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