2All C—C bond distances = 140 pm Benzeneempirical formula = CH9
3All C—C bond distances = 140 pm 140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene.9
4Figure: UNCaption:Benzene is actually a resonance hybrid between the two Kekule structures. This representation implies that the pi electrons are delocalized, with a bond order of 1.5 between adjacent carbon atoms. The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths.
6Thermochemical Measures of Stability heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene"Pt+3H2H°= – 208 kJ17
7Cyclic conjugation versus noncyclic conjugation 3H2Ptheat of hydrogenation = -208 kJ/mol (-49 kcal/mol)3H2Ptheat of hydrogenation = -337 kJ/mol (-85.8 kcal/mol)20
8Figure: 16-02Caption:The molar heats of hydrogenation and the relative energies of cyclohexene, 1,4-cyclohexadiene, 1,3-cyclohexadiene, and benzene.
9Resonance & Benzeneexpress the structure of benzene as a resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double bonds, but are delocalized over all six ring carbons.HH6
10Resonance & BenzeneCircle-in-a-ring notation stands for resonance description of benzene (hybrid of two resonance structures)14
11QuestionWhich of the following compounds has a double bond that is conjugated with the p system of the benzene ring?A) p-BenzyltolueneB) 2-Phenyl-1-deceneC) 3-PhenylcyclohexeneD) 3-Phenyl-1,4-pentadieneE) 2,4,6-trichloroanisole
12Figure: 16-01Caption:Benzene is a flat ring of sp2 hybrid carbon atoms with their unhybridized p orbitals all aligned and overlapping. The carbon-carbon bond lengths are all Å, and all the bond angles are exactly 120o.
13Question Which of the following has the smallest heat of combustion? A) B)C) D)
15Orbital Hybridization Model of Bonding in Benzene High electron density above and below plane of ring23
16Figure: 16-05Caption:Energy diagram of the molecular orbitals of benzene. Benzene's six pi electrons fill the three bonding orbitals, leaving the antibonding orbitals vacant.
17Hückel's RuleFrost's circle is a mnemonic that allows us to draw a diagram showing the relative energies of the p orbitals of a cyclic conjugated system.1) Draw a circle.2) Inscribe a regular polygon inside the circle so that one of its corners is at the bottom.3) Every point where a corner of the polygon touches the circle corresponds to a p electron energy level.4) The middle of the circle separates bonding and antibonding orbitals.7
18Frost's Circlep MOs of BenzeneBondingAntibonding8
19Benzene MOs 6 p AOs combine to give 6 MOs Antibonding orbitalsEnergyBonding orbitals6 p AOs combine to give 6 MOs3 MOs are bonding; 3 are antibonding6
20Benzene MOs All bonding MOs are filled Antibonding orbitalsEnergyBonding orbitalsAll bonding MOs are filledNo electrons in antibonding orbitals6
22Figure: 16-04Caption:The six pi molecular orbitals of benzene viewed from above. The number of nodal planes increases with energy, and there are two degenerate MOs at each intermediate energy level.
23Figure: 16-09Caption:In a cyclic conjugated system, the lowest-lying MO is filled with two electrons. Each of the additional shells consists of two degenerate MOs, with space for four electrons. If a molecule has (4N + 2) pi electrons, it will have a filled shell. If it has 4N electrons, there will be two unpaired electrons in two degenerate orbitals.
24Hückel's Rule: Annulenes the additional factor that influences aromaticity is the number of p electrons6
25Figure: UNCaption:Hydrocarbons with alternating double and single bonds are called annulenes. For example, benzene is a six-membered annulene, so it can be named -annulene. Cylobutadiene is -annulene, cyclooctatetraene is -annulene.
26Aromatic vs. “anti-aromatic” A. Deniz, et. al Aromatic vs. “anti-aromatic” A. Deniz, et. al., Science, 5 November 1999
27Question How many p electrons does the compound shown have? A) 5 B) 8
28Hückel's RuleAmong planar, monocyclic, completely conjugated polyenes, only those with 4N p electrons have resonance stability (i.e. They are aromatic; and they are also planar.)N 4N+20 21 62 103 144 187
29Hückel's RuleAmong planar, monocyclic, completely conjugated polyenes, only those with 4N p electrons have resonance stability (i.e. They are aromatic; and they are also planar.)N 4N+20 21 6 benzene!2 103 144 187
30QuestionWhat is the value of N of Huckel's rule for the cyclopentadienyl cation (shown)?A) N=1/2B) N=1/4C) N=1D) N=2
31Question Which is a true statement based on Huckel’s Rule. IIWhich is a true statement based on Huckel’s Rule.(Assume that both are planar and that vacant p-orbitals do not interupt conjugation.)A) I is aromatic and II is not.B) II is aromatic and I is not.C) I and II are aromatic.D) I and II are not aromatic.
32Hückel's Rule & molecular orbitals Hückel”s rule applies to: cyclic, planar, conjugated, polyenesthe p molecular orbitals of these compounds have a distinctive patternone p orbital is lowest in energy, another is highest in energy, and the others are arranged in pairs between the highest and the lowest7
33p-MOs of Benzene 6 p orbitals give 6 p orbitals AntibondingNon-bondingBenzeneBonding6 p orbitals give 6 p orbitals3 orbitals are bonding; 3 are antibonding8
34p-MOs of Benzene 6 p electrons fill all of the bonding orbitals AntibondingNon-bondingBenzeneBonding6 p electrons fill all of the bonding orbitalsall p antibonding orbitals are empty8
35Figure: 16-19Caption:Ultraviolet spectra of benzene and styrene
36http://www.youtube.com/watch?v=HPGE_GJkLJA Figure: 16-17-03UN1-3 Caption:Disubstituted benzenes are named using the prefixes ortho-, meta-, and para- to specify the substitution patterns. These terms are abbreviated o-, m-, and p-.
37Question How many isomers of dibromophenol are aromatic? A) 3 B) 4 E) none
38p-MOs of Cyclobutadiene (square planar) AntibondingNon-bondingCyclo- butadieneBonding4 p orbitals give 4p orbitals1 orbital is bonding, one is antibonding, and 2 are nonbonding8
39p-MOs of Cyclobutadiene (square planar) AntibondingNon-bondingCyclo- butadieneBonding4 p electrons; bonding orbital is filled; other 2 p electrons singly occupy two nonbonding orbitals8
40p-MOs of Cyclooctatetraene (square planar) AntibondingNon-bondingCyclo- octatetraeneBonding8 p orbitals give 8 p orbitals3 orbitals are bonding, 3 are antibonding, and 2 are nonbonding8
41p-MOs of Cyclooctatetraene (square planar) AntibondingNon-bondingCyclo- octatetraeneBonding8 p electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filled8
42p-Electron Requirement for Aromaticity 4 p electrons6 p electrons8 p electronsnot aromaticnot aromaticaromatic2
43Anti-aromatic (planar) Non-aromatic (non-planar) Figure: 16-08Caption:The polygon rule predicts that the MO energy diagram for these annulenes will resemble the polygonal shapes of the annulenes.Non-aromatic(non-planar)Aromatic
44Completely Conjugated Polyenes 6 p electrons; not completely conjugated6 p electrons; completely conjugatedHnot aromaticaromatic2
45Question The planar compound shown below is classified as A) non-aromaticB) aromaticC) anti-aromaticD) none of the above
46Annulenepredicted to be aromatic by Hückel's rule, but too much angle strain when planar and all double bonds are cis (therefore non-planar)10-sided regular polygon has angles of 144°16
47Annuleneincorporating two trans double bonds into the ring relieves angle strain but introduces van der Waals strain into the structure and causes the ring to be distorted from planarity16
48van der Waals strain between these two hydrogens Annulenevan der Waals strain between these two hydrogensincorporating two trans double bonds into the ring relieves angle strain but also introduces van der Waals strain into the structure and causes the ring to be non-planar16
49Annulene 14 p electrons satisfies Hückel's rule van der Waals strain between hydrogens inside the ring & thererfore non-planar16
50Annulene 16 p electrons does not satisfy Hückel's rule alternating short (134 pm) and long (146 pm) bondsnot aromatic16
51Annulene 18 p electrons satisfies Hückel's rule resonance energy = kJ/mol16
52Figure: 16-15Caption:Structures of diamond and graphite. Diamond is a lattice of tetrahedral carbon atoms linked in a rigid three-dimensional array. Graphite consists of planar layers of fused aromatic rings.
53Figure: 16-16Caption:Structure of C60 and a carbon nanotube. Each carbon in C60 is a bridgehead carbon for a five-membered ring and two six-membered rings.
55Figure: UNCaption:We can draw a five-membered ring of sp2 hybrid carbon atoms with all the unhybridized p orbitals lined up to form a continuous ring. With five pi electrons, this system would be neutral, but it would be a radical because of an odd number of electrons cannot all be paired. With four pi electrons Huckel's rule predicts the system to be antiaromatic. With six pi electrons, Huckel's rule predicts aromaticity.
56Figure: UNCaption:Cyclopentadiene is unusually acidic because loss of a proton converts the nonaromatic diene to the aromatic cyclopentadienyl anion.
57Figure: UNCaption: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.
58Figure: UNCaption:With conjugated systems such as cyclopendadienyl cation and anion, the resonance approach is a poor predictor of stability. The Huckel rule, based on molecular orbital theory, is a much better predictor of stability for these aromatic and antiaromatic systems.
59Figure: UNCaption:The cycloheptatrienyl cation is easily formed by treating the corresponding alcohol with dilute (0.01N) aqueous sulfuric acid.
60Figure: 16-18Caption:The mass spectrum of n-butylbenzene has its base peak at m/z = 91, corresponding to cleavage of a benzylic bond. The fragments are a benzyl cation and a propyl radical. The cation rearranges to the tropylium ion, detected at m/z = 91.
61Figure: UNCaption:Dianions of hydrocarbons are rare and are usually much more difficult to form. Cyclooctatetraene reacts with potassium metal, however, to form an aromatic dianion.
67Figure: 16-10Caption:Pyridine has six delocalized electrons in its pi system. The two non-bonding electrons on nitrogen are in an sp2 orbital, and they do not interact with the pi electrons of the ring.
68Figure: 16-11Caption:Pyridine is basic, with non-bonding electrons available to abstract a proton. The protonated pyridine (a pyridinium ion) is still aromatic.
70Figure: 16-12Caption:The pyrrole nitrogen atom is sp2 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.
71Figure: 16-13Caption:The pyrrole nitrogen atom must become sp3 hybridized to abstract a proton. This eliminates the unhybridized p orbital needed for aromaticity.
72QuestionWhich of the following compounds is best classified as an aromatic heterocycle?A) B)C) Aniline D) PyridineE) All of them
80Figure: UNCaption:Naphthalene is the simplest fused aromatic hydrocarbon, consisting of two fused benzene rings.
81Figure: UNCaption:As the number of aromatic ring increases, the resonance energy per ring continues to decrease and the compounds become more reactive.
82Benzene:Benzene is classified as a Group A, human carcinogen by the EPA.Increased incidence of leukemia has been observed in humans occupationally exposed to benzene.Chronic inhalation has caused various blood disorders, including reduced red blood cell count and aplastic anemia.Reproductive effects have been reported for women exposed to high levels by inhalation.Adverse effects on the developing fetus have been observed in animal tests.Benzene is an established human leukemogen, but the mechanism by which it produces leukemia remains unclear.Epidemiologic studies and case studies provide clear evidence of a causal association between exposure to benzene and acute nonlymphocytic leukemia (ANLL) and also suggest evidence for chronic nonlymphocytic leukemia (CNLL) and chronic lymphocytic leukemia (CLL). Other neoplastic conditions that are associated with an increased risk in humans are hematologic neoplasms, blood disorders such as preleukemia and aplastic anemia, Hodgkin's lymphoma, and myelodysplastic syndrome (MDS).Benzene is used as a constituent in motor fuels; as a solvent for fats, waxes, resins, oils, inks, paints, plastics, and rubber; in the extraction of oils from seeds and nuts; and in photogravure printing. It is also used as a chemical intermediate. Benzene is also used in the manufacture of detergents, explosives, pharmaceuticals, and dyestuffs."Classic" carcinogens, such as benzo[a]pyrene, aromatic amines, and aflatoxin, are thought to be activated to a single, ultimate carcinogenic metabolite. Typically these metabolites are highly electrophilic and bind strongly to DNA in a covalent fashion. Covalent binding of this type is readily measured as bound radioactivity. Classic carcinogens and their metabolites are also highly mutagenic in the Ames Salmonella test producing point mutations and small deletions.Benzene presents the exact opposite scenario. It does not form a single highly electrophilic metabolite. Its epoxide, benzene oxide, is highly unstable and rapidly rearranges to the major metabolite phenol.
83Benzene: Toxic Metabolites Benzene is an established human leukemogen, but the mechanism by which it produces leukemia remains unclear."Classic" carcinogens, such as benzo[a]pyrene, aromatic amines, and aflatoxin, are thought to be activated to a single, ultimate carcinogenic metabolite. Typically these metabolites are highly electrophilic and bind strongly to DNA in a covalent fashion. Covalent binding of this type is readily measured as bound radioactivity. Classic carcinogens and their metabolites are also highly mutagenic in the Ames Salmonella test producing point mutations and small deletions.Benzene presents the exact opposite scenario. It does not form a single highly electrophilic metabolite. Its epoxide, benzene oxide, is highly unstable and rapidly rearranges to the major metabolite phenol.
84Carcinogenic Mechanism Cooked meat products, regular consumption of which has been epidemiologically associated with increased levels of colon cancer (although this in itself does not prove carcinogenicity), have been shown to contain up to 4 ng/g of benzo[a]pyrene, and up to 5.5ng/g in fried chicken and 62.6ng/g in overcooked charcoal barbecued beef.Caption:Benzo[a]pyrene, one of the most thoroughly studied carcinogens, is formed whenever organic compounds undergo incomplete combustion.Carcinogenic Mechanism