Question

Which one of the following is not aromatic? (1) cyclopentadienyl anion (2) cyclohepta trienyl cation (3) cyclooctatetraene (4) triophene

Short answer

Cyclooctatetraene is not aromatic.

Step-by-step solution

- Define Aromaticity

According to Hückel's rule, a molecule is considered aromatic if it is cyclic, planar, fully conjugated, and has \(4n+2\) π-electrons, where n is a non-negative integer (0, 1, 2, ...).

- Analyze Cyclopentadienyl Anion

Cyclopentadienyl anion has 6 π-electrons (considering the lone pair on the carbon with a negative charge), satisfying the \(4n+2\) rule with n=1. It is also cyclic, planar, and fully conjugated. Hence, it is aromatic.

- Analyze Cyclohepta Trienyl Cation

Cyclohepta trienyl cation has 6 π-electrons. It follows Hückel's rule \(4n+2\) with n=1. It is cyclic, planar, and fully conjugated, making it aromatic.

- Analyze Cyclooctatetraene

Cyclooctatetraene has 8 π-electrons, which does not satisfy Hückel's \(4n+2\) rule. Instead, it follows \(4n\) rule with n=2. Additionally, it is not planar; it adopts a tub-shaped conformation to avoid antiaromaticity. Hence, it is non-aromatic.

- Analyze Thiophene

Thiophene is a five-membered ring with a sulfur atom. It has 6 π-electrons, satisfying the \(4n+2\) rule with n=1. It is also cyclic, planar, and fully conjugated. Thus, it is aromatic.

Key concepts

Hückel's rule

Hückel's rule is a fundamental principle in the field of organic chemistry relating to aromaticity. It states that for a molecule to be considered aromatic, it must meet the following criteria: it must be cyclic, planar, fully conjugated, and contain a total of \( 4n + 2 \) π-electrons, where n is a non-negative integer (0, 1, 2, ...).

A molecule's planarity allows the p orbitals to overlap continuously, creating a ring of delocalized π-electrons. The \( 4n + 2 \) formula ensures that the number of π-electrons is optimal for delocalization, leading to greater stability.

For example:

• When n = 0, \( 4 \times 0 + 2 = 2 \) π-electrons
• When n = 1, \( 4 \times 1 + 2 = 6 \) π-electrons
• When n = 2, \( 4 \times 2 + 2 = 10 \) π-electrons

These properties give aromatic compounds unique stability compared to non-aromatic or anti-aromatic compounds, making Hückel's rule a valuable tool for predicting aromaticity.

Cyclopentadienyl Anion

The cyclopentadienyl anion is a five-carbon ring with one carbon bearing a negative charge. This anion has 6 π-electrons from five p orbitals and one additional lone pair of electrons on the negatively charged carbon.

Applying Hückel's rule: \( 4n + 2 = 6 \)
Solving for n, we get n = 1, thus confirming aromaticity.

The key features include:

• Cyclic structure
• Planarity ensuring orbital overlap
• Fully conjugated system with alternating double bonds
• 6 π-electrons fitting the \( 4n + 2 \) rule

These properties allow the cyclopentadienyl anion to exhibit aromatic stability, essential for reactions like Diels-Alder and Friedel-Crafts.

Cyclohepta Trienyl Cation

The cyclohepta trienyl cation is a seven-membered ring with a positive charge. This cation possesses 6 π-electrons from the six p orbitals with one electron missing due to the positive charge.

Applying Hückel's rule: \( 4n + 2 = 6 \)
Solving for n, gives n = 1, fulfilling Hückel's criterion.

Characteristics include:

• Cyclic structure
• Planar configuration
• Fully conjugated alternating double bonds
• 6 π-electrons adhering to the \( 4n + 2 \) rule

These factors ensure the cyclohepta trienyl cation is aromatic, contributing to its reactivity and stability in organic reactions.

Cyclooctatetraene

Cyclooctatetraene is an eight-membered ring containing four double bonds, giving it 8 π-electrons. However, it does not satisfy Hückel's rule for aromaticity.

Analyzing its π-electrons: Eight π-electrons fit the formula \( 4n = 8 \) with n = 2, which means it follows the \( 4n \) rule rather than \( 4n + 2 \).

Key points about cyclooctatetraene:

• Non-planar, adopting a tub-shaped structure to avoid anti-aromaticity
• Failure to meet the \( 4n + 2 \) π-electron requirement

Therefore, cyclooctatetraene is non-aromatic due to its inability to maintain a planar structure and optimal π-electron delocalization, thus lacking the distinctive stability of aromatic compounds.