Organic Chemistry 6th Edition Paula Yurkanis Bruice Chapter 1 Electronic Structure and Bonding Acids and Bases © 2011 Pearson Education, Inc. 1 Organic Chemistry • Carbon-containing compounds were once considered “organ compounds” available only from living organisms. • The synthesis of the simple organic compound urea in 1828 showed that organic compounds can be prepared in the laboratory from non-living material. • Today, organic natural products are routinely synthesized in the laboratory. © 2011 Pearson Education, Inc. 2 Why Carbon? • Carbon neither gives up nor accepts electrons because it is in the center of the second periodic row. • Consequently, carbon forms bonds with other carbons and other atoms by sharing electrons. • The capacity of carbon to form bonds in this fashion makes it the building block of all living organisms. © 2011 Pearson Education, Inc. 3 Why Study Organic Chemistry? • Since carbon is the building block of all living organisms, a knowledge of Organic Chemistry is a prerequisite to understanding Biochemistry, Medicinal Chemistry, and Pharmacology. • Indeed, Organic Chemistry is a required course for studying Pharmacy, Medicine, and Dentistry. • Admission into these professional programs is highly dependent on your performance in Organic Chemistry. © 2011 Pearson Education, Inc. 4 Examples of Organic Compounds Used as Drugs Methotrexate, Anticancer Drug 5-Fluorouracil, Colon Cancer Drug Tamiflu, Influenza Drug AZT, HIV Drug 5 © 2011 Pearson Education, Inc. Examples of Organic Compounds Used as Drugs Haldol, Antipsychotic Elavil, Antidepressant Prozac, Antidepressant Viagra, Treats Erectile Dysfunction 6 © 2011 Pearson Education, Inc. The Structure of an Atom • An atom consists of electrons, positively charged protons, and neutral neutrons. • Electrons form chemical bonds. • Atomic number: numbers of protons in its nucleus • Mass number: the sum of the protons and neutrons of an atom • Isotopes have the same atomic number but different mass numbers. • The atomic weight: the average weighted mass of its atoms • Molecular weight: the sum of the atomic weights of all the atoms in the molecule © 2011 Pearson Education, Inc. 7 The Distribution of Electrons in an Atom • Quantum mechanics uses the mathematical equation of wave motions to characterize the motion of an electron around a nucleus. • Wave functions or orbitals tell us the energy of the electron and the volume of space around the nucleus where an electron is most likely to be found. • The atomic orbital closer to the nucleus has the lowest energy. • Degenerate orbitals have the same energy. © 2011 Pearson Education, Inc. 8 © 2011 Pearson Education, Inc. 9 The ground-state electronic configuration describes the orbitals occupied by the atom’s electrons with the lowest energy © 2011 Pearson Education, Inc. 10 The following principles determine which orbitals electrons occupy: • The Aufbau principle: an electron always goes to the available orbital with the lowest energy • The Pauli exclusion principle: only two electrons can occupy one atomic orbital and the two electrons have opposite spin • Hund’s rule: electrons will occupy empty degenerated orbitals before pairing up in the same orbital © 2011 Pearson Education, Inc. 11 Lewis’s theory: an atom will give up, accept, or share electrons in order to achieve a filled outer shell or an outer shell that contains eight electrons © 2011 Pearson Education, Inc. 12 Ionic Bonds Are Formed by the Transfer of Electrons Attractive forces between opposite charges are called electrostatic attractions © 2011 Pearson Education, Inc. 13 Covalent Bonds Are Formed by Sharing Electrons © 2011 Pearson Education, Inc. 14 © 2011 Pearson Education, Inc. 15 • Equal sharing of electrons: nonpolar covalent bond (e.g., H2) • Sharing of electrons between atoms of different electronegativities: polar covalent bond (e.g., HF) © 2011 Pearson Education, Inc. 16 A polar covalent bond has a slight positive charge on one end and a slight negative charge on the other © 2011 Pearson Education, Inc. 17 © 2011 Pearson Education, Inc. 18 A Polar Bond Has a Dipole Moment • A polar bond has a negative end and a positive end dipole moment (D) = m = e x d (e) : magnitude of the charge on the atom (d) : distance between the two charges © 2011 Pearson Education, Inc. 19 Electrostatic Potential Maps © 2011 Pearson Education, Inc. 20 Lewis Structure Formal charge = number of valence electrons – (number of lone pair electrons +1/2 number of bonding electrons) © 2011 Pearson Education, Inc. 21 Nitrogen has five valence electrons Carbon has four valence electrons Hydrogen has one valence electron and halogen has seven © 2011 Pearson Education, Inc. 22 Important Bond Numbers Neutral Cationic Anionic © 2011 Pearson Education, Inc. 23 Non-Octet Species • In the 3rd and 4th rows, expansion beyond the octet to 10 and 12 electrons is possible. Sulfuric Acid Periodic Acid Phosphoric Acid • Reactive species without an octet such as radicals, carbocations, carbenes, and electropositive atoms (boron, beryllium). Nitric Oxide Radical Radical, Mammalian Signaling Agent Carbocation © 2011 Pearson Education, Inc. Carbene Borane 24 © 2011 Pearson Education, Inc. 25 An orbital tells us the volume of space around the nucleus where an electron is most likely to be found The s Orbitals © 2011 Pearson Education, Inc. 26 The p Orbitals © 2011 Pearson Education, Inc. 27 Molecular Orbitals • Molecular orbitals belong to the whole molecule. • s bond: formed by overlapping of two s orbitals. • Bond strength/bond dissociation: energy required to break a bond or energy released to form a bond. © 2011 Pearson Education, Inc. 28 © 2011 Pearson Education, Inc. 29 In-phase overlap forms a bonding MO; out-of-phase overlap forms an antibonding MO: © 2011 Pearson Education, Inc. 30 Sigma bond (s) is formed by end-on overlap of two p orbitals: A s bond is stronger than a p bond © 2011 Pearson Education, Inc. 31 Pi bond (p) is formed by sideways overlap of two parallel p orbitals: © 2011 Pearson Education, Inc. 32 Bonding in Methane © 2011 Pearson Education, Inc. 33 Hybridization of One s and Three p Orbitals © 2011 Pearson Education, Inc. 34 The orbitals used in bond formation determine the bond angles • Tetrahedral bond angle: 109.5° • Electron pairs spread themselves into space as far from each other as possible © 2011 Pearson Education, Inc. 35 The Bonds in Ethane © 2011 Pearson Education, Inc. 36 Hybrid Orbitals of Ethane © 2011 Pearson Education, Inc. 37 Bonding in Ethene: A Double Bond © 2011 Pearson Education, Inc. 38 Bonding in Ethyne: A Triple Bond © 2011 Pearson Education, Inc. 39 Bonding in the Methyl Cation © 2011 Pearson Education, Inc. 40 Bonding in the Methyl Radical © 2011 Pearson Education, Inc. 41 Bonding in the Methyl Anion © 2011 Pearson Education, Inc. 42 Bonding in Water © 2011 Pearson Education, Inc. 43 Bonding in Ammonia and in the Ammonium Ion © 2011 Pearson Education, Inc. 44 Bonding in Hydrogen Halides © 2011 Pearson Education, Inc. 45 Summary • The shorter the bond, the stronger it is • The greater the electron density in the region of orbital overlap, the stronger is the bond • The more s character, the shorter and stronger is the bond • The more s character, the larger is the bond angle © 2011 Pearson Education, Inc. 46 Molecular Dipole Moment The vector sum of the magnitude and the direction of the individual bond dipole determines the overall dipole moment of a molecule © 2011 Pearson Education, Inc. 47 Brønsted–Lowry Acids and Bases • Acid donates a proton • Base accepts a proton • Strong reacts to give weak • The weaker the base, the stronger is its conjugate acid • Stable bases are weak bases © 2011 Pearson Education, Inc. 48 An Acid/Base Equilibrium [H 3O ][ A ] Ka [ H 2O][ AH ] LogKa pKa Ka: The acid dissociation constant. The stronger the acid, the larger its Ka value and the smaller its pKa value. © 2011 Pearson Education, Inc. 49 The Most Common Organic Acids Are Carboxylic Acids © 2011 Pearson Education, Inc. 50 © 2011 Pearson Education, Inc. 51 Protonated alcohols and protonated carboxylic acids are very strong acids © 2011 Pearson Education, Inc. 52 An amine can behave as an acid or as a base © 2011 Pearson Education, Inc. 53 © 2011 Pearson Education, Inc. 54 Strong Acids / Bases React to Form Weak Acids / Bases © 2011 Pearson Education, Inc. 55 The Structure of an Acid Affects Its Acidity • The weaker the base, the stronger is its conjugate acid • Stable bases are weak bases • The more stable the base, the stronger is its conjugate acid © 2011 Pearson Education, Inc. 56 The stability of a base is affected by its size and its electronegativity © 2011 Pearson Education, Inc. 57 © 2011 Pearson Education, Inc. 58 • When atoms are very different in size, the stronger acid will have its proton attached to the largest atom size overrides electronegativity © 2011 Pearson Education, Inc. 59 • When atoms are similar in size, the stronger acid will have its proton attached to the more electronegative atom © 2011 Pearson Education, Inc. 60 Substituents Affect the Strength of an Acid © 2011 Pearson Education, Inc. 61 • Inductive electron withdrawal increases the acidity of a conjugate acid © 2011 Pearson Education, Inc. 62 Acetic acid is more acidic than ethanol The delocalized electrons in acetic acid are shared by more than two atoms, thereby stabilizing the conjugated base © 2011 Pearson Education, Inc. 63 A Summary of the Factors That Determine Acid Strength 1. Size: As the atom attached to the hydrogen increases in size, the strength of the acid increases 2. Electronegativity © 2011 Pearson Education, Inc. 64 3. Hybridization 4. Inductive effect © 2011 Pearson Education, Inc. 65 5. Electron delocalization © 2011 Pearson Education, Inc. 66 Lewis Acids and Bases • Lewis acid: non-proton-donating acid; will accept two electrons • Lewis base: electron pair donors © 2011 Pearson Education, Inc. 67 Basicity and Drug Design: Methotrexate, Substituting Nitrogen for Oxygen 68 © 2011 Pearson Education, Inc. Acidity and Cosmetics: Skin Peel Agents • Skin Peels: Remove old skin with an acid to expose new young-looking skin. • The stronger the acid, the deeper the peel. • Examples of skin peel agents: Mild Peel Agents: Deep Peel Agent: 69 © 2011 Pearson Education, Inc.
