Chapter 16 Aromatic Compounds Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C: ratio to be 1:1. Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C 6 6. Other related compounds with low C: ratios had a pleasant smell, so they were classified as aromatic. Chapter 16: Aromatics Slide 16-2 1
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. C C C C C C Chapter 16: Aromatics Slide 16-3 Resonance Structure Each sp 2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. Chapter 16: Aromatics Slide 16-4 2
Unusual Reactions Alkene + KMnO 4 diol (addition) Benzene + KMnO 4 no reaction. Alkene + Br 2 /CCl 4 dibromide (addition) Benzene + Br 2 /CCl 4 no reaction. With FeCl 3 catalyst, Br 2 reacts with benzene to form bromobenzene + Br (substitution!). Double bonds remain. Chapter 16: Aromatics Slide 16-5 Unusual Stability ydrogenation of just one double bond in benzene is endothermic! Chapter 16: Aromatics Slide 16-6 3
Annulenes All cyclic conjugated hydrocarbons were proposed to be aromatic. owever, cyclobutadiene is so reactive that it dimerizes before it can be isolated. And cyclooctatetraene adds Br 2 readily. Look at MO s to explain aromaticity. Chapter 16: Aromatics Slide 16-7 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: Aromatics Slide 16-8 4
MO s for Benzene Chapter 16: Aromatics Slide 16-9 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: Aromatics Slide 16-10 5
MO s for Cyclobutadiene Chapter 16: Aromatics Slide 16-11 Energy Diagram for Cyclobutadiene Following und s rule, two electrons are in separate orbitals. This diradical would be very reactive. Chapter 16: Aromatics Slide 16-12 6
Polygon Rule The energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the bottom. Chapter 16: Aromatics Slide 16-13 Aromatic Requirements Structure must be cyclic with conjugated pi bonds. Each atom in the ring must have an unhybridized p orbital. The p orbitals must overlap continuously around the ring. (Usually planar structure.) Compound is more stable than its open-chain counterpart. Chapter 16: Aromatics Slide 16-14 7
Anti- and Nonaromatic Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart. Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. Chapter 16: Aromatics Slide 16-15 ückel s Rule If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 electrons, it is aromatic. If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic. Chapter 16: Aromatics Slide 16-16 8
[N]Annulenes [4]Annulene is antiaromatic (4N e - s) [8]Annulene would be antiaromatic, but it s not planar, so it s nonaromatic. [10]Annulene is aromatic except for the isomers that are not planar. Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar. Chapter 16: Aromatics Slide 16-17 MO Derivation of ückel s Rule Lowest energy MO has 2 electrons. Each filled shell has 4 electrons. Chapter 16: Aromatics Slide 16-18 9
Cyclopentadienyl Ions The cation has an empty p orbital, 4 electrons, so antiaromatic. The anion has a nonbonding pair of electrons in a p orbital, 6 e - s, aromatic. Chapter 16: Aromatics Slide 16-19 Acidity of Cyclopentadiene pk a of cyclopentadiene is 16, much more acidic than other hydrocarbons. Chapter 16: Aromatics Slide 16-20 10
Tropylium Ion The cycloheptatrienyl cation has 6 p electrons and an empty p orbital. Aromatic: more stable than open chain ion. O +, 2 O + Chapter 16: Aromatics Slide 16-21 Dianion of [8]Annulene Cyclooctatetraene easily forms a -2 ion. Ten electrons, continuous overlapping p orbitals, so it is aromatic. Chapter 16: Aromatics Slide 16-22 11
Pyridine eterocyclic aromatic compound. Nonbonding pair of electrons in sp 2 orbital, so weak base, pk b = 8.8. Chapter 16: Aromatics Slide 16-23 Pyrrole Also aromatic, but lone pair of electrons is delocalized, so much weaker base. Chapter 16: Aromatics Slide 16-24 12
Basic or Nonbasic? N N Pyrimidine has two basic nitrogens. N N Imidazole has one basic nitrogen and one nonbasic. N N N N Purine? Chapter 16: Aromatics Slide 16-25 Other eterocyclics Chapter 16: Aromatics Slide 16-26 13
Fused Ring ydrocarbons Naphthalene Anthracene Phenanthrene Chapter 16: Aromatics Slide 16-27 Reactivity of Polynuclear ydrocarbons As the number of aromatic rings increases, the resonance energy per ring decreases, so larger PA s will add Br 2. Br Br Br Br (mixture of cis and trans isomers) Chapter 16: Aromatics Slide 16-28 14
Larger Polynuclear Aromatic ydrocarbons Formed in combustion (tobacco smoke). Many are carcinogenic. Epoxides form, combine with DNA base. pyrene Chapter 16: Aromatics Slide 16-29 Allotropes of Carbon Amorphous: small particles of graphite; charcoal, soot, coal, carbon black. Diamond: a lattice of tetrahedral C s. Graphite: layers of fused aromatic rings. Chapter 16: Aromatics Slide 16-30 15
Diamond One giant molecule. Tetrahedral carbons. Sigma bonds, 1.54 Å. Electrical insulator. Chapter 16: Aromatics Slide 16-31 Graphite Planar layered structure. Layer of fused benzene rings, bonds: 1.415 Å. Only van der Waals forces between layers. Conducts electrical current parallel to layers. Chapter 16: Aromatics Slide 16-32 16
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: Aromatics Slide 16-33 Fused eterocyclic Compounds Common in nature, synthesized for drugs. Chapter 16: Aromatics Slide 16-34 17
Common Names of Benzene Derivatives O C 3 N2 OC 3 phenol toluene aniline anisole C C 2 O C C 3 O C O C O styrene acetophenone benzaldehyde benzoic acid Chapter 16: Aromatics Slide 16-35 Disubstituted Benzenes The prefixes ortho-, meta-, and para- are commonly used for the 1,2-, 1,3-, and 1,4- positions, respectively. Chapter 16: Aromatics Slide 16-36 18
3 or More Substituents Use the smallest possible numbers, but the carbon with a functional group is #1. O 2 N NO 2 O 2 N O NO 2 NO 2 1,3,5-trinitrobenzene NO 2 2,4,6-trinitrophenol Chapter 16: Aromatics Slide 16-37 Common Names for Disubstituted Benzenes C 3 C 3 O C O C 3 O C 3 3 C C 3 3 C m-xylene mesitylene o-toluic acid p-cresol Chapter 16: Aromatics Slide 16-38 19
Phenyl and Benzyl Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon. Br C 2 Br phenyl bromide benzyl bromide Chapter 16: Aromatics Slide 16-39 Physical Properties Melting points: More symmetrical than corresponding alkane, pack better into crystals, 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: Aromatics Slide 16-40 20
IR and NMR Spectroscopy C=C stretch absorption at 1600 cm -1. sp 2 C- stretch just above 3000 cm -1. 1 NMR at δ7-δ8 for s on aromatic ring. 13 C NMR at δ120-δ150, similar to alkene carbons. Chapter 16: Aromatics Slide 16-41 Mass Spectrometry Chapter 16: Aromatics Slide 16-42 21
UV Spectroscopy Chapter 16: Aromatics Slide 16-43 End of Chapter 16 omework: 27, 28, 31, 32, 34, 35, 37, 45, 46, 50 Chapter 16: Aromatics Slide 16-44 22