Problem 89 - Chemistry: The Central Science in SI Units, Global Edition

Question

Indole smells terrible in high concentrations but has a pleasant floral-like odor when highly diluted. Its structure is The molecule is planar, and the nitrogen is a very weak base, with \(K_{b}=2 \times 10^{-12}\). Explain how this information indicates that the indole molecule is aromatic.

Step-by-Step Solution

Answer

The indole molecule is aromatic because its structure is planar and contains a conjugated pi electron system, with 12 pi electrons satisfying Huckel's rule (4n+2, where n=2). The low basicity of the nitrogen atom in indole (\(K_{b} = 2 \times 10^{-12}\)) indicates that its lone pair of electrons is involved in resonance, further supporting its aromaticity. Additionally, the odor characteristics of indole, which range from unpleasant at high concentrations to pleasant and floral-like when highly diluted, can be considered an indirect indication of its aromatic nature.

Step 1: Understanding Aromaticity

Aromatic compounds are characterized by a planar, cyclic and conjugated structure where the delocalization of pi electrons leads to additional stability. To be aromatic, the molecule needs to satisfy Huckel's rule, which states that an aromatic molecule must have 4n+2 pi electrons, where n is a whole number.

Step 2: Examining the Structure of Indole

The structure of indole consists of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. The molecule is planar, meaning that all of its atoms lie in the same plane. The presence of alternating single and double bonds in both rings of indole indicates that the molecule has a conjugated system of pi electrons.

Step 3: Counting Pi Electrons in Indole

To determine whether indole is aromatic, we need to count the number of pi electrons in the molecule. In the benzene ring, there are 6 pi electrons from the 3 double bonds. In the pyrrole ring, there are 4 pi electrons from the 2 double bonds and 2 pi electrons from the lone pair of the nitrogen atom. This gives a total of 12 pi electrons, which indeed satisfies Huckel's rule (4n+2, where n=2).

Step 4: Considering the Nitrogen's Basicity

The nitrogen atom in indole is a very weak base, with a given \(K_{b} = 2 \times 10^{-12}\). In aromatic compounds, the lone pair of electrons on a heteroatom (such as nitrogen) is often involved in the molecule's resonance and is therefore less available to accept a proton. This is consistent with the low basicity of the nitrogen atom in indole.

Step 5: Odor Characteristics of Indole

Indole has a strong and unpleasant odor at high concentrations but a pleasant floral-like odor when highly diluted. Aromatic compounds often have characteristic odors due to their strong electron cloud, which can interact with receptors in our olfactory system. The odor characteristics of indole can be considered an indirect indication of its aromaticity.

Step 6: Conclusion

Based on the planar structure, conjugated pi electron system, the presence of 12 pi electrons satisfying Huckel's rule, low basicity of nitrogen, and odor characteristics, we can conclude that the indole molecule is indeed aromatic.

Key Concepts

Huckel's Rule

Huckel's rule is a fundamental principle in organic chemistry that helps determine whether a cyclic molecule will exhibit aromaticity. This rule states that for a molecule to be aromatic, it must have a specific number of pi electrons, following the formula \(4n+2\), where \(n\) is a non-negative integer.
In simpler terms, the number of pi electrons in an aromatic molecule will always be 2, 6, 10, 14, and so on.
Pi electrons are related to the double bonds within the molecule's structure, contributing to a stable electron delocalization.
This rule is key because molecules with aromatic character are notably more stable.

• Aromatic systems follow a cyclic and planar structure to allow electrons to delocalize effectively.
• The condition \(4n+2\) ensures the electrons are adequately spaced for ideal resonance.

In the case of indole, we can calculate its pi electrons to see if it meets this criterion. Indole has 12 pi electrons, which fits Huckel's rule with \(n=2\) (since \(4 \times 2 + 2=12\)).
Indole's satisfaction of Huckel's rule confirms its aromatic nature.

Aromatic Compounds

Aromatic compounds are a special class of organic molecules known for their stability and unique properties arising from electron delocalization. These compounds must be cyclic and planar, and they must have a conjugated system of pi electrons.
Aromaticity provides these molecules with increased stability compared to non-aromatic forms.

• This stability is due to electronic resonance within the cyclic arrangement of atoms.
• Common examples of aromatic compounds include benzene and naphthalene.

One notable property of many aromatic compounds is their distinct odors, which is why indole, an aromatic compound, can smell floral when diluted.
This scent characteristic is linked to how aromatic compounds interact with olfactory receptors.
In general, while indole may not smell pleasant in high concentrations, its diluted form shows the typical aromatic odor quality.

Pi Electrons

Pi electrons are the electrons that are found in pi bonds, which are a result of the sideways overlap of p orbitals in conjugated systems. In aromatic compounds, pi electrons play a crucial role in enabling resonance and delocalization, giving these molecules their characteristic stability.
Each double bond in a molecule contributes two pi electrons to the overall system.

• The benzene ring in indole donates 6 pi electrons from its 3 double bonds.
• The pyrrole ring contributes another 4 pi electrons from its 2 double bonds, plus 2 from the lone pair on nitrogen.

Therefore, in indole, we count a total of 12 pi electrons, which aligns perfectly with Huckel's rule.
These pi electrons create a delocalized electronic "cloud" over the molecule, which enhances stability and aromatic character.
By ensuring the electrons are shared across the rings, indole achieves increased stabilization.

Conjugated Systems

Conjugated systems refer to a series of alternating single and multiple bonds in a molecular structure. This setup allows for the overlap of p orbitals, thereby enabling electrons to be delocalized across the entire molecule. Delocalization in conjugated systems leads to aromatic stability.
Indole, for instance, contains a benzene and a pyrrole ring, both contributing to its conjugated system.

• Conjugation involves a seamless flow of electrons across the rings, conducted through alternating single and double bonds.
• This electron flow results in energy lowering and imparts additional stability to the molecule.

The conjugated nature of indole's structure aids in distributing the 12 pi electrons evenly across both cyclic components.
Indole's planarity enhances this delocalization, confirming its aromaticity.
This character not only contributes to its chemical stability but also influences its interaction with biological systems and sensory responses, such as odor detection.