Question

For a compound to be aromatic, all of the following must be true EXCEPT: a. the molecule is cyclic. b. the molecule contains4n+ 2\pielectrons. c. the molecule contains alternating single and double bonds. d. the molecule is planar.

Short answer

c. the molecule contains alternating single and double bonds.

Step-by-step solution

Understand Aromaticity

Aromatic compounds satisfy Huckel's Rule, which states the molecule must be cyclic, planar, have a fully conjugated system (alternating single and double bonds), and contain 4n + 2 (π) electrons where n is a non-negative integer.

Analyze each condition

a. The molecule is cyclic - This is a required condition for aromaticity. Aromatic molecules are cyclic. b. The molecule contains (4n + 2)π electrons - This is Huckel's Rule. Aromatic molecules have this feature. c. The molecule contains alternating single and double bonds - This is not explicitly required by Huckel’s Rule. Conjugation (a system of alternating π bonds) is required, but they do not strictly have to alternate. d. The molecule is planar - Aromatic molecules must be planar to allow for delocalization of π electrons.

Determine the exception

From the analysis, (c) is not a strict requirement because compounds can still follow Huckel's Rule through other forms of conjugation, not strictly through alternating single and double bonds.

Conclusion

All conditions except (c) must be true for a compound to be aromatic.

Key concepts

Huckel's Rule

To determine if a compound is aromatic, one of the most important criteria is following **Huckel's Rule**. This rule helps us to understand the unique stability of aromatic compounds. Here are the key elements of Huckel's Rule:

• The molecule must be cyclic, meaning it forms a ring structure.
• The molecule must be **planar**, allowing the p orbitals to overlap effectively.
• It has to have a fully **conjugated** π system (alternating single and double bonds all around the ring).
• The number of π electrons must fit the formula **4n + 2**, where n is a non-negative integer (0, 1, 2, 3, ...). This specific electron count is what gives aromatic molecules their extra stability.

Imagine molecules like benzene, where these criteria are met perfectly, giving it remarkable stability compared to other cyclic compounds.

Planarity

Another crucial aspect of aromatic compounds is their **planarity**, meaning all of the atoms in the molecule must lie in a single plane. Planarity is essential for the molecule to have effective overlap of p orbitals.

This overlap allows π electrons to delocalize over the entire ring, contributing to the molecule's stability. For example, benzene is planar, allowing for seamless overlap of its p orbitals, delocalizing the π electrons evenly over the ring.
Without planarity, the p orbitals cannot align properly, disrupting the necessary electron delocalization. As a result, the molecule would not be aromatic.

Conjugation

When we talk about **conjugation**, we're referring to the system of alternating single and double bonds within a molecule. This allows for p orbital overlap and the creation of a π system.

Conjugated systems have a cloud of π electrons that are delocalized across all the atoms involved in the conjugation. Take benzene as an example: it has alternating single and double bonds, forming a conjugated π system.
An important clarification is that, while alternating single and double bonds are common, it is the conjugation—the continuous overlap of p orbitals—that is crucial for aromaticity, not the strict alternation of bonds. Therefore, some aromatic compounds might have different patterns of electron delocalization but still maintain conjugation.