Problem 123 - A Complete Resource Book in Chemistry for JEE Main

Question

Cyclooctatetraene is not aromatic. The most important reason for this is that (a) it is a planar molecule (b) it has eight \(\pi\)-electrons (c) its structure cannot be described by more than the canonical forms (d) its structure is not that of regular octagon

Step-by-Step Solution

Answer

Cyclooctatetraene is not aromatic because it has eight \(\pi\)-electrons, not satisfying Hückel's rule (option b).

Step 1: Understanding Aromaticity

Aromatic compounds are cyclic, planar molecules with a continuous ring of p-orbitals and follow Hückel's rule (having \(4n + 2\) \(\pi\)-electrons, where \(n\) is an integer). This rule is known as Hückel's rule.

Step 2: Analyzing Cyclooctatetraene

Cyclooctatetraene has eight \(\pi\)-electrons. According to Hückel's rule, for a molecule to be aromatic, it needs to have \(4n + 2\) \(\pi\)-electrons. Eight does not satisfy the \(4n + 2\) rule because any \(n\) would result in an uneven number of electrons needed for aromaticity.

Step 3: Conclusion Based on Hückel's Rule

Since cyclooctatetraene has \(8\) \(\pi\)-electrons, it does not fit Hückel's rule for aromaticity. Option (b) is the correct reason it is not aromatic. The key reason is it does not have \(4n + 2\) \(\pi\)-electrons.

Key Concepts

Cyclooctatetraene

Cyclooctatetraene is a fascinating molecule, especially when exploring the topic of aromaticity. It is an organic compound composed of eight carbon atoms forming a ring, each connected by alternating single and double bonds. This ring formation might suggest similarity to benzene which is aromatic, but cyclooctatetraene tells a different story.
It defies expectations of aromaticity due to its lack of planarity, a key requirement for being aromatic. Unlike planar, aromatic compounds, cyclooctatetraene adopts a tub-like conformation. This non-planar shape means that its electrons are not delocalized in a continuous loop above and below the plane of the molecule, further separating it from typical aromatic compounds.

Hückel's rule

When determining if a molecule is aromatic, Hückel's rule offers a straightforward method. This rule states that a molecule must have \(4n + 2\) electrons to be considered aromatic, where \(n\) is any non-negative integer. This gives you numbers like 2, 6, 10, 14, etc., for electron counts.
In the case of cyclooctatetraene, the molecule has eight electrons. If we apply Hückel's rule, placing 8 into the formula \(4n + 2\), we cannot find a real integer \(n\) that satisfies this equation. The closest would be \(n=1\), which results in 6, or \(n=2\), which results in 10. Thus, cyclooctatetraene violates Hückel's rule. Its electron count simply does not allow it to be aromatic, no matter how we might wish otherwise.

Planarity in molecules

Planarity is a crucial feature in the structure of aromatic molecules. It refers to the molecule lying flat in a single plane. This structural feature allows p-orbitals to overlap effectively, which is important for electron delocalization, a characteristic trait of aromatic compounds.
Cyclooctatetraene, however, is not planar. It distorts into a boat shape which prevents all p-orbitals from continuously overlapping. This distortion prevents proper electron delocalization as seen in aromatic systems. As a result, cyclooctatetraene lacks the electron cloud typically seen in aromatic species, reinforcing its non-aromatic nature.

π-electron systems

The term -electron systems describes the presence of electrons in pi bonds which are typically found in conjugated systems. These electrons reside in p-orbitals that can overlap to allow electron delocalization, a hallmark of aromaticity if all other conditions are met.
In aromatic molecules, these -electrons form a stable, delocalized electron cloud over the entire molecule. However, for cyclooctatetraene, having eight -electrons breaks one of the major rules defined by Hückel. Because they do not follow the \(4n + 2\) rule, these -electrons fail to establish the aromatic stability seen in other compounds like benzene. Therefore, instead of stabilizing the molecule, in cyclooctatetraene, the -electrons contribute to it adopting a structure that avoids planarity and maintains non-aromatic behavior.