Arenes and Aromaticity(Benzene)

All CC bond distances = 140 pm140 pm140 pm140 pm140 pm140 pm140 pmBenzeneempirical formula = CH

All CC bond distances = 140 pm140 pm is the average between the CC single bond distance and the double bond distance in 1,3-butadiene.140 pm140 pm140 pm140 pm140 pm140 pm146 pm134 pm

Resonance & Benzene

Thermochemical Measures of Stabilityheat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene"H= 208 kJ+3H2Pt

Cyclic conjugation versus noncyclic conjugationheat of hydrogenation = -208 kJ/mol (-49 kcal/mol) heat of hydrogenation = -337 kJ/mol (-85.8 kcal/mol)3H2Pt3H2Pt

Resonance & 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.

Resonance & BenzeneCircle-in-a-ring notation stands for resonance description of benzene (hybrid of two resonance structures)

QuestionWhich 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-pentadiene E) 2,4,6-trichloroanisole

QuestionWhich of the following has the smallest heat of combustion?

A) B)

C) D)

The Molecular Orbitalsof Benzene

Orbital Hybridization Model of Bonding in BenzeneHigh electron density above and below plane of ring

Hckel'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.

Frost's Circle

Benzene MOs6 p AOs combine to give 6 MOs3 MOs are bonding; 3 are antibondingBonding orbitalsAntibonding orbitals

Benzene MOsAll bonding MOs are filledNo electrons in antibonding orbitalsBonding orbitalsAntibonding orbitals

Benzene MOs

Hckel's Rule:Annulenesthe additional factor that influences aromaticity is the number of p electrons

Aromatic vs. anti-aromaticA. Deniz, et. al., Science, 5 November 1999

QuestionHow many p electrons does the compound shown have? A)5B)8C)10D)12

Hckel's RuleAmong planar, monocyclic, completely conjugated polyenes, only those with 4N + 2 p electrons have resonance stability (i.e. They are aromatic; and they are also planar.)N 4N+20 21 6210314418

Hckel's RuleAmong planar, monocyclic, completely conjugated polyenes, only those with 4N + 2 p electrons have resonance stability (i.e. They are aromatic; and they are also planar.)N 4N+20 21 6benzene!210314418

QuestionWhat 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

QuestionWhich is a true statement based on Huckels 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.III

Hckel's Rule& molecular orbitalsHckels 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 lowest

p-MOs of Benzene6 p orbitals give 6 p orbitals3 orbitals are bonding; 3 are antibondingBenzeneAntibondingBondingNon-bonding

p-MOs of Benzene6 p electrons fill all of the bonding orbitalsall p antibonding orbitals are emptyBenzeneAntibondingBondingNon-bonding

QuestionHow many isomers of dibromophenol are aromatic?A)3B)4C)6D)8 E) none

p-MOs of Cyclobutadiene(square planar)4 p orbitals give 4p orbitals1 orbital is bonding, one is antibonding, and 2 are nonbondingCyclo- butadieneAntibondingBondingNon-bonding

p-MOs of Cyclobutadiene(square planar)4 p electrons; bonding orbital is filled; other 2 p electrons singly occupy two nonbonding orbitalsCyclo- butadieneAntibondingBondingNon-bonding

p-MOs of Cyclooctatetraene(square planar)8 p orbitals give 8 p orbitals3 orbitals are bonding, 3 are antibonding, and 2 are nonbondingAntibondingBondingCyclo- octatetraeneNon-bonding

p-MOs of Cyclooctatetraene(square planar)8 p electrons; 3 bonding orbitals are filled; 2 nonbonding orbitals are each half-filledAntibondingBondingCyclo- octatetraeneNon-bonding

p-Electron Requirement for Aromaticitynot aromaticaromaticnot aromatic4 p electrons6 p electrons8 p electrons

Completely Conjugated Polyenesaromatic6 p electrons; completely conjugatednot aromatic6 p electrons; not completely conjugated

QuestionThe compound shown is classified as

A)non-aromaticB)aromaticC)anti-aromaticD)none of the above

[10]Annulenepredicted to be aromatic by Hckel'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

[10]Annuleneincorporating 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 planarity

[10]Annuleneincorporating 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-planarvan der Waals strain between these two hydrogens

[14]Annulene14 p electrons satisfies Hckel's rulevan der Waals strain between hydrogens inside the ring & thererfore non-planarHHHH

[16]Annulene16 p electrons does not satisfy Hckel's rulealternating short (134 pm) and long (146 pm) bondsnot aromatic

[18]Annulene18 p electrons satisfies Hckel's ruleresonance energy = - 418 kJ/mol

Aromatic Ions

Cycloheptatrienyl Cation6 electrons delocalized over 7 carbonspositive charge dispersed over 7 carbonsvery stable carbocationalso called tropylium cation

Cycloheptatrienyl Cation

Cycloheptatrienyl CationTropylium cation is so stable that tropylium bromide is ionic rather than covalent.mp 203 C; soluble in water; insoluble in diethyl etherBrIonicCovalent

Cyclopentadienide Anion6 electrons delocalized over 5 carbonsnegative charge dispersed over 5 carbonsstabilized anion

Cyclopentadienide Anion

Acidity of CyclopentadieneCyclopentadiene is unusually acidic for a hydrocarbon.Increased acidity is due to stability of cyclopentadienide anion.pKa = 16Ka = 10-16

Electron Delocalization in Cyclopentadienide Anion

Compare Acidities ofCyclopentadiene and CycloheptatrienepKa = 16Ka = 10-16pKa = 36Ka = 10-36

Compare Acidities ofCyclopentadiene and CycloheptatrieneAromatic anion6 electronsAntiaromatic anion 8 electrons

Cyclopropenyl CationN = 04N +2 = 2 electronsalso written as

Cyclooctatetraene Dianionn = 24n +2 = 10 electronsHHHHHHHHHHHHHHHH2also written as

Heterocyclic Aromatic Compounds

Aromatic Heterocyclic CompoundsA heterocycleis a cyclic compound in whichone or more of the ring atoms is an atom other than carbon.

Furan Is Aromatic

Pyridine Is Aromatic

Pyrrole Is Aromatic

QuestionWhich of the following compounds is best classified as an aromatic heterocycle?

A) B)

C) AnilineD) Pyridine

E) All of them

Examples of Important NitrogenHeterocyclic Aromatic Compounds

399Figure: 16-00-03UN

Caption: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.

51720Figure: 16-02

Caption:The molar heats of hydrogenation and the relative energies of cyclohexene, 1,4-cyclohexadiene, 1,3-cyclohexadiene, and benzene.

614Figure: 16-01

Caption: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 1.397 , and all the bond angles are exactly 120o.

2623Figure: 16-05

Caption:Energy diagram of the molecular orbitals of benzene. Benzene's six pi electrons fill the three bonding orbitals, leaving the antibonding orbitals vacant.

78666Figure: 16-04

Caption: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.

Figure: 16-09

Caption: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.

6Figure: 16-03-01UN

Caption:Hydrocarbons with alternating double and single bonds are called annulenes. For example, benzene is a six-membered annulene, so it can be named [6]-annulene. Cylobutadiene is [4]-annulene, cyclooctatetraene is [8]-annulene.

7778888882Figure: 16-08

Caption:The polygon rule predicts that the MO energy diagram for these annulenes will resemble the polygonal shapes of the annulenes.

2161616161616Figure: 16-15

Caption: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.

Figure: 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.

Figure: 16-19

Caption:Ultraviolet spectra of benzene and styrene

Figure: 16-18

Caption: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.

21Figure: 16-09-02UN

Caption: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.

Figure: 16-09-03UN

Caption:Cyclopentadiene is unusually acidic because loss of a proton converts the nonaromatic diene to the aromatic cyclopentadienyl anion.

Figure: 16-09-06UN

Caption: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.

Figure: 16-09-07UN

Caption: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.

Figure: 16-09-10UN

Caption:The cycloheptatrienyl cation is easily formed by treating the corresponding alcohol with dilute (0.01N) aqueous sulfuric acid.

Figure: 16-09-13UN

Caption:Dianions of hydrocarbons are rare and are usually much more difficult to form. Cyclooctatetraene reacts with potassium metal, however, to form an aromatic dianion.

222223222222262727282930Figure: 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-.

Figure: 16-14

Caption:Pyrrole, furan, and thiophene are isoelectronic. In furan and thiophene, the pyrrole N-H bond is replaced by a nonbonding pair of electrons in the sp2 hybrid orbital.

Figure: 16-10

Caption: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.

Figure: 16-11

Caption:Pyridine is basic, with non-bonding electrons available to abstract a proton. The protonated pyridine (a pyridinium ion) is still aromatic.

Figure: 16-12

Caption: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.

Figure: 16-13

Caption:The pyrrole nitrogen atom must become sp3 hybridized to abstract a proton. This eliminates the unhybridized p orbital needed for aromaticity.

Figure: 16-14-08UN

Caption:Naphthalene is the simplest fused aromatic hydrocarbon, consisting of two fused benzene rings.

Figure: 16-14-09UN

Caption:As the number of aromatic ring increases, the resonance energy per ring continues to decrease and the compounds become more reactive.

Figure: 16-14-13UN

Caption:Benzo[a]pyrene, one of the most thoroughly studied carcinogens, is formed whenever organic compounds undergo incomplete combustion.