(3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 Chapter 12 Conjugated and Aromatic Molecules from Organic Chemistry by Robert C. Neuman, Jr. Professor of Chemistry, emeritus University of California, Riverside [email protected] <http://web.chem.ucsb.edu/~neuman/orgchembyneuman/> Chapter Outline of the Book ************************************************************************************** I. Foundations 1. Organic Molecules and Chemical Bonding 2. Alkanes and Cycloalkanes 3. Haloalkanes, Alcohols, Ethers, and Amines 4. Stereochemistry 5. Organic Spectrometry II. Reactions, Mechanisms, Multiple Bonds 6. Organic Reactions *(Not yet Posted) 7. Reactions of Haloalkanes, Alcohols, and Amines. Nucleophilic Substitution 8. Alkenes and Alkynes 9. Formation of Alkenes and Alkynes. Elimination Reactions 10. Alkenes and Alkynes. Addition Reactions 11. Free Radical Addition and Substitution Reactions III. Conjugation, Electronic Effects, Carbonyl Groups 12. Conjugated and Aromatic Molecules 13. Carbonyl Compounds. Ketones, Aldehydes, and Carboxylic Acids 14. Substituent Effects 15. Carbonyl Compounds. Esters, Amides, and Related Molecules IV. Carbonyl and Pericyclic Reactions and Mechanisms 16. Carbonyl Compounds. Addition and Substitution Reactions 17. Oxidation and Reduction Reactions 18. Reactions of Enolate Ions and Enols 19. Cyclization and Pericyclic Reactions *(Not yet Posted) V. Bioorganic Compounds 20. Carbohydrates 21. Lipids 22. Peptides, Proteins, and α−Amino Acids 23. Nucleic Acids ************************************************************************************** *Note: Chapters marked with an (*) are not yet posted. 0 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 12: Conjugated and Aromatic Molecules 12.1 Conjugated Molecules 12-4 1,3-Butadiene (12.1A) 12-4 Atomic Orbital Overlap in 1,3-Butadiene Molecular Orbitals The Bonding M.O.'s Conformations Other Alternating Multiple Bonds Pentadienes (12.1B) 12-7 1,3-Pentadiene 1,4-Pentadiene 1,2-Pentadiene. Stability of Conjugated Systems (12.1C) 12-9 Heats of Hydrogenation of Pentadienes Heats of Hydrogenation of Butadienes Aromatic Molecules (12.1D) 12-11 12.2 Reactivity of Conjugated Systems 12-11 Addition of H-Cl to 1,3-Butadiene (12.2A) 12-11 Products Mechanism Delocalized Carbocation Resonance Structures (12.2B) 12-14 Carbocation Resonance Structures Meaning of Resonance Structures Meaning of The Double Headed Arrow Other Reactions with Delocalized Intermediates (12.2C) 12-16 Acid Catalyzed Hydration Electrophilic Halogenation Free Radical Addition of H-Br. 12.3 Writing Resonance Structures 12-18 A General Procedure (12.3A) 12-19 Carbocations (C+) (12.3B) 12-19 Carbanions (C-) (12.3C) 12-20 . Radicals (C ) (12.3D) 12-20 12.4 More on Delocalized Systems 12-21 Localized vs. Delocalized Intermediates (12.4A) 12-21 Reactions other than Addition (12.4B) 12-21 Solvolysis of Haloalkenes Radical Halogenation Conjugated Systems with Heteroatoms (12.4C) 12-23 Resonance Forms with Heteroatoms Relative Importance of Resonance Forms (continued) 1 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 12.5 Benzenoid Aromatic Molecules 12-25 Benzene (12.5A) 12-25 Reactivity Stability 1 H NMR Spectra The Real Structure of Benzene (12.5B) 12-27 Benzene Geometry Benzene Resonance Structures Benzene Molecular Orbitals Benzene MO's, Resonance, and Unusual Properties (12.5C) 12-30 Chemical Reactivity Stability 1 H NMR Chemical Shifts Substituted Benzenes (12.5D) 12-32 12.6 Nomenclature of Benzenoid Aromatic Molecules 12-32 Monocyclic Arenes (12.6A) 12-33 Systematic Nomenclature Common Nomenclature ortho, meta, and para (12.6B) 12-34 The Phenyl Group The Benzyl Group Polycyclic Arenes (12.6C) 12-36 12.7 Aromatic Systems without Benzene Rings 12-37 Annulenes (12.7A) 12-37 Aromatic and Nonaromatic Annulenes Cyclobutadiene and Cyclooctatetraene. Resonance Structures Do Not Tell the Story MO Diagrams for C4H4, C6H6 and C8H8 (12.7B) 12-39 Cyclooctatetraene Cyclobutadiene. Aromatic Annulenes Besides Benzene (12.7C) 12-40 Hückel's Rule Other Annulenes Heteroaromatic Systems (12.7D) 12-41 Pyridine Pyrrole Furan and Thiophene Purine and Pyrimidine Aromatic Ions (12.7E) 12-43 Cycloheptatrienyl Cation Cyclopentadienyl Anion Cyclopropenyl Cation. Cyclooctadienyl Dianion (continued) 2 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 12.8 Making Substituted Benzenes 12-44 Electrophilic Aromatic Substitution Mechanism (12.8A) 12-44 Electrophiles Arenium Ion Formation Deprotonation of the Arenium Ion Reactions with Substituted Benzenes Formation of the Electrophile (12.8B) 12-47 Halogenation Sulfonation Nitration. Alkylation NH2 and OH Groups on Arenes (12.8C) 12-53 Synthesis of Aniline (Ph-NH2) Synthesis of Phenol (Ph-OH) Acidity of Arene OH Groups Basicity of Arene NH2 Groups 12.9 Spectrometry of Conjugated and Aromatic Molecules 12-57 NMR Spectral Data (12.9A) 12-57 1 H Chemical Shifts 13 C Chemical Shifts UV-Visible Spectral Data (12.9B) 12-58 Conjugated Polyenes The Electron Excitation Process Arenes The Use of UV-Visible Data Infrared Spectrometry (12.9C) 12-61 Supplemental Appendix to "Aromatic Ions (Section 12.7E)" 12-63 3 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 12: Conjugated and Aromatic Molecules •Conjugated Molecules •Reactivity of Conjugated Molecules •Writing Resonance Structures •More on Delocalized Systems •Benzenoid Aromatic Molecules •Nomenclature of Benzenoid Aromatic Molecules •Aromatic Systems without Benzene Rings •Making Substituted Benzenes •Spectrometry of Conjugated and Aromatic Molecules 12.1 Conjugated Molecules Conjugated molecules have π electrons that are not localized in individual double or triple bonds. Rather their π electrons are delocalized throughout an extended π system. We will see later that aromatic molecules are a special class of conjgated molecules. 1,3-Butadiene (12.1A) We show the simplest arrangement of multiple bonds that leads to π electron delocalization and conjugation in Figure 12.001 where two C=C bonds are separated by a C-C single bond. Figure 12.001 The simplest example of a molecule with that arrangement of C=C bonds is 1,3-butadiene. with the structure CH =CH-CH=CH . 2 2 Atomic Orbital Overlap in 1,3-Butadiene. We focus on different aspects of the atomic orbitals in 1,3-butadiene in Figure 12.002 [next page] in order to show why its π electrons are delocalized. The first structure shows 1,3-butadiene as we normally write it. The two C=C bonds appear to be localized between C1 and C2 and between C3 and C4. Without further expanation that formula implies the localized π MO's that we show in the second structure. The first structure shows 1,3-butadiene as we normally write it. The two C=C bonds appear 4 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 Figure 12.002 to be localized between C1 and C2 and between C3 and C4. Without further expanation that formula implies the localized π MO's that we show in the second structure. We learned in Chapter 8 that localized π MO's of C=C bonds result from overlap of the individual 2p atomic orbitals on sp2 C's that we show in the third structure. However, this third structure reveals that not only can we overlap the 2p orbitals on C1 and C2, and on C3 and C4, but we can also overlap those on C2 and C3. This possibility for extended overlap of all four 2p orbitals leads to delocalization of π electron density across all four carbons as we represent in the fourth structure of Figure 12.002. Because all four 2p orbitals can overlap, the resultant π molecular orbitals in 1,3- butadiene are more complex than those arising from overlap of two 2p AO's on adjacent C's. Molecular Orbitals. We learned in Chapter 8 that overlap of two 2p AO's in a molecule such as CH2=CH2 gives one bonding and one antibonding π MO (Figure 12.003). Figure 12.003 5 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 The two electrons in the π bond are normally in the lowest energy bonding MO. Since the number of molecular orbitals that form as the result of atomic orbital overlap is always equal to the number of overlapping AO's, overlap of the four 2p AO's in 1,3- butadiene leads to the formation of four molecular orbitals (Figure 12.004). Figure 12.004 We show these 4 MO's here without explaining their mathematical origins. Since the two MO's of lowest energy contain the 4 π electrons, we call them bonding MO's. The two higher energy MO's without electrons are called antibonding MO's. The Bonding M.O.'s. The lowest energy π MO has two lobes that lie above and below a plane containing the carbon skeleton. This single MO with two lobes extends over all four carbon atoms of the molecule and contains two π electrons. The remaining two π electrons are in the next higher energy bonding MO that consists of four lobes as we show in Figure 12.004. Although this second bonding π MO looks like the second structure in Figure 12.002, it has a very different meaning. We drew the structure in Figure 12.002 to represent the localized bonding π MO's of the two C=C bonds. In contrast, the second bonding π MO in Figure 12.004 is a single MO that is made up of four lobes. The lowest energy π MO (Figure 12.004) includes all four C atoms of the two double bonds. As a result, the 2 electrons in that MO are delocalized over four carbons rather than just localized between two C's of a single C=C. We will see shortly that this delocalization has important effects on both the stability and chemical reactions of this conjugated molecule. 6 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 Conformations. Conjugated dienes such as 1,3-butadiene have two different planar conformations like those we show in Figure 12.005. Figure 12.005 We designate them as the s-trans and s-cis conformations (or s-E and s-Z conformations) where the terms cis (Z) and trans (E) refer to the orientations of the two C=C bonds with respect to the intervening C2-C3 single bond. While the s-trans conformation of 1,3- butadiene is more stable than the s-cis conformation by 10 to 13 kJ/mol, both conformations have delocalized π electrons. Other Alternating Multiple Bonds. Conjugation like we have just described is not limited to the interaction of just two C=C bonds, nor is it limited only to C=C bonds. Whenever two or more multiple bonds of any type (eg. C=C, C=O, C=N, C≡C, C≡N, etc.) alternate with single bonds in a pattern such as "single-multiple-single-multiple-single- multiple" etc., those multiple bonds are usually conjugated and have delocalized π electrons. We show examples of various conjugated molecules in Figure 12.006. Figure 12.006 Pentadienes (12.1B) In order to emphasize the structural requirements for conjugation, we compare three isomeric pentadienes in Figure 12.007 [next page]. Each of these pentadienes has two C=C bonds, but you can see that the relative positions of the two C=C bonds are different in each molecule. 1,3-Pentadiene. Of these three pentadienes, 1,3-pentadiene is the only molecule that is conjugated. Since its two C=C bonds are separated by just one single bond, all four 2p 7 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 orbitals can simultaneously interact. As a result, it has four π MO's that are completely equivalent to those that we showed in Figure 12.004 for 1,3-butadiene. Figure 12.007 The C=C bond between C3 and C4 can be either cis or trans. In fact, cis-1,3-pentadiene and trans-1,3-pentadiene are discrete isomers, but both are conjugated. We will discuss differences between them below. (Each of these cis and trans isomers can also have s-trans and s-cis conformations). 1,4-Pentadiene. In contrast to cis and trans-1,3-pentadiene, the two C=C bonds in 1,4- pentadiene are separated by two C-C single bonds (and an intervening CH group) (Figure 2 12.007). As a result, the two sets of 2p orbitals that overlap to form the two C=C bonds cannot interact with each other so we say that these C=C bonds are isolated. Each C=C bond has a localized bonding π MO that contains the two π electrons of that C=C (Figure 12.007). 1,2-Pentadiene. Even though the two C=C bonds in 1,2-pentadiene share the same C, they cannot interact with each other. The C that is common to each of these C=C bonds is sp hybridized so it has two 2p orbitals that are perpendicular (orthogonal) to each other as we described for molecules of this type in Chapter 8. As a result, the two π bonds in this compound are also orthogonal (perpendicular) to each other and do not overlap. The π electrons in each C=C bond are localized in their respective 8 (3/94)(9,10/96)(3,4,5/04) Neuman Chapter 12 C=C bonding MO's. Two such orthogonal double bonds attached to the same carbon are said to be cumulated and cumulated systems are not conjugated. Cumulated dienes like 1,2- pentadiene are commonly called allenes. Stability of Conjugated Systems (12.1C) Conjugation affects the chemical and physical properties of the molecules containing those conjugated bonds. Molecules with conjugated multiple bonds are generally more stable than comparable molecules with unconjugated multiple bonds. We can see this extra stability due to conjugation when we compare heats of hydrogenation of related conjugated and unconjugated systems as we show in the next section. Heats of Hydrogenation of Pentadienes. We show heats of hydrogenation for three of the pentadienes that we just discussed in the reactions below: CH2=CH-CH2-CH=CH2 + 2H2 → CH3CH2CH2CH2CH3 + 253 kJ/mol 1,4-pentadiene pentane CH2=CH-CH=CH-CH3 + 2H2 → CH3CH2CH2CH2CH3 + 223 kJ/mol trans-1,3-pentadiene pentane CH2=CH-CH=CH-CH3 + 2H2 → CH3CH2CH2CH2CH3 + 229 kJ/mol cis-1,3-pentadiene pentane You can see that more heat is given off when we hydrogenate the unconjugated diene 1,4- pentadiene than when we hydrogenate either of the two conjugated 1,3-pentadienes. This is a quantitative demonstration that conjugated cis and trans-1,3-pentadiene are more stable than unconjugated 1,4-pentadiene (Figure 12.008). Figure 12.008 9