Organic Chemistry, 7 L. G. Wade, Jr. 2010, Prentice Hall
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1 Organic Chemistry, 7 th Edition L. G. Wade, Jr. Chapter 16 Aromatic Compounds 2010, Prentice Hall
2 Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C 6H 6. He named it benzin. Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic. Chapter 16 2
3 Kekulé Structure Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested. Failed to explain existence of only one isomer of 1,2-dichlorobenzene. H H C C H C C H C C H H Chapter 16 3
4 Resonance Structures of Benzene Benzene is actually a resonance hybrid between the two Kekulé structures. The C C bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond bond lengths (bond order 1.5). Benzene's resonance can be represented by drawing a circle inside id the six-membered ring as a combined representation. ti Chapter 16 4
5 Structure of Benzene Each sp 2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. The six pi electrons are delocalized over the six carbons. Chapter 16 5
6 Unusual Addition of Bromine to Benzene When bromine adds to benzene, a catalyst such as FeBr 3 is needed. The reaction that occurs is the substitution of a hydrogen by bromine. Addition of Br 2 to the double bond is not observed. Chapter 16 6
7 Resonance Energy Benzene does not have the predicted heat of hydrogenation of -359 kj/mol. The observed heat of hydrogenation is -208 kj/mol, a difference of 151 kj. This difference between the predicted and the observed value is called the resonance energy. Chapter 16 7
8 Molar Heats of Hydrogenation Chapter 16 8
9 Annulenes Annulenes are hydrocarbons with alternating single and double bonds. Benzene is a six-membered annulene, so it can be named [6]-annulene. Cylobutadiene is [4]-annulene, cyclooctatetraene is [8]-annulene. Chapter 16 9
10 Annulenes All cyclic conjugated hydrocarbons were proposed p to be aromatic. However, cyclobutadiene is so reactive that it dimerizes before it can be isolated. Cyclooctatetraene adds Br 2 readily to the double bonds. Molecular orbitals can explain aromaticity. Chapter 16 10
11 MO Rules for Benzene Six overlapping p orbitals must form six molecular orbitals. Three will be bonding, three antibonding. Lowest energy MO will have all bonding interactions, no nodes. As energy of MO increases, the number of nodes increases. Chapter 16 11
12 MO s for Benzene Highest molecular l orbital Lowest molecular orbital Chapter 16 12
13 First MO of Benzene The first MO of benzene is entirely bonding with no nodes. It has very low energy because it has six bonding interactions and the electrons are delocalized over all six carbon atoms. Chapter 16 13
14 Intermediate MO of Benzene The intermediate levels are degenerate (equal in energy) with two orbitals at each energy level. Both π 2 and π 3 have one nodal plane. Chapter 16 14
15 All Antibonding MO of Benzene The all-antibonding π * 6 has three nodal planes. Each pair of adjacent p orbitals is out of phase and interacts destructively. Chapter 16 15
16 Energy Diagram for Benzene The six electrons fill three bonding pi orbitals. All bonding orbitals are filled ( closed shell ), an extremely stable arrangement. Chapter 16 16
17 MO s for Cyclobutadiene Chapter 16 17
18 Electronic Energy Diagram for Cyclobutadiene Following Hund s rule, two electrons are in separate nonbonding molecular orbitals. This diradical would be very reactive. Chapter 16 18
19 Polygon Rule The energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the bottom. Chapter 16 19
20 Aromatic Requirements Structure must be cyclic with conjugated pi pbods bonds. Each atom in the ring must have an unhybridized p orbital (sp 2 or sp). The p orbitals must overlap continuously around the ring. Structure must be planar (or close to planar for effective overlap to occur) Delocalization of the pi electrons over the ring must lower the electronic energy. Chapter 16 20
21 Anti- and Nonaromatic Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but electron delocalization increases its electronic energy. Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. Chapter 16 21
22 Hückel s Rule Once the aromatic criteria is met, Huckel s rule applies. If the number of pi electrons is (4N + 2) the compound is aromatic (where N is an integer) If the number of pi electrons is (4N) the compound is antiaromatic. Chapter 16 22
23 Orbital Overlap of Cyclooctatetraene Cyclooctatetraene ae e assumes a nonplanar a tub conformation that avoids most of the overlap between the adjacent pi bonds. Huckel's rule simply does not apply. Chapter 16 23
24 Annulenes [4]Annulene is antiaromatic. [8]Annulene would be antiaromatic, but it s not planar, so it s nonaromatic. [10]Annulene is aromatic except for the isomers that are nonplanar. Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar. Chapter 16 24
25 MO Derivation of Hückel s Rule Aromatic compounds have (4N + 2) electrons and the orbitals are filled. Antiaromatic compounds have only 4N electrons and has unpaired electrons in two degenerate orbitals. Chapter 16 25
26 Cyclopentadienyl Ions The cation has an empty p orbital, 4 electrons, so it is antiaromatic. The anion has a nonbonding pair of electrons in a p orbital, 6 electrons, it is aromatic. Chapter 16 26
27 Deprotonation of Cyclopentadiene By deprotonating the sp 3 carbon of cyclopentadiene, the electrons in the p orbitals can be delocalized over all five carbon atoms and the compound would be aromatic. Cyclopentadiene is acidic because deprotonation will convert it to an aromatic ion. Chapter 16 27
28 Orbital View of the Deprotonation of Cyclopentadiene Deprotonation will allow the overlap of all the p orbitals in the molecule. Cyclopentadiene is not necessarily as stable as benzene and it reacts readily with electrophiles. Chapter 16 28
29 Cyclopentadienyl Cation Huckel s rule predicts that the cyclopentadienyl cation, with four pi electrons, is antiaromatic. In agreement with this prediction, the cyclopentadienyl cation is not easily formed. Chapter 16 29
30 Resonance Forms of Cyclopentadienyl Ions Chapter 16 30
31 Tropylium Ion aromatic The cycloheptatrienyl cation has 6 pi electrons and an empty p orbital. The cycloheptatrienyl cation is easily formed by treating ti the corresponding alcohol l with dilute (0.01N) 01N) aqueous sulfuric acid. The cycloheptatrienyl y cation is commonly known as the tropylium ion. Chapter 16 31
32 Cyclooctatetraene Dianion Cyclooctatetraene t t reacts with potassium metal to form an aromatic dianion. The dianion has 10 pi electrons and is aromatic. Chapter 16 32
33 Which of the following is an aromatic compound? Non-aromatic Aromatic There is an sp 3 carbon in All carbons are sp 3 the ring, delocalization will hybridized and it obeys not be complete. Huckel s rule. Chapter 16 33
34 Pyridine Pi System Pyridine has six delocalized electrons in its pi system. The two non-bonding electrons on nitrogen are in an sp 2 orbital, and they do not interact with the pi electrons of the ring. Chapter 16 34
35 Pyridine Pyridine is basic, with a pair non-bonding electrons available to abstract a proton. The protonated pyridine (the pyridinium ion) is still aromatic. Chapter 16 35
36 Pyrrole Pi System The pyrrole nitrogen atom is sp 2 hybridized with a lone pair of electrons in the p orbital. This p orbital overlaps with the p orbitals of the carbon atoms to form a continuous ring. Pyrrole is aromatic because it has 6 pi electrons (N =1) 1). Chapter 16 36
37 Pyrrole Also aromatic, but lone pair of electrons is delocalized, so much weaker base. Chapter 16 37
38 Basic or Nonbasic? N N Pyrimidine has two basic nitrogens. N N H Not basic Imidazole has one basic nitrogen and one nonbasic. Not basic N N H N N Only one of purine s nitrogens is not basic. Chapter 16 38
39 Other Heterocyclics Chapter 16 39
40 Is the molecule below aromatic, anti-aromatic or non-aromatic? H N N N Aromatic Chapter 16 40
41 Naphthalene Fused rings share 2 atoms and the bond between them. Naphthalene is the simplest fused aromatic hydrocarbon. Chapter 16 41
42 Fused Ring Hydrocarbons Chapter 16 42
43 Polynuclear Aromatic Hydrocarbons H Br Br H Br H Br H As the number of aromatic rings increases, the resonance energy per ring decreases, so larger polynuclear aromatic hydrocarbons will add Br 2. Chapter 16 43
44 Larger Polynuclear Aromatic Hydrocarbons Formed in combustion (tobacco smoke). Many are carcinogenic. Epoxides form, combine with DNA base. pyrene Chapter 16 44
45 Allotropes opes of Carbon Amorphous: small particles of graphite; charcoal, soot, coal, carbon black. Diamond: a lattice of tetrahedral C s Cs. Graphite: layers of fused aromatic rings Chapter 16 45
46 Diamond One giant molecule. Tetrahedral carbons. Sigma bonds, 1.54 Å. Electrical insulator. Chapter 16 46
47 Graphite Planar layered structure. Layer of fused benzene rings, bonds: Å. Only van der Waals forces between layers. Conducts electrical current parallel to layers. Chapter 16 47
48 Some New Allotropes Fullerenes: 5- and 6-membered rings arranged to form a soccer ball structure. Nanotubes: half of a C 60 sphere fused to a cylinder of fused aromatic rings. Chapter 16 48
49 Fused Heterocyclic Compounds Common in nature, synthesized for drugs. Chapter 16 49
50 Common Names of Benzene Derivatives Chapter 16 50
51 Disubstituted t Benzenes Numbers can also be used to identify the relationship between the groups; ortho- is 1,2-disubstituted, meta- is 1,3, and para- is 1,4. Chapter 16 51
52 Three or More Substituents Use the smallest possible numbers, but the carbon with a functional group is #1. Chapter 16 52
53 Common Names for Disubstituted Benzenes O CH 3 CH 3 C OH CH 3 OH CH 3 H 3 C CH 3 H 3 C m-xylene mesitylene o-toluic acid p-cresol Chapter 16 53
54 Phenyl and Benzyl Br CH 2 Br phenyl bromide benzyl bromide Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon. Chapter 16 54
55 Physical Properties of Aromatic Compounds Melting points: More symmetrical than corresponding alkane, a pack better into crystals, s, so higher melting points. Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes. Density: More dense than nonaromatics, less dense than water. Solubility: Generally insoluble in water. Chapter 16 55
56 IR and NMR Spectroscopy C C stretch absorption at 1600 cm -1. sp 2 C H stretch just above 3000 cm HNMRatδ7 δ7 δ8 δ8 forh s Hs on aromatic ring. 13 C NMR at δ120 δ150, similar to alkene carbons. Chapter 16 56
57 Mass Spectrometry Chapter 16 57
58 UV Spectroscopy Chapter 16 58
59 Chapter 16 59
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