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Aromatic Cpds

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    KOT 222 ORGANIC CHEMISTRY II

    CHAPTER 16

    AROMATIC COMPOUNDS

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    Benzene

    An aromatic compound.

    Isolated in 1825 by Michael Faraday whodetermined C:H ratio to be 1:1.

    Synthesized in 1834 by Eilhard Mitscherlich

    who determined molecular formula to beC6H6.

    Other related compounds with low C:H ratios

    had a pleasant smell, so they were classifiedas aromatic.

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    Kekul Structure of Benzene

    Proposed in 1866 by Friedrich Kekul, shortlyafter multiple bonds were suggested.

    Showing localized double-bonds.

    Failed to explain existence of only one isomer of1,2-dichlorobenzene.

    Cl

    Cl

    Cl

    ClX

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    Resonance Structure of Benzene

    Benzene is actually a resonance hybrid between

    the two Kekul structures.The pi electrons are delocalized, with a bond

    order of 1.5 between adjacent carbon atoms

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    Orbital Representation of Benzene

    Benzene is a flat ring of sp2 hybrid carbon atomswith their unhybridized porbitals all aligned andoverlapping.

    The conjugation and delocalization of theelectrons give greater stability.

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    Unusual Reactions of Benzene

    Benzene as a cyclic conjugated triene is predicted toreact as polyenes.

    But, its reactions are unusual.

    Alkene + KMnO4 diol (addition)Benzene + KMnO4 no reaction.

    Alkene + Br2/CCl4 dibromide (addition)

    Benzene + Br2/CCl4

    no reaction.With FeCl3 catalyst, Br2 reacts with

    benzene to form bromobenzene + HBr(substitution!). Double bonds remain.

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    Unusual Stabilities of Benzene

    The hydrogenation of the first double bond of

    benzene is endothermic.

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    Annulenes

    Cyclic hydrocarbons with alternating single anddouble bonds (uncharged, even no. of C atoms).

    Assumptions:

    All are aromatic.

    Have similar stabilities asbenzene.

    But, not all are aromatic.

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    Six overlapping patomicorbitals form six MOs.

    Three will be bonding,

    three antibonding.The intermediate energy

    levels are degenerate,

    two MOs at each level.Lowest energy MO will

    have all bonding

    interactions, no nodes.As energy of MO

    increases, the numberof nodes increases.

    The MOs of Benzene

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    Energy Diagram for Benzene

    The six electrons fill three bonding pi orbitals.

    All bonding orbitals are filled (closed shell), an

    extremely stable arrangement.

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    The MOs of Cyclobutadiene

    There are four MOs: the lowest-energy bonding orbital,the highest-energy antibonding orbital, and twodegenerate nonbonding orbitals.

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    Energy Diagram for Cyclobutadiene

    Four pi electrons fill the MOs.

    Following Hunds rule, two electrons are inseparate degenerate orbitals.

    This diradical with the highest-lying electrons innonbonding MOs would be very reactive.

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    Polygon Rule

    The energy diagram for an annulene has thesame polygonal shape as the cyclic compoundwith one vertex (all-bonding MO) at the bottom.

    The nonbonding line cuts horizontally throughthe center of the polygon.

    The pi electrons fill the MOs follows the aufbau

    principle and Hunds rule.

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    Aromatic Requirements

    1. Structure must be cyclic withconjugatedpi bonds.

    2. Each atom in the ring must havean unhybridized porbital.

    3. The unhybridized porbitals mustoverlap to form a continuousring. (Usually planar structure.)

    4. Delocalization of pi electronsover the ring must lower theelectronic energy

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    Anti- and Nonaromatic

    Antiaromatic compoundsare cyclic, conjugated, withoverlapping p orbitals around

    the ring, but the energy of thecompound is greater than itsopen-chain counterpart.

    antiaromatic

    Nonaromatic compounds

    do not have a continuous ring

    of overlapping porbitals andmay be nonplanar.

    CH3

    CH3

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    Hckels Rule

    If the compound has a planar and continuousring of overlapping porbitals and has 4N+ 2electrons, it is aromatic.

    If the compound has a planar and continuousring of overlapping porbitals and has 4Nelectrons, it is antiaromatic.

    Otherwise, the system/compound isnonaromatic.

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    Benzene Six pi electrons.

    (4N+2) system

    Aromatic

    Cyclobutadiene Four pi electrons.

    (4N) system

    Antiromatic

    Cyclooctatetraene(4N) system

    AntiromaticX Nonaromatic

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    Larger Annulenes

    Larger 4Nannulenes are not antiaromaticbecause they are flexible enough to becomenonplanar.

    [12] annulene [16] annulene

    Larger 4N+2annulenes depend on whether themolecule can adopt the necessary planarconformation.

    All-cis

    nonaromatic

    H H

    Two trans

    nonaromatic

    naphthalene

    aromatic

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    MO Derivation of Hckels Rule

    In a cyclic conjugated system, the lowest-energyMO is filled with two electrons.

    Each of the additional shells has two degenerate

    MOs, with space for four electrons.

    A diradical, unstable.

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    Aromatic Ions

    Hckels rule also applies to systems having oddnumbers of C atoms and bearing positive ornegative charges.

    Cyclopentadienyl Ions:The cation has an empty porbital, 4 electrons,

    so antiaromatic.

    The anion has a nonbonding pair of electrons ina porbital, 6 e-s, aromatic.

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    Acidity of Cyclopentadiene

    Unusually acidic (pKa of 16).

    Loss of a proton converts the nonaromatic dieneto the aromatic cyclopentadienyl anion.

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    Cyclopentadienyl cation

    Huckels rule predicts that the cyclopentadienylcation, with four pi electrons, is antiaromatic.

    The cyclopentadienyl cation is not easily formed.

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    Cycloheptatrienyl Ions

    The cycloheptatrienyl cation (tropylium ion) iseasily formed.

    H OH

    H+ , H2O

    H

    +

    The cycloheptatrienyl anion is difficult to form.

    HH

    B

    H

    6 pi electron,(4N+2) system,aromatic

    8 pi electron,

    4Nsystem,antiaromatic

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    Cyclooctatetraene Dianion

    Cyclooctatetraene is nonaromatic.

    Its dianion is easily prepared with planar, regularoctagonal structure aromatic.

    Continuous overlapping

    p orbitals.

    (4N+2) system.

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    Heterocyclic Aromatic Compounds

    A heterocyclic compound is a cyclic compound in whichone or more of the ring atoms is an atom other thancarbon

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    Pyridine

    Aromatic nitrogen analogue of benzene. It 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 thepi electrons of the ring.

    These electrons are in thesp2 orbital perpendicular to

    the porbital.

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    Protonation of pyridine:

    Pyridine is basic, with non-bonding electronsavailable to abstract a proton.

    aromatic aromatic

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    Pyrrole

    An aromatic five-membered heterocycle.Nonbonding electrons on N atom involve in the

    pi bonding system, so much weaker base.

    Nounhybridizedporbitalneeded foraromaticity

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    Basic or Nonbasic?

    N atom with it nonbonding electrons involve inthe pi bonding system is nonbasic.

    NN Pyrimidine has two basicnitrogens.

    N N H Imidazole has one basicnitrogen and one nonbasic.

    N

    N

    N

    N

    H

    Purine?

    3 basic,1 nonbasic

    Most nonbasic nitrogens

    have three single bonds.

    Most basic nitrogenshave a double bond in

    the ring.

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    Other Heterocyclics

    All are aromatic

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    Polynuclear Aromatic Hydrocarbons

    Compounds composed of two or more fusedbenzene rings.

    Naphthalene

    Anthracene Phenanthrene

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    Reactivity of

    Polynuclear HydrocarbonsAs the number of aromatic rings increases, the

    resonance energy per ring decreases.

    These large PAHs can undergo addition reactions.

    BrH

    H Br

    1

    2

    3

    45

    6

    7

    8 9

    10

    Br

    H

    BrH

    1

    2

    3

    4

    5

    6

    7

    8

    9

    10

    (mixture of cis and trans isomers)

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    Larger Polynuclear

    Aromatic HydrocarbonsFormed in combustion (tobacco smoke).

    Many are carcinogenic.Epoxides form, combine with DNA base.

    pyrene

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    Allotropes of Carbon

    Amorphous: small particles of graphite; charcoal,soot, coal, carbon black.

    Diamond: a lattice of tetrahedral Cs.

    Graphite: layers of fused aromatic rings.

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    Some New Allotropes of Carbon

    Fullerenes: 5- and 6-membered rings arrangedto form a soccer ball structure.

    Nanotubes: half of a C60 sphere fused to acylinder of fused aromatic rings.

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    Fused Heterocyclic Compounds

    Common in nature, synthesized for drugs.

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    Common Names of

    Benzene Derivatives

    OH OCH3NH2CH3

    phenol toluene aniline anisole

    C

    H

    CH2 C

    O

    CH3C

    O

    HC

    O

    OH

    styrene acetophenone benzaldehyde benzoic acid

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    Disubstituted Benzenes

    The prefixes ortho-, meta-, and para- are commonlyused for the 1,2-, 1,3-, and 1,4- positions, respectively.

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    3 or More Substituents

    Use the smallest possible numbers, but

    the carbon with a functional group that define thebase name is #1.

    NO2

    NO2

    O2N

    1,3,5-trinitrobenzene

    NO2

    NO2

    O2N

    OH

    2,4,6-trinitrophenol

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    Common Names forSubstituted Benzenes

    CH3

    CH3

    CH3

    CH3H3C

    CH3

    CO OH

    OH

    H3Cm-xylene mesitylene o-toluic acid p-cresol

    Ph l d B l

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    Phenyl and Benzyl

    Benzene ring as a substituent on anothermolecule is called as phenyl group.

    CH2 C C CH3

    CH2 C C CH3Ph

    1-phenyl-2-butyne

    Benzyl group: seven-carbon unit consisting of abenzene ring and a methylene group.

    Br

    phenyl bromide

    Ph sical Properties of Ben ene

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    Physical Properties of Benzene

    and Its DerivativesMelting points: More symmetrical than

    corresponding alkane, pack better intocrystals, so higher melting points.

    Boiling points: Dependent on dipolemoment, so ortho> meta> para, fordisubstituted benzenes.

    Density: More dense than nonaromatics,

    less dense than water.Solubility: Generally insoluble in water.

    IR d NMR S t

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    IR and NMR Spectroscopy

    C=C stretch absorption at 1600 cm-1.

    sp2 C-H stretch just above 3000 cm-1.

    1

    H NMR at 7-8 for Hs on aromatic ring.13C NMR at 120-150, similar to alkene

    carbons.

    Mass Spectrometry

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    Mass Spectrometry

    The most common fragmentation ofalkylbenzene derivative is the cleavage of abenzylic bond.

    UV Spectroscopy

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    UV Spectroscopy

    Benzene has three absorptions.

    max at 184 nm

    Moderateband

    Characteristicband

    Additional conjugateddouble bond cause

    bathochromic shift.

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