what is the effect of annulation on aromaticity ? Related: Craig’s Ru...
According to energetic, electron delocalization, and geometrical indices, angular benzo-annelation increases, whereas linear benzo-annelation decreases, the extent of the local aromaticity of the central ring containing heteroatoms.
The effect of benzo-annelation on the local aromaticity of the central ring of acridine (1), 9H-carbazole (2), dibenzofuran (3), and dibenzothiophene (4) was analyzed by means of the energy effects (ef), pairwise energy effects (pef), multicenter delocalization index (MCI), electron density at ring critical points (ρ(rC)), harmonic oscillator model of aromaticity (HOMA), and nucleus independent chemical shifts (NICS). According to energetic, electron delocalization, and geometrical indices, angular benzo-annelation increases, whereas linear benzo-annelation decreases, the extent of the local aromaticity of the central ring containing heteroatoms. The local aromaticity of the central heterocyclic ring in the examined molecules can significantly vary by applying different modes of benzo-annelation. The NICS values do not always support the results obtained by the other aromaticity indices and, in some cases, lead to completely opposite conclusions.
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what is the effect of annulation on aromaticity ? Related: Craig’s Ru...
Effect of Annulation on Aromaticity:
Annulation refers to the process of fusing two or more rings together to form a larger ring system. This process can have a significant effect on the aromaticity of the resulting compound. Aromaticity is a property of certain organic compounds that exhibit a high degree of stability and unique reactivity due to the presence of a conjugated π-electron system.
1. Introduction to Aromaticity:
Aromatic compounds possess a cyclic, planar, and conjugated π-electron system, which fulfills the following criteria:
- The compound must be cyclic, meaning it must form a complete ring of atoms.
- The compound must be planar, ensuring that all atoms lie in the same plane.
- The compound must possess a continuous π-electron system, where the π-electrons are delocalized over the entire ring.
- The compound must follow Hückel's rule, which states that the number of π-electrons in the system must be 4n + 2, where n is an integer.
2. Craig's Rule:
Craig's rule, proposed by James Craig, provides a way to determine the aromaticity of annulated ring systems. According to this rule, a ring that is fused to an aromatic ring will also be aromatic if the number of π-electrons in the fused ring plus the number of π-electrons in the aromatic ring satisfies Hückel's rule (4n + 2).
3. Effect of Annulation on Aromaticity:
When two or more rings are fused together, the resulting compound may or may not be aromatic. The effect of annulation on aromaticity depends on the number of π-electrons present in the fused ring system.
- If the fused ring system has 4n + 2 π-electrons, it will be aromatic regardless of the aromaticity of the individual rings. This is known as the "through-space stabilization" effect, where the aromaticity of one ring stabilizes the other ring(s) through conjugation.
- If the fused ring system does not have 4n + 2 π-electrons, it will not be aromatic. The aromaticity of the individual rings is disrupted due to the lack of conjugation between them.
4. Examples:
- Benzene fused with a five-membered ring (cyclopentadienone) results in a six-membered aromatic ring system called benzofuran. Both the benzene and cyclopentadienone rings individually have 6 π-electrons, and their fusion satisfies Hückel's rule (6 + 6 = 12 = 4n + 2). Therefore, benzofuran is aromatic.
- Benzene fused with a six-membered ring (cyclohexene) results in a seven-membered non-aromatic ring system called naphthalene. The benzene ring has 6 π-electrons, and the cyclohexene ring has 4 π-electrons. The total number of π-electrons in naphthalene is 10, which does not satisfy Hückel's rule (10 ≠ 4n + 2). Hence, naphthalene is not aromatic.
Conclusion:
Annulation can have a profound impact on the aromaticity of a compound. Fusing rings together can either maintain or disrupt aromaticity depending on