SNS - Organic Chemistry - Aromatic Compounds

Question 1

Aromatic

Answer 1

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

Question 2

Huckel’s Rule

Answer 2

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

Question 3

Anti-Aromatic Compounds

Answer 3

Cyclic conjugated polyenes possessing 4n electrons

Are cyclic conjugated polyenes that are destabilised

Question 4

Nomenclature

Group Name

Answer 4

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

Question 5

Nomenclature

Benzene Group

Answer 5

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

Question 6

Nomenclature

Benzyl

Answer 6

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

Question 7

Nomenclature

Substituted benzene rings

Answer 7

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

Question 8

Physical Properties

Answer 8

Generally similar to those of other hydrocarbons

Question 9

Chemical Properties

Answer 9

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

Question 10

Aromatic Compounds

Reactions

Answer 10

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

Question 11

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Answer 11

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

Question 12

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Halogenation

Answer 12

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

Question 13

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Sulphonation

Answer 13

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

Question 14

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Nitration

Answer 14

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

Question 15

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Acylation

Answer 15

Friedel-Crafts Reaction

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

Usually cataylsed by Lewis acids such as AlCl3

Question 16

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Answer 16

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

Question 17

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Strength of Substituent Effect By Class

Answer 17

1. Activating - greatest effect
2. Deactivating, Ortho/Para Directing
3. Deactivating, Meta Directing - least effect

Question 18

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Activating

Answer 18

Ortho/Para Directing

Electron Donating, tend to enhance substitution

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

Question 19

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Deactivating - Ortho/Para Directing

Answer 19

Tend to inhibit substitution, electron withdrawing

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

Question 20

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Deactivating - Meta Directing

Answer 20

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

Question 21

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

Carbonyl Compounds

Answer 21

Deactivating, Meta

Question 22

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

COOH

Answer 22

Deactivating, Meta

Question 23

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

COOR

Answer 23

Deactivating, Meta

Question 24

Aromatic Compounds

Reactions

Electrophillic Aromatic Substitution

Substituent Effects

COR

Answer 24

Deactivating, Meta

Question 25

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects CHO

Answer 25

Deactivating, Meta

Question 26

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects NO2

Answer 26

Deactivating, Meta

Question 27

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects SO3H

Answer 27

Deactivating, Meta

Question 28

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects NH2

Answer 28

Activating

Question 29

Aromatic Compounds Reactions Electrophillic Aromatic Substitution Substituent Effects Cl

Answer 29

Deactivating, Ortho/para

Question 30

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects Br

Answer 30

Deactivating, Ortho/para

Question 31

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects NR2

Answer 31

Activating

Question 32

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects OH

Answer 32

Activating

Question 33

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects NHCOR

Answer 33

Activating

Question 34

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects OR

Answer 34

Activating

Question 35

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects OCOR

Answer 35

Activating

Question 36

Aromatic Compounds ## Footnote Reactions Electrophillic Aromatic Substitution Substituent Effects R

Answer 36

Activating

Question 37

Aromatic Compounds Reactions Reduction

Answer 37

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