which is aromatic? - ncsu/ch223... · 14 aromatic pi electronswhich is aromatic? select the...

Download Which is aromatic? - ncsu/CH223... · 14 aromatic pi electronsWhich is aromatic? Select the aromatic

Post on 22-Aug-2018

217 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • 1

    1

    Which is aromatic?1. 12. 2

    3. 3

    4. 4

    How many are aromatic?

    Assume all are planar.

    Note: all formal chargesare shown, but notlone pairs.

    A

    2

    Which is aromatic?14 aromatic pi electrons

    Select the aromatic one(s)

    B (like Al)relatively stable w/ sextet of valence electrons

    B sp2 hybridized

    A

    all atoms possess pure p-orbital (continuous p-system)

    the other p bond is

    one p bond is w/ aromatic p-systemn

    X

    3

    Dr. Kay Sandberg

    Last lecture

    This lecture

    Reactions of benzene derivatives

    Aromaticity

    Birch Reduction

    Free-radical substitution (via NBS) review

    What makes a system aromatic

    Alkenylbenzene reactions

    Benzylic chemistry

    4

    Kay Sandberg

    Regioselectivity of free-radical halogenation

    3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    Section 11.12

    3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    d

    d

    dd

    3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    H Br

    BrH

    Radical character

    shared by

    benzylic,

    ortho & para carbons

    5

    N

    O

    O

    Br

    (N-bromosuccinimide)

    Kay Sandberg

    Regioselectivity of free-radical halogenation

    Br3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    NBS

    NBS Regioselectivie reagent

    for allylic (chap 10) and benzylic (chap 11)

    bromination.6

    Kay Sandberg

    3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    Section 11.12

    d

    d

    d

    d

    Br

    LQ #1) Draw the substitution product that would

    result if the Br bonded to the para carbon

    instead of the benzylic carbon.

    Even though we can draw this product on paper,

    it never forms in the flask. Do you know why you

    should never propose this as a major product?

    Radical character shared by

    benzylic, ortho & para carbons

  • 2

    7

    Oxidation numbers

    Section 11.13

    Na2Cr2O7

    H2O, H2SO4, heat

    Kay Sandberg

    KMnO4

    H2O, heat

    Cr2O72-

    -2x 7

    -14+ = -2+12

    x 2

    +6

    Cr(VI)

    Oxidation: benzene derivatives

    8

    Mn(VII)

    1. 2. 3. 4. 5. 6. 7. 8.

    0% 0% 0% 0%0%0%0%0%

    1. 1

    2. 2

    3. 3

    4. 4

    5. 5

    6. 6

    7. 7

    8. 8

    Section 11.13

    Na2Cr2O7

    H2O, H2SO4, heat

    Kay Sandberg

    KMnO4

    H2O, heat

    Cr2O72-

    -2x 7

    -14+ = -2+12

    x 2

    +6

    Cr(VI)

    Oxidation: benzene derivatives

    Click Manganeses ox #

    10

    9

    Alkane & benzene

    Section 11.13

    RR'

    RCH2CH2R'

    Na2Cr2O7

    H2O, H2SO4, heat

    Kay Sandberg

    Na2Cr2O7

    H2O, H2SO4, heat

    Oxidation: alkylbenzene

    No reaction

    No reaction

    10

    Alkylbenzene oxidation

    Section 11.13

    Kay Sandberg

    Na2Cr2O7

    H2O, H2SO4, heat

    C

    R

    HH

    Oxidation: alkylbenzene

    C

    O

    OH

    C

    O

    OH

    O2N

    C

    O

    OH

    C

    O

    HO

    CH3O2N1. KMnO4, H2O,

    2. H2O, H+

    CH3CH

    CH3

    CH3

    Na2Cr2O7

    H2O, H2SO4, heat

    CH3C

    CH3

    CH3

    CH3

    Na2Cr2O7

    H2O, H2SO4, heat

    C

    O

    OH

    C

    CH3

    CH3

    CH3

    benzoic acid

    11

    Toluene vs benzene

    Section 11.13

    Kay Sandberg

    liver enzymeC

    H

    HH

    O2cytochrome P-450

    Oxidation: alkylbenzene

    C

    O

    OH

    Toluenewater insoluble

    Benzoic acidwater soluble

    liver enzyme

    O2cytochrome P-450

    o

    carcinogen12

    Oxidation reagent flash cards

    Section 11.13 Kay SandbergOxidation: alkylbenzene

    C

    O

    OH

    Benzoic acid

    1. KMnO4, H2O

    2. acid workup

    C

    O

    OH

    Benzoic acid

    Na2Cr2O7

    H2O, H2SO4

    C

    H

    HR

    C

    R'

    HR

    C

    methyl, 1o, 2o benzylic carbons possess benzylic H

    1o benzylic C

    2o benzylic C

  • 3

    13

    Benzylic reactions

    NBS

    Kay Sandberg

    C

    H

    HH C

    O

    OH

    TolueneBenzoic acid

    KMnO4

    Benzylic bromide

    Na2Cr2O7

    H2O, H2SO4

    C

    R'

    HR

    C

    Reactions involving a benzylic radical intermediate.

    2o benzylic C (or toluene or 1o benzylic)

    peroxide

    C

    Br

    R'R

    14

    3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    Dr. Kay Sandberg

    C

    C

    C

    H

    H

    H

    H

    H

    d

    d

    dd

    Allylic radical vs benzylic radical

    Alike in that both are stabilized by resonance.

    Different in that substitution can occur at all

    radical character carbons of allylic system,

    compared to only at benzylic position.

    d d

    C

    C

    C

    H

    H

    H

    H

    H

    15

    C

    C

    C

    H

    H

    H

    H

    H

    Dr. Kay Sandberg

    C

    C

    C

    H

    H

    H

    H

    H

    3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    Allylic cation vs benzylic cation

    Alike in that both are stabilized by resonance.

    Different in that substitution can occur at all

    positive character carbons of allylic system,

    compared to only at benzylic position?

    d+ d+ d+

    d+

    d+

    d+

    3 2

    1

    65

    4 C

    H

    H

    HH

    H

    H H

    16

    Kay Sandberg

    Section 10.2Allylic carbocations: Rates of solvolysis

    C C

    C

    H3C

    CH3

    H

    H

    H

    3o Allylic cation

    C C

    C Cl

    H3C CH3

    H

    H

    H

    H3C

    C Cl

    H3C CH3

    H3C

    C

    H3C

    CH3

    3o carbocation

    H

    O

    H

    O

    100 x faster

    SN1

    SN1

    17

    Kay Sandberg

    Section 11.14Benzylic carbocations: Rates of solvolysis

    C

    H3C

    CH3

    3o benzylic cation

    C Cl

    H3C CH3

    H

    O

    600 x faster

    SN1

    H3C

    C Cl

    H3C CH3

    H3C

    C

    H3C

    CH3

    3o carbocation

    H

    O

    SN1

    18

    - +

    Kay SandbergSection 11.14

    Stability of benzyl cation

    C

    H

    H

    3 2

    1

    65

    4 C

    H

    H

    3 2

    1

    65

    4 C

    H

    H

    3 2

    1

    65

    4 C

    H

    H

    Resonance delocalized:

    benzylic

    ortho

    para

    ortho

    ROY G B

  • 4

    19

    Kay Sandberg

    Section 11.14Stability of benzyl cation

    Resonance delocalized:

    C

    H

    H

    3 2

    1

    65

    4 C

    H

    H

    3 2

    1

    65

    4 C

    H

    H

    3 2

    1

    65

    4 C

    H

    H

    benzylic

    ortho

    para

    ortho

    C

    H

    H

    d+d+

    d+

    d+

    20

    Orbital description of benzylic cation

    Kay Sandberg

    Section 11.14Stabilization due to overlap of

    benzylic pure p orbital

    with the extended p system of ring

    Benzyl cation

    21

    Kay Sandberg

    EC C

    C

    R

    R

    H

    H

    H

    R

    C

    R

    R

    R

    C

    R

    H

    R

    C

    H

    H

    H

    C

    H

    H

    CH H2C vinyl cation (least stable)

    allylic cation

    carbocation stabilities

    C C

    C

    R

    R

    C

    H

    C

    CC

    H

    HH

    H benzylic cation (most stable)

    22

    C C

    C

    H3C

    CH3

    H

    H

    H

    O

    C C

    C

    H3C

    CH3

    H

    H

    H

    d+

    d+

    Kay Sandberg

    Section 10.2Allylic carbocations: Rates of solvolysis

    C C

    C

    H3C

    CH3

    H

    H

    H

    Allylic cation:

    C C

    C Cl

    H3C CH3

    H

    H

    H

    HO

    SN1 C C

    C

    H3C

    CH3

    H

    H

    H

    C C

    C

    H3C

    CH3

    H

    H

    HO

    PT

    HO

    PT

    23

    Kay Sandberg

    Section 11.14

    Benzylic cation

    C

    H3C

    CH3C Cl

    H3C CH3

    H

    OSN1

    C d+d+

    d+

    d+

    OH

    24

    Kay Sandberg

    Section 11.14

    H

    O

    C

    H3C

    CH3

    O CH2

    CH3

    -H+ (PT)

    still aromatic

    C d+d+

    d+

    d+

    C

    H3C

    CH3

    O

    -H+ (PT)

    C

    HO

    LQ #2) Draw product resulting

    from attack of the ortho C.

    C

    H3C

    CH3

    O H H

    HH

    H

    Again, you would never propose

    this to be a product in practice.

  • 5

    25

    Kay Sandberg

    Section 11.14Unlike allylic halides where nucleophilic

    attack occurs at different carbons bearing

    a partial positive charge, benzylic halides

    only give one substitution product.

    Benzyl cation

    C

    H3C

    CH3C Cl

    H3C CH3

    H

    OSN1

    C

    H3C

    CH3

    O CH2

    CH3

    -H+Attack at ortho or

    para positions would

    destroy aromaticity

    Only

    substitution

    product

    C d+d+

    d+

    d+

    26

    Birch reduction What

    Kay Sandberg

    Section 11.11Birch Reduction

    arene re