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O CH H Benzene Benzaldehyde Toluene - chem. fractional distillation of coal tar yields benzene,...

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  • Benzene Benzaldehyde

    O

    H

    Toluene

    CH3

    aromatic: usually fragrant ; benzaldehyde: from cherries, peaches, almonds; benzene: coal distillate; toluene: Tolu balsam

    Aromatic; benzene-like, six-membered ring with three double bonds

    Ch.15 Benzene and Aromaticity

  • O

    HO

    CH3

    H

    H H

    Estrone

    aromatic compounds; benzene-like structure

    N

    N

    Cl

    OH3C

    Diazepam(tranquilizer)

    O NCH3

    HO

    H HHO

    H

    Morphine

    benzene: toxic, cause bone-marrow depression prolonged exposure leukopenia (lowered white blood cell count)

  • - come from coal and petroleum- coal: complex mixture, arrays of benzene-like rings- thermal breakdown of coal at 1000oC in the absence of air boils off volatile products coal tar- fractional distillation of coal tar yields benzene, toluene, xylene, naphthalene, etc.

    15.1 Sources of Aromatic Hydrocarbons

    - petroleum: contains few aromatic compounds, consists largely of alkanesbut, during petroleum refining, aromatic molecules are formed (500oC, high pressure); eg) heptane is converted to toluene by dehydrogenation and cyclization

  • Benzene Toluene

    CH3

    Xylenes

    CH3H3C

    Indene

    Naphthalene Biphenyl Anthracene Phenanthracene

    Some aromatic hydrocarbons found in coal tar

  • CH3

    OH

    NH2

    C

    CHO

    COOH

    CN

    CH3CH3

    O

    CH3

    Toluene

    Phenol

    Aniline

    Acetophenone

    Benzaldehyde

    Benzoic acid

    Benzonitrile

    ortho-Xylene

    Cumene Styrene

    15.2 Nomenclature- a lot of common names are used

  • Br NO2

    Bromobenzene Nitrobenzene

    Et

    Ethylbenzene

    Monosubstituted benzenes:- benzene as parent name- as a substituent: C6H5- (phenyl), C6H5CH2- (benzyl),

    A phenyl group 2-Phenylheptane

    2

    1

    A benzyl group

  • Cl Me

    ortho-Dichlorobenzene meta-Xylene

    Cl

    Me CHO

    Cl

    para-Chlorobenzaldehyde

    Disubstituted benzenes:- ortho (o), meta (m), para (p)

    X

    ortho

    meta

    para

    meta

    ortho

  • Br CH3

    CH31

    24

    4-Bromo-1,2-dimethylbenzene

    Benzenes with more than two substitents- numbering: lowest numbers possible, alphabetically

    O2N Cl

    NO21

    24

    2-Chloro-1,4-dinitrobenzene

  • Common names as parent names: with the principal substituent assumed to be on carbon 1

    O2N NO212

    4

    2,4,6-Trinitrotoluene

    CH3

    NO2

    Br Br12

    2,6-Dibromophenol

    OH

    Cl COOH

    m-Chlorobenzoic acid

  • H

    HH

    HH

    H Iron

    Br2

    Br

    HH

    HH

    H

    H

    HH

    BrH

    Br

    all six hydrogens are equivalent

    Not formed

    H

    H

    + HBr

    15.3 Structure and Stability of Benzene

    - benzene is relatively unreactive toward most reagents that react with alkenes

    - benzene reacts with Br2 in the presence of iron catalyst: substitution product but not addition product

  • OsO4

    H3O+

    HBr

    OH

    OH

    OH

    Br

    Benzene does not undergo electrophilic addition reactions.

    OsO4

    H3O+

    HBr

    No reaction

    No reaction

    No reaction

    Why is benzene unreactive compared with other alkenes?Why does benzene give a substitution product rather than an addition product on reaction with Br2?

  • -118 kJ/mol

    -230 kJ/mol-356 kJ/mol(expected)

    150 kJ/mol(difference)

    -206 kJ/mol(actual)

    Heat of hydrogenations

    - benzene has 150 kJ/mol extra stability

  • H

    HH

    HH

    H

    H

    HH

    H H

    H

    1.5 bonds average

    The Resonance Proposal

    - resonance stabilization in benzene: more stable, less reactive than normal isolated alkenes

    bond length: C-C (154 pm), C=C (134 pm)but benzene: 139 pm (all C-C bonds are equivalent)

  • 15.4 Molecular Orbital Description of Benzene

    structural properties of benzene: - flat, symmetrical, regular hexagon- bond angle: 120o, sp2- six orbitals are equivalent

    C

    C

    C C

    C

    H

    H H

    C H

    HH

  • 2 3

    1

    4* 5

    *

    6*

    Antibonding

    Bonding

    Nonbonding

    six p- atomic orbitals

    six benzene molecular orbitals

    Benzene molecular orbitals

    Degenerated: orbitals with same energy level (2 and 3, 4 and 5)

  • 15.5 Aromaticity and the Hckel 4n + 2 Rule

    review of benzene: - a cyclic conjugated molecule- unusually stable- planar, regular hexagon- substitution reaction (not addition reaction)- resonance hybrid

    Hckel 4n + 2 Rule: needed to complete a description of aromaticity

    For a planar, monocyclic system of conjugation;- aromatic: 4n + 2 electrons, n = 0, 1, 2, 3... (2, 6, 10, 14...), stabilized- antiaromatic: 4n electrons, (4, 8, 12, 16...), unstabilized

  • Cyclobutadiene: 4 electrons, antiaromatic, highly reactive

    -78oC

    Diels-Alder

    Benzene: 6 electrons, aromatic

    Cyclooctatetraene: 8 electrons, antiaromatic

    H 147 pm

    134 pm

    5.7 ppm- like alkene, reactive to Br2, KMnO4, HCl- not planar, tube shaped

  • 15.6 Aromatic Ions

    Hckel 4n + 2 Rule: applicable to ionic molecules

    cyclopentadienyl anion cycloheptatrienyl cation

    6 electrons: aromatic cation, anion

  • HHpKa = 16

    Cyclopentadiene: acidic C-H bond

    cyclopentadienylanion

    cyclopentadienylradical

    cyclopentadienylcation

    6 electrons 5 electrons 4 electrons

    aromatic antiaromatic(highly reactive)

  • cycloheptatrienylanion

    cycloheptatrienylradical

    cycloheptatrienylcation

    8 electrons 7 electrons 6 electronsaromaticantiaromatic

    (highly reactive)

    Cycloheptatrienyl cation: aromatic, 6 electrons

  • H H H

    + HBrBr2

    Br-

    Cycloheptatrienyl cation: all 7 carbons are equally charged

  • Dianion aromatic

    2 K2-

    2K+ 2-

    10 electronsaromatic

  • 15.7 Aromatic Heterocycles: Pyridine and Pyrrole

    6 electrons: heteroaromatic rings

    C

    C

    C C

    C

    H

    H H

    N

    HHN1

    2

    34 lone pair in

    sp2 orbital

    sp2 hybridPyridine

  • 6 electrons: heteroaromatic rings

    NH

    N H

    6 electrons

    lone pair in p orbital

    sp2 hybridPyrrole

  • other heteroaromatic rings

    S

    6 electrons

    lone pair in p orbital

    sp2 hybrid

    SThiophene

    OFuran

    O

    6 electrons

    lone pair in p orbital

    sp2 hybrid

  • 15.8 Why 4n + 2?

    - energy levels of 6 electron system

    2 3

    1

    4* 5

    *

    6*

    six p- atomic orbitals

    Ener

    gy

    degenerate energy levels need 4 electrons

    need 2 electrons

  • - energy levels of cyclopentadienyl molecular orbitals

    2 3

    1

    4* 5

    *

    five p- atomic orbitals

    Ener

    gy

    2 3

    1

    4* 5

    *

    2 3

    1

    4* 5

    *

    cation radical anion

  • - Increase by 4 electrons to fill the bonding orbitals for extended cyclic aromatic systems

    bonding orbitals

    antibonding orbitals

    10 electrons

    14 electrons

  • 15.9 Naphthalene: A Polycyclic Aromatic Compound

    Naphthalene Benz[a]pyreneAnthracene Coronenecancer suspect agent

    many different resonance forms

  • C60 Fullerene and Carbon Nanotube

  • Br2

    Feheat

    Br

    + HBr

    1

    electrophilic aromatic substitution

    C

    C

    C C

    C

    H

    H H

    C

    H

    C C

    CH

    C H

    HH

    10 electrons: aromatic, fully delocalized throughout both rings

    Azulene

  • 15.10 Spectroscopy of Aromatic CompoundsIR Spectroscopy

    810-840 cm-1p-Disubstituted735-770 cm-1o-Disubstituted

    690-710 cm-1

    810-850 cm-1m-Disubstituted

    690-710 cm-1

    730-770 cm-1Monosubstituted

    - strong690-900 cm-1

    - complex motion of ring - 1500, 1600 bands are intense

    1450-1600 cm-1

    - weak1660-2000 cm-1C-H out of plane stretching

    - low intensity- occurs left of typical saturated C-H band

    3030 cm-1

    C-H stretching

  • UV Spectroscopy

    255-275 nmless intense205 nmintense

    NMR Spectroscopy

    110-140 ppm13C NMR6.5-8.0 ppm1H NMR

    ring current: low field shifts (vinylic protons, 4.5-6.5 ppm)

  • Aromatic ring current: aromatic protons are deshielded by induced magnetic field caused by delocalized electrons circulating in the molecular orbitals of the aromatic ring

  • systems: C=C, C=O

    induced magnetic field

    proton deshielded by induced field

    H0

  • Inside protons of aromatic ring: shielded, upfield shift

    HH

    HHH

    H

    HH

    H

    H

    HH H

    H

    H

    H

    HH

    [18]Annulene Inside H: -3.0

    outside H: 9.3

    O O

    HH

    4.41

  • - ring current is characteristic of all Hckel aromatic molecules and is a good test of aromaticity

    H 147 pm

    134 pm

    5.7 ppmH

    HH

    HH

    H

    7.37 ppm

    H

    HH

    HH

    CH2

    2.3-3.0 ppm

    R

    - benzylic position: downfield shift

  • CH3 Cl

    128.4

    21.3137.7

    129.3128.5

    138.8127.6128.4

    125.6 125.4

    133.7 128.1

    126.0

    13C NMR

    110-140 ppm13C NMR6.5-8.0 ppm1H NMR

  • - intense C-H out-of-plane bending600-900

    - benzylic protons- aryl protons

    2.3-3.

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