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III Aromatic Compounds
Aromatic Compounds: Compounds that resemble benzene in structure and
chemical behavior (terms comes from fragrant odors)
Benzene: - Cyclic Compound
- Six-Membered Ring
- ONLY Six Hydrogens
- Six Carbons
- Three Double Bonds
Representations:
Natural occurring aromatic systems:
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3.1 Structure and Bonding of Benzene
Unexpected Stability:
- Very stable
- Very resistant to chemical changes
Example Halogination:
Example Hydrogenation:
- Shorter than expected carbon-carbon bond length
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Bonding:
Benzene is based on 6 Carbons and 6 Hydrogens
In 1865 Kekulé suggested 2 resonance structures for Benzene:
We are thinking of Benzene as a Resonance Hybrid:
Also suggested structures for Benzene:
Dewar Benzene Ladenburg Benzene
Remember Rules for drawing resonance structures:
1) ONLY electrons move
2) The ONLY electrons that can move are electrons and non-bonding electrons
3) The total number of electrons in the molecules does not change
Electrons can move in the following way:
1) Move π electrons toward a positive charge or a π bond
2) Move a non-bonding electron pair towards a π bond
3) Move a single non-bonding electron toward a π bond
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Properties:
- Clear liquid
- Planar
- Bond angle 120º
- All carbons are sp2 hybridized
- Delocalized electrons
What is the reason for the stability and chemical inertness of Benzene?
Aromaticity:
Criteria for aromaticity:
1) The molecule MUST by CYCLIC and PLANAR
2) The ring MUST contain ONLY sp2 hybridized carbons that can form a
delocalized system
3) The number of π electrons MUST be 4n+2 (Hückel Rule)
⇒ Ring systems with 6, 10, 14 etc number of electrons are aromatic
Examples:
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Examples of systems that are NOT aromatic:
3.2 Polycyclic and Heterocyclic Aromatic Compounds
Polycyclic Aromatic Compounds: Compounds based on more than one ring
systems that are fused
Examples:
Heterocyclic Aromatic Compounds: Compounds that contain a heteroatom
Examples:
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Q1: Why are pyridine and furan aromatic?
Examples of heterocyclic aromatic systems:
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3.3 Nomenclature
Problem: Aromatic compounds have OFTEN trivial names and do not follow
IUPAC nomenclature
3.3.1 Monosubstituted Aromatics
Toluene:
Phenol:
Aniline:
Benzoic Acid:
Benzaldehyde:
Chlorobenzene:
Nitrobenzene:
Styrene:
Anisole:
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3.3.2 Disubstituted Benzenes
The ortho, meta, and para nomenclature:
ortho-Xylene:
meta-Xylene:
para-Xylene:
para-Chlorotoluene
meta-Nitrobenzoic acid
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3.3.3 Polysubstituted Benzenes
2,4,6-Trinitrotoluene (TNT):
1,2,4-Trichlorobenzene:
3-ethyl-2-methylanisole:
2,4,6-Trinitrophenol:
1-Bromo-2-chloro-4-iodobenzene:
3.3.4 Aromatic Compounds Designated by Prefixes
Phenyl:
Benzyl:
Example 2-Methyl-5-phenyl-2-pentene:
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3.4 Electrophilic Aromatic Substitutions
Electrophilic aromatic substitutions are the characteristic reactions of aromatic
compounds
Aromatic compounds are electron rich (π-cloud)
⇒ Electron deficient species (electrophiles) are attracted to aromatic compounds
In an electrophilic aromatic substitution reaction, and electrophile is put
on the ring and an H+ comes off the ring
Examples:
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3.4.1 Mechanism
General Mechanism:
Important: Resonance structures
Energy Diagram of electrophilic aromatic substitutions versus additions:
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3.4.2 Halogenations
Brominations, Chlorinations, and Iodinations
Generation of the electrophile:
Substitutions:
3.4.3 Nitrations
Mechanism:
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3.4.4 Sulfonations
Mechanism:
3.4.5 Friedel Crafts Acylations
an Acyl Group an Alkyl Group
Example of a Friedel Crafts Acylation
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Mechanism:
3.4.6 Friedel Crafts Alkylations
Example:
Mechanism:
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3.5 Structural Effects on Substitutions
Rate: Substituents can influence the rate of reaction
Example Nitration:
Activating Group: Group that increases the reactivity of an aromatic
compound to electrophilic substitution
Deactivating Group: Group that decreases the reactivity of an aromatic
compound to electrophilic substitution
Orientation: Substituents can influence the orientation of a reaction
Example:
Compounds that are ortho-para directors:
Compounds that are meta directors
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- All Activating Substituents are Ortho/Para Directors
- All Weakly Deactivating Substituents are Ortho/Para Directors
- All Deactivating Substituents (except the Halogens) are Meta Directors
⇒ in all these cases the reason is stability of the Carbocation Intermediate
→ The more stable the Carbocation, the better
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3.5.1 Interpretation
Substituents can be categorized into Electron Donating Substituents (Groups)
and Electron Withdrawing Substituents (Groups)
Electron Donating Substituents (Groups) increase the reactivity of the benzene
ring toward electrophilic aromatic substitution
Electron Withdrawing Substituents (Groups) decrease the reactivity of the
benzene toward electrophilic aromatic substitution
There are two ways how a substituent can donate (or withdrawal) electrons into
(from) an aromatic ring system
(A) Resonance
(B) Inductive Effect
3.5.1.1 Resonance Electron Donation and Withdrawal (M-Effect)
Example of Donation:
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Example of Withdrawal:
3.5.1.2 Inductive Electron Donation and Withdrawal (I-Effect)
If a substituent bonded to a benzene ring is LESS electron withdrawing than a
hydrogen, the electrons in the σ bond that attaches the substituent to the ring will
move towards the ring ⇒ They INDUCTIVELY donate electrons to the ring
Inductive Electron Withdrawal other way around
Examples:
I and M effects can work against each other
Example Halogens:
Q2. Which one wins?
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Putting everything together:
- The more resonance structures the intermediate has, the more stable it is
- The orientation is based on the stability of the Carbocation
→ Inductive donation = ortho/para directors
→ Resonance donation = ortho/para directors (one more resonance form)
→ Positive or partial positive charge on substituent = meta directors
All substituents that donate electrons into the ring either inductively or
by resonance are ortho/para directors
All substituents that cannot donate electrons into the ring either
inductively or by resonance are meta directors
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3.6 Other Reactions on Aromatic Compounds
Benzyl carbocation:
Oxidation:
Diazotization:
Sandmeyer Reaction:
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3.7 Designing a Synthesis: Mono and Disubstituted Benzenes
Examples:
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Summary of Chapter 3:
Aromatic Compounds and Benzene
Aromaticity
4n+2
Planar
only sp2 and sp
Poly and Hetero Aromatic Compounds
Electrophilic Aromatic Substitutions
Mechanisms
Examples
Inductive Effect
Resonance
Activating Groups
Deactivating Groups
EWG
EDG
meta Directors
ortho/para directors
Reactions on Side-Chains
Designing of a Synthesis