aromaticity and benzene

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Organic Chemistry OnLine ©2000 © 2000, Paul R. Young University of Illinois at Chicago, All Rights Reserved Aromaticity

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reaction and mechanism for aromaticity and benzene.

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  • Organic Chemistry OnLine 2000

    2000, Paul R. YoungUniversity of Illinois at Chicago, All Rights Reserved

    Aromaticity

  • Organic Chemistry OnLine 2000

    Aromaticity: Derivatives of Benzene

    H

    H

    H

    H

    H

    H

  • Organic Chemistry OnLine 2000

    very slow reaction

    no reaction

    no reaction HCl

    Br2/CCl4

    H2/Pt

    Benzene has four degrees of unsaturation, but undergoesnone of the common reactions of alkenes...

    H2/Pt

    C6H6

  • Organic Chemistry OnLine 2000

    Heat of Hydrogenation(energy released during reduction with H2/Pt)

    Reactant Product H

    Cyclohexene Cyclohexane 28.6 kcal/mole1,3-Cyclohexadiene Cyclohexane 55.4 kcal/moleBenzene Cyclohexane 49.8 kcal/mole

    H2/Pt

    86 kcal/mole expected; 50 kcal/mole observed36 kcal/mole more stable than expected

  • Isomers of C6H6

  • Organic Chemistry OnLine 2000

    Ladenburg benzene

    H

    H

    H

    H

    H

    H

    Kekul benzene

    C6H6 isomers as possiblestructures for benzene...

  • Organic Chemistry OnLine 2000

    4 isomers expected for Kekul benzene

    Br

    Br

    Br

    Br

    Br

    Br

    Br

    Br

    3 dibromo isomersBr2/FeBr3

    Benzene

  • Organic Chemistry OnLine 2000

    Br

    Br

    Br

    Br

    long bond short bond

  • Organic Chemistry OnLine 2000

    Br

    Br

    Br

    Br

    BrBr

    ...three dibromo isomers are predicted.

    Ladenburg benzene

  • Organic Chemistry OnLine 2000

    3 isomers expected

    Br

    Br

    Br

    Br

    Br

    Br

    Br

    Br

    3 isomersBr2/FeBr3

    identical

    Benzene

  • Organic Chemistry OnLine 2000

    Br

    Br

    Br

    Br

    Kekul suggested that the two possible 1,2-isomers were in rapid equilibrium...

  • Organic Chemistry OnLine 2000

    Br

    Br

    Br

    Br

    Br

    Br

    Br

    Bror

    Br

    Br

    ...today we recognize the two structures Kekuldrew as simple resonance forms.

  • Organic Chemistry OnLine 2000

    The conjugated system in the benzene ring results in theformation of a continuous -cloud, above and below theplane of the ring.

    An electrostatic potential mapof benzene showing enhancedelectron density above and

    below the ring plane.

  • y 1

    y 2 y 3

    y 4* y 5*

    y 6*The six p-orbitals of benzenecombine to form six -molecularorbitals of increasing energy.

    The six bonding electronsoccupy the three lowestenergy molecular orbitals.

  • Organic Chemistry OnLine 2000

    y 6 *

    y 4 * y 5 *

    y 2 y 3

    y 1

    Calculated shapes forthe -molecularorbitals of benzene...

  • Organic Chemistry OnLine 2000

    H

    H

    H

    H

    H

    H

    CH3H

    H

    H

    H

    H

    CH3H

    CH3H

    H

    H

    NH2H

    H

    H

    H

    H

    OH

    H

    H

    H

    H

    H

    NO2H

    H

    H

    H

    H

    benzene toluene meta-xylene

    aniline phenol nitrobenzene

    Representative Aromatic Compounds...

  • Organic Chemistry OnLine 2000

    napthalene anthracene

    phenanthrene

    Some Polycyclic Aromatic Compounds...

  • Organic Chemistry OnLine 2000

    napthalene anthracene

    phenanthrene O

    DNA damage

    Some Polycyclic Aromatic Compounds...

  • Organic Chemistry OnLine 2000

    HO O CH2 C

    H

    NH2

    COOH

    I

    I

    I

    I

    Thyroxin

    DDT

    CHC

    Cl

    Cl

    Cl

    Cl

    Cl

    Some Other NotableAromatic Compounds...

  • Organic Chemistry OnLine 2000

    Br CH2CH3 CH3

    bromobenzene ethylbenzene methylbenzeneor toluene

    Nomenclature

    1. Monosubstituted benzene derivatives are names as otherhydrocarbons, using benzene as the parent name.

  • Organic Chemistry OnLine 2000

    2. Disubstituted benzenes are named using ortho-, para- andmeta- to describe the substitution pattern (1,2 1,4 and 1,3respectively) or simply by numbering the substituents.

    Nomenclature

    1. Monosubstituted benzene derivatives are names as otherhydrocarbons, using benzene as the parent name.

    Br

    Br

    CH2CH3

    CH2CH3

    CH3

    H3C

    ortho-dibromobenzeneor 1,2-dibromobenzene

    meta-diethylbenzeneor 1,3-diethylbenzene

    para-dimethylbenzene,1,4-dimethylbenzene

    or para-xylene

  • Organic Chemistry OnLine 2000

    3. Substituents are numbered to give the lowest possiblenumber sequence at the first point of difference, assigningpriorities alphabetically if there is a tie.

    CH2CH3

    CH2CH3

    Cl

    1-chloro-3,5-diethylbenzene

    1 3

    5

    2. Disubstituted benzenes are named using ortho-, para- andmeta- to describe the substitution pattern (1,2 1,4 and 1,3respectively) or simply by numbering the substituents.

    Nomenclature

    1. Monosubstituted benzene derivatives are names as otherhydrocarbons, using benzene as the parent name.

  • Organic Chemistry OnLine 2000

    4. When a benzene ring is a substituent on another chain, itis referred to as a phenyl group.

    1-phenylheptane

    1

    3. Substituents are numbered to give the lowest possiblenumber sequence at the first point of difference, assigningpriorities alphabetically if there is a tie.

    2. Disubstituted benzenes are named using ortho-, para- andmeta- to describe the substitution pattern (1,2 1,4 and 1,3respectively) or simply by numbering the substituents.

    Nomenclature

    1. Monosubstituted benzene derivatives are names as otherhydrocarbons, using benzene as the parent name.

  • Organic Chemistry OnLine 2000

    CH3H

    H

    NO2

    H

    H

    CH3CH3

    H

    H

    Br

    H

    CH3H

    Cl

    H

    H

    H

    ...provide IUPAC names for each of the molecules shown above.

  • Organic Chemistry OnLine 2000

    CH3H

    H

    NO2

    H

    H

    CH3CH3

    H

    H

    Br

    H

    CH3H

    Cl

    H

    H

    H

    4-bromo-1,2-dimethylbenzene

  • Organic Chemistry OnLine 2000

    CH3H

    H

    NO2

    H

    H

    CH3CH3

    H

    H

    Br

    H

    CH3H

    Cl

    H

    H

    H

    4-bromo-1,2-dimethylbenzene1-chloro-3-methylbenzene or 3-chlorotoluene

  • Organic Chemistry OnLine 2000

    CH3H

    H

    NO2

    H

    H

    CH3CH3

    H

    H

    Br

    H

    CH3H

    Cl

    H

    H

    H

    4-bromo-1,2-dimethylbenzene

    1-methyl-4-nitrobenzene or 4-nitrotoluene

    1-chloro-3-methylbenzene or 3-chlorotoluene

  • Organic Chemistry OnLine 2000

    CH2CH3

    Cl

    H

    NO2

    H

    H

    Cl

    CH2CH3

    H

    NO2

    H

    H

    Cl

    H

    CH2CH3NO2

    H

    H

    ...provide IUPAC names for each of the molecules shown above.

  • Organic Chemistry OnLine 2000

    CH2CH3

    Cl

    H

    NO2

    H

    H

    Cl

    CH2CH3

    H

    NO2

    H

    H

    Cl

    H

    CH2CH3NO2

    H

    H

    1

    2

    4

    2-chloro-1-ethyl-4-nitrobenzene

  • Organic Chemistry OnLine 2000

    CH2CH3

    Cl

    H

    NO2

    H

    H

    Cl

    CH2CH3

    H

    NO2

    H

    H

    Cl

    H

    CH2CH3NO2

    H

    H

    1-chloro-2-ethyl-4-nitrobenzene

    1

    2

    4

    1

    2

    4

    2-chloro-1-ethyl-4-nitrobenzene

  • Organic Chemistry OnLine 2000

    CH2CH3

    Cl

    H

    NO2

    H

    H

    Cl

    CH2CH3

    H

    NO2

    H

    H

    Cl

    H

    CH2CH3NO2

    H

    H

    2-chloro-1-ethyl-4-nitrobenzene1-chloro-2-ethyl-4-nitrobenzene

    4-chloro-2-ethyl-1-nitrobenzene

    1

    2

    4

    1

    2

    4

    1

    2

    4

  • Organic Chemistry OnLine 2000

    CH3

    Cl

    H

    NO2

    H

    H

    Cl

    CH3

    H

    NO2

    H

    H

    Cl

    H

    CH3NO2

    H

    H

    ...name as derivatives of toluene.

  • Organic Chemistry OnLine 2000

    CH3

    Cl

    H

    NO2

    H

    H

    Cl

    CH3

    H

    NO2

    H

    H

    Cl

    H

    CH3NO2

    H

    H

    2-chloro-4-nitrotoluene

  • Organic Chemistry OnLine 2000

    CH3

    Cl

    H

    NO2

    H

    H

    Cl

    CH3

    H

    NO2

    H

    H

    Cl

    H

    CH3NO2

    H

    H

    2-chloro-4-nitrotoluene 2-chloro-5-nitrotoluene

  • Organic Chemistry OnLine 2000

    CH3

    Cl

    H

    NO2

    H

    H

    Cl

    CH3

    H

    NO2

    H

    H

    Cl

    H

    CH3NO2

    H

    H

    2-chloro-4-nitrotoluene 2-chloro-5-nitrotoluene

    5-chloro-2-nitrotoluene

  • Organic Chemistry OnLine 2000

    The resonance description of benzene will explain thegeometry of the molecule and the isomer distribution

    of benzene derivatives, but does not explain theunusual stability of benzene and its derivatives.

    The stability of benzene is suggested to arise from thefact that the conjugated system is planar andcontains 4n + 2 electrons (with n = 1), and it issuggested that all compounds having planar,

    conjugated systems containing 4n + 2 electrons willshare this stability. This property, described

    originally by Hckel, is referred to as aromaticity.

  • Organic Chemistry OnLine 2000

    4n + 2 Rule

  • 2,3-Dimethylcyclobutadiene

  • 2,3-Dimethylcyclobutadiene

    Planar...

  • 2,3-Dimethylcyclobutadiene

    Planar...4 p electrons,

  • 2,3-Dimethylcyclobutadiene

    Planar...4 p electrons,therefore not aromatic (antiaromatic)

  • 2,3-Dimethylcyclobutadiene

    Planar...4 p electrons,therefore not aromatic (antiaromatic)

  • Organic Chemistry OnLine 2000

    4 p electronsantiaromatic

    4n + 2 Rule

  • Benzene

  • Benzene

    Planar...

  • Benzene

    Planar...6 p electrons,

  • Benzene

    Planar...6 p electrons,therefore aromatic

  • Benzene

    Planar...6 p electrons,therefore aromatic

  • Organic Chemistry OnLine 2000

    4 p electronsnot aromatic

    6 p electronsaromatic

    4n + 2 Rule

  • Cyclooctatetraene

  • Cyclooctatetraene

    Not planar...

  • Cyclooctatetraene

    Not planar...8 p electrons,

  • Cyclooctatetraene

    Not planar...8 p electrons,therefore not aromatic

  • Cyclooctatetraene

  • Cyclooctatetraene

  • Organic Chemistry OnLine 2000

    4 p electronsnot aromatic

    6 p electronsaromatic

    8 p electronsnot aromatic

    4n + 2 Rule

  • Cyclopentadienyl Anion

  • Cyclopentadienyl Anion

    Planar...

  • Cyclopentadienyl Anion

    Planar...6 p electrons,

  • Cyclopentadienyl Anion

    Planar...6 p electrons,therefore aromatic

  • Cyclopentadienyl Anion

    Planar...6 p electrons,therefore aromatic

  • Organic Chemistry OnLine 2000

    4 p electronsnot aromatic

    6 p electronsaromatic

    8 p electronsnot aromatic

    6 p electronsaromatic

    4n + 2 Rule

  • Cycloheptatrienyl Cation

    +

  • Cycloheptatrienyl Cation

    +

    Planar...

  • Cycloheptatrienyl Cation

    +

    Planar...6 p electrons,

  • Cycloheptatrienyl Cation

    +

    Planar...6 p electrons,therefore aromatic

  • Cycloheptatrienyl Cation

    +

    Planar...6 p electrons,therefore aromatic

  • Organic Chemistry OnLine 2000

    4 p electronsnot aromatic

    6 p electronsaromatic

    4n + 2 Rule

  • Organic Chemistry OnLine 2000

    4n + 2 Rule

  • Organic Chemistry OnLine 2000

    N

    N N

    N

    NH2

    HNO

    O

    CH3

    H

    NH

    Adenine Uracil

    4n + 2 Rule

  • Organic Chemistry OnLine 2000

    N

    N N

    N

    NH2

    H

    4n + 2 Rule

  • Organic Chemistry OnLine 2000

    N

    N N

    N

    NH2

    H

    4n + 2 Rule

    Adenine:10 electrons

    Aromatic

  • Organic Chemistry OnLine 2000

    N

    N N

    N

    NH2

    H

    Adenine:10 electrons

    Aromatic

    4n + 2 Rule

    The lone pairs on thenitrogens are orthogonal to

    the -system and do notparticipate in themolecular orbital.

  • Organic Chemistry OnLine 2000

    N

    N N

    N

    NH2

    H

    Adenine:10 electrons

    Aromatic

    4n + 2 Rule

    NO

    O

    CH3

    H

    NH

  • Organic Chemistry OnLine 2000

    NO

    O

    CH3

    H

    NH

    N

    N N

    N

    NH2

    H

    Adenine:10 electrons

    Aromatic

    4n + 2 Rule

  • Organic Chemistry OnLine 2000

    NO

    O

    CH3

    H

    NH

    N

    N N

    N

    NH2

    H

    Adenine:10 electrons

    Aromatic

    4n + 2 Rule

    Uracil:6 electrons

    Aromatic

  • Organic Chemistry OnLine 2000

    CH2

    CH3CH CH2

    CH3

    CH CH2H3C

    H3C

    CH2

    N

    N N

    N

    CH2CH2COOHCOOH

    Fe

    Heme

    4n + 2 Rule

  • Organic Chemistry OnLine 2000

    CH2

    CH3CH CH2

    CH3

    CH CH2H3C

    H3C

    CH2

    N

    N N

    N

    CH2CH2COOHCOOH

    Fe

    4n + 2 Rule

  • Organic Chemistry OnLine 2000

    CH2

    CH3CH CH2

    CH3

    CH CH2H3C

    H3C

    CH2

    N

    N N

    N

    CH2CH2COOHCOOH

    Fe

    4n + 2 Rule

    Heme: 22 electrons; Aromatic (n = 5)

  • Heme

  • Heme

    Planar...

  • Heme

    Planar...22 p electrons,

  • Heme

    Planar...22 p electrons,therefore aromatic

  • Organic Chemistry OnLine 2000

    Buckminsterfullerene

    C60 with 31 double bonds

  • Buckminsterfullerene

  • Buckminsterfullerene

    62 p electrons, (4n + 2 with n = 15)

  • Buckminsterfullerene

    62 p electrons, (4n + 2 with n = 15)not planar with considerable ring strain...

  • Buckminsterfullerene

    62 p electrons, (4n + 2 with n = 15)not planar with considerable ring strain...the molecule appears aromatic, but is much morereactive than benzene.

  • Organic Chemistry OnLine 2000

    Would you predict cyclodecapentaene to be anaromatic compound?

    Cyclodecapentaene

  • Organic Chemistry OnLine 2000

    Would you predict cyclodecapentaene to be anaromatic compound?

    Cyclodecapentaene

    ...consider thegeometry of the

    two centralhydrogens.

  • Organic Chemistry OnLine 2000

    Cyclodecapentaene

    HH

    Would you predict cyclodecapentaene to be anaromatic compound?

  • Steric repulsion between the opposing hydrogens forces the system out of planarity, therefore the molecule is not aromatic.

  • Organic Chemistry OnLine 2000

    Azulene, C10H8, is a deep blue hydrocarbon with a large dipolemagnetic moment.

    a. is azulene aromatic using the Hckel definition?b. utilize resonance forms for azulene to explain the largedipole moment.

  • Organic Chemistry OnLine 2000

    10 p -electronsAromatic

    Azulene, C10H8, is a deep blue hydrocarbon with a large dipolemagnetic moment.

    a. is azulene aromatic using the Hckel definition?b. utilize resonance forms for azulene to explain the largedipole moment.

  • Organic Chemistry OnLine 2000

    The two major (neutral) resonance forms for azulene...

  • Organic Chemistry OnLine 2000

    The two major (neutral) resonance forms for azulene...

    ...and the very stable Zwiterionic form.

  • Organic Chemistry OnLine 2000

    The two major (neutral) resonance forms for azulene...

    ...and the very stable Zwiterionic form.

    cycloheptatrienecation-like

  • Organic Chemistry OnLine 2000

    The two major (neutral) resonance forms for azulene...

    ...and the very stable Zwiterionic form.

    cycloheptatrienecation-like

    cyclopentadieneanion-like

  • Organic Chemistry OnLine 2000

    dd

    d

    d

    d

    d

    dd

    The contribution from the Zwitterionic resonance formleads to the presence of a large dipole moment, which

    is associated with color in conjugated molecules.

  • Organic Chemistry OnLine 2000

    dd

    d

    d

    d

    d

    dd

    Contrast the electron density map of azulene with that ofnapthalene, which has no significant Zwitterionic component.

  • Organic Chemistry OnLine 2000

    stable very unstable

    2

    Account for the differences in reactivity between pentalene (anunknown compound) and the well-known pentalene dianion:

  • Organic Chemistry OnLine 2000

    8 -electrons;not aromatic

    10 -electrons;aromatic

    2

  • Organic Chemistry OnLine 2000

    Reactions of Aromatic Sidechains

    1. Oxidation with neutral MnO4-

    2. Allylic bromination with NBS

  • Organic Chemistry OnLine 2000

    Oxidation withNeutral MnO4-

    CH3

    CH2CH2CH3

    MnO4-/H2O, heat

    MnO4-/H2O, heat

    MnO4-/H2O, heat

  • Organic Chemistry OnLine 2000

    CH3

    CH2CH2CH3

    MnO4-/H2O, heat

    MnO4-/H2O, heat

    MnO4-/H2O, heat

    COOH

    ...benzylic carbonmust have at least

    one hydrogen.

  • Organic Chemistry OnLine 2000

    MnO4-/H2O, heat

    CH3

    CH3

    MnO4-/H2O, heat

  • Organic Chemistry OnLine 2000

    MnO4-/H2O, heat

    COOH

    COOH

    CH3

    CH3

    MnO4-/H2O, heat

    COOH

    COOH

  • Organic Chemistry OnLine 2000

    MnO4-/H2O, heat

    COOH

    COOH

    CH3

    CH3

    MnO4-/H2O, heat

    COOH

    COOH

  • Organic Chemistry OnLine 2000

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    CH2CH2CH3

    CH3 Free RadicalBromination of Alkyl

    Side-Chains

  • Organic Chemistry OnLine 2000

    O

    OO

    O

    Benzoyl Peroxide (PhCO 2 )2 N Br

    O

    O N-bromo-succinimide

    Br

    CH2

    H

    CH2HBr +

    NBSCH2

    Br

    O

    O2

    Br

  • Organic Chemistry OnLine 2000

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    CH2CH2CH3

    CH3 Free RadicalBromination of Alkyl

    Side-Chains

  • Organic Chemistry OnLine 2000

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    CH2CH2CH3

    CH3

    Br

    CHCH2CH3Br

    CH2 Br

  • Organic Chemistry OnLine 2000

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    CH2CH2CH3

    CH3

    Br

    CHCH2CH3Br

    CH2 Br

  • Organic Chemistry OnLine 2000

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    NBS/CCl4"radical initiator"

    CH2CH2CH3

    CH3

    Br

    CHCH2CH3Br

    CH2 Br

  • Organic Chemistry OnLine 2000

    Dissolving Metal Reduction of Benzene DerivativesThe Birch Reduction

    Li/NH3

  • Organic Chemistry OnLine 2000

    Li/NH3

    CH3Li/NH3

    CH3

    OCH3Li/NH3

    OCH3

    With substituted arenes,the Birch Reduction will

    proceed to place thedouble bond on a ring

    carbon which is attachedto electron donating

    substituents...

    Dissolving Metal Reduction of Benzene DerivativesThe Birch Reduction

  • Organic Chemistry OnLine 2000

    Li/NH3

    CH3Li/NH3

    CH3

    OCH3Li/NH3

    OCH3

    Dissolving Metal Reduction of Benzene DerivativesThe Birch Reduction

  • Organic Chemistry OnLine 2000

    Li/NH3

    CH3Li/NH3

    CH3

    OCH3Li/NH3

    OCH3

    Dissolving Metal Reduction of Benzene DerivativesThe Birch Reduction

  • Organic Chemistry OnLine 2000

    MnO4-/H2O

    Predict the products of the following reactions.

    Li/NH3

    NBS/CCl4"radical initiator"

  • Organic Chemistry OnLine 2000

    Br

    Br

    MnO4-/H2O

    Predict the products of the following reactions.

    Li/NH3

    NBS/CCl4"radical initiator"

  • Organic Chemistry OnLine 2000

    COOH

    HOOC

    Br

    Br

    MnO4-/H2O

    Predict the products of the following reactions.

    Li/NH3

    NBS/CCl4"radical initiator"

  • Organic Chemistry OnLine 2000

    COOH

    HOOC

    Br

    Br

    MnO4-/H2O

    Predict the products of the following reactions.

    Li/NH3

    NBS/CCl4"radical initiator"