SNS - Organic Chemistry - Aromatic Compounds Flashcards
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
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
Anti-Aromatic Compounds
Cyclic conjugated polyenes possessing 4n electrons
Are cyclic conjugated polyenes that are destabilised
Nomenclature
Group Name
- Referred to as aryl compounds or arenes and are represented by the symbol Ar
Nomenclature
Benzene Group
The benzene group is called a phenyl group (Ph) when named as a substituent
Nomenclature
Benzyl
The term benzyl refers to a toluene molecule substituted at the methyl position
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
Physical Properties
Generally similar to those of other hydrocarbons
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
Aromatic Compounds
Reactions
- Electrophilic Aromatic Substitution: (a) Halogenation, (b) Sulphonation, (c) Nitration, (d) Acylation
- Reduction
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
- Acylation
- Halogenation
- Nitration
- Sulphonation
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
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
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
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
