Organic Chemistry Jin Hongwei College of Chemical

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Organic Chemistry

• Jin Hongwei
• College of Chemical Engineering and
• Materials Science
• jhwei828_at_zjut.edu.cn

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Chapter Five Aromatic Hydrocarbons and Aromaticity

• Isomerism and Nomenclature of Aromatic
  Hydrocarbons.
• Structure and Stability of Benzene.
• Physical Properties of Monocyclic Aromatic
  Hydrocarbons.
• Chemical Properties of Monocyclic Aromatic
  Hydrocarbons.
• Chemical Properties of Polycyclic Aromatic
  Hydrocarbons.
• Aromaticity and the Huckel Rule.

3
Introduction(1)

• In 1834 the German chemist Eilhardt Mitscherlich
  (University of Berlin) firstly synthesized
  benzene by heating benzoic acid with calicum
  oxide. Using vapor density measurements,
  Mitscherlich further showed that benzene has the
  molecular formula C6H6
• The molecular formula itself was surprising.
  Benzene has only as many hydrogen atoms as it has
  carbon atoms, it should be a highly unsaturated
  compound. Eventually, chemists began to recognize
  that benzene does not show the behavior expected
  of a highly unsaturated compound.

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Introduction(2)

• During the latter part of the nineteenth century
  the Kekule Couper-Butlerov theory of valence was
  systematically applied to all known organic
  compounds. Organic compounds were classified as
  being either aliphatic or aromatic.
• To be classified as aliphatic meant that the
  chemical behavior of a compound was fatlike.
• To be classified as aromatic meant that the
  compound had a low hydrogen-carbon ratio and that
  it was fragrant.

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Isomerism and Nomenclature of Aromatic
Hydrocarbons(2)

• Disubstituted benzenes are named using one of the
  prefixes ortho(o), meta(m), or para(p).
• An ortho-disubstituted benzene has its two
  substituents in a 1,2 relationship on the ring a
  meta-disubstituted benzene has its two
  substituents in a 1,3 relationship and a
  para-disubstituented benzene has its substituents
  in a 1,4 relationship. For example

BACK
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Isomerism and Nomenclature of Aromatic
Hydrocarbons(1)

• Monosubstituted benzene are systematically named
  in the same manner as other hydrocarbons, with
  benzene as the parent name. For example
• If the alkyl substituent has more than six
  carbons, or has carbon-carbon double bond and
  triple bond, the compound is named as a
  phenyl-substituted alkane, alkene or alkyne. For
  example

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Structure and Stability of Benzene(1)

• In 1865, August Kekule, the originator of the
  structual theory, proposed the first definite
  structure for benzene, a structure that is still
  used today. Kekule suggested that the carbon
  atoms of benzene are in a ring, that they are
  bonded to each other by alternating single and
  double bonds, and that one hydrogen atom is
  attached to each carbon atom.
• The fact that the bond angles of the carbon atoms
  in the benzene ring are all 120o strongly
  suggests that the carbon atoms are sp2
  hydridized.

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Structure and Stability of Benzene(2)

• Although benzene is clearly unsaturated, it is
  much more stable than other alkenes, and it fails
  to undergo typical alkene reactions. For example
• We can get a quantitative idea of benzenes
  stability from the heats of hydrogenation.

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Chemical Properties of Monocyclic
AromaticHydrocarbons(1)

• Chemistry of Benzene Electrophilic Aromatic
  Substitution.
• The most common reaction of aromatic compounds is
  electrophilic aromatic substitution. That is, an
  electrophile (E) react with an aromatic ring and
  substitutes for one of the hydrogens
• Many different substituents can be introduced
  onto the aromatic ring by electrophilic
  substitution reactions. By choosing the proper
  reagents, its possible to halogenate the
  aromatic ring, nitrate it, sulfonate it, alkylate
  it, or acylate it.
• 
  Halogenation
• Nitration
  Sulfonation
• Alkylation
  
  Acylation

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(2)

• Aromatic Halogenation
• A. Bromination of Aromatic Rings
• A benzene ring , with its six pelectrons in a
  cyclic conjugated system, is a site of electron
  density. Thus, benzene acts as an electron donor
  (a Lewis base, or nucleophile) in most of its
  chemistry, and most of its reactions take place
  with electron acceptors (Lewis acids, or
  electrophiles). For example, benzene react with
  Br2 in the presence of FeBr3 as catalyst to yield
  the substitution product bromobenzene.

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(3)

• The mechanism of the electrophilic bromination of
  benzene.

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(4)

• Aromatic Halogenation
• B. Chlorination and Iodination of Aromatic Rings
• Chlorine and iodine can be introduced into
  aromatic rings by electrophilic substitution
  reactions, but fluorine is too reactive, and only
  poor yields of monofluoroaromatic products are
  obtained by direct fluorination. For example

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(5)

• Aromatic Nitration
• Aromatic rings can be nitrated by reaction with a
  mixture of concentrated nitric and sulfuric
  acids. The electrophile in this reaction is the
  nitronium ion, NO2, which is generated from HNO3
  by protonation and loss of water. The nitronium
  ion react with benzene to yield a carboncation
  intermediate in much the same way as Br. Loss of
  H from this intermediate gives the product,
  nitrobenzene.

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(6)

• Aromatic Sulfonation
• Aromatic rings can be sulfonated by reaction with
  fuming sulfuric acid, a mixture of H2SO4 and SO3.
  The reactive electrophile is either HSO3 or SO3,
  depending on reaction conditions. Substitution
  occurs by the same two-step mechanism seen
  previously for bromination and nitration.

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(7)

• Alkylation of Aromatic Rings The Friedel-Crafts
  Reaction
• One of the most useful of all electrophilic
  aromatic substitution reactions is alkylation,
  the attachment of an alkyl group to the benzene
  ring.
• For example
• The Friedel-Crafts alkylation reaction is an
  electrophilic aromatic substitution in which the
  electrophile is a carbocation, R. Aluminum
  chloride catalyzes the reaction by helping the
  alkyl halide to ionize in much the same way that
  FeBr3 catalyzes aromatic brominations by
  polarizing Br2 . Loss of a proton then completes
  the reaction.

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Chemical Properties of Monocyclic
AromaticHydrocarbons(8)

• The mechanism of the Friedel-Crafts alkylation
  reaction
• Give the structures of the major products of the
  following reactions
• How to prepare propylbenzene by Friedel-Crafts
  reaction?

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(9)

• An acyl group, -COR, is introduced onto the ring
  when an aromatic compound reacts with a
  carboxylic acid chloride, RCOCl, in the presence
  of AlCl3. For example, reaction of benzene with
  acetyl chloride yields the ketone, acetophenone.
• The mechanism of Friedel-Crafts acylation

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(10)

• How to prepare propylbenzene by Friedel-Crafts
  reaction?
• By contrast, the Friedel-Crafts acylation of
  benzene with propanoyl chloride produces a ketone
  with an unrearranged carbon chain in excellent
  yield.
• This ketone can then be reduced to propylbenzene
  by several methods. One general method-called the
  Clemmensen reduction-consists of refluxing the
  ketone with hydrochloric acid containing
  amalgamated zinc.

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(11)

• Substituent Effects in Substituted Aromatic Rings
• Only one product can form when an electrophilic
  substitution occurs on benzene, but when what
  would happen if we were to carry out a reaction
  on an aromatic ring that already has a
  substituent?
• A substituent already present on the ring has two
  effects
• 1. A substituent affects the reactivity of the
  aromatic ring. Some substituents activate the
  ring, making it more reactive than benzene, and
  some deactivate the ring, making it less reactive
  than benzene.
• For example
• Reactive rate 1000 1
  0.033 6?10-8
• of nitration

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(12)

• Substituent Effects in Substituted Aromatic Rings
• 2. Substituents affect the orientation of the
  reaction. The three possible disubstituted
  products-ortho, meta, and para- are usually not
  formed in equal amounts. Instead, the nature of
  the substituent already present on the benzene
  ring determines the position of the second
  substitution. For example
  Orientation of Nitration in Substitued Benzenes
• Product ()
  Product()
• Ortho Meta
  Para Ortho
  Meta Para
• Meta-directing deactivators
  Ortho- and para-directing deactivators
• -N(CH3)3 2 87
  11 -F 13
  1 86
• -NO2 7
  91 2 -Cl
  35 1 64
• -COOH 22 76 2
  -Br 43
  1 56
• -CN 17 81
  2 -I 45
  1 54
• -COOCH3 28 66
  6 Ortho- and para-directing
  activators
• -COCH3 26 72
  2 -CH3 63
  3 34
• -CHO 19 72 9
  -OH 50
  0 50
• 
  -NHCOCH3 19
  2 79

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(13)

• Substituent Effects in Substituted Aromatic Rings
• Substituents can be classified into three groups
• Ortho-and para-directing activators, ortho-and
  para-directing deactivators, and meta-directing
  deactivators.
• Ortho-and para- ortho-and
  para- Meta-directing
• directing activators
  directing deactivators
• 
  deactivators

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(14)

• An Explanation of Substituent Effects(1)
• Activation and Deactivation of Aromatic Rings
• The common feature of all activating groups is
  that they donate electrons to the ring, thereby
  stabilizing the carbocation intermediate from
  electrophilic addition and causing it to form
  faster.
• The common feature of all deactivating groups is
  that they withdraw electrons from the ring,
  thereby destabilizing the carbocation
  intermediate from electrophilic addition and
  causing it to form more slowly.

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(15)

• An Explanation of Substituent Effects(2)
• Ortho- and Para- Directing Activators Alkyl
  Groups
• Inductive and resonance effects account for the
  directing ability of substituents as well as for
  their activating or deactivating ability. Take
  alkyl groups, for example, which have an
  electron-donating inductive effect and behave as
  ortho and para directors. The results of toluene
  nitration are shown as below

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(15)

• An Explanation of Substituent Effects(3)
• Ortho- and Para- Directing Activators OH and NH2
• Hydroxyl, alkoxyl, and amino groups are also
  ortho-para activators, but for a different reason
  than for alkyl groups. Hydroxyl, alkoxyl, and
  amino groups have a strong, electron-donating
  resonance effect that is most pronounced at the
  ortho and para positions and outweighs a weaker
  electron-withdrawing inductive effect. When
  phenol is nitrated, only ortho and para attack is
  observed

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(16)

• An Explanation of Substituent Effects(4)
• Ortho- and Para- Directing Deactivators Halogens
• Halogens are deactivating because their stronger
  electron-withdrawing inductive effect outweighs
  their weaker electron-donating resonance effect.
  Though weak, that electron-donating resonance
  effect is felt only at the ortho and para
  positions.

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(17)

• An Explanation of Substituent Effects(5)
• Meta- Directing Deactivators
• Meta-directing deactivators act through a
  combination of inductive and resonance effects
  that reinforce each other. Inductively, both
  ortho and para intermediates are destabilized
  because a resonance form places the positive
  charge of the carbocation intermediate directly
  on the ring carbon atom that bears the
  deactivating group. At the same time, resonance
  electron withdrawal is also felt at the ortho and
  para positions. Reaction with an electrophilic
  therefore occurs at the meta position.

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(18)

• Trisubstituted Benzenes Additivity of Effects
• Further electrophilic substitution of a
  disubstituted benzene is governed by the same
  resonance and inductive effects just discussed.
  The only difference is that its necessary to
  consider the additive effects of two different
  groups. In practice, three rules are usually
  sufficient
• Rule 1. If the directing effects of the two
  groups reinforce each other, there is no problem.
• Rule 2. If the directing effects of the two
  groups oppose each other, the more powerful
  activating group has the dominant influence, but
  mixtures of products often result.
• Rule 3. Further substitution rarely occurs
  between the two groups in a meta- disubstituted
  compound because this site is too hindered.
• Some examples

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(19)

• Synthesis of Substituted Benzenes
• One of the surest ways to learn organic chemistry
  is to work synthesis problems. The ability to
  plan a successful multistep synthesis of a
  complex molecule requires a working knowledge of
  the uses and limitations of many hundreds of
  organic reactions. Not only must you know which
  reactions to use, you must also know when to use
  them. The order in which reactions are carried
  out often critical to the success of the overall
  scheme.
• The ability to plan a sequence of reactions in
  the right order is particularly valuable in the
  synthesis of substituted aromatic rings, where
  the introduction of a new substituent is strongly
  affected by the directing effects of other
  substituents. Planning synthesis of substituted
  aromatic compounds is therefore an excellent way
  to gain facility with the many reactions learned
  in the past few chapters. Some examples

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(20)

• Reduction of Aromatic Compounds
• To hydrogenate an aromatic ring, its necessary
  to use a platinum catalyst with hydrogen gas at
  several hundred atmospheres pressure. For
  example
• Oxidation of Benzene

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Chemical Properties of Monocyclic Aromatic
Hydrocarbons(21)

• Oxidation of Alkylbenzene Side Chains
• Alkyl side chains are readily attacked by
  oxidizing agents and are converted into carboxyl
  groups, -COOH. For example
• Bromination of Alkylbenzene Side Chains
• 
  BACK

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Chemical Properties of Polycyclic Aromatic
Hydrocarbons(1)

• Polycyclic aromatic hydrocarbons have two or more
  benzene rings fused together. For example
• Naphthalene
  Anthracene Phenanthrene
• Reactions of Naphthalene

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Chemical Properties of Polycyclic Aromatic
Hydrocarbons(2)

• Substituent Effects in Substituted Naphthalene

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Aromaticity and the Huckel Rule

• In 1931 the Germen physicist Erich Huckel carried
  out a series of mathematical calculations based
  on the theory of molecular orbital. Huckels rule
  is concerned with compounds containing one planar
  ring in which each atom has a p orbital as in
  benzene. His calculations show that planar
  monocyclic rings containing 4n2 pelectrons,
  where n0, 1, 2, 3,, and so on, delocalized
  electrons should be aromatic. For example

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Additional problems of chapter five (1)

• 3.1 Give IUPAC names for the following compounds
• (a)
  (b)
• (c)
  (d)
• 3.2 Predict the major product(s) of the following
  reactions
• (a)
• (b)
• (c)

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Additional problems of chapter five (2)

• 3.3 At what position, and on what ring, would you
  expect the following substances to undergo
  electrophilic substitution?
• (a) (b)
  (c)
• (d) (e)
  (f)
• (g) (h)
  (i)
• 3.4 How would you synthesize the following
  substances starting from benzene?
• (a) (b)
  (c) (d)

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Additional problems of chapter five (3)

• 3.4 Which would you expect to be aromatic
  compounds according to Huckel 4n2 rule?
• (a) (b)
  (c) (d)
  (e)
• (f) (g)
  (h) (i)