21 Benzene

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    21-1

    Chapter 21, Benzeneand and the Concept of

    Aromaticity

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    21-2

    Benzene - Kekul

    In 1872, August Kekul proposed the following

    structure for benzene.

    This structure, however, did not account for the

    unusual chemical reactivity of benzene.

    CH

    CH

    CH

    CHC

    H

    C

    HC

    C

    CC

    C

    C

    H

    HHH

    HH

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    Benzene - Resonance

    We often represent benzene as a hybrid of two

    equivalent Kekul structures. Each makes an equal contribution to the hybrid and

    thus the C-C bonds are neither double nor single, butsomething in between.

    Benzene as a hybrid of two equ ivalen tcontributing structures

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    Benzene - Resonance Model

    The concepts of hybridization of atomic orbitals

    and the theory of resonance, developed in the1930s, provided the first adequate description ofbenzenes structure.

    The carbon skeleton is a planar regular hexagon.

    All C-C-C and H-C-C bond angles 120.

    sp2-sp

    2

    sp2-1s109 pm

    120

    120

    120

    139 pm

    C

    C

    C

    C

    C

    C H

    H H

    H

    H H

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    The Pi System of Benzene

    (a) The carbon framework with the six 2p orbitals.

    (b) Overlap of the parallel 2p orbitals forms one torusabove the plane of the ring and another below it

    this orbital represents the lowest-lying pi-bondingmolecular orbital.

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    Benzene-Molecular Orbital Model

    The molecular orbital representation of the pi

    bonding in benzene.

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    Orbitals of the pi System of BenzeneNumber ofnodal surfaces

    0

    1

    2

    3

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    Benzene - Resonance

    Resonance energy:The difference in energy

    between a resonance hybrid in which theelectrons are delocalized

    and

    the most stable one of its hypotheticalcontributing structures in which electrons arelocalized on particular atoms and in particularbonds.

    One way to estimate the resonance energy of benzeneis to compare the heats of hydrogenation of benzeneand cyclohexene.

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    Benzene- Resonance Energy

    Experimentaldata

    Model

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    Concept of Aromaticity

    The underlying criteria for aromaticity were

    recognized in the early 1930s by Erich Hckel,based on molecular orbital (MO) calculations.

    To be aromatic, a compound must

    Be cyclic. Have one p orbital on each atom of the ring.

    Be planar or nearly planar so that there is continuousor nearly continuous overlap of all p orbitals of the

    ring. Have a closed loop of (4n + 2) pi electrons in the cyclic

    arrangement of p orbitals.

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    Frost Circles

    Frost circle: A graphic method for determining

    the relative order of pi MOs in planar, fullyconjugated monocyclic compounds.

    Inscribe in a circle a polygon of the same number ofsides as the ring to be examined such that one of the

    vertices of the polygon is at the bottom of the circle.

    The relative energies of the MOs in the ring are givenby where the vertices of the polygon touch the circle.

    Those MOs

    Below the horizontal line through the center of the ringare bonding MOs.

    on the horizontal line are nonbonding MOs.

    above the horizontal line are antibonding MOs.

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    Frost Circles

    Frost circles describing the MOs for monocyclic,

    planar, fully conjugated four-, five-, and six-memberedrings.

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    Relationship of hexa-1,3,5-triene to benzene

    How does the linear triene relate

    to benzene?

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    Relationship of hexa-1,3,5-triene to benzene

    ?

    R l i hi f h 3 i b

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    Relationship of hexa-1,3,5-triene to benzene

    ?

    Look at orbitals 2 and 3.

    p2

    p3

    Curvearound

    Antibonding,destabilizing

    Bonding,stabilizing

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    Aromatic Hydrocarbons

    Annulene:A cyclic hydrocarbon with a

    continuous alternation of single and doublebonds.

    [14]Annulene is aromatic according to Hckels

    criteria.

    [14]Annulene

    (aromatic)

    HH

    H H

    H

    H

    H

    H

    HH

    H

    H

    H H

    n = 3

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    Aromatic Hydrocarbons

    [18]Annulene is also aromatic.

    [18]Annulene

    (aromatic)

    H

    H

    H

    H

    H

    H

    H

    H

    HH

    H

    H

    H

    H

    H

    H

    H

    H

    n = 4

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    Aromatic Hydrocarbons

    According to Hckels criteria, [10]annulene should be

    aromatic; it has been found, however, that it is not. Nonbonded interactions between the two hydrogens

    that point inward toward the center of the ring forcethe ring into a nonplanar conformation in which

    overlap of the ten 2p orbitals is no longer continuous.

    [10]Annulene

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    Aromatic Hydrocarbons

    What is remarkable relative to [10]annulene is that if

    the two hydrogens facing inward toward the center ofthe ring are replaced by a methylene (CH2) group, thering is able to assume a conformation close enough toplanar that it becomes aromatic.

    CH2

    Bridged [10]annulene

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    Antiaromatic Hydrocarbons

    Antiaromatic hydrocarbon:A monocyclic, planar,

    fully conjugated hydrocarbon with 4n pielectrons (4, 8, 12, 16, 20...).

    An antiaromatic hydrocarbon is especially unstablerelative to an open-chain fully conjugated hydrocarbon

    of the same number of carbon atoms.

    Cyclobutadiene is antiaromatic.

    In the ground-state electron configuration of thismolecule, two electrons fill the p

    1

    bonding MO.

    The remaining two electrons lie in the p2 and p3nonbonding MOs.

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    Cyclobutadiene

    The ground state of planar cyclobutadiene has two

    unpaired electrons, which make it highly unstable andreactive.

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    Cyclooctatetraene

    Cyclooctatetraene, with 8 pi electrons is not aromatic;

    it shows reactions typical of alkenes. X-ray studies show that the most stable conformation

    is a nonplanar tub conformation.

    Although overlap of 2p orbitals occurs to form pi

    bonds, there is only minimal overlap between sets of2p orbitals because they are not parallel.

    viewed from above viewed through an edge

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    Cyclooctatetraene

    MO energy diagram for a planar conformation of

    cyclooctatetraene.

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    Heterocyclic Aromatics

    Heterocyclic compound: A compound that

    contains more than one kind of atom in a ring. In organic chemistry, the term refers to a ring with one

    or more atoms that differ from carbon.

    Pyridine and pyrimidine are heterocyclic analogsof benzene; each is aromatic.

    Pyridine

    N

    N

    N

    Pyrimidine

    1

    2

    3

    4

    5

    6

    12

    3 4

    5

    6

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    Pyridine

    The nitrogen atom of

    pyridine is sp2

    hybridized.

    The unshared pair ofelectrons lies in an sp2

    hybrid orbital and is nota part of the six pielectrons of thearomatic system (thearomatic sextet).

    Resonance energy ofpyridine is134 kJ (32kcal)/mol.

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    Furan and Pyrrole

    The oxygen atom of furan is sp2 hybridized.

    one unshared pairs of electrons on oxygen lies in anunhybridized 2p orbital and is a part of the aromaticsextet.

    The other unshared pair lies in an sp2 hybrid orbital

    and is not a part of the aromatic system. The resonance energy of furan is 67 kJ (16 kcal)/mol.

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    Other Heterocyclics

    Purine

    Indole

    N

    N

    NN

    N

    H

    H

    N

    H

    CH2 CH2 NH 2

    Serotonin(a neurotransmitter)

    HO

    Caffeine

    N

    NN

    N

    O

    O

    H3 C

    CH3

    CH3

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    Aromatic Hydrocarbon Ions

    Any neutral, monocyclic, unsaturated

    hydrocarbon with an odd number of carbonsmust have at least one CH2 group and, therefore,cannot be aromatic.

    Cyclopropene, for example, has the correct number of

    pi electrons to be aromatic, 4(0) + 2 = 2, but does nothave a closed loop of 2p orbitals.

    Cyclopropene Cyclopentadiene Cycloheptatriene

    CH2 CH2CH2

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    Cyclopropenyl Cation

    If, however, the CH2 group of cyclopropene is

    transformed into a CH+

    group in which carbon is sp2

    hybridized and has a vacant 2p orbital, the overlap oforbitals is continuous and the cation is aromatic.

    Cyclopropenyl cation represented as a hybridof three equ ivalen t contributin g structures

    +

    H

    H

    H

    H

    H

    H

    H

    H

    H+

    +

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    Cyclopropenyl Cation

    When 3-chlorocyclopropene is treated with SbCl5, it

    forms a stable salt.

    This chemical behavior is to be contrasted with that of5-chloro-1,3-cyclopentadiene, which cannot be made

    to form a stable salt.

    +

    Cyclopropenyl

    hexachloroantimonate

    +

    3-Chloro-

    cyclopropene

    H

    H

    ClSbCl5 SbCl6

    -

    Antimony(V)

    chloride(a Lewis acid)

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    Cyclopentadienyl Cation

    If planar cyclopentadienyl cation were to exist, it wouldhave 4 pi electrons and be antiaromatic.

    Note that we can draw five equivalent contributingstructures for the cyclopentadienyl cation. Yet this

    cation is not aromatic because it has only 4 pielectrons.

    Cyclope ntadienylte trafluoroborate

    ++

    5-Chloro-1,3-cyclopentadiene

    H

    ClHAgBF4 BF4 - + AgCl

    C C

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    Cyclopentadienyl Anion, C5H5-

    To convert cyclopentadiene to an aromatic ion, it

    is necessary to convert the CH2 group to a CHgroup in which carbon becomes sp2 hybridizedand has 2 electrons in its unhybridized 2p orbital.

    H

    HH

    HH

    the origin of the 6 pi electronsin the cyclopentadienyl anion

    Cyclopentadienyl anion(aromatic)

    HH

    HH

    H:

    H

    H

    H

    HH

    n = 1

    C l di l A i C H

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    Cyclopentadienyl Anion, C5H5-

    As seen in the Frost circle, the six pi electrons of

    cyclopentadienyl anion occupy the p1, p2, and p3molecular orbitals, all of which are bonding.

    C l di l A i C H

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    Cyclopentadienyl Anion, C5H5-

    The pKa of cyclopentadiene is 16.

    In aqueous NaOH, it is in equilibrium with its sodiumsalt.

    It is converted completely to its anion by very strongbases such as NaNH2 , NaH, and LDA.

    pKa15.7pKa16.0

    Na+

    + H2OH

    H

    H

    H

    H

    CH2 + NaOH :

    C l h i l C i C H

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    Cycloheptatrienyl Cation, C7H7+

    Cycloheptatriene forms an aromatic cation by

    conversion of its CH2 group to a CH+ group withits sp2 carbon having a vacant 2p orbital.

    +

    Cyclohep tatrienyl cation(Tropylium ion)

    (aromatic)

    H

    HH

    H

    H

    HH

    H

    HH

    H

    H

    HH

    +

    N l t

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    Nomenclature

    Monosubstituted alkylbenzenes are named as

    derivatives of benzene. Many common names are retained.

    Toluene CumeneEthylbe nzene Styrene

    Phenol Aniline Benzoic acid Anisole

    COOHNH2 OCH3OH

    Benzaldehyde

    CHO

    N l t

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    Nomenclature

    Benzyl and phenyl groups

    (Z)-2-Phenyl-

    2-butene

    4-(3-Me thoxyphenyl)-

    2-butanone

    1-Phenyl-1-pentanone

    O OH3CO

    Ph

    Benzene Phenyl group, Ph- Toluene Benzyl group, Bn-

    CH3 CH2

    Di b tit t d B

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    Disubstituted Benzenes

    Locate two groups by numbers or by the

    locators ortho (1,2-), meta (1,3-), and para (1,4-). Where one group imparts a special name, name the

    compound as a derivative of that molecule.

    CH3

    Br

    COOH

    NO2

    Cl

    NH2

    CH3

    CH3

    2-Nitrobenzoic acid

    (o-Nitrobenzoic acid )

    3-Chloroaniline

    (m-Chloroaniline)

    4-Bromotoluene

    (p-Bromotoluene)

    m-Xylene

    Di b tit t d B

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    Disubstituted Benzenes

    Where neither group imparts a special name, locate

    the groups and list them in alphabetical order.CH2 CH3

    Cl

    NO2Br

    1-Bromo-2-nitrobenzene(o-Bromonitrobenzene)

    1-Chloro-4-ethylbenzene(p-Chloroethylbenzene)

    1

    2

    3

    4 2

    1

    P l b tit t d D i ti

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    Polysubstituted Derivatives

    If one group imparts a special name, name the

    molecule as a derivative of that compound. If no group imparts a special name, list them in

    alphabetical order, giving them the lowest set ofnumbers.

    CH3

    NO2

    OH

    Br

    Br

    NO2

    CH2 CH3

    Br

    4

    2

    1

    6

    4

    21

    4

    1 2

    4-Chloro-2-nitro-

    toluene

    2,4,6-Tribromo-

    phenol2-Bromo-1-ethyl-4-

    nitrobenzene

    Br

    Cl

    Ph l

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    Phenols

    The functional group of a phenol is an -OH group

    bonded to a benzene ring.

    1,2-Benzenediol(Catechol)

    1,4-Benzenediol(Hydroquinone )

    3-Methylphenol(m-Cresol)

    Phenol

    OH OHOHOH

    OHCH3

    OH

    Acidit of Phenols

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    Acidity of Phenols

    Phenols are significantly more acidic than

    alcohols.

    OH H2 O

    CH3CH2 OH H2 O

    O-

    CH3CH2 O-

    H3 O+

    H3 O+

    pKa = 9.95+

    +

    +

    + pKa = 15.9

    Acidity of Phenols

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    Acidity of Phenols

    Separation of water-

    insoluble phenolsfrom water-insolublealcohols.

    Acidity of Phenols (Resonance)

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    Acidity of Phenols (Resonance)

    The greater acidity of phenols compared with alcohols

    is due to the greater stability of the phenoxide ionrelative to an alkoxide ion.

    These 2 Kekulstructures are

    equivalent

    HH

    OO O O

    H

    O

    These three contrib uting s tructuresdelocalize the negative charge

    onto carbon atoms of the rin g

    H

    OO O O

    H

    O

    Phenol Subsitituents (Inductive Effect)

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    Phenol Subsitituents (Inductive Effect)

    Alkyl and halogen substituents effect acidities by

    inductive effects: Alkyl groups are electron-releasing.

    Halogens are electron-withdrawing.

    p-ChororophenolpKa 9.18

    m-ChlorophenolpKa 8.85

    PhenolpKa 9.95m-

    CresolpKa 10.01p-

    CresolpKa 10.17

    OH OH OH OH OH

    CH3CH3

    Cl

    Cl

    Phenol Subsitituents(Resonance Inductiion)

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    Phenol Subsitituents(Resonance, Inductiion)

    Nitro groups increase the acidity of phenols by both an

    electron-withdrawing inductive effect and a resonanceeffect.OH

    NO2

    OH OH

    NO2PhenolpKa 9.95

    p-NitrophenolpKa 7.15

    m-NitrophenolpKa 8.28

    Acidity of Phenols

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    Acidity of Phenols

    Part of the acid-strengthening effect of -NO2 is due to

    its electron-withdrawing inductive effect. In addition, -NO2 substituents in the ortho and para

    positions help to delocalize the negative charge.

    + +

    delocalization of negative

    charge onto oxygen furtherincreases the resonancestabilization of phenoxide ion

    O

    O O

    N

    O

    O

    NO

    Synthesis: Alkyl Aryl Ethers

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    Synthesis: Alkyl-Aryl Ethers

    Alkyl-aryl ethers can be prepared by the

    Williamson ether synthesis: but only using phenoxide salts and haloalkanes.

    haloarenes cannot be used because they areunreactive to SN2 reactions.

    no reaction+X RO-Na

    +

    Synthesis: Alkyl Aryl Ethers

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    Synthesis: Alkyl-Aryl Ethers

    OH CH2 =CHCH2 ClNaOH, H2 O, CH2 Cl2

    OCH2 CH=CH2

    Phenyl 2-prope nyl ether

    (Allyl phenyl ether)

    +

    Phenol 3-Chloropropene(Allyl chloride)

    OH

    O

    O

    CH3 OSOCH3NaOH, H2 O, CH2 Cl2

    OCH3 Na2 SO4+

    Methyl phenyl e ther(Anisole)

    +

    Phenol Dimethyl sulfate

    Synthesis: Kolbe Carboxylation

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    Synthesis: Kolbe Carboxylation

    Phenoxide ions react with carbon dioxide to give

    a carboxylate salt.

    OH

    NaOH

    H2

    O

    O-Na

    +

    CO2

    H2

    O

    OH

    CO-Na

    +O

    HCl

    H2 O

    OH O

    COH

    Phenol Sodiumphenoxide

    Sodium salicylate S alicylic acid

    Mechanism: Kolbe Carboxylation

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    Mechanism: Kolbe Carboxylation

    The mechanism begins by nucleophilic addition of the

    phenoxide ion to a carbonyl group of CO2.

    O

    C

    O

    O

    OC

    H

    OO OH

    CO

    O

    A cyclohexadienone

    intermediate

    +

    Sodium

    phenoxide

    Salicylate anion

    keto-enol

    tautomerism

    (1) (2)

    Go back to aromatic

    structure

    Synthesis: Quinones

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    Synthesis: Quinones

    Because of the presence of the electron-donating

    -OH group, phenols are susceptible to oxidationby a variety of strong oxidizing agents.

    H2 CrO4

    Phenol 1,4-Benzoquinone(p-Quinone)

    O

    O

    OH

    Quinones

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    Quinones

    OH

    OH K2Cr2O7

    OH

    OH

    H2SO

    4

    K2Cr2O7

    H2SO

    4

    O

    O

    O

    O

    1,4-Benzoquinone(p-Quinone)

    1,2-Benzenediol(Catechol)

    1,2-Benzoquinone(o-Quinone)

    1,4-Benzenediol(Hydroquinone)

    Quinones

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    Quinones

    Readily reduced to hydroquinones.

    1,4-Benzoquinone(p-Quinone)

    (reduction)

    1,4-Benzenediol(Hydroquinone)

    O

    O

    OH

    OH

    Na2 S2O4 , H2 O

    Coenzyme Q

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    Coenzyme Q

    Coenzyme Q is a carrier of electrons in the

    respiratory chain.O

    O

    MeO

    HMeO MeO

    MeO

    OH

    OH

    Hn n

    Coenzyme Q(oxidized form)

    Coenzyme Q(reduced form)

    reduction

    oxidation

    Benzylic Oxidation

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    Benzylic Oxidation

    Benzene is unaffected by strong oxidizing agents

    such as H2CrO4 and KMnO4 Halogen and nitro substituents are also unaffected by

    these reagents.

    An alkyl group with at least one hydrogen on its

    benzylic carbon is oxidized to a carboxyl group.

    2-Chloro-4-nitrotoluene 2-Chloro-4-nitrobenzoic acid

    H2 CrO4

    O2N Cl

    CH3

    O2N Cl

    COOH

    Benzylic Oxidation

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    Benzylic Oxidation

    If there is more than one alkyl group on the benzene

    ring, each is oxidized to a -COOH group.

    1,4-Dimethylbenzene(p-xylene)

    1,4-Be nzenedicarboxylic acid(terephthalic acid)

    CH3H2 SO4

    K2 Cr2 O7H3 C COH

    O

    HOC

    O

    Benzylic Chlorination

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    Benzylic Chlorination

    Chlorination and bromination occur by a radical

    chain mechanism.CH3

    Cl2+

    CH2Cl

    HCl+

    Toluene

    heator light

    Benzyl chloride

    ( PhCO2) 2 , CCl4

    NBS

    Br

    Ethylbenzene 1-Bromo-1-phenylethane(racemic)

    Mechanism: Benzylic Reactions

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    Mechanism: Benzylic Reactions

    Benzylic radicals (and cations also) are easily

    formed because of the resonance stabilization ofthese intermediates.

    The benzyl radical is a hybrid of five contributingstructures.

    C

    C

    C

    C C

    Benzylic Halogenation

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    Benzylic Halogenation

    Benzylic bromination is highly regioselective.

    Benzylic chlorination is less regioselective.

    (PhCO2 )2 , CCl4

    NBSBr

    Ethylbenzene 1-Bromo-1-phenylethane(the only product formed)

    Cl

    Cl2 +

    heator light

    1-Chloro-2-phenylethane

    (10%)

    Ethylbenzene

    +

    1-Chloro-1-phenylethane

    (90%)

    Cl

    Hydrogenolysis

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    Hydrogenolysis

    Hydrogenolysis:Cleavage of a single bond by H2

    Benzylic ethers are unique in that they are cleavedunder conditions of catalytic hydrogenation.

    O H2Pd/ C

    OH

    Me+

    Benzyl butyl ether Toluene1-Butanol

    +

    this bondis cleaved

    Synthesis, Protecting Group: Benzyl Ethers

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    Synthesis, Protecting Group: Benzyl Ethers

    The value of benzyl ethers is as protecting

    groups for the OH groups of alcohols andphenols.

    To carry out hydroboration/oxidation of this alkene,the phenolic -OH must first be protected; it is acidic

    enough to react with BH3 and destroy the reagent.

    OH

    1. ClCH2 Ph

    O Ph

    2. BH3 THF

    Et 3N 3. H2 O2/ NaOH

    H2Pd/ C

    O Ph

    OH

    OH

    OH

    2-(2-Propenyl)phenol

    (2-Allylphenol)

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