5. Benzene and Aromaticity

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5. Benzene and Aromaticity
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Aromatic Compounds

• The term Aromatic is used to refer to the class
  of compounds structurally related to Benzene.
• The first discovered of these compounds were
  fragrant substances but the term aromatic, though
  still used, is not applicable to the vast
  majority of these compounds

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• The common names of some substituted aromatics
  are so firmly entrenched in the literature that
  they must be memorized

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15.2 Naming Aromatic Compounds

• Monosubstituted benzenes are named by first
  naming the substituent and following this with
  the word benzene

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Naming Alkyl Substituted Benzenes

• Alkyl benzenes are named in one of two different
  ways
• If the alkyl group contains 6 or fewer carbons,
  then the compound is named as an alkyl
  substituted benzene
• If the alkyl group contains more than 6 carbons
  then the compound is named as phenyl substituted
  alkane

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Naming Benzenes With More Than Two Substituents

• Choose numbers to get lowest possible values
• List substituents alphabetically with hyphenated
  numbers
• Common names, such as toluene can serve as root
  name

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Naming Disubstituted Benzenes

• Relative positions on a benzene ring are
  indicated by the following prefixes
• ortho- (o) on adjacent carbons (1,2)
• meta- (m) separated by one carbon (1,3)
• para- (p) separated by two carbons (1,4)
• Also used to describe reaction patterns
  (reaction occurs at the para position)

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Complete the Following Examples
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Structure of Benzene

• The actual structure of benzene lies somewhere
  between the two resonance forms pictured below

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Experimental Observations That Lead To This
Resonance Picture of Benzene

• All its C-C bonds are the same length 139 pm
  between single (154 pm) and double (134 pm) bonds
• Electron density in all six C-C bonds is
  identical
• Structure is planar, hexagonal

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Molecular Orbital Description of the Resonance in
Benzene

• Each C is sp2 hybridized and has a p orbital
  perpendicular to the plane of the six-membered
  ring. Each p orbital has one electron in it.
  This makes it impossible to identify 3 localized
  double bonds in benzene
• .

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Consequence of Resonance Stability

• The resonance stability of benzene is so very
  substantial that benzene shows none of the
  characteristic chemical behavior of other alkenes
• Alkene Br2/CCl4 ? dibromide (addition
  product)Benzene Br2/CCl4 ? no reaction.
• Alkene HBr ? Bromoalkane (addition
  product)Benzene HBr ? no reaction.
• The reason that benzene does not take part in any
  electrophilic addition rxns. is that, to do so,
  would destroy its stable conjugated system. An
  energetically unfavorable situation.

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Please Recall the General Mechanism for Aromatic
Substition

-
Br
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Heats of Hydrogenation as Indicators of Resonance
Stability of Benzene

• The addition of H2 to CC normally gives off
  about 118 kJ/mol 3 double bonds should give off
  356kJ/mol
• Benzene has 3 double bonds but gives off only 206
  kJ/mol on reacting with 3 H2 molecules
• Therefore it is about 150 kJ more stability
  than a regular alkene having s set of three
  double bonds

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Reactions of Aromatic Compounds

• Electrophilic addition reactions,
• so common amongst normal alkenes, do not occur
  with aromatics, in spite of the fact that each
  aromatic ring contains three double bonds.
• The reason for this is that these reactions break
  the double bond and this would mean that the very
  stable aromatic system would be disrupted.
• Instead, the characteristic reactions of
  aromatics are electrophilic substitution
  reactions rather that addition because these
  retain the very stable cyclic aromatic system

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Electrophilic Addition and Electrophilic
Substitution
E base-
ElectrophilicAddition
Electrophilic Substitution
base-
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Aromatic Addition Compared to Aromatic Substition
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All Electrophilic Aromatic Substitution Reactions
take place by the same General Mechanism.

• Aromatics (benzene) are less reactive towards
  electrophiles then are normal alkenes.
• Consequently, a catalyst is usually needed to
  convert the electrophile containing reagent
  into a stronger electrophile.
• The catalyst needed to react molecular bromine
  (Br2) with benzene is ferric bromide. FeBr3
  basically turns the weaker electrophile, Br2,
  into the stronger electrophile, Br
• FeBr3 is a Lewis Acid and accepts an electron
  pair from Br2 and thereby puts a strong positive
  charge on the end Bromine atom.

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Generalized Mechanism for Electrophilic Aromatic
Substitution cont.

• Once generated. the stronger electrophile gets
  attacked by the pi electrons of the aromatic
  system, forming an intermediate, resonance
  stabilized, carbocation.
• Finally, the carbocation stabilizes itself by
  loosing a ring H and regenerating the stable
  cyclic conjugated system, with the electrophile
  on the ring where the H used to be. See next
  slide.

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?
FeBr4- Br
?
?-
Br
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?
FeBr4- Br
?
?-
Br
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Aromatic Chlorination

• Chlorine and iodine (but not fluorine, which is
  too reactive) can substitute on an aromatic ring.
  Each requires a special catalyst or promoter to
  generate a sufficiently strong electrophile
• Chlorination follows the same mechanism as
  bromination and requires FeCl3 catalyst

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Aromatic Iodination

• Iodine (I2) must be oxidized with Cu2 or
  peroxide to form the more powerful electrophile,
  I

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Aromatic Nitration

• The combination of nitric acid and sulfuric acid
  produces the electrophile NO2 (nitronium ion)
• It reacts with benzene to produce nitrobenzene

HNO3
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Nitroaromatics are Important for Two Reasons

• Nitroaromatics are important in themselves and
  also the nitro group can be converted into other
  functional groups that couldnt be placed on the
  aromatic ring directly
• For example, reduction of the nitro group by
  stannous chloride yields the corresponding amine

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Aromatic Sulfonation

• The combination of sulfuric acid and sulfur
  trioxide (SO3) produces the electrophile HSO3
• Its reaction with benzene produces
  benzenesulfonic acid

SO3
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Importance of Aromatic Sulfonic Acids

• Aromatic Sulfonic Acids are valuable
  intermediates in the preparation of dyes and
  pharmaceuticals.
• Aromatic Sulfonic Acids are the precursors needed
  for the synthesis of Sulfa Drugs such as
  sulfanilamide.These were among the first useful
  antibiotics known and credited with saving
  countless lives during W.W.II

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Aromatic Sulfonic Acids are also important for
the further chemistry that they can undergo

• When sulfonic acids are mixed with sodium
  hydroxide at elevated temperatures a net
  replacement of the sulfonic group by the hydroxyl
  group results.
• This constitutes one of the few methods for
  preparing phenols.

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16.3 Alkylation of Aromatic Rings The
FriedelCrafts Reaction

• Aromatic substitution of a R for an aromatic
  proton (H)
• Aluminum trichloride, a Lewis Acid catalyst,
  promotes the formation of the (R) carbocation

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Limitations of the Friedel-Crafts Alkylation

• Only alkyl halides can be used (F, Cl, I, Br)
• Aryl halides and vinylic halides do not react
  (their carbocations are too hard to form)
• This rxn will not work with rings containing an
  amino group or a strongly electron-withdrawing
  deactivating group

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Control Problems with F/C Alkylations

• Unwanted multiple alkylations can occur because
  the first alkylation is activating. That is to
  say, once the first alkyl group substitutes on
  the ring the monosubstituted benzene is more
  reactive than benzene itself and consequently
  more likely to be substituted with another alkyl
  group

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Carbocation Rearrangements During Alkylation

• The last problem associated with F/C Alkylation
  is the possible rearrangement of the intermediate
  carbocation to a more stable carbocation
• These rearrangements usually involve hydride (H-)
  or alkide (R-) shifts

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16.4 Acylation of Aromatic Rings

• Reaction of an acid chloride (RCOCl) in the
  presence of AlCl3 catalyst with an aromatic ring
  substitutes an acyl group, ?COR , on to the
  aromatic ring
• Benzene with acetyl chloride yields acetophenone

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Mechanism of Friedel-Crafts Acylation

• Similar to alkylation
• Reactive electrophile resonance-stabilized acyl
  cation
• An acyl cation does not rearrange

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Electrophilic Aromatic Substitution of a
Monosubstituted Benzene

• What effects does a substituent already present
  on a benzene ring have on the electrophilic
  substitution of a second group?
• Reactivity Some monosubstituted benzenes are
  more reactive that benzene towards further
  electrophilic aromatic substitution (activating
  substituents) some monosubstituted benzenes are
  less reactive (deactivating substituents)
• Orientation A substituent that is already on a
  benzene ring directs the position of any incoming
  groups

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Reactivity Activating Substituents

• Activating Substituents these activate a
  benzene ring towards further substitution by
  donating electron density into the aromatic ring.
  Donating electon density into the ring increases
  the reaction rate by stabilizing the intermediate
  carbocation.

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Reactivity Deactivating Substituents

• Deactivating Substituents these deactivate a
  benzene ring towards further substitution by
  withdrawing electron density from the aromatic
  ring. Withdrawing electon density from the ring
  decreases the reaction rate by destabilizing the
  intermediate carbocation

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Orientation

• The second effect that the substituent of a
  monosubstituted benzene can have on further
  electrophilic aromatic substitution is to direct
  incoming electrophiles to particular positions on
  the aromatic ring. Substituents are either ortho
  para directors or they are meta directors.
  Combining this information with the reactivity
  characteristics of a substituent we find that all
  substituents can be classified into one of three
  groups
• Ortho Para Activators
• Meta Deactivators
• Ortho Para Deactivators

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Ortho-Para Activating Groups

• Please recall that activating groups increase the
  e- density of the aromatic ring. These
  substituents also direct incoming groups to the
  ortho and para positions as only these positions
  afford a resonance structure for the intermediate
  carbocation in which the positive charge is on
  the ring carbon to which the e- donating group is
  bonded a very stable situation. The increased
  stability of this resonance structure favors
  substitution in these positions. The electron
  donating substituents may stabilize the positive
  charge by the inductive effect or by resonance.
  See Next Slide for Example

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Meta Deactivators

• Recall that deactivating groups withdraw e-
  density from the aromatic ring. All members of
  this group except for the halogens direct
  incoming groups to the meta position for it is
  only in this position that resonance structures
  for the intermediate carbocation do not place the
  positive charge on the ring carbon to which the
  e- withdrawing group is bonded (an unstable
  situation). Avoiding this extremely unstable
  situation is what makes the meta position the
  most highly favored (most stable).

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Ortho-Para Deactivating Groups

• Recall that halogens deactivate aromatic rings by
  inductive withdrawal of e- density. In addition
  to this ability, all halogens possess nonbonded
  e-s that can be used to resonance-stabilize a
  positive charge on an adjacent carbon. It is
  this ability that make halogens ortho-para
  directors. If the incoming group attaches to
  either the ortho or para position, one of the
  resonance structures for the intermediate
  carbocation places the positive charge on a ring
  carbon to which the halogen is bonded. This
  allows the halogens to resonance-stabilize the
  positive charge.

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16.5 Substituent Effects in Aromatic Rings
Summarized

• Substituents already present on an aromatic ring
  can cause the aromatic compound to be (much) more
  or (much) less reactive than benzene
• Substituents also direct the orientation of
  incoming groups on to the aromatic ring
• ortho- and para-directing activators, ortho- and
  para-directing deactivators, and meta-directing
  deactivators

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16.7 Trisubstituted Benzenes Additivity of
Effects

• How does one predict the orientation of a third
  group coming in to a disubstituted benzene
• If the directing effects of the two groups are
  the same, the result is additive

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Substituents with Opposite Effects

• If the directing effects of two groups oppose
  each other, the more powerful activating group
  decides the principal outcome

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Meta-Disubstituted Compounds

• Substitution between two groups in a
  meta-disubstituted compound rarely occurs because
  the site is too sterically hindered
• To make aromatic rings with three adjacent
  substituents, it is best to start with an
  ortho-disubstituted compound

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16.10 Oxidation of Aromatic Compounds

• Alkyl side chains can be oxidized to ?CO2H by
  strong reagents such as KMnO4 and Na2Cr2O7 if
  they have a C-H next to the ring
• Converts an alkylbenzene into a benzoic acid,
  Ar?R ? Ar?CO2H

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16.11 Reduction of Aromatic Compounds

• Aromatic rings are inert to catalytic
  hydrogenation under conditions that reduce alkene
  double bonds
• Can selectively reduce an alkene double bond in
  the presence of an aromatic ring
• Reduction of an aromatic ring requires more
  powerful reducing conditions (high pressure or
  rhodium catalysts)

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Reduction of Aryl Alkyl Ketones

• Aromatic ring activates neighboring carbonyl
  group toward reduction
• Ketone is converted into an alkylbenzene by
  catalytic hydrogenation over Pd catalyst

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16.12 Synthesis Strategies

• These syntheses require planning and
  consideration of alternative routes
• Work through the practice problems in this
  section following the general guidelines for
  synthesis