Explain why each compound is aromatic, antiaromatic, or nonaromatic.
(g)
(h)

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 Huckel's rule, which states that the molecule must have (4n + 2) π-electrons, where n is an integer.
2. Step 2: Analyze compound (g), cytosine. Cytosine is a six-membered ring containing nitrogen and oxygen atoms. The ring is cyclic and planar due to sp2 hybridization of the atoms in the ring. The conjugated π-electron system includes the double bonds and lone pairs on the heteroatoms. Count the π-electrons to determine if it satisfies Huckel's rule.
3. Step 3: For compound (g), cytosine, consider the lone pairs on the nitrogen and oxygen atoms. Not all lone pairs contribute to aromaticity; only those in conjugation with the π-system are considered. Verify if the total π-electron count is (4n + 2). If it satisfies Huckel's rule, the compound is aromatic; if not, it is nonaromatic or antiaromatic.
4. Step 4: Analyze compound (h), a six-membered ring with oxygen. The ring is cyclic and planar, and it has alternating double bonds, indicating a conjugated π-electron system. Count the π-electrons, including the lone pair on oxygen, to determine if it satisfies Huckel's rule.
5. Step 5: For compound (h), verify if the lone pair on oxygen contributes to the conjugated π-system. If the total π-electron count satisfies Huckel's rule, the compound is aromatic. If it does not satisfy Huckel's rule, determine if the compound is antiaromatic (4n π-electrons) or nonaromatic (does not meet the criteria for 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 in the ring. This instability often results in higher reactivity compared to nonaromatic compounds. An example of an antiaromatic compound is cyclobutadiene.

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 typically behave like aliphatic compounds. An example is cyclohexane.