Aromaticity is an important concept in organic chemistry. Compounds containing aromatic rings exhibi — Organic Chemistry Chemistry Question
Aromaticity
Aromaticity is an important concept in organic chemistry. Compounds containing aromatic rings exhibit characteristic physico-chemical properties and reactivities. A simple rule, the Hückel rule helps us to identify aromatic structures. This rule postulates that a cyclic conjugated system is aromatic if the number of p electrons participating in the delocalized π-bonds is 4n+2, where n is a nonnegative integer. The rule can be extended to cover polycyclic and fused ring systems.
Give examples for aromatic structures, where n = 0, 1, 2.
Model Answer
n = 0: cyclopropenyl cation [VISUAL]; n = 1: benzene [VISUAL]; n = 2: naphthalene [VISUAL]
Show that linearly fused, originally aromatic rings (where the individual rings are added stepwise to the chain via 2 carbon atoms) are Hückel aromatic. Example: [VISUAL]
Model Answer
Fusion of an aromatic ring with 6 π electrons to a linear, aromatic chain, which has 4n+2 π electrons implies that we will lose two aromatic carbon atoms, hence the total number of π electrons is: 4n + 2 + 6 – 2 = 4(n + 1) + 2, i.e., the Hückel condition is satisfied.
Show that fusion of two such fused chains via 2 carbon atoms also results in Hückel aromatic structures. Example: [VISUAL]
Model Answer
Fusion of two linear chains of fused aromatics via 2 carbon atoms means that the total number of π electrons is 4n1 + 2 + 4n2 + 2 – 2 = 4(n1 + n2) + 2, consequently the structure is aromatic according to Hückel.
Give a criterion for a general fusion of two aromatic hydrocarbons to yield a Hückel aromatic system. Example: [VISUAL]
Model Answer
A possible criterion can be that the fusion should involve 4n+2 aromatic carbon atoms, where n is a nonnegative integer.
Show with a counterexample that the Hückel rule is not a necessary condition for aromaticity.
Model Answer
For example coronene. [VISUAL]
Cyclopentadiene has pKa = 18, cyclopentene has pKa = 45. Explain the difference.
Model Answer
The conjugate base of cyclopentadiene is aromatic.
Tautomerism may also result in aromatic structures. Show how the following compound may undergo interconversion to yield an aromatic structure. [VISUAL]
Model Answer
[VISUAL] The conversion of isoxazol-5-one into its aromatic enol form (5-hydroxyisoxazole).
However, some of the following aromatic structures prefer to rearrange into structures not featuring an aromatic ring. Find these structures. Where do these non-aromatic structures have essential roles? Give additional examples of tautomerism, where the molecule prefers to lose its aromatic character. [VISUAL]
Model Answer
The first is cytosine, the second is uracil. The essential role of these tautomeric forms is to participate in the hydrogen bonds of the nucleic acid chains. The aromatic tautomers would form fewer hydrogen bonds within the DNA base-pairs. Additional examples are guanine and thymine. (Adenine remains aromatic.)