Organic Chemistry Chapter One Review - 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 compounds hav...

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 Figure 1.2 Relative energies and order of filling of orbitals 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. (a) C O (c) C O (b) C O (d) C O 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. H H C H C C C C H C H H (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. 2- carbonate ion CO3 - acetate ion CH 3 COO acetone CH 3 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. N N O 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 WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER TWO Acids and Bases Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-1 Arrhenius Acids and Bases In 1884, Svante Arrhenius proposed these definitions: – Acid: A substance that dissolves in water to produce hydronium ions (H3O+). – Base: A substance that dissolves in water to produce hydroxide ions (OH–). – This definition of an acid is a slight modification of the original Arrhenius definition, which was that an acid produces H+ in aqueous solution. – Today we know that H+ reacts immediately with a water molecule to give a hydronium ion H3O+. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-2 Arrhenius Acids and Bases When HCl, for example, dissolves in water, it reacts with water to give a hydronium ion and a chloride ion. Insert equation top page 41 We use curved arrows to show the change in position of electron pairs during this reaction. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-3 Arrhenius Acids and Bases With bases, the situation is slightly different. – Many bases are metal hydroxides such as KOH, NaOH, Mg(OH)2, and Ca(OH)2. – These compounds are ionic solids and, when they dissolve in water, their ions merely separate. – Bases that are not metal hydroxides produce OH– by reacting with water molecules. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-4 Arrhenius Acids and Bases – We use curved arrows to show the transfer of a proton from water to ammonia. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-5 Brønsted-Lowry Acids & Bases Acid: A proton donor. Base: A proton acceptor. Conjugate base: The species formed from an acid when an acid donates a proton to a base. Conjugate acid: The species formed from a base when the base accepts a proton from an acid. – Acid-base reaction: A proton-transfer reaction. – Conjugate acid-base pair: Any pair of molecules or ions that can be interconverted by transfer of a proton. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-6 Conjugate Acids & Bases We illustrate these relationships by the reaction of hydrogen chloride with water: Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-7 Brønsted-Lowry Acids & Bases Brønsted-Lowry definitions do not require water as a reactant. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-8 Brønsted-Lowry Acids & Bases We use curved arrows to show the flow of electrons that occurs in the transfer of a proton from acetic acid to ammonia. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-9 Brønsted-Lowry Acids & Bases Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-10 Brønsted-Lowry Acids & Bases Note the following about the conjugate acid-base pairs in Table 2.1: 1. An acid can be positively charged, neutral, or negatively charged; examples of each type are H3O+, H2CO3, and H2PO4–. 2. A base can be negatively charged or neutral; examples are OH–, Cl–, and NH3. 3. Acids are classified a monoprotic, diprotic, or triprotic depending on the number of protons that each may give up; examples are HCl, H2CO3, and H3PO4. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-11 Brønsted-Lowry Acids & Bases Carbonic acid, for example, can give up one proton to become bicarbonate ion, and then the second proton to become carbonate ion. 4. Several molecules and ions appear in both the acid and conjugate base columns; that is, each can function as both an acid and as a base. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-12 Brønsted-Lowry Acids & Bases 5. There is an inverse relationship between the strength of an acid and the strength of its conjugate base. – The stronger the acid, the weaker its conjugate base. – HI, for example, is the strongest acid in Table 2.1 and its conjugate base, I–, is the weakest base in the table. – CH3COOH (acetic acid) is a stronger acid that H2CO3 (carbonic acid); conversely, CH3COO– (acetate ion) is a weaker base that HCO3– (bicarbonate ion). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-13 Acid and Base Strength Strong acid: One that reacts completely or almost completely with water to form H3O+ ions. Strong base: One that reacts completely or almost completely with water to form OH– ions. – Here are the six most common strong acids and the four most common strong bases. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-14 Acid and Base Strength Weak acid: A substance that only partially dissociates in water to produce H3O+ ions. – Acetic acid, for example, is a weak acid; in water, only 4 out every 1000 molecules are converted to acetate ions. Weak base: A substance that only partially dissociates in water to produce OH– ions. – ammonia, for example, is a weak base. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-15 Acid-Base Reactions Acetic acid is incompletely ionized in aqueous solution. The equation for the ionization of a weak acid, HA, is: Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-16 pKa Values for Some Organic and Inorganic Acids Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-17 Acid-Base Equilibria To determine the position of equilibrium in an acid-base reaction: – Identify the two acids in the equilibrium; one on the left and one on the right. – Use the information in Table 2.2 to determine which is the stronger acid and which is the weaker acid. – Remember that the stronger acid gives the weaker conjugate base, and the weaker acid gives the stronger conjugate base. – The stronger acid reacts with the stronger base to give the weaker acid and weaker base. – Equilibrium lies on the side of the weaker acid and the weaker base. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-18 Acid-Base Equilibrium Equilibrium in the following acid-base reaction lies to the right, on the side of the weaker acid and the weaker base. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-19 Structure and Acidity The most important factor in determining the relative acidity of an organic acid is the relative stability of the anion, A–, formed when the acid, HA, transfers a proton to a base. We consider these four factors: 1. The electronegativity of the atom bonded to H in HA. 2. Resonance stabilization of A–. 3. The inductive effect. 4. The size and delocalization of charge in A–. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-20 Structure and Acidity Electronegativity of the atom bearing the negative charge. Within a period – The greater the electronegativity of the atom bearing the negative charge, the more strongly its electrons are held. – The more strongly electrons are held, the more stable the anion A–. – The more stable the anion A–, the greater the acidity of the acid HA. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-21 Structure and Acidity Resonance delocalization of the charge on A– – Compare the acidity of a carboxylic acid and an alcohol, both of which contain an –OH group. – Carboxylic acids are weak acids. Values of pKa for most unsubstituted carboxylic acids fall within the range of 4 to 5. – Alcohols are very weak acids. Values of pKa for most alcohols fall within the range of 15 to 18. – How do we account for the fact that carboxylic acids are stronger acids than alcohols? Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-22 Structure and Acidity – The greater the resonance stabilization of the anion, the more acidic the compound. – There is no resonance stabilization in an alkoxide anion. – We can write two equivalent contributing structures for the carboxylate anion; the negative charge is delocalized evenly over the two oxygen atoms. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-23 Structure and Acidity Inductive polarization of electron density transmitted through covalent bonds caused by a nearby atom of higher electronegativity Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-24 Structure and Acidity The more stable the anion A–, the greater the acidity of the acid HA. – The larger the volume over which the charge on an anion (or cation) is delocalized, the greater the stability of the anion (or cation). – When considering the relative acidities of the hydrogen halides, (HI > HBr > HCl > HF), we need to consider the relative stabilities of the halide ions. – Recall from general chemistry that atomic size is a periodic property. – For main group elements, atomic radii increase going down a group and increase going across a period. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-25 Structure and Acidity – For the halogens, iodine has the largest atomic radii, fluorine has the smallest I > Br > Cl > F. – Anions are always larger than the atoms from which they are derived. For anions, nuclear charge is unchanged but the added electron(s) introduce new repulsions and the electron cloud swells. – Among the halide ions, I– has the largest atomic radius, and F– has the smallest atomic radius. – Thus, HI is the strongest acid because the negative charge on iodide ion is delocalized over a larger volume than the negative charge on chloride, etc. – The result of both resonance and the inductive effect is due to the delocalization of charge. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-26 Lewis Acids and Bases Lewis acid: Any molecule or ion that can form a new covalent bond by accepting a pair of electrons. Lewis Base: Any molecule or ion that can form a new covalent bond by donating a pair of electrons. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-27 Lewis Acids and Bases – When HCl dissolves in water, for example, the strongest available Lewis base is an H2O molecule, and the following proton-transfer reaction takes place. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-28 Lewis Acids and Bases – When HCl dissolves in methanol, the strongest available Lewis base is a CH3OH molecule, and the following proton-transfer reaction takes place. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-29 Lewis Acids and Bases Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-30 Lewis Acids and Bases Another type of Lewis acid we will encounter in later chapters is an organic cation in which a carbon is bonded to only three atoms and bears a positive formal charge. – such carbon cations are called carbocations Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-31 Acids and Bases End Chapter 2 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 2-32 WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER THREE Alkanes and Cycloalkanes Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-1 Structure Hydrocarbon: A compound composed only of carbon and hydrogen. Saturated hydrocarbon: A hydrocarbon containing only carbon-carbon single bonds. Alkane: A saturated hydrocarbon whose carbons are arranged in an open chain. Aliphatic hydrocarbon: An alternative name for an alkane. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-2 Organic Chemistry Figure 3.1 The Four Classes of Hydrocarbons Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-3 Structure of Alkanes Shape – Tetrahedral geometry (all carbons are sp3 hybridized). – All bond angles are approximately 109.5°. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-4 Representing Alkanes – Line-angle formula: an abbreviated way to draw structural formulas. Each line represents a single bond. Each line ending represents a CH3 group. Each vertex (angle) represents a carbon atom. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-5 Alkanes Alkanes have the general formula CnH2n+2 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-6 Constitutional Isomers Constitutional isomers: Compounds with the same molecular formula but a different connectivity of their atoms. – There are two constitutional isomers with the molecular formula C4H10. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-7 Constitutional Isomerism – The potential for constitutional isomerism from just the elements carbon and hydrogen is enormous. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-8 IUPAC Nomenclature – Suffix -ane specifies an alkane. – Prefix tells the number of carbon atoms. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-9 IUPAC Nomenclature Parent name The longest carbon chain. Substituent: A group bonded to the parent chain. – Alkyl group: A substituent derived by removal of a hydrogen from an alkane; given the symbol R-. – CH4 becomes CH3- (methyl). – CH3CH3 becomes CH3CH2- (ethyl). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-10 IUPAC Nomenclature Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-11 IUPAC Nomenclature 1. The name of an alkane with an unbranched chain consists of a prefix and the suffix -ane. 2. For branched alkanes, the parent chain is the longest chain of carbon atoms. 3. Each substituent is given a name and a number. 4. If there is one substituent, number the chain from the end that gives it the lower number. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-12 IUPAC Nomenclature 5. If there are two or more identical substituents, number the parent chain from the end that gives the lower number to the substituent encountered first. The number of times the substituent occurs is indicated by the prefixes di-, tri-, tetra-, and so on. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-13 IUPAC Nomenclature 6. If there are two or more different substituents, – list them in alphabetical order. – number from the end of the chain that gives the substituent encountered first the lower number. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-14 IUPAC Nomenclature 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization. Iso, as in isopropyl, is included in alphabetization. In the following example, the alphabetizing names are ethyl and methyl. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-15 Classification of Carbons – Primary (1°): a C bonded to one other carbon. – Secondary (2°): a C bonded to two other carbons. – Tertiary (3°): a C bonded to three other carbons. – Quaternary (4°): a C bonded to four other carbons. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-16 Cycloalkanes General formula CnH2n – Five- and six-membered rings are the most common. Structure and nomenclature – Prefix the name of the corresponding open-chain alkane with cyclo-, name each substituent on the ring. – If only one substituent, no need to give it a number. – If two substituents, number the ring from the substituent of lower alphabetical order. – If three or more substituents, number the ring to give them the lowest set of numbers, and then list them in alphabetical order. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-17 Cycloalkanes Commonly written as line-angle formulas – examples: Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-18 IUPAC- A General System prefix-infix-suffix – Prefix tells the number of carbon atoms in the parent. – Infix tells the nature of the carbon-carbon bonds. – Suffix tells the class of compound. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-19 IUPAC- A General System prefix-infix-suffix – Prefix tells the number of carbon atoms in the parent. – Infix tells the nature of the carbon-carbon bonds. – Suffix tells the class of compound. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-20 IUPAC - a general system prop-en-e = propene eth-an-ol = ethanol but-an-one = butanone but-an-al = butanal but-an-oic acid = butanoic acid cyclohex-an-ol = cyclohexanol eth-yn-e = ethyne eth-an-amine = ethanamine Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-21 Conformation Conformation: Any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond. – Staggered conformation: A conformation about a carbon-carbon single bond where the atoms on one carbon are as far apart as possible from the atoms on an adjacent carbon. On the right is a Newman projection formula. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-22 Conformation – Eclipsed conformation: A conformation about a carbon- carbon single bond in which the atoms on one carbon are as close as possible to the atoms on an adjacent carbon. – The lowest energy conformation of an alkane is a fully staggered conformation. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-23 Conformations Torsional strain: Strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation. – Also called eclipsed interaction strain. – The torsional strain between staggered and eclipsed ethane is approximately 12.6 kJ/mol(3.0 kcal/mol). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-24 Cycloalkanes Cyclopentane – In planar cyclopentane, all C-C-C bond angles are 108°, which differ only slightly from the tetrahedral angle of 109.5°. – Consequently there is little angle strain. – Angle strain: Strain that arises when a bond angle is either compressed or expanded compared with its optimal value. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-25 Cycloalkanes Cyclopentane (cont'd) – In planar cyclopentane, there are 10 fully eclipsed C-H bonds creating a torsional strain of approximately 42 kJ/mol (10 kcal/mol). – Puckering to an “envelope” conformation relieves part of this strain – In an envelope conformation, eclipsed interactions are reduced but angle strain is increased slightly (105°). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-26 Cycloalkanes Cyclohexane – The most stable conformation is a puckered conformation called a chair conformation. – In a chair conformation, all bond angles are approx. 109.5° and all bonds on adjacent carbons are staggered. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-27 Cycloalkanes Chair cyclohexane – Six C–H bonds are equatorial and six are axial. – An equatorial bond extends from the ring roughly perpendicular to the imaginary axis of the ring. – An axial bond extends from the ring roughly parallel to the imaginary axis of the ring. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-28 Cyclohexane Boat conformation: A puckered conformation in which carbons 1 & 4 are bent toward each other. – A boat conformation is less stable than a chair conformation by 27 kJ/mol (6.5 kcal/mol). – Torsional strain is created by four sets of eclipsed hydrogen interactions. – Steric strain (nonbonded interaction strain) is created by one set of flagpole interactions. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-29 Cyclohexane Following is a structural formula and ball-and-stick model of cholestanol, a close relative of cholesterol. Describe the conformation of each ring in cholestanol. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-30 Cyclohexane chair cyclohexane (cont'd) – There are two equivalent chair conformations. – The alternative chair conformation interconvert via a boat conformation. – All C–H bonds that are equatorial in one chair are axial in the alternative chair, and vice versa. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-31 Methylcyclohexane – A group equatorial in one chair is axial in the alternative chair. – The two chairs are no longer of equal stability. They differ by 7.28 kJ/mol (1.74 kcal/mol) Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-32 Cis-trans Isomerism Cis-trans isomers have – The same molecular formula. – The same connectivity of their atoms. – An arrangement of atoms in space that cannot be interconverted by rotation about sigma bonds. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-33 Cis-trans isomerism – A cyclopentane ring is commonly viewed through an edge or from above. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-34 Cis-trans isomerism – A cyclohexane ring is commonly viewed as a planar hexagon viewed from the side or from above. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-35 Cis-trans isomerism – Or we can represent a cyclohexane as a chair conformation. – In viewing chair conformations, groups equatorial in one chair are axial in the alternative chair. – For trans-1,4-dimethylcyclohexane, the diequatorial chair is more stable than the diaxial chair. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-36 Cis-trans isomerism – For cis-1,4-dimethylcyclohexane, the alternative chairs are of equal stability. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-37 Cis-trans isomerism – Problem 3.11: Draw the alternative chair conformations of this trisubstituted cyclohexane, and state which conformation is the more stable. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-38 Conformation of Cyclohexanes Problem: Draw alternative chair conformations of each substituted cycloalkane, and state which conformation of each is the more stable. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-39 Physical Properties Alkanes are nonpolar compounds and have only weak interactions between their molecules. Dispersion forces: Weak intermolecular forces of attraction resulting from interaction of temporary induced dipoles. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-40 Physical Properties Boiling point – Low-molecular-weight alkanes (1 to 4 carbons) are gases at room temperature; e.g., methane, propane, butane. – Higher-molecular-weight alkanes (5 to 17 carbons) are liquids at room temperature (e.g., hexane, decane, gasoline, kerosene). – High-molecular-weight alkanes (18 or more carbons) are white, waxy semisolids or solids at room temperature (e.g., paraffin wax). Density – Average density is about 0.7 g/mL. – Liquid and solid alkanes float on water. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-41 Physical Properties Constitutional isomers are different compounds and have different physical properties. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-42 Reactions of Alkanes Oxidation is the basis for the use of alkanes as energy sources for heat and power. – Heat of combustion: the heat released when one mole of a substance is oxidized to carbon dioxide and water. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-43 Sources of Alkanes Natural gas – 90–95% methane, 5-10% ethane Petroleum – Gases (bp below 20 °C) – Naphthas, including gasoline (bp 20–200 °C) – Kerosene (bp 175–275 °C) – Fuel oil (bp 250–400 °C) – Lubricating oils (bp above 350 °C) – Asphalt (residue after distillation) Coal Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-44 Sources of Alkanes Figure 3.13 Fractional distillation of petroleum. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-45 Synthesis Gas – Synthesis gas: A mixture of carbon monoxide and hydrogen in varying proportions, depending on how it is produced. – Methanol and acetic acid are produced from synthesis gas. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-46 Methane Economy If the United States moves from a petroleum economy to a natural gas economy as some advocate, synthesis gas and synthesis gas- derived methanol will become key building blocks. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-47 Alkanes and Cycloalkanes End Chapter 3 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-48 WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER FOUR Alkenes and Alkynes Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-1 Unsaturated Hydrocarbons Unsaturated Hydrocarbon: A hydrocarbon that contains one or more carbon-carbon double or triple bonds or benzene-like rings. – Alkene: contains a carbon-carbon double bond and has the general formula CnH2n. – Alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-2 Unsaturated Hydrocarbons Arene: benzene and its derivatives (Ch 9) Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-3 Benzene and Phenyl Groups We do not study benzene and its derivatives until Chapter 9. – However, we show structural formulas of compounds containing a phenyl group before that time. – The phenyl group is not reactive under any of the conditions we describe in chapters 5-8. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-4 Structure of Alkenes The two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-5 Structure of Alkenes Figure 4.1 According to the orbital overlap model, a double bond consists of one s bond formed by overlap of sp2 hybrid orbitals and one p bond formed by overlap of parallel 2p orbitals. Rotating by 90°breaks the pi bond. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-6 Cis-Trans Isomerism Because of restricted rotation about a C–C double bond, groups on the carbons of a double bond are either cis or trans to each other. – Because of nonbonded interaction strain between alkyl substituents on the same side of the double bond, a trans alkene is more stable than an isomeric cis alkene. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-7 Structure of Alkynes The functional group of an alkyne is a carbon- carbon triple bond. A triple bond consists of: – One s bond formed by the overlap of sp hybrid orbitals. – Two p bonds formed by the overlap of sets of parallel 2p orbitals. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-8 Nomenclature of Alkenes IUPAC Nomenclature of alkenes – Use the infix -en- to show the presence of a carbon-carbon double bond. – Number the parent chain to give the first carbon of the double bond the lower number. – Follow IUPAC rules for numbering and naming substituents. – For a cycloalkene, number the atoms of the ring beginning with the two carbons of the double bond. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-9 Nomenclature of Alkenes IUPAC nomenclature of alkenes Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-10 Nomenclature of Alkenes Some alkenes, particularly low-molecular- weight alkenes, are known almost exclusively by their common names. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-11 Nomenclature of Alkynes IUPAC nomenclature of alkynes – Use the infix -yne to show the presence of a carbon-carbon triple bond. – Number the parent chain to give the first carbon of the triple bond the lower number. – Follow IUPAC rules for numbering and naming substituents. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-12 Configuration: Cis-Trans The cis-trans system: Configuration is determined by the orientation of atoms of the main chain. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-13 Configuration: E,Z To Assign an E,Z configuration, first assign a priority to the substituents on each carbon of the double bond. – If the groups of higher priority are on the same side of the double bond, the configuration is Z (German: zusammen, together). – If the groups of higher priority are on opposite sides of the double bond, the configuration is E (German: entgegen, opposite). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-14 Configuration: E,Z Priority rules 1. Priority is based on atomic number; the higher the atomic number, the higher the priority. 2. If priority cannot be assigned on the basis of the atoms bonded directly to the double bond, look to the next set of atoms; priority is assigned at the first point of difference. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-15 Configuration - E,Z 3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-16 Configuration - E,Z Example: Name each alkene by the E,Z system and specify its configuration. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-17 Cis-Trans Isomerism – The configuration of the double bond in cyclopropene through cycloheptene must be cis. These rings are not large enough to accommodate a trans double bond. – Cyclooctene is the smallest cycloalkene that can accommodate a trans double bond. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-18 Cis-Trans Isomerism Dienes, trienes, and polyenes – For an alkene with n carbon-carbon double bonds, each of which can show cis-trans isomerism, 2n cis-trans isomers are possible. – Consider 2,4-heptadiene; it has four cis-trans isomers, two of which are drawn here. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-19 Cis-Trans Isomerism – Vitamin A has five C-C double bonds, four of which can show cis-trans isomerism. – Vitamin A is the all-trans isomer. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-20 Physical Properties Alkenes and alkynes are nonpolar compounds. – The only attractive forces between their molecules are dispersion forces. The physical properties of alkenes and alkynes are similar to those of alkanes with similar carbon skeletons. – Those that are liquid at room temperature are less dense than water (1.0 g/mL). – They dissolve in each other and in nonpolar organic solvents. – They are insoluble in water. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-21 The Acidity of Terminal Alkynes One of the major differences between the chemistry of alkenes and alkynes is that the hydrogen atom of a terminal alkyne is sufficiently acidic (pKa 25) that it an be removed by a strong base such as sodium amide, NaNH2, to give an alkyne anion. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-22 The Acidity of Terminal Alkynes Because water (pKa 15.7) is a stronger acid than acetylene or a terminal alkyne, hydroxide ion is not a strong enough base for an acid-base reaction with a terminal alkyne. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-23 The Acidity of Terminal Alkynes The pKa values for alkane and alkene hydrogens are so large that no base is strong enough to remove a hydrogen from either of these classes of compounds. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-24 Problem 4.39 Zoapatanol is found in the leaves and twigs of Montanoa tomentoa. (a) Specify the configuration of each carbon-carbon double bond. (b) How many cis-trans isomers are possible for this compound. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-25 Problem 4.40 Pyrethrin II and pyrethrosin are derived from plants of the chrysanthemum family. (a) In each label all C—C double bonds about which cis/trans isomerism is possible. (b) How many cis-trans isomers are possible for each structural formula? Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-26 The Acidity of Terminal Alkynes How do we account for the acidity of a terminal alkyne compared to an alkane or alkene? To answer this question, we must focus on the stability of the anion derived from each class of hydrocarbon. The lone pair of electrons of an alkane anion lies in an sp3 hybrid orbital, which has 25% s character. For an alkene anion, the lone pair lies in an sp2 hybrid orbital, which has 33% s character. For a terminal alkyne anion, the lone pair lies in an sp hybrid orbital, which has 50% s character. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-27 The Acidity of Terminal Alkynes The greater the s character of the orbital bearing the negative charge, the greater the stability of the anion, and thus the greater acidic the hydrogen removed. Of the series of compounds alkane, alkene, and alkyne, the carbon of a terminal alkyne is the most electronegative (50% s character). Therefore, a terminal alkyne anion is the most stable of the series, and a terminal alkyne is the most acidic. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-28 Alkenes and Alkynes End Chapter 4 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-29  INTRODUCTION TO ORGANIC CHEMISTRY Sixth Edition William H. Brown Thomas Poon Chapter 5 Reactions of Alkenes and Alkynes This slide deck contains animations. Please disable animations if they cause issues with your device. Copyright ©2021 John Wiley & Sons, Inc. Reactions of Alkenes Copyright ©2021 John Wiley & Sons, Inc. 2 Energy Diagram Energy diagram: A graph of the energy changes that occur during a chemical reaction; energy is plotted on the y-axis. Reaction progress on the x-axis. Figure 5.1 An energy diagram for a one-step exothermic reaction of C and A-B to give C-A and B. Copyright ©2021 John Wiley & Sons, Inc. 3 Energy Diagram (Cont’d 1) Transition state: An unstable species of maximum energy formed during the course of a reaction; a maximum on an energy diagram. Activation energy Ea: The difference in energy between the reactants and the transition state. o Ea determines the rate of reaction. o If the Ea is large, very few molecular collisions occur with sufficient energy to reach the transition state, and the reaction is slow. o If the Ea is small, many collisions generate sufficient energy to reach the transition state, and the reaction is fast. Copyright ©2021 John Wiley & Sons, Inc. 4 Energy Diagram (Cont’d 2) Also shown on an energy diagram are: o Heat of reaction DH: The difference in energy between reactants and products. o Exothermic reaction: A reaction in which the energy of the products is lower than the energy of the reactants; a reaction in which heat is liberated. o Endothermic: A reaction in which the energy of the products is higher than the energy of the reactants: a reaction in which heat is absorbed. Copyright ©2021 John Wiley & Sons, Inc. 5 Energy Diagram (Cont’d 3) Figure 5.2 Energy diagram for a two-step exothermic reaction involving formation of a reaction intermediate. Copyright ©2021 John Wiley & Sons, Inc. 6 Energy Diagram (Cont’d 4) o Reaction intermediate: An energy minimum between two transition steps. Intermediates are highly reactive and rarely, if ever, can one be isolated. o Rate-determining step: The step in a reaction sequence that crosses the highest energy barrier; the slowest step in a multistep reaction. Copyright ©2021 John Wiley & Sons, Inc. 7 Mechanism Patterns Before we discuss any particular reactions and their mechanisms, let us first analyze several of the common patterns to be seen in the mechanisms we will encounter. Pattern 1: Add a Proton. In Section 2.2, we saw how curved arrows can be used to show how a proton-transfer reaction takes place. In this example, curved arrows show the redistribution of valence electrons and the formation of a new covalent bond when a proton is transferred from acetic acid to ammonia. Copyright ©2021 John Wiley & Sons, Inc. 8 Mechanism Patterns (Cont’d 1) Pattern 1: Add a proton. In this example, a proton is added across the pi bond of a C—C double bond to form a new C—H bond. Adding a proton is typical of all reactions that are catalyzed by an acid. Copyright ©2021 John Wiley & Sons, Inc. 9 Mechanism Patterns (Cont’d 2) Pattern 1: Add a proton. While it is most accurate to show proton transfer from H3O+ in aqueous solution, we will often simplify the equation to show just the proton H+ and the formation of a new covalent bond. Copyright ©2021 John Wiley & Sons, Inc. 10 Mechanism Patterns (Cont’d 3) Pattern 2: Take a proton away. Reversing Pattern 1 corresponds to taking a proton away. The mechanism for taking a proton away is similar to that for adding a proton, except that we focus our attention on the compound that loses the proton instead of the compound that adds a proton. Copyright ©2021 John Wiley & Sons, Inc. 11 Mechanism Patterns (Cont’d 4) Pattern 3: Reaction of an electrophile and a nucleophile to form a new covalent bond. Electrophile: an electron-poor species that can accept a pair of electrons to form a new covalent bond; a Lewis acid. Nucleophile: an electron-rich species that can donate a pair of electrons to form a new covalent bond; a Lewis base. Copyright ©2021 John Wiley & Sons, Inc. 12 Mechanism Patterns (Cont’d 5) Pattern 4: Rearrangement of a bond. A rearrangement occurs when the electrons of a sigma bond break the bond from one atom and form a new covalent bond to an adjacent atom. Copyright ©2021 John Wiley & Sons, Inc. 13 Mechanism Patterns (Cont’d 6) Pattern 5: Break a bond to form a stable molecule or ion. A carbocation can be formed when a chemical species breaks off from a molecule, taking the electrons from the former single bond with it. The chemical species formed is called the leaving group. The bond breaks because doing so forms one or more stable ions or molecules. Copyright ©2021 John Wiley & Sons, Inc. 14 Electrophilic Additions to Alkenes Addition of hydrogen halides (HCl, HBr, HI) Br CH3-CH=CH2 + HBr CH3-CH-CH3 Addition of water (H2O/H2SO4) Hydration OH H2SO4 CH3-CH=CH2 + H 2O CH3-CH-CH3 Addition of halogens (Cl2, Br2) Halogenation Br Br CH3-CH=CH2 + Br2 CH3-CH-CH2 Copyright ©2021 John Wiley & Sons, Inc. 15 Addition of Hydrogen Halides Carried out with the pure reagents or in a polar solvent such as acetic acid. Addition is regioselective. o Regioselective reaction: A reaction in which one direction of bond forming or bond breaking occurs in preference to all other directions. o Markovnikov’s rule: In additions of HX to a double bond, H adds to the carbon with the greater number of hydrogens already bonded to it. Copyright ©2021 John Wiley & Sons, Inc. 16 Regioselectivity Markovnikov’s rule is but one example of regioselectivity. We will see more examples in this and later chapters. Copyright ©2021 John Wiley & Sons, Inc. 17 Markovnikov’s Rule Problem: Complete these reactions by predicting the major product formed in each reaction. (a) + HBr (b) H2SO4 + H2O H2SO4 (c) + H2O (d) + HBr Copyright ©2021 John Wiley & Sons, Inc. 18 Addition of HCl to 2-Butene A two-step mechanism o Step 1: Add a proton. Formation of a sec-butyl cation, a 2° carbocation intermediate. o Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond. Reaction of the sec-butyl cation (an electrophile) with chloride ion (a nucleophile) completes the reaction. Copyright ©2021 John Wiley & Sons, Inc. 19 Addition of HCl to 2-Butene (Cont’d) Figure 5.4 Energy diagram for the two-step exothermic addition of HCl to 2-butene. Copyright ©2021 John Wiley & Sons, Inc. 20 Carbocations Figure 5.3 The structure of the tert-butyl cation. Copyright ©2021 John Wiley & Sons, Inc. 21 Carbocations (Cont’d 1) Carbocation: A species containing a carbon atom that has three bonds to it, six electrons in its valence shell, and bears a positive charge. o Bond angles about the positively charged carbon are approximately 120°. o Carbon uses sp2 hybrid orbitals to form a sigma bond to each attached group. o The unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons Carbocations are: o Electrophiles: that is, they are electron-loving. o Lewis acids: that is, they are electron-pair acceptors. Copyright ©2021 John Wiley & Sons, Inc. 22 Carbocations (Cont’d 2) o A 3° carbocation is more stable than a 2° carbocation, and requires a lower activation energy for its formation. o A 2° carbocation is, in turn, more stable than a 1° carbocation, and requires a lower activation energy for its formation. o Methyl and 1° carbocations are so unstable that they are never observed in solution. Copyright ©2021 John Wiley & Sons, Inc. 23 Relative Stability of Carbocations Inductive effect: The polarization of the electron density of a covalent bond as a result of the high electronegativity of a nearby atom. o The electronegativity of a carbon atom bearing a positive charge exerts an electron-withdrawing inductive effect that polarizes electrons of adjacent sigma bonds toward it. o Thus, the positive charge of a carbocation is not localized on the trivalent carbon, but rather is delocalized over nearby atoms as well. o The larger the area over which the positive charge is delocalized, the greater the stability of the cation. Copyright ©2021 John Wiley & Sons, Inc. 24 The Inductive Effect Figure 5.5 3˚ Carbocations are more stable and require a lower activation energy for their formation than 2° carbocations. 1° and methyl carbocations are so difficult to form that they are never observed in solution or in any of the reactions we will discuss. Copyright ©2021 John Wiley & Sons, Inc. 25 Copyright ©2021 John Wiley & Son, Inc. 26 Copyright ©2021 John Wiley & Sons, Inc. 27 Markovnikov’s Rule (Cont’d 1) The chemical basis for the regioselectivity embodied in Markovnikov’s rule lies in the relative stabilities of carbocation intermediates. The reason why the proton of H—X adds to the less substituted carbon of the double bond is that this mode of addition leads to the more stable carbocation intermediate. Copyright ©2021 John Wiley & Sons, Inc. 28 Carbocation Rearrangements As we have seen, the product of electrophilic addition to an alkene involves rupture of a pi bond and formation of two new sigma bonds in its place. In the following addition only 17% of the expected product is formed. Rearrangement: A reaction in which the connectivity of atoms in the product is different from that in the starting material. Copyright ©2021 John Wiley & Sons, Inc. 29 Carbocation Rearrangements (Cont’d 1) In the rearrangements we examine, typically either an alkyl group or a hydrogen atom migrates with its bonding electrons from an atom to an adjacent electron-deficient atom as illustrated in the following mechanism. The key step in this type of rearrangement is called a 1,2-shift. Copyright ©2021 John Wiley & Sons, Inc. 30 Carbocation Rearrangements (Cont’d 2) Step 1: Add a proton. Proton transfer from HCl to the alkene to give a 2° carbocation intermediate. Step 2: Rearrangement of a bond. Migration of a methyl group with its bonding electrons from the adjacent carbon gives a more stable 3° carbocation. Copyright ©2021 John Wiley & Sons, Inc. 31 Carbocation Rearrangements (Cont’d 3) Step 3: Reaction of a nucleophile and an electrophile to form a new covalent bond. Reaction of the 3° carbocation (an electrophile and a Lewis acid) with chloride ion (a nucleophile and a Lewis base) gives the rearranged product. Copyright ©2021 John Wiley & Sons, Inc. 32 Addition of H2O to an Alkene Addition of H2O to an alkene is called hydration. o Acid-catalyzed hydration of an alkene is regioselective: hydrogen adds preferentially to the less substituted carbon of the double bond. Thus H–OH adds to alkenes in accordance with Markovnikov's rule. Copyright ©2021 John Wiley & Sons, Inc. 33 Addition of H2O to an Alkene (Cont’d 1) Step 1: Add a proton. Proton transfer from the acid catalyst (H3O+) to propene gives a 2° carbocation intermediate. Copyright ©2021 John Wiley & Sons, Inc. 34 Addition of H2O to an Alkene (Cont’d 2) Step 2: Reaction of a nucleophile and an electrophile to form a new covalent bond. Reaction of the carbocation intermediate with water completes the valence shell of carbon and gives an oxonium ion. Copyright ©2021 John Wiley & Sons, Inc. 35 Addition of H2O to an Alkene (Cont’d 3) Step 3: Take a proton away. Proton transfer from the oxonium ion to water gives the alcohol and regenerates the acid catalyst. Copyright ©2021 John Wiley & Sons, Inc. 36 Addition of H2O to an Alkene (Cont’d 4) Problem: Account for the fact that the acid-catalyzed hydration of alkenes can be used to prepare both 2° and 3° alcohols but, with the exception of ethanol, it cannot be used to prepare 1° alcohols. OH OH H2SO4 + + H O 2 not formed Copyright ©2021 John Wiley & Sons, Inc. 37 Addition of H2O to an Alkene (Cont’d 5) Problem: Draw the structural formula of an alkene that undergoes acid-catalyzed hydration to give each alcohol as the major product. More that one alkene may give each alcohol as the major product. OH OH (b) (a) OH OH (c) (d) Copyright ©2021 John Wiley & Sons, Inc. 38 Carbocation Rearrangements (Cont’d 4) Rearrangements also occur during the acid-catalyzed hydration of alkenes, especially where the carbocation formed in the first step can rearrange to a more stable carbocation. Copyright ©2021 John Wiley & Sons, Inc. 39 Carbocation Rearrangements (Cont’d 5) Problem: Propose a mechanism for the following transformation. Copyright ©2021 John Wiley & Sons, Inc. 40 Carbocations—Summary The carbon bearing a positive charge is sp2 hybridized with bond angles of 120° about it. The order of carbocation stability is 3°>2°>1°. Carbocations are stabilized by the electron-withdrawing inductive effect of the positively charged carbon. Methyl and 1° carbocations are so unstable that they are never formed in solution. Carbocations may undergo rearrangement by a 1,2-shift, when the rearranged carbocation is more stable than the original carbocation. The most commonly observed pattern of rearrangement is from a 2° to 3° carbocation. Copyright ©2021 John Wiley & Sons, Inc. 41 Carbocations—Summary (Cont’d) Carbocation intermediates undergo three types of reactions: 1. Rearrangement by a 1,2-shift to a more stable carbocation. 2. Addition of nucleophile to form a new covalent bond (e.g., halide ion, H2O). 3. Loss of a proton to give an alkene (the reverse of the first step in both the addition of HX and the acid- catalyzed hydration of an alkene). Copyright ©2021 John Wiley & Sons, Inc. 42 Addition of Cl2 and Br2 Problem: Treatment of 2-methylpropene with methanol in the presence of an acid catalyst gives tert-butylmethyl ether. At one time this compound was added to gasoline to increase octane rating. Due to environmental concerns, however, it is no longer used for this purpose. Propose a mechanism for this reaction. Copyright ©2021 John Wiley & Sons, Inc. 43 Addition of Cl2 and Br2 (Cont’d 1) Carried out with either the pure reagents or in an inert solvent such as CH2Cl2. Copyright ©2021 John Wiley & Sons, Inc. 44 Addition of Cl2 and Br2 (Cont’d 2) Addition is stereoselective. Stereoselective reaction: A reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed. Addition to a cycloalkene, for example, gives only a trans product. The reaction occurs with anti stereoselectivity. Copyright ©2021 John Wiley & Sons, Inc. 45 Addition of Cl2 and Br2 (Cont’d 3) Step 1: Reaction of a nucleophile and an electrophile to form a new covalent bond. Reaction of the pi bond (a nucleophile) with bromine (an electrophile) gives a bridged bromonium ion intermediate Copyright ©2021 John Wiley & Sons, Inc. 46 Addition of Cl2 and Br2 (Cont’d 4) Step 2: Reaction of a nucleophile, and an electrophile to form a new covalent bond. Attack of bromide ion from the side opposite the bridged bromonium ion opens the three- membered ring. Copyright ©2021 John Wiley & Sons, Inc. 47 Addition of Cl2 and Br2 (Cont’d 5) The addition of chlorine or bromine to cyclohexene and its derivatives gives a trans-diaxial product because only axial positions on adjacent carbon atoms are anti and coplanar. The initial trans-diaxial conformation is in equilibrium with the more stable trans-diequatorial conformation. Copyright ©2021 John Wiley & Sons, Inc. 48 Hydroboration-Oxidation The result of hydroboration followed by oxidation of an alkene is hydration of the carbon-carbon double bond. Because –H adds to the more substituted carbon of the double bond and –OH adds to the less substituted carbon, we refer to the regiochemistry of hydroboration/oxidation as anti- Markovnikov hydration. Copyright ©2021 John Wiley & Sons, Inc. 49 Copyright ©2021 John Wiley & Son, Inc. 50 Reduction of Alkenes Alkenes react with H2 in the presence of a transition metal catalyst to give alkanes. o The most commonly used catalysts are Pd, Pt, and Ni. o The reaction is called catalytic reduction or catalytic hydrogenation. Copyright ©2021 John Wiley & Sons, Inc. 51 Reduction of Alkenes (Cont’d 1) o The most common pattern is syn addition of hydrogens; both hydrogens add to the same face of the double bond. o Catalytic reduction is predominantly syn stereoselective. Copyright ©2021 John Wiley & Sons, Inc. 52 Catalytic Reduction of an Alkene Figure 5.6 Syn addition of H2 to an alkene involving a transition metal catalyst. a) H2 and the alkene are absorbed on the catalyst. b) One H is transferred forming a new C-H bond. c) The second H is transferred. The alkane is released. Copyright ©2021 John Wiley & Sons, Inc. 53 Copyright ©2021 John Wiley & Son, Inc. 54 Copyright ©2021 John Wiley & Sons, Inc. 55 Heats of Hydrogenation Copyright ©2021 John Wiley & Sons, Inc. 56 Heats of Hydrogenation (Cont’d 1) Reduction involves net conversion of a weaker pi bond to a stronger sigma bond. The greater the degree of substitution of a double bond, the lower its heat of hydrogenation. o The greater the degree of substitution, the more stable the double bond. The heat of hydrogenation of a trans alkene is lower than that of an isomeric cis alkene. o A trans alkene is more stable than its isomeric cis alkene. o The difference is due to nonbonded interaction strain in the cis alkene. Copyright ©2021 John Wiley & Sons, Inc. 57 Heats of Hydrogenation (Cont’d 2) o Figure 5.7 Heats of hydrogenation of cis-2-butene and trans-2-butene. o trans-2-butene is more stable than cis-2-butene by 4.2 kJ/mol. Copyright ©2021 John Wiley & Sons, Inc. 58 Reactions of Alkynes (Cont’d 1) As we saw in Chapter 4, one of the major differences between the chemistry of alkanes, alkenes, and alkynes is that terminal alkynes are weak acids. Copyright ©2021 John Wiley & Sons, Inc. 59 Alkylation of Terminal Alkynes Treatment of a 1-alkyne with a very strong base such as sodium amide, NaNH2, converts the alkyne to an acetylide anion. Copyright ©2021 John Wiley & Sons, Inc. 60 Acetylide Anions in Synthesis An acetylide anion is both a strong base and a nucleophile. As a nucleophile, it can donate a pair of electrons to an electrophilic carbon atom and form a new carbon-carbon bond. In this example, the electrophile is the partially positive carbon of chloromethane. As the new carbon-carbon bond is formed, the carbon-halogen bond is broken. Because an alkyl group is added to the original alkyne, this reaction is called alkylation. Copyright ©2021 John Wiley & Sons, Inc. 61 Acetylide Anions in Synthesis (Cont’d 1) The importance of alkylation of acetylide anions is that the two- carbon molecule acetylene can be used to create larger carbon skeletons. For reasons we will discuss fully in Chapter 7, this type of alkylation is successful only for methyl and primary alkyl halides (CH3X and RCH2X). Copyright ©2021 John Wiley & Sons, Inc. 62 Reduction of Alkynes (Cont’d 2) Treatment of an alkyne with H2 in the presence of a transition metal catalyst, most commonly Pd, Pt, or Ni, results in addition of two moles of H2 and conversion of the alkyne to an alkane. Copyright ©2021 John Wiley & Sons, Inc. 63 Reduction of Alkynes (Cont’d 3) By the proper choice of catalyst it is possible to stop the reaction at the addition of one mole of H2. The most commonly used catalyst for this purpose is the Lindlar catalyst, which consists of finely powdered palladium metal deposited on solid calcium carbonate that has been specially modified with lead salts. Reduction (hydrogenation) of alkynes over Lindlar catalyst is syn stereoselective, the two hydrogens are added from the same face of the triple bond to give a cis alkene. Copyright ©2021 John Wiley & Sons, Inc. 64 Reduction of Alkynes (Cont’d 4) Problem: Starting with acetylene and any other necessary reagents, propose a synthesis for each of the following compounds. Any compound made in one part of the problem may be used as a starting material for another part of the problem. (a) 1-Butyne (b) 1-Butene (c) 1-Butanol (d) 2-Butanol (e) 3-Hexyne (f) cis-3-Hexene (g) Hexane (h) 3-Hexanol (i) 3,4-Dibromohexane Copyright ©2021 John Wiley & Sons, Inc. 65 Reactions of Alkenes (Cont’d 2) End Chapter 5 Copyright ©2021 John Wiley & Sons, Inc. 66 Copyright Copyright © 2021 John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that permitted in Section 117 of the 1976 United States Act without the express written permission of the copyright owner is unlawful. Request for further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. The purchaser may make back- up copies for his/her own use only and not for distribution or resale. The Publisher assumes no responsibility for errors, omissions, or damages, caused by the use of these programs or from the use of the information contained herein. Copyright ©2021 John Wiley & Sons, Inc. 67