BENZENE - AROMATIC COMPOUNDS Aliphatic Compounds:

BENZENE - AROMATIC COMPOUNDSAliphatic Compounds: Open chain (acyclic) and those cycliccompounds with similar chemical properties. A typicalreaction type of unsaturated aliphatic compounds:electrophilic addition.Aromatic Compounds:Benzene, C6H6, and otherunsaturated compounds thatresemble it in chemicalbehavior. The aromaticproperties of benzene are thosethat distinguish it from aliphatichydrocarbons.orBenzeneWhat are these properties?Properties of Aromatic Compounds -- Cyclic and each atom in the ring is a π-center (uses a patomic orbital to form π-type bonds),ie, sp2 or sp. Ring is flat or nearly so.1

High degree of unsaturation but resistant to additionreactions --- generally undergo electrophilicsubstitution (an electrophilic reagent replaces ahydrogen [usually] attached to the ring). Unusually stable. π-Electrons delocalized above and below plane of ring.Benzene Molecular Orbitalsσ-Orbitals —Each carbon is sp2 hybridized. The 18 carbon sp2 atomicorbitals [(3/carbon)x(6 carbons)] and 6 hydrogen s atomicorbitals interact to form 12 σ and 12 σ* molecular orbitals.The 24 electrons from the atomic orbitals occupy the 12 σorbitals giving 12 σ bonds.HHHCCCCCCHH a σ-orbital;containing 2 electrons,a σ-bondH2

π-Orbitals —π-Type orbitals form by the parallel overlap of p atomicorbitals.P orbitals have twolobes. One lobe hasa positive amplitudeand the other anegative amplitude –metaphorically, a hilland a valley. If twoadjacent p orbitalsare in phase – hillnext to hill and valleynext to valley – theywill overlap to form amolecular orbital in aconstructive way: onehill will join with theother and one valleywill join with theother. Electrons willbe found whereverthe amplitude 0.This situation– bonding – is shownin the figure to theright.Amplitudes of Two p OrbitalsArranged to Interact In-Phase0.50.40.30.20.1C-0.1-0.2-0.3-0.4-0.53C

On the other hand, iftwo adjacent porbitals are out ofphase – hill next tovalley and valleynext to hill – theywill overlap in adestructive way: thehills and valleys willbe leveled and insome places theamplitude will bezero. [The placeswhere the amplitudeis 0 is called anode.] Electronsare not found atnodes. Thissituation– antibonding – isshown in the figureto the right.Amplitudes of Two p OrbitalsArranged to Interact 0.5Finally, it sometimeshappens that the p orbital of a particular atom contributesnothing to a molecular orbital. The amplitude of themolecular orbital at that atom is zero. This is a nonbondingsituation for that molecular orbital for that atom.4

Before we consider benzene, let’s look at a couple ofsimpler compounds (which happen to be aliphatic) that haveπ-bonds.EthyleneThe π and π* orbitals in ethylene are formed by a combiningof the 2p atomic orbitals of the carbons. The π orbital isbonding between the two carbons – electrons in this orbitalhave less energy than they would have in an atomic 2porbital. The π* orbital is antibonding – electrons in thisorbital have more energy than they would have in an atomic2p orbital.H2CH2CCH2 H2CIn-phasecombinationleads to ds to π*.H2CπCH2bonding5

1, 3-ButadieneAt first glance we might think that this would be just like twoethylenes hooked together: a π and π* orbital betweencarbons 1 and 2 and also between carbons 3 and 4.However, this overlooks the interaction that is possiblebetween the p orbital on carbon-2 and the one on carbon-3.In fact, the four atomic p orbitals can interact with fourdifferent phase arrangements giving four π-type molecularorbitals. This is shown in the figure below. Each of the patomic orbitals has the same energy and each contains oneelectron. Each can interact with a neighbor in a bondingway (the orbitals are in phase, CC , and can overlap), anantibonding way (the orbitals are out of phase, CC , andcancel each other out between the atoms), or a nonbondingway (the neighboring p orbital makes no contribution to themolecular orbital that will form: CC ). The result of theinteraction of the four p orbitals in 1,3-butadiene is theformation of four molecular orbitals, Ψ1 through Ψ4. This isshown schematically in the diagram below.6

H2CCH CH CH2H2CCH CH CH2H2CH2CΨ4Ψ3H2CCH CH CH2H2CCH CH CH2ECH CH CH2Ψ2H2CCH CH CH2CH CH CH2Ψ1H2CCH CH CH2The orbital of lowest energy is Ψ1; it is bonding betweeneach of the carbons. The next higher in energy is Ψ2; it isbonding between C-1 and C-2 and between C-3 and C-4,but antibonding between C-2 and C-3 because the orbitalchanges phase here. Both of these orbitals are bondingorbitals, overall: they are of lower energy than the isolatedatomic p orbitals. [Since electrons in an atomic p orbital arenonbonding, the energy level of the p orbitals is called thenonbonding energy level.] Ψ3 and Ψ4 are antibondingorbitals, overall: they are of higher energy than the isolatedatomic p orbitals.The four electrons from the atomic p orbitals go into Ψ1 andΨ2, the two bonding π orbitals, because these are of lowestenergy and each, like any orbital, can accommodate twoelectrons.7

Finally, it might be noted that the conjugated 1,3-butadienesystem is more stable than an isolated diene because thetotal energy of the four electrons in Ψ1 and Ψ2 turns out tobe less than the energy of four electrons in two isolated πbonds.BenzeneEach carbon "starts" with a p atomic orbital containing oneelectron. These orbitals are perpendicular to the ring, butparallel to each other. These atomic “basis” orbitals areshown in the figure below. Each p orbital interacts("overlaps") with two neighbors. This gives rise to six π-typeorbitals, Ψ1 through Ψ6. Ψ1 through Ψ3 are bonding orbitals;Ψ4 through Ψ6 are antibonding. It turns out that Ψ1 is thelowest energy orbital. Ψ2 and Ψ3 are degenerate, i.e. theyhave the same energy, and it is higher than the energy ofΨ1. The electrons in the three occupied bonding orbitalsgive rise to one doughnut of electron density above the ringand one below, containing a total of 6 π electrons. Thisaromatic electronic delocalization results in considerablestabilization – more than is observed in the case ofconjugated aliphatic compounds.8

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Because of the π-electrons, benzene, and other aromaticcompounds, frequently act as Lewis bases or nucleophiles;thus, they are suscepable to electrophilic attack. Becauseof the stability associated with the delocalized electrons, thisfeature tends to be retained in the products; consequently,these reactions are usually substitutions, not additions.EE How stable is benzene?Compared to what?Compared to isolated π-bonds, and conjugated, nonaromatic π-systems.10H

Heats of Hydrogenation of Cyclic Unsaturated MoleculesReactantProduct Hohn x Hoh "Stability"(C6H12)cyclohexene cyclohexane 28.6kcal28.6kcal0kcal1,3-cyclohexadienecyclohexane 55.457.21.8benzenecyclohexane 49.885.836Is aromatic stability shared by all monocyclic compoundswith conjugated π systems?etc.?No.Aromatic stability seems to be associated with***completely filled π bonding orbitalscompletely filled or empty π non-bonding orbitalscompletely vacant π antibonding orbitals.Do I need to do Huckel MO calculations to tell what πorbitals exist in one of these molecules and whether or notthey are filled?11

No.Use the polygon mnemonic to find the orbitals, and fill themwith the π electrons in order of increasing energy.THE POLYGON MNEMONICDraw the molecule as a regular polygon and rotate it around its center soone apex is pointing down. Each apex corresponds to a π molecular orbital.The center of the polygon corresponds to the non-bonding energy level(same energy as an isolated p atomic orbital); orbitals of lower energy arebonding; orbitals of higher energy are anti-bonding.12

ondinganti-bondingbondingAromaticNot aromaticnon-planar,ordinary alkeneNot aromatictoo much anglestrain to be planarNot aromatictoo much stericstrain to be planarHHAromatic13

Is this the simplest way to predict whether or not amonocyclic conjugated polyene is aromatic?No.Well.If a monocyclic conjugated polyene consisting only of πcenters is planar, it will be aromatic if it contains 4n 2 π electrons, wheren is 0, 1, 2 .it will be anti-aromatic if it contains 4n π electrons.14

Some examples --4n 2 π Electrons: Aromatic Moietiesn 0, 2 π electronsHn 1, 6 π electronsHHH HO-t-Bu AgClHHSO4-K BF4- H2OAgBF4HHHClHHH OH - K O-t-Budilute H SO24pKa 16N HNpyrrolepyridinen 2, 10 π electrons 2 Ko2K8 π electrons,tub shaped,not aromatic10 π electrons,planar, aromatic15

4n π Electrons: Non- or Anti-Aromatic Moietiesn 1, 4 π electrons, antiaromaticHHn 2, 8 π electrons, antiaromatic if planarHHnon-aromatic,tub shapedbaseH H-baseanti-aromaticpKa 50n 3, 12 π electrons, antiaromatic if planarnon-aromatic, non-planar16

Polycyclic (polynuclear) Aromatic HydrocarbonsExamples , carcinogenic,found in cigarette smokeThese compounds are aromatic, but do not have as muchstabilization per ring as benzene. Therefore, although theyundergo electrophilic substitutions, they are more likely thanbenzene to undergo some addition reactions, especially ifthis results in benzene rings in the products.17

HXHY XY XYYHH XSources of Aromatic CompoundsAromatic hydrocarbons may be obtained from --1)petroleum – not rich in aromatics, but has some.2)reforming cycloalkanes from petroleum, egCH3CH3heat, catalysthigh pressure18 3 H2

3)coal tar and coal gas.Coal derives from plants which have suffered partial decayand been subjected to heat and pressure.plants --- peat --- lignite --- bituminous (soft) coal --- --- anthracite (hard)Bituminous coal is a good source of aromatic compounds.Destructive distillation of it forms coal gas, coal tar, andcoke. Coal tar is rich in aromatic hydrocarbons, bases (eg,pyridine), and phenolic (Ar-OH) compounds. [Ar- isshorthand for an aromatic ring, just as R- is shorthand for analkyl group.]bituminous heat,coalno airnonvolatilevolatilecoke(blast furnaces,fuel)coal gasand coal tarRich in aromatic compounds:benzene, toluene, xylenes, etc.19

orbital have more energy than they would have in an atomic 2p orbital. 6 1, 3-Butadiene . volatile coal gas and coal tar Rich in aromatic compounds: benzene, toluene, xylenes, etc. 3) coal tar and coal gas. Coal derives from plants which have suffer