molecules Problem: Following is a structural formula of benzene,C6H6, which we will study in Chapter 9. (a) Using VSEPR, predict each H-C-C and C-C-C bond angle in benzene. (b) Predict the shape of a benzene molecule. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-31 Polar and Nonpolar Molecules A molecule with polar bonds is nonpolar if: – The vector sum of its bonds dipoles is zero (that is, the bond dipoles cancel each other). – Carbon dioxide has two polar covalent bonds and because of its geometry, is a nonpolar molecule. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-32 Polar and Nonpolar Molecules A water molecule has two polar covalent bonds and, because of its geometry, is a polar molecule. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-33 Polar and Nonpolar Molecules An ammonia molecule has three polar covalent bonds, and because of its geometry, is a polar molecule. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-34 Polar and Nonpolar Molecules – Chloromethane and formaldehyde are polar molecules. – Acetylene is a nonpolar molecule. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-35 Resonance A way to describe molecules and ions for which no single Lewis structure provides a truly accurate representation. Figure 1.11 Three Lewis structures for the carbonate ion. Each implies that one carbon-carbon bond is different from the other two. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-36 Resonance Linus Pauling - 1930s – Many molecules and ions are best described by writing two or more Lewis structures. – Individual Lewis structures are called contributing structures. – Connect individual contributing structures by a double-headed arrow. – The molecule or ion is a hybrid of the various contributing structures. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-37 Resonance Figure 1.12 The carbonate ion as a hybrid of three equivalent contributing structures. Curved arrows show the redistribution of valence electrons between one contributing structure and the next. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-38 Resonance Curved arrow: A symbol used to show the redistribution of valence electrons. In using curved arrows, there are only two allowed types of electron redistribution: – from a bond to an adjacent atom. – from an atom to an adjacent bond. Electron pushing by the use of curved arrows is a survival skill in organic chemistry. – learn it well! Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-39 Resonance All acceptable contributing structures must: 1. Have the same number of valence electrons. 2. Obey the rules of covalent bonding. – No more than 2 electrons in the valence shell of H. – No more than 8 electrons in the valence shell of a 2nd period element. – 3rd period elements may have up to 12 electrons in their valence shells. 3. Differ only in distribution of valence electrons. 4. Have the same total number of paired and unpaired electrons. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-40 Resonance Examples of ions and a molecule best represented as resonance hybrids. Draw contributing structures for each resonance hybrid. carbonate ion 2- CO3 acetate ion CH3 COO - acetone CH3 COCH3 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-41 Resonance Problem: Nitrous oxide, N2O, laughing gas, is a colorless, nontoxic, tasteless, and odorless gas. Because it is soluble in vegetable oils (fats), it is used as a propellant in whipped toppings. (a) How many valence electrons are present in nitrous oxide? (b) Write two equivalent contributing structures for this molecule. The connectivity is N—N—O. Be certain to show formal charges, if any are present. (c) Explain why the following is not an acceptable contributing structure. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-42 Shapes of Atomic Orbitals Figure 1.13 All s orbitals have the shape of a sphere, with its center at the nucleus. – Of the s orbitals, a 1s orbital is the smallest, a 2s orbital is larger, and a 3s orbital is larger still. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-43 Shapes of Atomic Orbitals Figure 1.14 A p orbital consists of two lobes arranged in a straight line with the center at the nucleus. – p orbitals come in sets of three: 2px, 2py, and 2pz. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-44 Orbital Overlap Model of Bonding A covalent bond forms when a portion of an atomic orbital of one atom overlaps a portion of an atomic orbital of another atom. Figure 1.15 In forming the covalent bond in H– H, for example, there is overlap of the 1s orbitals of each hydrogen. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-45 Hybrid Orbitals A Problem: – Overlap of a 2s atomic orbitals of one atom and 2p atomic orbitals of another atom would give bond angles of approximately 90°. – Instead, we observe bond angles of approximately 109.5°, 120°, and 180°. A Solution – Hybridization of atomic orbitals. – 2nd row elements use sp3, sp2, and sp hybrid orbitals for bonding. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-46 Hybrid Orbitals We study three types of hybrid atomic orbitals: – sp3 one s atomic orbital + three p atomic orbitals give four sp3 hybrid orbitals. – sp2 one s atomic orbital + two p atomic orbitals give three sp2 hybrid orbitals. – sp one s atomic orbital + one p atomic orbital give two sp hybrid orbitals. Overlap of hybrid orbitals can form two types of covalent bonds, depending on the geometry of the overlap: –  bonds are formed by “direct” overlap. –  bonds are formed by “parallel” overlap. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-47 sp3 Hybrid Orbitals – Each sp3 hybrid orbital has two lobes of unequal size. – The four sp3 hybrid orbitals are directed toward the corners of a regular tetrahedron at angles of 109.5°. – Figure 1.16 sp3 hybrid orbitals. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-48 sp3 Hybrid Orbitals Figure 1.17 Orbital overlap pictures of methane, ammonia, and water. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-49 sp2 Hybrid Orbitals Figure 1.18 A single sp2 hybrid orbital has two lobes of unequal size: – The three sp2 hybrid orbitals are directed toward the corners of an equilateral triangle at angles of 120°. – The unhybridized 2p orbital is perpendicular to the plane of the three sp2 hybrid orbitals. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-50 sp2 Hybrid Orbitals Figure 1.19 Covalent bonding in ethylene. Ethylene is a planar molecule. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-51 sp2 Hybrid Orbitals Figure 1.20 A carbon-oxygen double bond consists of one sigma (s) bond and one pi (p) bond. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-52 sp Hybrid Orbitals Figure 1.21 A single sp hybrid orbital has two lobes of unequal size. – The two sp hybrid orbitals lie in a line at an angle of 180°. – The two unhybridized 2p orbitals are perpendicular to each other and to the line through the two sp hybrid orbitals. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-53 sp Hybrid Orbitals – Figure 1.22 Covalent bonding in acetylene. A carbon-carbon triple bond consists of one sigma (s) bond and two pi (p) bonds. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-54 Hybrid Orbitals Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-55 Functional Groups Functional Group: An atom or group of atoms within a molecule that shows a characteristic set of physical and chemical properties. Functional groups are important for three reasons, they are: – The units by which we divide organic compounds into classes. – The sites of characteristic chemical reactions. – The basis for naming organic compounds. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-56 Functional Groups Alcohol: A compound that contains an – OH (hydroxyl group) bonded to a tetrahedral carbon atom. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-57 Functional Groups Amine: A compound that contains an amino group: a nitrogen atom bonded to one, two, or three carbon atoms. – Amines are classified as 1°, 2°, and 3° according to the number of carbon atoms bonded directly to the nitrogen atom. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-58 Functional Groups Carbonyl group (C=O) of aldehydes and ketones. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-59 Functional Groups Carboxyl group of carboxylic acids. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-60 Covalent Bonding & Shapes of Molecules End Chapter 1 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 1-61