The following molecules and ions are grouped by similar structures. Classify each as aromatic, antiaromatic, or nonaromatic. For the aromatic and antiaromatic species, give the number of pi electrons in the ring.
(c)

Verified step by step guidance

1. Step 1: Analyze the structure of each molecule or ion. Determine whether the ring is cyclic, planar, and conjugated (i.e., has alternating single and double bonds or lone pairs that can participate in resonance). These are prerequisites for aromaticity or antiaromaticity.
2. Step 2: Apply Hückel's rule for aromaticity, which states that a molecule is aromatic if it has (4n + 2) π electrons, where n is a non-negative integer. Count the π electrons in the ring for each structure.
3. Step 3: For antiaromatic species, check if the molecule has 4n π electrons, which leads to instability due to electron repulsion. If neither (4n + 2) nor 4n π electrons are present, the molecule is nonaromatic.
4. Step 4: For each structure, consider the effect of substituents (e.g., lone pairs, charges) on the π electron count. For example, a negatively charged substituent may contribute two π electrons, while a positively charged substituent may remove electrons from the π system.
5. Step 5: Classify each structure as aromatic, antiaromatic, or nonaromatic based on the π electron count and the criteria above. For aromatic and antiaromatic species, specify the number of π electrons in the ring.

Key Concepts

Aromaticity

Aromatic compounds are cyclic, planar molecules with a ring of p-orbitals that allows for delocalization of pi electrons. According to Hückel's rule, a molecule is aromatic if it contains 4n + 2 pi electrons, where n is a non-negative integer. This delocalization contributes to the stability and unique reactivity of aromatic compounds.

Antiaromaticity

Antiaromatic compounds are also cyclic and planar but contain 4n pi electrons, which leads to instability due to the unfavorable electron delocalization. This instability results in higher reactivity compared to nonaromatic compounds. Understanding the distinction between aromatic and antiaromatic systems is crucial for predicting the behavior of these molecules.

Nonaromaticity

Nonaromatic compounds do not meet the criteria for aromaticity or antiaromaticity. They may be acyclic, lack planarity, or have an insufficient number of pi electrons for delocalization. Nonaromatic compounds typically exhibit standard reactivity patterns and do not benefit from the stability associated with aromatic systems.