(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.
(ii) For aromatic molecules, solve for n in Hückel’s rule. For all other molecules, explain which rule of aromaticity is being broken.
(b)

Verified step by step guidance

1. Identify the structure: The molecule is a five-membered ring with a nitrogen atom and two double bonds, known as pyrrole.
2. Determine the number of π-electrons: In pyrrole, the nitrogen atom contributes two electrons from its lone pair to the π-system, making a total of 6 π-electrons (4 from the double bonds and 2 from the nitrogen).
3. Apply Hückel's rule: For a molecule to be aromatic, it must have (4n + 2) π-electrons, where n is a non-negative integer. Check if 6 fits this formula.
4. Solve for n in Hückel's rule: Set up the equation 4n + 2 = 6 and solve for n to determine if the molecule is aromatic.
5. Conclude the classification: If the molecule satisfies Hückel's rule with an integer value of n, it is aromatic. Otherwise, determine if it is nonaromatic or antiaromatic based on the failure to meet the criteria of planarity or conjugation.

Key Concepts

Aromaticity

Aromaticity is a property of cyclic, planar structures with a ring of resonance bonds that leads to enhanced stability. For a molecule to be aromatic, it must follow Hückel's rule, which requires a certain number of π-electrons (4n + 2, where n is a non-negative integer) in the conjugated system. This concept is crucial for determining whether a molecule is aromatic, nonaromatic, or antiaromatic.

Hückel's Rule

Hückel's rule is used to determine if a planar ring molecule will have aromatic properties. According to this rule, a molecule is aromatic if it has 4n + 2 π-electrons, where n is a non-negative integer. This rule helps in identifying the stability and reactivity of the molecule, as aromatic compounds are generally more stable due to delocalized electrons.

Nonaromatic and Antiaromatic Compounds

Nonaromatic compounds do not meet the criteria for aromaticity, often due to lack of planarity or an incorrect number of π-electrons. Antiaromatic compounds, on the other hand, are cyclic and planar but have 4n π-electrons, leading to instability. Understanding these distinctions is essential for classifying molecules and predicting their chemical behavior.