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Chapter 17: Benzene and Aromaticity TNT 8-methyl-N-vanillyl-6-nonenamide (Capsaicin) Buckminsterfullerene.

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Presentation on theme: "Chapter 17: Benzene and Aromaticity TNT 8-methyl-N-vanillyl-6-nonenamide (Capsaicin) Buckminsterfullerene."— Presentation transcript:

1 Chapter 17: Benzene and Aromaticity TNT 8-methyl-N-vanillyl-6-nonenamide (Capsaicin) Buckminsterfullerene

2 Key points & objectives: Aromatic molecules are cyclic, conjugated, flat, and unusually stable 4n + 2 electrons (n = 0, 1, 2, …. Hydrocarbon aromatics: benzene, naphthalene, anthracene, toluene, xylylene Heterocyclics: pyridine, pyrimidine, imidazole, pyrrole, thiophene, furan, indole…. Molecular orbitals using Frost diagrams (inscribed circles) Ring current deshields NMR signals – downfield

3 Benzene 1 st isolated by Michael Faraday in 1825 From “Benzoin,” corrupt form of the Arabic "luban jawi” for the “frankincense of Java”

4 Frankincense Boswellia sacra triterpene Cancer drug anti-inflammatory hepatotoxicity Not “aromatic” in the technical sense

5 Aromatic-fragrant myrth Commiphora myrrha tree Antiseptic, embalming agent, incense

6 Cinnamon Diabetes Antimicrobial antioxidant (2E)-3-phenylprop-2-enal cinnamaldehyde

7 8-methyl-N-vanillyl-6-nonenamide (Capsaicin) Capsaicin 16,000,000 Scovilles psoriasis relieve the pain of peripheral neuropathypainneuropathy trigger apoptosis in human colon and lung cancerapoptosislung cancer

8 Vanilla Tincture (ethanol extract) of vanilla aphrodisiacaphrodisiac and a remedy for feversfevers catecholaminescatecholamines (including adrenaline)adrenaline addictive

9 Aromatic molecules Flat Conjugated (4n +2) pi electrons Unusually stable Ring current (deshielding protons)

10 Anesthetics & analgesics Advil, and Motrin

11 Sunscreens Only complete UVA block

12 12 Very high energy radiation (UVC) is currently blocked by the ozone layer (ozone hole issue) High energy radiation (UVB) does the most immediate damage (sunburns) But lower energy radiation (UVA) can penetrate deeper into the skin, leading to long term damage Source: N.A. Shaath. The Chemistry of Sunscreens. In: Lowe NJ, Shaath NA, Pathak MA, editors. Sunscreens, development, evaluation, and regulatory aspects. New York: Marcel Dekker; p Skin Damage

13 Sources and Names of Aromatic Hydrocarbons From high temperature distillation of coal tar Heating petroleum at high temperature and pressure over a catalyst

14 Aromatics are less reactive than Alkenes

15 Aromatics Nomeclature

16

17 Agent orange

18 Polychlorinatedbiphenyls PCB’s Thermally stable, electrically insulating heat transfer liquid Casting wax for lost wax process for making metal things

19 Mueller 1948 Nobel Prize in Medicine dichlorodiphenyltrichloroethane Malaria mosquito

20 Thermodynamic stability of benzene: Heats of Hydrogenation

21 Monosubstituted Benzenes Most monosubstituted aromatics are named using -benzene as the parent name preceded by the substituent name (as a prefix; all one word): fluorobenzene fluoro nitroethyl nitrobenzeneethylbenzene

22 Alkyl-substituted Benzenes Alkyl substituted benzenes are named according to the length of the carbon chain of the alkyl group. With six carbons or fewer in the alkyl chain, they are named as ‘alkylbenzene.’ e.g., propylbenzene:

23 Alkyl-substituted Benzenes With more than six carbons in the alkyl chain, they are named as a ‘phenylalkane,’ where the benzene ring is named as a substituent (phenyl) on the alkane chain e.g., 4-phenylnonane 4-phenylnonane

24 The Benzyl Group The benzyl group is a common name for a methyl substituted benzene (toluene) having substitution for one of the hydrogens on the methyl group. the benzyl group benzyl bromide benzyl alcohol

25 Common Names of Subs. Benzenes There are a number of nonsystematic (common) names commonly used for certain monosubstituted benzenes (see next slide) These ten common names should be memorized. These common names are used as base names when naming more their more highly substituted derivatives. Examples of these will be given later.

26 Mono-substituted Benzene Nomenclature: Common Names

27 Disubstituted Benzenes Disubstituted benzenes can be named in one of two ways. Each method describes the relative positions of the two groups on the benzene ring. Systematic numbering of the aromatic ring. Using the prefixes ortho-, meta-, or para-. When numbering the ring carbons, carbon # 1 is always a substituted carbon. The substituents are listed alphabetically.

28 Disubstituted Benzenes ortho- (abbreviated o- )= 1,2-disubstituted (two groups on adjacent carbons on the ring)

29 Disubstituted Benzenes meta- (abbreviated m- )= 1,3-disubstituted (two groups having one unsubstituted carbon between them)

30 Disubstituted Benzenes para-(abbreviated p- )= 1,4-disubstituted (two groups on opposite sides of the ring)

31 Disubstituted Benzenes When one of the substituents changes the base name, either o-, m-, and p- or numbers may be used to indicate the position of the other substituent. Carbon # 1 is always the carbon bearing the substituent that changes the base name. p-bromoaniline or 4-bromoaniline o-chlorophenol or 2-chlorophenol

32 Common Names of Disubs. Benzenes There are a few nonsystematic (common) names for disubstituted benzenes that you should be familiar with:

33 Disubstituted Benzenes Relative positions on a benzene ring ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4) Describes reaction patterns (“occurs at the para position”)

34 Polysubstituted Benzenes Polysubstituted benzenes must be named by numbering the position of each substituent on the ring (with more than two substituents, o-, m-, and p-can NOT be used.) The numbering is carried out to give the substituents the lowest possible numbers. Carbon #1 always has a substituent. List the substituents alphabetically with their appropriate #s. 2-ethyl- 1- fluoro - 4-nitrobenzene

35 Polysubstituted Aromatics having a Common base name Common names of the monosubstituted benzenes are used as parent names for polysubstituted aromatics when one of the substituents changes the base name. For such rings with common names, the carbon bearing the substituent responsible for the common name is always carbon #1. The substitutents are listed in alphabetical order. 5-bromo-2-chlorotoluene toluene chloro bromo

36 Polysubstituted Benzenes bromo-2-ethyl-1-nitrobenzene5-bromo-2-chlorophenol

37 bromo-6-chloro-4-nitrotoluene1-bromo-3-chloro-2-ethyl-5-nitrobenzene

38 38 A benzene substituent is called a phenyl group, and it can be abbreviated in a structure as “Ph-”. Therefore, benzene can be represented as PhH, and phenol would be PhOH. Naming Benzene as a Substituent

39 Polycyclic Aromatic Hydrocarbons (PAH) Metabolic byproducts of benzo [a] pyrene react with DNA to form adducts, leading to carcinogenesis (cancer).

40 40 Naphthalene Orbitals Three resonance forms and delocalized electrons

41 41 Figure C NMR Absorptions of Dibromobenzenes The number of signals (lines) in the 13 C NMR spectrum of a disubstituted benzene with two identical groups indicates whether they are ortho, meta, or para to each other.

42 42 Figure 17.5 Drugs that Contain a Benzene Ring

43 Heterocyclic Aromatics

44

45 45 Pyridine A six-membered heterocycle with a nitrogen atom in its ring  electron structure resembles benzene (6 electrons) The nitrogen lone pair electrons are not part of the aromatic system (perpendicular orbital) Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity

46 Protonation of Pyrroles and Pyridines

47 47 Pyrrole A five-membered heterocycle with one nitrogen  electron system similar to that of cyclopentadienyl anion Four sp 2 -hybridized carbons with 4 p orbitals perpendicular to the ring and 4 p electrons Nitrogen atom is sp 2 -hybridized, and lone pair of electrons occupies a p orbital (6  electrons) Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base

48

49 Structure and Stability of Benzene: Molecular Orbital Theory Benzene reacts slowly with Br 2 to give bromobenzene (where Br replaces H) This is substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier

50 50 Heats of Hydrogenation as Indicators of Stability The addition of H 2 to C=C normally gives off about 118 kJ/mol – 3 double bonds would give off 356kJ/mol Two conjugated double bonds in cyclohexadiene add 2 H 2 to give off 230 kJ/mol Benzene has 3 unsaturation sites but gives off only 206 kJ/mol on reacting with 3 H 2 molecules Therefore it has about 150 kJ more “stability” than an isolated set of three double bonds

51 32 kcal/mole

52 52 Benzene’s Unusual Structure 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 C–C–C bond angles 120° Each C is sp 2 and has a p orbital perpendicular to the plane of the six-membered ring

53 53 Four structural criteria must be satisfied for a compound to be aromatic: 1. A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. The Criteria for Aromaticity

54 54 2. A molecule must be planar. All adjacent p orbitals must be aligned so that the  electron density can be delocalized. Since cyclooctatetraene is nonplanar and not aromatic, it undergoes addition reactions just like those of other alkenes. The Criteria for Aromaticity

55 55 3. A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. The Criteria for Aromaticity

56 56 4. A molecule must satisfy Hückel’s rule, and contain a particular number of  electrons. Benzene is aromatic and especially stable because it contains 6  electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4  electrons. Hückel's rule: The Criteria for Aromaticity

57 Hückel’s rule refers to the number of  electrons, not the number of atoms in a particular ring.

58 58 Why 4n +2? When electrons fill the various molecular orbitals, it takes two electrons (one pair) to fill the lowest-lying orbital and four electrons (two pairs) to fill each of n succeeding energy levels This is a total of 4n + 2

59 59 The combination of two p orbitals can be constructive—that is, with like phases interacting—or destructive, that is, with opposite phases interacting. When two p orbitals of similar phase overlap side-by-side, a  bonding molecular orbital results. When two p orbitals of opposite phase overlap side-by-side, a  * antibonding orbital results. Bonding and Antibonding Orbitals

60 60 Two atomic p orbitals combine to form two molecular orbitals. The bonding  MO is lower in energy than the two p orbitals. The  * antibonding MO is higher in energy because a destabilizing node results, which pushes nuclei apart when orbitals of opposite phase combine. Figure 17.8 Formation of π and π* M olecular Orbitals

61 61 Since each of the six carbon atoms in benzene has a p orbital, six atomic p orbitals combine to form six  MOs. Figure 17.9  Molecular Orbitals for Benzene

62 62 Inscribed Polygon Method of Predicting Aromaticity

63 63 This method works for all monocyclic completely conjugated systems regardless of ring size. The total number of MOs always equals the number of vertices of the polygon. The inscribed polygon method is consistent with Hückel's 4n + 2 rule—there is always one lowest energy bonding MO that can hold two  electrons and the other bonding MOs come in degenerate pairs that can hold a total of four  electrons. Inscribed Polygon Method of Predicting Aromaticity

64 64 Figure Inscribed Polygon Method of Predicting Aromaticity

65 65 Buckminsterfullerene (C 60 ) is a third elemental form of carbon. Buckminsterfullerene is completely conjugated, but it is not aromatic since it is not planar (CAREFULL!!!) It undergoes addition reactions with electrophiles in much the same way as ordinary alkenes. Buckminsterfullerene—Is it Aromatic?

66 66 Compounds With 4n  Electrons Are Not Aromatic (May be Antiaromatic) Planar, cyclic molecules with 4 n  electrons are much less stable than expected (antiaromatic) They will distort out of plane and behave like ordinary alkenes 4- and 8-electron compounds are not delocalized (single and double bonds) Cyclobutadiene is so unstable that it dimerizes by a self-Diels-Alder reaction at low temperature Cyclooctatetraene has four double bonds, reacting with Br 2, KMnO 4, and HCl as if it were four alkenes

67 67 Aromatic Ions The 4n + 2 rule applies to ions as well as neutral species Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic The key feature of both is that they contain 6  electrons in a ring of continuous p orbitals

68 68 Aromaticity of the Cyclopentadienyl Anion 1,3-Cyclopentadiene contains conjugated double bonds joined by a CH 2 that blocks delocalization Removal of H + at the CH 2 produces a cyclic 6-electron system, which is stable Removal of H - or H generates nonaromatic 4 and 5 electron systems Relatively acidic (pKa = 16) because the anion is stable

69 69 Cycloheptatriene Cycloheptatriene has 3 conjugated double bonds joined by a CH 2 Removal of “H - ” leaves the cation The cation has 6 electrons and is aromatic

70 70 1 H NMR spectroscopy readily indicates whether a compound is aromatic. The protons on sp 2 hybridized carbons in aromatic hydrocarbons are highly deshielded and absorb at 6.5–8 ppm, whereas hydrocarbons that are not aromatic absorb at 4.5–6 ppm. NMR and Aromaticity

71 71 Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4n + 2  electrons. Hydrocarbons containing a single ring with alternating double and single bonds are called annulenes. To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene. Larger Aromatic Rings

72 72 [10]-Annulene has 10  electrons, which satisfies Hückel's rule, but a planar molecule would place the two H atoms inside the ring too close to each other. Thus, the ring puckers to relieve this strain. Since [10]-annulene is not planar, the 10  electrons cannot delocalize over the entire ring and it is not aromatic. Hückel’s Rule and Number of  Electrons

73 Biochemically Relevant Aromatics Amino Acids

74 Biologically Relevant Aromatics NADH NAD+ Nicotinamide adeine dinucleotide, the biolgical hydrogenator


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