struttura del benzene il benzene pi gentilucci/gentilucci_cap9...struttura del benzene il benzene...

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  • 1

    Struttura del benzene

    Il benzene pi stabile di un triene coniugato

    Gentilucci- Composti aromatici e reattivit

    Aromaticity (Huckel).

    All compounds having

    planar, conjugated

    systems containing

    4n + 2 electrons

    Delocalizzazione

    elettronica nel

    benzene

    Gentilucci- Composti aromatici e reattivit

  • 2

    6 pz orbitals 3 bonding orbitals +

    3 antibonding

    The in phase overlap of the six

    p-orbitals forms the

    continuous -system

    Gentilucci- Composti aromatici e reattivit

    Aromaticity. It is also essential that the carbon skeleton be

    planar.

    Cyclodecapentaene has 10 -electrons, but is not aromatic

    because the ring hydrogens force the -system out of

    planarity.

    In [18]annulene (18 -electrons) there is sufficient

    room for the central six hydrogen atoms to fit

    within the middle of the ring system.

    anthracen phenantren

    Gentilucci- Composti aromatici e reattivit

  • 3

    Heteroaromatic

    compounds

    Purinic and

    Pyrimidinic bases

    Gentilucci- Composti aromatici e reattivit

    Arenes: Nomenclature

    1. Benzene is the parent

    name; when a benzene ring

    is a substituent: "phenyl"

    group.

    2. Disubstituted benzenes are named using ortho-, para- and meta- to describe the

    substitution pattern (1,2 1,4 and 1,3 respectively) or simply by numbering the

    substituents.

    3. Substituents are numbered to give the lowest possible number sequence at the

    first point of difference, assigning priorities alphabetically if there is a "tie".

    Gentilucci- Composti aromatici e reattivit

  • 4

    4. There are also a large number of common (or "trivial") names for arenes

    Gentilucci- Composti aromatici e reattivit

    Gentilucci- Composti aromatici e reattivit

    ?

  • 5

    The role of the FeBr3 is to complex the bromine to form a bromonium cation-like

    species which is the actual electrophilic agent.

    Bromination

    Br2 + benzene,

    catalyzed by FeBr3.

    In general, these reactions require a Lewis acid catalyst.

    Electrophilic aromatic substitution,

    An electrophile reacts with the ring, forming a new bond to a ring carbon with the

    loss of one hydrogen.

    Gentilucci- Composti aromatici e reattivit

    Since the ring is a conjugated system, the cationic charge which forms on the

    adjacent carbon is delocalized over the ring, ortho- and para- to the position

    where the electrophile bonded.

    Loss of H+ from the sigma-complex regenerates the aromatic -system.

    This electrophile first

    forms a loose complex

    with the -cloud, which

    rearranges to a cationic

    -complex, in which the

    electrophile is directly

    bonded to a ring carbon.

    -complex

    Gentilucci- Composti aromatici e reattivit

  • 6

    -complex

    Gentilucci- Composti aromatici e reattivit

    Chlorination

    proceeds by a similar

    mechanism.

    Iodination

    I2/CuCl2 is typically

    utilized to generate the

    electrophilic I+ cation

    Nitration

    using a mixture of

    nitric and sulfuric acids

    to generate the

    electrophile NO2+.

    Gentilucci- Composti aromatici e reattivit

  • 7

    Sulfonation

    Fuming sulfuric acid

    (H2SO4 saturated with

    SO3) to generate SO3H+.

    Friedel-Crafts

    alkylation.

    A Lewis acid complexes

    with an alkyl halide to

    give a species with

    electrophilic character

    on the carbon of the

    alkyl halide.

    Gentilucci- Composti aromatici e reattivit

    Gentilucci- Composti aromatici e reattivit

    (this means that an alkyl halide such as 1-bromopropane is not

    suitable in this reaction, since it would be prone to rearrange to

    the more stable isopropyl carbocation)

    These limitations can be circumvented by using A derivative

    of the Friedel-Crafts alkylation is the Friedel-Crafts acylation

    5.

  • 8

    Friedel-Crafts acylation: the arene is converted to an aryl ketone. An acylium ion-

    like species is formed by reaction of an acid halide, or an acid anhydride, with the

    Lewis acid.

    Gentilucci- Composti aromatici e reattivit

    Substitution of substitutedbenzenes

    Gentilucci- Composti aromatici e reattivit

    fast

    slow

    slow

  • 9

    Kinetic and orientation effects

    When electrophilic aromatic substitution occurs on a ring already bearing one or

    more substituent, the nature of that substituent will impact both the rate of the

    reaction and the regiochemistry of the reaction.

    Arenes with activating

    substituents will react faster

    than benzene itself,

    Arenes with deactivating

    substituents will react more

    slowly.

    Further, substituents are

    grouped into two categories;

    ortho- para- directing,

    meta-directing.

    Gentilucci- Composti aromatici e reattivit

    An activating substituent releases electron density into the ring either inductively,

    or through resonance (the electrophile is, after all, looking for electrons).

    Ortho-and para- orientation effect is seen by considering the family of resonance

    forms; these clearly show enhanced electron density localized ortho-and para- to the

    point of attachment.

    CH3

    Inductive effect Conjugative effect

    Gentilucci- Composti aromatici e reattivit

  • 10

    Deactivating, meta-directing substituents such as the nitro group can be seen

    to function by removing electron density from the ring ortho- and para- to

    themselves, leaving only the meta-positions with sufficient electron density to

    support the electrophilic (electron-seeking) reaction.

    Gentilucci- Composti aromatici e reattivit

    Halogens: deactivating, ortho-and para- directing

    Halogens are somewhat unique in that they deactivate inductively (and are

    therefore less reactive than benzene), but they direct ortho-and para- since they

    enhance the electron density at these positions by resonance, as shown below

    Gentilucci- Composti aromatici e reattivit

  • 11

    Gentilucci- Composti aromatici e reattivit

    meta-directing

    substituents don't

    really activate the

    meta-positions,

    they deactivate

    everywhere else.

    Gentilucci- Composti aromatici e reattivit

  • 12

    Multiple substituents on a ring.

    The effects are generally either

    cumulative, or the most strongly

    activating substituent ultimately

    directs the regiochemistry.

    Gentilucci- Composti aromatici e reattivit

    Exercise: electrophilic substitution of toluene

    Pyridoxal-phosphate (PLP, pyridoxal-5'-phosphate) is a coenzyme in all

    transaminations, and in some decarboxylation and dehydration reactions of amino

    acids.

    Gentilucci- Composti aromatici e reattivit

  • 13

    Reactions of Aryl Side-Chains

    Oxidation. Arenes having an alkyl side-chain with at least one benzylic hydrogen

    will undergo oxidation by neutral MnO4- to give the corresponding benzoic acid.

    Gentilucc i- Composti aromatici e reattivit

    As with all reactions

    of MnO4-, the

    reaction involves

    radical

    intermediates and

    side reactions are

    common.

    Gentilucc i- Composti aromatici e reattivit

    Radical helogenation.

    Since benzyl radicals are

    quite stable, radical

    bromination occurs quite

    rapidly on alkyl benzenes.

    The reaction often utilize

    NBS (N-bromosuccinimide)

    in CCl4 in the presence of a

    "radical initiator" to

    generate the bromine

    radical.

  • 14

    Reduction

    Since arenes are resistant to

    catalytic reduction, alkene side-

    chains can be specifically

    reduced to the alkane without

    reducing the ring.

    The ring can be reduced with Pt

    or Pd at high temperatures and

    pressure, although Rh will

    catalyze the reduction under

    very mild conditions.

    Nitro groups are reduced to

    the corresponding amine.

    Aryl ketones are smoothly

    reduced catalytically to the

    corresponding alkane

    (alternative to the Friedel-

    Crafts alkylation).

    Gentilucc i- Composti aromatici e reattivit

    Nucleophilic Aromatic Substitution

    Arenes having strongly electron-

    withdrawing groups on the ring, and at

    least one potential leaving group (e.g.

    an halogen) can undergo substitution by

    strong nucleophiles,

    Gentilucc i- Composti aromatici e reattivit

    This substitution cannot be

    occurring by a simple SN1 or

    SN2.

    The pathway for SN2 is blocked

    by the ring, and the SN1

    mechanism would involve the

    generation of an unstable aryl

    cation on a ring which is

    already extremely electron

    deficient.

    The reaction involves an addition-elimination pathway.

    The negative charge

    is delocalized