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Introduction of Other Aromatic Compounds

Several unsaturated cyclic compounds have the exceptional properties which we now consider as a feature of "aromatic" systems. The following examples are illustrative:

Compound

 Structural Formula

Reaction with Br2

Thermodynamic
Stabilization

1,3-Cyclopentadiene

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Addition ( 0 ºC )

Slight

1,3,5-Cycloheptatriene

Addition ( 0 ºC )

Slight

1,3,5,7-Cyclooctatetraene

Addition ( 0 ºC )

Slight

Benzene

Substitution

Large

Pyridine

Substitution

Large

Furan

Substitution ( 0 ºC )

Moderate

Pyrrole

Substitution

Moderate

 

Generally the properties associated with alkenes are contained by the starting first three compounds (cyclic polyenes). With bromine each reacts readily to give addition products, as do most alkenes. The Conjugation offsets this increase in energy by a small amount (4-6 kcal/mol). Thermodynamic change on introducing double bonds into the carbon atom ring is also typical of alkenes (a destabilization of ca. 26 kcal/mol for each double bond).

Apart from first three compounds remaining four compounds display very distinct properties and are considered aromatic. The pyridine and Benzene are comparatively unreactive with bromine requiring heat and/or catalysts to force reaction, result of which is substitution rather than addition. The pyrrole and Furan react more fastly with bromine, but they also give the substitution products. To favor substitution rather than addition this tendency suggests that parent unsaturated ring system has exceptional stability. This conclusion is supported by the thermodynamic measurements. Enhanced stability, considered as aromatic stabilization, ranges to 36 kcal/mol for benzene from a low of 16 kcal/mol for furan.

Factors Required for Aromaticity

  • A planar (or near planar) cycle of sp2hybridized atoms, p-orbitals of that are oriented parallel to each other. These overlapping the p-orbitals generate an array of π-molecular orbitals.
  • Through 4n+2 electrons (where n is an integer or zero) these π-orbitals are occupied. This requirement is known asHückel Rule. All aromatic compounds discussed above have 6 π-electrons (n=1).

Annulenes

Annulenes are the monocyclic compounds made up of alternating conjugated double bonds. 1, 3, 5, 7-cyclooctatetraene and Benzene are instances of annulenes; they are named annulene and annulene respectively, as per the general nomenclature system where the number of pi-electrons in an annulene is designated by a number in brackets. Some annulenes are aromatic (for example benzene), but many are not because of non-planarity or a failure to satisfy the Hückel Rule. Compounds classified as annulenes (a Hückel Rule system) serve to demonstrate these factors.

Barrelene

The Hückel rule's formulation prompted organic chemists to consider the possible aromaticity of various unusual unsaturated hydrocarbons. One this type of compound was the 6 π-electron bicyclic structure, presently known as the barrelene. Even though in barrelene the π-bonds are not coplanar, it was believed that the transannular overlap might still lead to aromatic stabilization. 

Aromatic Ionsd

The Carbanions and carbocations may also show aromatic stabilization.

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