Aromatic Compounds

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Chapter 5
Aromatic Compounds
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Aromatic Compounds

• Aromatic compound A term used to classify
  benzene and its derivatives containing 6-membered
  rings with 3 double bonds
• Arene An aromatic hydrocarbon
• Aryl group A group derived from an aromatic
  compound by removal of an H give the symbol Ar-

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Structure of Benzene The Kekulé Proposal

• C6H6
• Six-numbered ring, with three double bonds

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Structure of BenzeneThe Resonance Proposal

• In benzene, each carbon-carbon connection is an
  average of 1.5 bonds, midway between a single
  bond and a double bond

• All carbon-carbon bonds in benzene are the same
  length, 0.139 nm long
• Most C-C single bonds have lengths near 0.154 nm
• Most CC double bonds are about 0.134 nm long

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• An orbital view of benzene shows the situation
  more clearly, emphasizing the cyclic conjugation
  of the benzene molecule and the equivalence of
  the six carbon-carbon bonds
• The carbon skeleton is a regular hexagon, with
  all C-C-C and H-C-C bond angles 120
• Each the six carbon atoms has a p orbital that
  can overlap equally well neighboring p orbitals
  on both sides

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Naming Aromatic Compounds

• Monosubstituted alkylbenzenes are named as
  derivatives of benzene
• The same as other hydrocarbons, only with
  benzene used as the parent chain
• But many common names are retained

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• Disubstituted Benzenes Locate the 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
• where neither group imparts a special name,
  locate the groups and list them in alphabetical
  order

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• Benzenes with more than two substituents are
  named by numbering the position of each
  substituent on the ring so that the lowest
  possible number are used

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• Practice

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Electrophilic Aromatic Substitution Reactions
Bromination

• The most characteristic reaction of aromatic
  compounds is electrophilic aromatic substitution
• An electron-poor reagent (an electrophile, E)
  reacts with the electron-rich aromatic ring (a
  nucleophile) and substitutes for one of the ring
  hydrogens

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• Many different substituents can be introduced
  onto the aromatic ring by electrophilic
  substitution
• Halogenate the aromatic ring (substitute a
  halogen -F, -Cl,
• Nitrate (nitro group -NO2)
• Sulfonate (sulfonic acid group -SO3H)
• Alkylate (alkyl group -R)
• Acylate (acyl group -COR)

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• All these reactions take place by a simple
  mechanism
• Bromination of benzene
• FeBr3 as catalyst

Substitution Addition
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• The aromatic rings are less reactive toward
  electrophiles than alkenes are
• Br2 in CH2Cl2 solution reacts instantly with most
  alkenes but does not react with benzene
• A catalyst such as FeBr3 is needed
• The catalyst makes the molecule Br2 more
  electrophilic by reacting with it to give FeBr4-
  and Br
• FeBr3 Br2 ?
  FeBr4- Br
• A base (FeBr4-) removes H from the
  bromine-bearing carbon to yield the neutral
  aromatic substitution product

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Other Electrophilic Aromatic Substitution
Reactions
Chlorination and Iodination
(Antiallergy)
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Nitration The electrophile is the nitronium
ion, NO2, generated as reaction of nitric
acid The two-step nitration/reduction sequence is
a key part of the industrial synthesis of many
dyes and pharmaceutical agents
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• Sulfonation
• Aromatic rings react with so-called fuming
  sulfuric acid, a mixture of SO3 and H2SO4
• The reactive electrophile is HSO3
• A key step in the synthesis of such compounds as
  the sulfa drug family of antibiotics

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The Friedel-Crafts Alkylation and Acylation
Reactions

• Alkylation
• Friedel-Crafts alkylation forms a new C-C bond
  between a benzene ring and an alkyl group
• Catalyst AlCl3
• Electrophile is a carboncation, R
• Friedel-Crafts reactions do not succeed on
  aromatic rings substituted by the groups NO2,
  -COR

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• Acylation
• Friedel-Crafts acylation forms a new C-C bond
  between a benzene ring and an acyl group

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Substituent Effects in Electrophilic Aromatic
Substitution

• Substituents already present on an aromatic ring
  have two effects
• Substituents affect the reactivity of the
  aromatic ring
• Activate
• Deactivate
• Substituents affect the orientation of the
  reaction
• Ortho, meta, and para

(deactivator)
(activator)
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(activator) ortho-para directing
(deactivator) meta directing
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An Explanation of Substituent Effects

• Activating and deactivating effects in aromatic
  rings
• Activating groups donate electrons to the ring,
  thereby making the ring more electron-rich,
  stabilizing the carbocation intermediate, and
  lowering the activation energy for its formation
• Deactivating groups withdraw electrons from the
  ring, thereby making the ring more electron-poor,
  destabilizing the carbocation intermediate, and
  raising the activation energy for its formation

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d-
H
d-
d
O?H
Cl
d
d-
C O
d
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Orienting effects in aromatic rings Ortho and
para directors
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Orienting effects in aromatic rings Meta
directors
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Oxidation and Reduction of Aromatic Compounds

• Benzene is unaffected by strong oxidizing agents
  such as H2CrO4 and KMnO4
• halogen and nitro substituents are unaffected by
  these reagents
• Alkyl groups attached to aromatic ring are
  readily attacked by oxidizing agents and are
  converted into carboxyl group (-COOH)

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K2Cr2O7
H2SO4
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• Benzene is also inert to reduction under typical
  hydrogenation condition
• If high temperature and pressures are used

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Other Aromatic Compounds

• Polycyclic aromatic hydrocarbons (PAHs) contain
  two or more aromatic rings, each pair of which
  shares two ring carbons

(Carcinogenic substances)
(in mothballs)
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Parabens

• Parabens are a class of chemicals widely used as
  preservatives by cosmetic and pharmaceutical
  industries
• They are becoming increasingly controversial,
  however, because they have been found in breast
  cancer tumors (an average of 20 ng/g of tissue)
• Parabens have also displayed the ability to
  slightly mimic estrogen (a hormone known to play
  a role in the development of breast cancer). No
  effective direct links between parabens and
  cancer have been established, however
• Another concern is that the estrogen-mimic aspect
  of parabens may be a factor in the increasing
  prevalence of early puberty in girls
• Studies indicate that methylparaben applied on
  the skin may react with UVB leading to increased
  skin aging and DNA damage

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The concept of Aromaticity

• Heterocyclic compound contains one or more atoms
  other than carbon in a ring
• Pyridine and pyrimidine are heterocyclic analogs
  of benzene. Each is aromatic.

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• Aromatic character
• Have an uninterrupted (continued) cloud of
  delocalized p electrons circling above and below
  the plane of the molecule. (planar and cyclic)
• Have one 2p orbital on each atom of the ring
• Have six p electrons (3 pairs), or odd number of
  pairs of p electrons

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Organic Synthesis

• The only trick to devising an organic synthesis
  is to work backward
• What is the immediate precursor of that product
• Work back again, one step at a time
• Until a suitable starting material is found

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PRACTICE
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Step 1
Step 2
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Step 3
a
b
Solution
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