Explain why each compound or ion should be aromatic, antiaromatic, or nonaromatic.
(a)
(b)
(c)

Verified step by step guidance

1. Step 1: Recall the criteria for aromaticity. A compound is aromatic if it satisfies the following conditions: (a) It is cyclic, (b) It is planar, (c) It has a conjugated π-electron system, and (d) It follows Hückel's rule, which states that the molecule must have (4n + 2) π-electrons, where n is a non-negative integer.
2. Step 2: Analyze compound (a), the cyclononatetraene cation. This molecule is cyclic and conjugated, but it has 8 π-electrons (from 4 double bonds). Since 8 does not satisfy Hückel's rule (4n + 2), the molecule is antiaromatic if it is planar. If it is non-planar, it would be nonaromatic.
3. Step 3: Analyze compound (b), the cyclononatetraene anion. This molecule is cyclic and conjugated, and it has 10 π-electrons (from 4 double bonds and 2 electrons from the negative charge). Since 10 satisfies Hückel's rule (4n + 2, where n = 2), the molecule is aromatic if it is planar.
4. Step 4: Analyze compound (c), the [16]annulene dianion. This molecule is cyclic and conjugated, and it has 18 π-electrons (from 8 double bonds and 2 electrons from the negative charges). Since 18 satisfies Hückel's rule (4n + 2, where n = 4), the molecule is aromatic if it is planar. However, steric hindrance due to the hydrogen atoms inside the ring may affect planarity.
5. Step 5: Summarize the findings. Compound (a) is antiaromatic if planar, compound (b) is aromatic if planar, and compound (c) is aromatic if planar but may face steric hindrance affecting its aromaticity.

Key Concepts

Aromaticity

Aromatic compounds are cyclic, planar molecules with a ring of resonance that follow Hückel's rule, which states they must have 4n + 2 π electrons (where n is a non-negative integer). This delocalization of electrons contributes to their stability and unique chemical properties. Common examples include benzene and its derivatives.

Antiaromaticity

Antiaromatic compounds are also cyclic and planar but contain 4n π electrons, leading to destabilization due to the presence of electron-electron repulsion. This instability often results in higher reactivity compared to nonaromatic compounds. Cyclobutadiene is a classic example of an antiaromatic compound.

Nonaromaticity

Nonaromatic compounds do not meet the criteria for aromaticity or antiaromaticity. They may be acyclic, lack planarity, or have an insufficient number of π electrons. These compounds do not exhibit the special stability associated with aromatic systems, and their reactivity is typically similar to that of aliphatic compounds.