(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.
(a)

Verified step by step guidance

Identify the structure: The image shows a four-membered ring with a nitrogen atom, which is a cyclobutadiene with a nitrogen atom replacing one of the carbon atoms.

Determine the number of π-electrons: In this structure, there are two π-electrons from the double bond.

Apply Hückel's rule: Aromatic compounds must have (4n + 2) π-electrons, where n is a non-negative integer. Check if 2 fits this formula.

Check planarity and conjugation: For a molecule to be aromatic, it must be planar and fully conjugated. Cyclobutadiene is planar and conjugated.

Classify the molecule: Since the molecule has 4 π-electrons, which fits the 4n rule (n=1), it is antiaromatic. Anti-aromatic compounds are planar, cyclic, and have 4n π-electrons.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Aromaticity

Aromaticity refers to a property of cyclic compounds that exhibit enhanced stability due to the delocalization of π electrons across the ring structure. For a molecule to be classified as aromatic, it must be cyclic, planar, fully conjugated, and follow Hückel's rule, which states that the number of π electrons must equal 4n + 2, where n is a non-negative integer.

Hückel's Rule

Hückel's rule is a criterion used to determine the aromaticity of a compound. It states that a planar, cyclic molecule is aromatic if it contains 4n + 2 π electrons in its conjugated system. This rule helps in identifying aromatic compounds by calculating the total number of π electrons and checking if it fits the formula, thus providing a quantitative measure of aromatic stability.

Anti-aromaticity

Anti-aromaticity is a property of cyclic compounds that leads to instability due to the presence of 4n π electrons, which is contrary to Hückel's rule. Such compounds are typically less stable than their non-aromatic counterparts because the electron delocalization does not confer the same level of stabilization. Identifying anti-aromatic compounds involves recognizing the cyclic structure and counting the π electrons to see if they fit the 4n pattern.

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Related Practice

Textbook Question

(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.

(d)

Textbook Question

Predict the products of the following reactions.

(a)