AROMATIC COMPOUNDS By PUAN AZDUWIN BINTI KHASRI
Criteria for Aromaticity 1. A compound must have an uninterrupted cyclic cloud of 𝜋electrons above and below the plane of the molecule 2. The p cloud must contain an odd number of pairs of pelectrons. BENZENE Benzene is an aromatic compound because it is cyclic and planar, every carbon in the ring has a p orbital, and the 𝜋 cloud contains three pairs of 𝜋 electrons.
Hückel’s Rule For a planar, cyclic compound to be aromatic, its uninterrupted p cloud must contain (4n + 2) p electrons, where n is any whole number
Monocyclic hydrocarbons with alternating single and double bonds are called annulenes: Cyclobutadiene and cyclooctatetraene are NOT AROMATIC, because they have an even number of p electron pairs
not aromatic not aromatic aromatic Cyclopentadienedoes not have an uninterrupted ring of p orbital-bearing atoms Cyclopentadienylcationhas an even number of p electron pairs Cyclopentadienyl anion has an uninterrupted ring of p orbital-bearing atoms and an odd number of p electron pairs
The resonance hybrid shows that all the carbons in the cyclopentadienyl anion are equivalent
These compounds consist of fused benzene rings and are aromatic: Any compound consisting of fused benzene rings is aromatic
Aromatic Heterocyclic Compounds A compound does not have to be a hydrocarbon to be aromatic. A HETEROCYCLIC compound has ring atoms other than carbon Example:Heterocyclic Compounds
Antiaromaticity A compound is antiaromatic if it is a planar, cyclic, continuous loop of porbitals with an even number of pairs of p electrons Antiaromatic compounds are highly unstable, but the nonplanar versions are stable A compound is classified as being antiaromatic if it fulfills the first criterion for aromaticity but does not fulfill the second criterion.
Nomenclature of Monosubstituted Benzenes Some are named by attaching “benzene” after the name of the substituent:
A benzene substituent is called phenyl. A benzene substituent with a methylene group is called benzyl.
Electrophilic Aromatic Substitution Reactions • Halogenation • Nitration • Sulfonation • Friedel–Crafts acylation • Friedel–Crafts alkylation
General Mechanism for Electrophilic Aromatic Substitution of Benzene Carbocation intermediate
1. Halogenationof Benzene LEWIS ACID CATALYST LEWIS ACID CATALYST
Lewis acid weakens the Br–Br (or Cl–Cl) bond, which makes the halogen a better electrophile:
B: Bromide or Benzene Mechanism for bromination The catalyst is regenerated:
2. Nitration of Benzene Nitration of benzene with nitric acid requires sulfuric acid as a catalyst.
Nitronium ion formation: Mechanism for Nitration;
3.Sulfonation of Benzene Fuming sulfuric acid (a solution of in sulfuric acid) or concentrated sulfuric acid is used to sulfonate aromatic rings
Mechanism for sulfonation Sulfonation of benzene is a reversible reaction. Mechanism for desulfonation
Reaction coordinate diagram for the sulfonation the desulfonation SULFONATION A-B RATE DETERMINING STEP Has a smaller rate constant (Higher energy hill, thus slower reaction) than B-C sulfonation of benzene B DESULFONATION B-A RATE DETERMINING STEP C-B has a smaller rate constant than B-A (because once B is formed, its easier for B to get to C and proceed to A) C A Desulfonation of benzenesulfonic acid
4.Friedel–Crafts Acylation Reactions Either an acyl halide or an acid anhydride can be used for Friedel–Crafts acylation.
Mechanism for Friedel–Crafts acylation: Must be carried out with more than one equivalent of AlCl3:
5.Friedel–Crafts Alkylation of Benzene The Friedel–Crafts alkylation reaction substitutes an alkyl group for a hydrogen.
However, 100% of the 2-methyl-2-phenylbutane product can be obtained if a bulky alkyl halide is used:
Friedel–Crafts alkylation will not produce a good yield of an alkylbenzene containing a straight-chain group, because the carbocation will rearrange: Acylium ions, however, do not rearrange:
Methodologies Used for the Reduction Step There are more general methods available to reduce a ketone carbonyl group to a methylene group
Using Coupling Reactions to Alkylate Benzene The Gilman reagent: The Stille reaction: The Suzuki reaction:
The resulting halide product can undergo a nucleophilic substitution reaction:
Oxidation of an alkyl group bonded to a benzene ring Provided that a hydrogen is bonded to the benzylic carbon,
The same reagent that oxidizes alkyl substituents will oxidize benzylic alcohols:
However, aldehydes or ketones can be generated if a milder oxidizing agent is used:
It is possible to selectively reduce just one of the two nitro groups: