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  • Reactions of Aromatic Compounds

    Aromatic compounds are stabilized by this “aromatic stabilization” energy

    Due to this stabilization, normal SN2 reactions observed with alkanes

    do not occur with aromatic compounds

    (SN2 reactions never occur on sp2 hybridized carbons!)

    In addition, the double bonds of the aromatic group do not behave similar to alkene reactions

  • Aromatic Substitution

    While aromatic compounds do not react through addition reactions seen earlier

    Br2

    Br

    Br Br2 FeBr3

    Br

    With an appropriate catalyst, benzene will react with bromine

    The product is a substitution, not an addition

    (the bromine has substituted for a hydrogen)

    The product is still aromatic

  • Electrophilic Aromatic Substitution

    Aromatic compounds react through a unique substitution type reaction

    Initially an electrophile reacts with the aromatic compound to generate an arenium ion

    (also called sigma complex)

    The arenium ion has lost aromatic stabilization

    (one of the carbons of the ring no longer has a conjugated p orbital)

  • Electrophilic Aromatic Substitution

    In a second step, the arenium ion loses a proton to regenerate the aromatic stabilization

    The product is thus a substitution

    (the electrophile has substituted for a hydrogen)

    and is called an Electrophilic Aromatic Substitution

  • Energy Profile

    Potential energy

    Reaction Coordinate

    Starting material

    Transition states

    Transition

    states

    Intermediate

    Products

    E H

    E

    The rate-limiting step is therefore the formation of the arenium ion

  • The properties of this arenium ion therefore control electrophilic aromatic substitutions

    (just like any reaction consider the stability of the intermediate

    formed in the rate limiting step)

    1)

    The rate will be faster for anything that stabilizes the arenium ion

    2)

    The regiochemistry will be controlled by the stability of the arenium ion

    The properties of the arenium ion will predict the outcome of

    electrophilic aromatic substitution chemistry

  • Bromination

    To brominate an aromatic ring need to generate an electrophilic source of bromine

    In practice typically add a Lewis acid (e.g. FeBr3) to bromine

    In presence of aromatic ring this electrophilic bromine source will react

    Br2 FeBr3 Br

    H Br

  • With unsubstituted benzene the position of reaction is arbitrary

    With a monosubstituted aromatic ring, however, can obtain three possible products

    How to predict which is favored?

    Controlled by stability of arenium ion intermediate

  • The stability is different depending upon placement of carbocation

  • Alkyl groups are electron donating

    Therefore toluene will favor electrophilic substitution at ortho/para positions

    (only ortho/para substitution places carbocation adjacent to alkyl group on ring)

    Often obtain more para than ortho due to sterics

  • In addition to orientational control, substituents affect reactivity

    CH3 NO2

    As the aromatic ring acquires more electron density, the arenium ion will be more stable

  • Toluene therefore reacts faster in an electrophilic aromatic substitution than benzene

    The alkyl group is called an activating group

    (it activates the ring for a faster rate)

    Any substituent that increases the rate for an electrophilic aromatic substitution

    is called an “activating” substituent

    Substituents that lower the rate for an electrophilic aromatic substitution

    are called “deactivating” subsituents

    (nitro is one example of a deactivating group)

    These are groups that lower the electron density of the aromatic ring

  • Factors that affect Activators/Deactivators

    There are in general two mechanisms that can affect a substituent

    1)

    Inductive

    Substituents that are more electronegative than carbon

    will inductively pull electron density out of the ring

    2)

    Resonance

    Substituents that have a lone pair of electrons adjacent to the ring

    can donate electron density into the ring through resonance

    O CH3 O CH3

    O CH3 O CH3

  • Many substituents will have both inductive and resonance effects

    Need to balance the effects

    R

    Alkyl substituents

    inductively donate,

    no resonance effects,

    therefore activators

    Electronegative atoms with lone pair of electrons have opposing effects

    Inductively withdrawing

    therefore deactivating

    Resonance donating,

    therefore activating

    Z Z

    when a neutral O or N is directly bonded to a benzene ring,

    the resonance effect dominates and the net effect is activating

    when a halogen is bonded to a benzene ring,

    the inductive effect dominates and the net effect is deactivating

  • Other Deactivating Groups

    There are two other main classes of deactivating groups

    1)  A conjugated system where both inductive and resonance effects

    pull electron density from the ring

    Y Z

    Whenever Z is more electronegative than Y as seen in structure

    N O

    O

    O N

    2)

    A formal positive charge is placed directly adjacent to ring

    N CH3

    H3C CH3

  • Activating vs. Deactivating Ability can be compared

    The substituents can be compared relatively

  • Orientational Control Can Be Predicted

    1)

    All activating groups favor ortho/para substitution

    2)  Deactivating groups with a lone pair of electrons adjacent to the ring

    favor ortho/para substitution

    (halogens are in this category)

    3)

    Other deactivating groups favor meta substitution

  • With deactivating groups besides halogens, the favored substitution is meta

    Can be predicted by stability of arenium ion

    Only the meta substitution does not place a carbocation adjacent to EWG

  • All positions with a deactivating group are slower than benzene

    * The meta position is the LEAST disfavored

    Consider reaction coordinate for the first, rate-determining step

  • Multiple Substituents

    How to determine orientation of electrophilic aromatic substitution

    if there is more than one substituent?

    A few rules to consider:

    1)

    The effects are cumulative

    2)  The stronger substituent according to the relative effects

    will be correspondingly more important for directing effects

    3)  When given a choice, a new substituent typically

    will not go ortho to two other substituents

  • Consider some examples:

    ortho/para director

    CH3

    CN Br2 FeBr3meta director

    CH3 CN

    Br

    H3C CN Br2 FeBr3

    ortho/para director

    meta director

    H3C CN

    Br

    Br2 FeBr3

    H3C

    OCH3

    ortho/para director

    ortho/para director

    H3C

    OCH3

    Br

    Stronger director wins

    Stronger director wins

  • Reactivity of aromatic ring can affect amount of reaction

    In reactions with strongly activated rings, polyhalogenation occurs

    With either phenol or aniline, the reaction will proceed

    until all ortho/para positions are reacted when catalyst is used

  • With these highly activated ring systems, catalyst is not necessary

    Reaction will only proceed with strongly activated ring systems,

    still need catalyst for less activated aromatic rings,

    but with phenol or aniline reaction of one halogen can occur with no catalyst present

  • Other Electrophiles Beside Bromine

    Chlorination reaction is similar to bromination

    Often use Lewis acid with chlorine substituents to avoid cross contamination

    (e.g. often use AlCl3 for chlorination but FeBr3 for bromination)

    Also with strongly activated rings obtain polychlorination products with catalyst

  • Iodination requires a stronger oxidizing reagent

    The reagents are a method to generate iodonium ion in situ

    The iodonium can then react as the electrophile

  • Fluorine is much harder to add in an electrophilic addition

    There is not an easy method to generate F+

    Typically to add fluorine to an aromatic ring a diazonium route is used

    This chemistry will be dealt in more detail in chapter 19

  • Other Common Electrophiles Besides Halogens

    Nitration

    Adding a nitro group to an aromatic ring is a convenient and useful reaction

    Need to generate a source of nitronium ion

  • An advantage of nitration is the nitro group can be reduced to an amine

    Allows

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