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Flashcards in SNS - Organic Chemistry - Aromatic Compounds Deck (37):
1


Aromatic

Any unusually stable ring system - cyclic congugated polyenes that possess 4n + 2 pi electrons and adopt planar conformations to allow maximum overlap of conjugated pi orbitals

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Huckel's Rule

Important indicator of aromaticity

Is the criterion that aromatic compounds must possess 4n + 2 pi electrons where n is any non-negative integer. Therefore can be 4, 6, 10, 14, 18 etc

 

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Anti-Aromatic Compounds

Cyclic conjugated polyenes possessing 4n electrons

Are cyclic conjugated polyenes that are destabilised

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Nomenclature

Group Name

 

  • Referred to as aryl compounds or arenes and are represented by the symbol Ar

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Nomenclature

Benzene Group

The benzene group is called a phenyl group (Ph) when named as a substituent

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Nomenclature

Benzyl

The term benzyl refers to a toluene molecule substituted at the methyl position

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Nomenclature

Substituted benzene rings

Named as alkyl benzenes with the substituents numbered to produce the lowest possible sequence.

A 1,2- disubstituted benzene ring is called ortho

A 1,3- disubstituted benzene ring is called meta

A 1,4- disubstiuted compound is called para

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Physical Properties

Generally similar to those of other hydrocarbons

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Chemical Properties

Significantly affected by aromaticity and thus different from those of other hydrocarbons

The characteristic planar shape of benzene permits the ring's six pi orbitals to overlap, delocalising the electron density.

All six carbon atoms are sp2 hybridised and each of the six orbitals overlaps equally with its neighbours. As a result, the delocalised electrons form two pi electron clouds above and below the plane of the ring, stabilising the molecule and making it fairly unreactive

Therefore, benzene and other aromatic compounds don't undergo addition reactions as do alkenes

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Aromatic Compounds

Reactions

  1. Electrophilic Aromatic Substitution: (a) Halogenation, (b) Sulphonation, (c) Nitration, (d) Acylation
  2. Reduction

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

The most important reaction of aromatic compounds.

An electrophile replaces a proton on an aromatic ring to produce a substituted aromatic compound

  1. Acylation
  2. Halogenation
  3. Nitration
  4. Sulphonation

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution


Halogenation

Aromatic rings react with bromine and chlorine in the presence of a Lewis acid such as FeCl3, FeBr3 or AlCl3 to produce monosubstituted products in good yield

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Sulphonation

Aromatic rings react with fuming sulphuric acid (mixture of sulphuric acid and sulphur trioxide) to form sulphonic acids

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Nitration

A mixture of nitric acid and sulphuric acid used to create the nitronium ion NO2+ which is a strong electrophile

This reacts with aromatic rings to produce nitro compounds

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Acylation

Friedel-Crafts Reaction

A carbocation electrophile, usually an acyl group, is incorporated into the aromatic ring

Usually cataylsed by Lewis acids such as AlCl3

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Substiutuents on the aromatic ring have a strong influence on the susceptibility of the ring to electrophilic aromatic substitution and strongly affect what position on the ring an incoming electrophile is most likely to attack

Substituents can be grouped into three classes:

  1. Activating
  2. Deactivating, Ortho/para directing
  3. Deactivating, Meta directing

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Strength of Substituent Effect By Class

1. Activating - greatest effect

2. Deactivating, Ortho/Para Directing

3. Deactivating, Meta Directing - least effect

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Activating

Ortho/Para Directing

Electron Donating, tend to enhance substitution

  1. NH2
  2. NR2
  3. OH
  4. NHCOR
  5. OR
  6. OCOR
  7. R

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Deactivating - Ortho/Para Directing

Tend to inhibit substitution, electron withdrawing

  1. F
  2. Cl
  3. Br
  4. I

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Deactivating - Meta Directing

  1. NO2
  2. SO3H
  3. Carbonyl compounds including COOH, COOR, COR and CHO

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Carbonyl Compounds

Deactivating, Meta

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

COOH

Deactivating, Meta

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

COOR

Deactivating, Meta

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

COR

Deactivating, Meta

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

CHO

Deactivating, Meta

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

NO2

Deactivating, Meta

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

SO3H

Deactivating, Meta

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

NH2

Activating

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Cl

Deactivating, Ortho/para

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Br

Deactivating, Ortho/para

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

NR2

Activating

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

OH

Activating

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

NHCOR

Activating

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

OR

 

Activating

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

OCOR

Activating

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Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

R

Activating

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Aromatic Compounds

Reactions

Reduction

Benzene rings can be reduced by catalytic hydrogenation under vigorous conditions (high temp and pressure) to yield cyclohexane

Ruthenium and rhodium on carbon are the most common catalysts

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