Chapter 15 Benzene and Aromaticity

Chapter 15 Benzene and Aromaticity

Aromatic Compounds Aromatic Originally used to describe fragrant substances Refers to a class of compounds that meets Hückel criteria for aromaticity 2

Aromatic Compounds Aromatic Originally used to describe fragrant substances Refers to a class of compounds that meets Hückel criteria for aromaticity 3

Aromatic Compounds The Hückel 4n + 2 Rule Developed by Erich Hückel in 1931 States that a molecule can be aromatic only if: It has a planar, monocyclic system of conjugation It contains a total of 4n + 2 molecules n = 0,1,2,3 4n electrons are considered antiaromatic 4

Aromatic Compounds: Source Coal and petroleum are the major sources of simple aromatic compounds Coal primarily comprises of large arrays of conjoined benzene-like rings When heated to 1000 C, coal thermally breaks down to yield coal tar Coal https://grist.files.wordpress.com/2013/09/lump-o-coal.jpg A representative structure of bituminous coal Proc. Natl. Acad. Sci. USA 79, 3365 (1982) 5

Aromatic Compounds: Source Fractional distillation of coal tar yields many aromatic compounds 2016 Cengage Learning. 6

Aromatic Compounds: Source Petroleum primarily comprises alkenes and few aromatic compounds Formation of more aromatic molecules occur when alkanes are passed over a catalyst at high pressure and temperature Petroleum http://www.investigroup.com/wp-content/uploads/2014/07/5.jpg 7

Aromatic Compounds: Nomenclature Aromatic compounds naming system uses: Nonsystematic names 2016 Cengage Learning. 8

Aromatic Compounds: Nomenclature Aromatic compounds naming system uses: Nonsystematic names International Union of Pure and Applied Chemistry (IUPAC) Rules Allows use of widely used names 9

Aromatic Compounds: Nomenclature International Union of Pure and Applied Chemistry (IUPAC) Rules Monosubstituted benzenes have systematic names with benzene being the parent name 2016 Cengage Learning. 10

Aromatic Compounds: Nomenclature International Union of Pure and Applied Chemistry (IUPAC) Rules Arenes are alkyl-substituted benzenes Alkyl-substituent benzenes are smaller than the ring (<6 carbons) Phenyl-substituted benzenes Phenyl-substituted benzenes are larger than the ring (>7 carbons) The term phenyl (Ph or Φ) is used in substituent benzene ring C 6 H 5 The term benzyl is used for the C 6 H 5 CH 2 group 2016 Cengage Learning. 11

Aromatic Compounds: Nomenclature International Union of Pure and Applied Chemistry (IUPAC) Rules Disubstituted Benzenes Names based on the placement of substituents Ortho- is 1,2 disubstituted Meta- is 1,3 disubstituted Para- is 1,4 disubstituted Provides clarity in the discussion of reactions 2016 Cengage Learning. 12

Aromatic Compounds: Nomenclature International Union of Pure and Applied Chemistry (IUPAC) Rules Disubstituted Benzenes Names based on the placement of substituents Ortho (o), meta (m), and para (p)» Provide clarity in the discussion of reactions 2016 Cengage Learning. 13

Aromatic Compounds: Nomenclature International Union of Pure and Applied Chemistry (IUPAC) Rules Benzenes +2 or more substituents Numbers with the lowest possible values are chosen List substituents alphabetically with hyphenated numbers Common names, such as toluene can serve as root name(as in TNT) 2016 Cengage Learning. 14

Worked Example Provide the IUPAC name for the following compound Solution: The compound is 1-Ethyl-2,4-dinitrobenzene Substituents on trisubstituted rings receive the lowest possible numbers 2016 Cengage Learning. 15

STRUCTURE AND STABILITY OF BENZENE 16

Aromatic Compounds: Stability of Benzene The reactivity of benzene is much lesser than that of alkenes despite having six fewer hydrogens Benzene - C 6 H 6 Cycloalkane - C 6 H 12 2016 Cengage Learning. 17

Aromatic Compounds: Stability of Benzene Comparison of the heats of hydrogenation proves the stability of benzene Remember Heat of Hydrogenation is the heat produced when alkene is reduced to an alkane Alkene with lower (less negative) value is more stable Reduction is exothermic (converting weaker pi bond to stronger sigma bond) Depends on degree of substitution of double bond (greater substitution, lower heat of hydrogenation) Trans alkene is lower than cis alkene 2016 Cengage Learning. 18

Aromatic Compounds: Stability of Benzene Comparison of the heats of hydrogenation proves the stability of benzene 2016 Cengage Learning. 19

Aromatic Compounds: Structure of Benzene All its C-C bonds are the same length: 139 pm between single (154 pm) and double (134 pm) bonds Electron density in all six C-C bonds is identical Structure is planar, hexagonal 1.39 Å 2016 Cengage Learning. 20

Aromatic Compounds: Structure of Benzene Carbon atoms and p orbitals in benzene are equivalent Impossible to define three localized bonds in which a given p orbital overlaps only one neighboring p orbital All electrons move freely in the entire ring due to equal overlap of all p orbitals 1.39 Å 2016 Cengage Learning. 21

Aromatic Compounds: Structure of Benzene Structure is in resonance Resonance influences its rate of reactivity 1.39 Å 2016 Cengage Learning. 22

Aromatic Compounds: Structure of Benzene Benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon carbon bonds The ring does not indicate the number of electrons in the ring but is a reminder of the delocalized structure 2016 Cengage Learning. 23

Aromatic Compounds: Structure of Benzene Molecular orbital description of benzene The 6 p-orbitals combine to give: Three bonding orbitals with 6 electrons Three antibonding with no electrons Orbitals with the same energy are degenerate 2016 Cengage Learning. 24

Aromatic Compounds Observations about benzene and benzene like aromatic compounds Unusually stable - Heat of hydrogenation 150 kj/mol less negative than a hypothetical cyclic triene Planar hexagon - Bond angles are 120, carboncarbon bond length is 139 pm Undergoes substitution rather than electrophilic addition Resonance hybrid with structure between two line-bond structures 25

AROMATICITY AND THE HÜCKEL 4N+2 RULE 26

Aromatic Compounds: Hückel Rule States that a molecule can be aromatic only if: It has a planar, monocyclic system of conjugation It contains a total of 4n + 2 electrons n = 0,1,2,3 Antiaromatic if 4n electrons are considered 27

Aromatic Compounds: Hückel Rule Does molecule contain (4n+2) or 4n pi electrons Cyclobutadiene Four pi electrons Antiaromatic It reacts readily and exhibits none of the properties corresponding to aromaticity It dimerizes by a Diels-Alder reaction at 78 C 2016 Cengage Learning. 28

Aromatic Compounds: Hückel Rule Does molecule contain (4n+2) or 4n pi electrons Benzene possesses six electrons (4n + 2 = 6 when n = 1) Aromatic 2016 Cengage Learning. 29

Aromatic Compounds: Hückel Rule Does molecule contain (4n+2) or 4n pi electrons Cyclooctatetraene possesses eight electrons Not aromatic Comprises four double bonds 2016 Cengage Learning. 30

Aromatic Compounds: Stability and Molecular Orbital Theory Molecular orbitals for cyclic conjugated molecules Always contain a single lowest-lying MO Above lowest MO, MOs come in degenerate pairs Energy Levels of the Six Benzene Molecular Orbitals 2016 Cengage Learning. 31

Worked Example To be aromatic, a molecule must have 4n + 2 electrons and must have a planar, monocyclic system of conjugation Explain why cyclodecapentaene has resisted all attempts at synthesis though it has fulfilled only one of the above criteria 32

Worked Example Solution: Cyclodecapentaene possesses 4n + 2 (n = 2) but is not flat If cyclodecapentaene were flat, the starred hydrogen atoms would crowd each other across the ring To avoid this interaction, the ring system is distorted from planarity 2016 Cengage Learning. 33

Aromatic Ions The 4n + 2 rule applies to ions as well as neutral substances Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic 2016 Cengage Learning. 34

Aromatic Ions How are ions aromatic? Starting with a neutral saturated hydrocarbon Remove one hydrogen from the saturated CH 2 Rehybridize the carbon from sp 3 to sp 2 Result is a fully conjugated product with a p orbital on every product 2016 Cengage Learning. 35

Aromatic Ions Methods to remove hydrogen from saturated CH 2 Removing the hydrogen with both electrons (H: ) from the C H bond results in a carbocation Removing the hydrogen with one electron (H ) from the C H bond results in a carbon radical Removing the hydrogen without any electrons (H + ) from the C H bond results in a carbanion 2016 Cengage Learning. 36

Aromatic Ions: Cyclopentadienyl Anion Disadvantages of the four- -electron cyclopentadienyl cation and the five- cyclopentadienyl radical Highly reactive Difficult to prepare Not stable enough for aromatic systems Advantages of using the six- -electron cyclopentadienyl cation Easily prepared Extremely stable pk a =16 2016 Cengage Learning. Acidicty of a hydrogen atom 37

Worked Example Cyclooctatetraene readily reacts with potassium metal to form the stable cyclooctatetraene dianion, C 8 H 8 2 Explain why this reaction occurs so easily Determine the geometry for the cyclooctatetraene dianion 2016 Cengage Learning. 38

Worked Example Solution: When cyclooctatetrene accepts two electrons, it becomes a (4n + 2) electron aromatic ion Cyclooctatetraenyl dianion is planar with a carbon carbon bond angle of 135, that of a regular octagon 2016 Cengage Learning. 39

AROMATIC HETEROCYCLES: PYRIDINE AND PYRROLE 40

Aromatic Heterocycles Heterocycle: Cyclic compound that comprises atoms of two or more elements in its ring Carbon along with nitrogen, oxygen, or sulfur Aromatic compounds can have elements other than carbon in the ring 41

Aromatic Heterocycles: Pyridine Six-membered heterocycle with a nitrogen atom in its ring Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity 2016 Cengage Learning. 42

Aromatic Heterocycles: Pyridine The nitrogen lone pair electrons are not part of the aromatic system (perpendicular orbital) The structure of pyridine is quite similar to that of benzene All five sp 2 -hybridized ions possess a p orbital perpendicular with one to the plane of the ring Each p orbital comprises one electron The nitrogen atom is also sp 2 -hybridized and possesses one electron in a p orbital 2016 Cengage Learning. 43

Aromatic Heterocycles: Pyrimidine Pyrimidine comprises two nitrogen atoms in a six-membered, unsaturated ring The sp 2 -hybridized nitrogen atoms share an electron each to the aromatic system 2016 Cengage Learning. 44

Aromatic Heterocycles: Pyrrole and Imidazole 2016 Cengage Learning. 45

Aromatic Heterocycles: Pyrimidine and Imidazole Significant in biological chemistry Pyrimidine is the parent ring system present in cytosine, thymine, and uracil Histidine contains an aromatic imidazole ring 2016 Cengage Learning. 46

Worked Example Draw an orbital picture of Furan to show how the molecule is aromatic 2016 Cengage Learning. 47

Worked Example Solution: Oxygen contributes two lone-pair electron from a p orbital perpendicular to the plane of the ring It possesses 6 electrons on a cyclic, conjugated system; it is aromatic Furan is an oxygen analog of pyrrole 2016 Cengage Learning. 48

POLYCYCLIC AROMATIC COMPOUNDS 49

Polycyclic Aromatic Compounds Hückel rule is relevant only to monocyclic compounds Aromaticity can also be applied to polycyclic aromatic compounds 2016 Cengage Learning. 50

Polycyclic Aromatic Compounds: Naphthalene Orbitals Three resonance forms and delocalized electrons Naphthalene and other polycyclic aromatic hydrocarbons possess certain chemical properties that correspond to aromaticity Heat of hydrogenation in naphthalene is approximately 250 kj/mol 2016 Cengage Learning. 51

Polycyclic Aromatic Compounds: Naphthalene Orbitals Comparison of the heats of hydrogenation Naphthalene is approximately 250 kj/mol 2016 Cengage Learning. 52

Polycyclic Aromatic Compounds: Naphthalene Naphthalene possesses a cyclic, conjugated electron system p orbital overlap is present along the ten-carbon periphery of the molecule and across the central bond Aromaticity is due to the electron delocalization caused by the presence of ten electrons (Hückel number) 2016 Cengage Learning. 53

Polycyclic Aromatic Compounds: Analogs of Naphthelene Quinolone, isoquinolone, and purine have pyridine-like nitrogens that share one electron Indole and purine have pyrrole-like nitrogens that share two electrons 2016 Cengage Learning. 54

Worked Example Azulene, a beautiful blue hydrocarbon, is an isomer of naphthalene Determine whether it is an aromatic Draw a second resonance form of azulene in addition to the form shown below 2016 Cengage Learning. 55

Worked Example Solution: Azulene is an aromatic because it has a conjugated cyclic electron system containing ten electrons (a Hückel number) 2016 Cengage Learning. 56

SPECTROSCOPY OF AROMATIC COMPOUNDS 57

Spectroscopy of Aromatic Compounds: IR Infrared Spectroscopy C H stretching absorption is seen at 3030 cm 1 Usually of low intensity Left of typical saturated C-H stretch A series of peaks are present between 1450 and 1600 cm 1 Caused by the complex molecular motions of the ring 2016 Cengage Learning. 58

Spectroscopy of Aromatic Compounds: Ultraviolet Spectroscopy Presence of a conjugated system makes ultraviolet spectroscopy possible Intense absorption occurs near 205 nm Less intense absorption occurs between 255 nm and 275 nm 2016 Cengage Learning. 59

Spectroscopy of Aromatic Compounds: NMR Spectroscopy The aromatic ring shields hydrogens Absorption occurs between 6.5 and 8.5 δ The ring current is responsible for the difference in chemical shift between aromatic and vinylic protons Ring current is the magnetic field caused by the circulation of delocalized electrons when the aromatic ring is perpendicular to a strong magnetic field The effective magnetic field is greater than the applied field 60

Spectroscopy of Aromatic Compounds: NMR Spectroscopy 2016 Cengage Learning. 61

Spectroscopy of Aromatic Compounds: NMR Spectroscopy Aromatic protons appear as two doublets at 7.04 and 7.37 δ Benzylic methyl protons appear as a sharp singlet at 2.26 δ 2016 Cengage Learning. 62

Spectroscopy of Aromatic Compounds: 13 C NMR Spectroscopy Carbons in aromatic ring absorb between 110 and 140 δ Shift is distinct from alkane carbons but in same range as alkene carbons 2016 Cengage Learning. 63

Spectroscopy of Aromatic Compounds: 13 C NMR Spectroscopy Mode of substitution influences the formation of two, three, or four resonances in the proton-decoupled 13 C NMR spectrum 2016 Cengage Learning. 64

Spectroscopy of Aromatic Compounds: 13 C NMR Spectroscopy The Proton-Decoupled 13 C NMR Spectra of the Three Isomers of Dichlorobenzene 2016 Cengage Learning. 65

Summary The term aromatic refers to the class of compounds that are structurally similar to benzene The Hückel rule states that in order to be aromatic, a molecule must possess 4n + 2 electrons, where n = 0,1,2,3, and so on Apart from IUPAC terms, disubstituted benzenes are also called ortho, meta, or para derivatives The C 6 H 5 unit is called a phenyl group The C 6 H 5 CH 2 unit is called a benzyl group 66

Summary Planar, cyclic, conjugated molecules with other numbers of electrons are antiaromatic Pyridine and pyrimidine are six-membered, nitrogen containing, aromatic heterocycles 67

Give the shape of the benzene molecule. a)tetrahedral b)bent c) Trigonal pyramidal d)planar 68

Give the shape of the benzene molecule. a) Tetrahedral b) Bent c) Trigonal pyramidal d) Planar Explanation: All six carbons and six hydrogens are in the same plane. 69

Give the hybridization of each carbon in benzene. a) sp b) sp 2 c) sp 3 d) sp 4 70

Give the hybridization of each carbon in benzene. a) sp b) sp 2 c) sp 3 d) sp 4 Explanation: Each carbon in benzene is sp 2 hybridized. 71

Give the bond angle of the atoms in benzene. a)45 b)60 c) 90 d)109.5 e)120 72

Give the bond angle of the atoms in benzene. a) 45 b) 60 c) 90 d) 109.5 e) 120 Explanation: The carbons are trigonal planar with angles of 120. 73

Classify a)aromatic b)antiaromatic c) Nonaromatic d)acyclic 74

Classify a) Aromatic b) Antiaromatic c) Nonaromatic d) Acyclic Explanation: The compound gives a whole number for N in Hückel s rule (4N + 2 = 6, N = 1). 75

Classify a)aromatic b)antiaromatic c) Nonaromatic d)acyclic 76

Classify a) Aromatic b) Antiaromatic c) Nonaromatic d) Acyclic Explanation: The compound is cyclic and has continuous delocalized electrons, but does not give a whole number for Hückel s rule (4N + 2 = 8, N = 3/2). 77

Classify a)aromatic b)antiaromatic c) Nonaromatic d)acyclic 78

Classify a) Aromatic b) Antiaromatic c) Nonaromatic d) Acyclic Explanation: This cyclic compound does not have a continuous, overlapping ring of p orbitals and is nonaromatic. 79

Name N H a)pyridine b)pyrrole c) Pyrimidine d)imidazole 80

Name N H a) Pyridine b) Pyrrole c) Pyrimidine d) Imidazole Explanation: Pyrrole is a heterocyclic aromatic compound. 81

Name N N a)imidazole b)pyrimidine c) Pyridine d)purine e)furan 82

Name N N a) Imidazole b) Pyrimidine c) Pyridine d) Purine e) Furan Explanation: Pyrimidine is an aromatic compound with nitrogens in the 1 and 3 positions. 83

Classify O a)aromatic b)antiaromatic c) Nonaromatic d)acyclic 84

Classify O a) Aromatic b) Antiaromatic c) Nonaromatic d) Acyclic Explanation: Furan is a heterocyclic aromatic compound. 85

Name S a)pyrimidine b)imidazole c) Purine d)furan e)thiophene 86

Name S a) Pyrimidine b) Imidazole c) Purine d) Furan e) Thiophene Explanation: Thiophene is a heterocyclic aromatic compound. 87

Name a)anthracene b)naphthalene c) Phenanthrene d)benzene 88

Name a) Anthracene b) Naphthalene c) Phenanthrene d) Benzene Explanation: Naphthalene contains two benzene rings fused together. 89

Name a)anthracene b)naphthalene c) Phenanthrene d)benzene 90

Name a) Anthracene b) Naphthalene c) Phenanthrene d) Benzene Explanation: Anthracene contains three benzene rings fused together. 91

Name N H a)purine b)indole c) Benzimidazole d)quinoline 92

Name N H a) Purine b) Indole c) Benzimidazole d) Quinoline Explanation: Indole contains a benzene ring with a five-membered ring fused to it. 93

N Name N H a)purine b)indole c) Benzimidazole d)quinoline 94

N Name N H a) Purine b) Indole c) Benzimidazole d) Quinoline Explanation: Benzimidazole contains a benzene ring with an imidazole fused to it. 95

Cl NH 2 Name Br a)4-bromo-3-chloroaniline b)4-bromo-3-chlorophenol c) 4-Bromo-3-chloroanisole d)1-bromo-2-chloro-4-aniline e)1-bromo-2-chloro-4-phenol 96

Cl NH 2 Name Br a) 4-Bromo-3-chloroaniline b) 4-Bromo-3-chlorophenol c) 4-Bromo-3-chloroanisole d) 1-Bromo-2-chloro-4-aniline e) 1-Bromo-2-chloro-4-phenol Explanation: Aniline is the parent compound. The NH 2 is at position one. 97

CH 3 Name OH a) p-methylphenol b) m-methylphenol c) o-methylphenol d) 4-Methylphenol e) 3-Methylphenol 98

CH 3 Name OH a) p-methylphenol b) m-methylphenol c) o-methylphenol d) 4-Methylphenol e) 3-Methylphenol Explanation: The groups are on adjacent carbons, which is ortho. 99

Name O C NO 2 H a)3-amino-5-benzaldehyde b)5-amino-3-benzaldehyde c) 3-Aminobenzaldehyde d)5-nitro-3-benzaldehyde e)3-nitrobenzaldehyde 100

Name O C NO 2 H a) 3-Amino-5-benzaldehyde b) 5-Amino-3-benzaldehyde c) 3-Aminobenzaldehyde d) 5-Nitro-3-benzaldehyde e) 3-Nitrobenzaldehyde Explanation: Benzaldehyde is the parent compound. 101

NO 2 HO Name NO 2 a)1,3-dinitrophenol b)1-hydroxy-2,4-dinitrobenzene c) 2,4-Dinitrobenzen-1-ol d)2,4-dinitrophenol e)4,6-dinitrophenol 102

NO 2 HO Name NO 2 a) 1,3-Dinitrophenol b) 1-Hydroxy-2,4-dinitrobenzene c) 2,4-Dinitrobenzen-1-ol d) 2,4-Dinitrophenol e) 4,6-Dinitrophenol Explanation: Phenol is the parent compound. 103

Name C 6 H 5 CH 2 CH 2 C CCH 3 a)1-phenylpent-3-yne b)5-phenylpent-2-yne c) 4-Phenylpent-2-yne d)1-phenylbut-2-yne e)1-phenylbut-3-yne 104

Name C 6 H 5 CH 2 CH 2 C CCH 3 a)1-phenylpent-3-yne b)5-phenylpent-2-yne c) 4-Phenylpent-2-yne d)1-phenylbut-2-yne e)1-phenylbut-3-yne 105