Question

Indole is an aromatic heterocycle that has a benzene ring fused to a pyrrole ring. Draw an orbital picture of indole. (a) How many \(\pi\) electrons does indole have? (b) What is the electronic relationship of indole to naphthalene?

Short answer

Indole has 10 π electrons and is electronically similar to naphthalene.

Step-by-step solution

Understand the Indole Structure

Indole consists of two fused rings: a benzene ring and a pyrrole ring. Each carbon in the benzene ring has a p-orbital perpendicular to the plane of the ring. The pyrrole has a nitrogen with a lone pair also in a p-orbital, which can delocalize into the aromatic system.

Counting π Electrons in Benzene

A benzene ring has 6 π electrons. Each carbon atom contributes one π electron through its p-orbital.

Counting π Electrons in Pyrrole

Pyrrole contributes 4 π electrons: one from each of the four carbon atoms and the lone pair from nitrogen which also participates in the π system.

Total π Electrons in Indole

Add the π electrons from benzene and pyrrole. Indole has 10 π electrons: 6 from benzene and 4 from pyrrole.

Electronic Relationship of Indole to Naphthalene

Naphthalene, another aromatic compound, also has 10 π electrons due to its two fused benzene rings. Indole is electronically similar to naphthalene because both have the same number of π electrons, contributing to their stability and aromaticity.

Key concepts

Aromaticity

Aromaticity is a fascinating concept that explains why certain cyclic molecules are especially stable. This stability comes from the molecule having delocalized electrons above and below its plane. In simpler terms, aromatic molecules have a "cloud" of electrons that circle around the ring.

A molecule must meet specific criteria to be considered aromatic:

• It must be cyclic (a ring structure).
• Every atom in the ring must have a p-orbital, allowing for electron delocalization.
• It must follow Hückel's rule, which means it has \(4n + 2\) π electrons, with \(n\) being an integer.

Indole fits these criteria by having 10 π electrons, making it aromatic and thus stable.

Heterocycle

The term heterocycle refers to a ring structure that contains at least one atom other than carbon. In organic chemistry, this is quite important, as these non-carbon atoms often contribute unique properties to the molecule.

Indole is a classic example of a heterocycle, where the non-carbon atom is nitrogen. This nitrogen is part of the pyrrole ring, fused with a benzene ring.

• The aromatic nature of the indole's heterocycle leads to its distinct chemical behavior.
• Nitrogen's presence allows it to donate electrons, contributing to indole's aromatic system.

Understanding heterocycles is crucial for explaining the behavior of many biological molecules, like DNA nucleotides.

Pi Electrons

Pi electrons are the electrons that reside in the p-orbitals and participate in pi bonds. These electrons play a key role in the stability and reactivity of aromatic compounds.

In indole, the π electrons are found in both the benzene and pyrrole rings. Here's how they add up:

• The benzene ring provides 6 π electrons, with each carbon contributing one electron.
• The pyrrole ring contributes 4 π electrons: one from each carbon and two from the nitrogen’s lone pair.

This totaling of 10 π electrons confirms indole's aromaticity and electronic similarity to other aromatic compounds like naphthalene.

Naphthalene

Naphthalene is a well-known aromatic hydrocarbon consisting of two fused benzene rings. It serves as a benchmark for understanding other aromatic systems like indole. Just like indole, naphthalene has 10 π electrons, which contribute to its stability.

The electronic relationship between indole and naphthalene is founded on this shared count of π electrons.

• The same number of π electrons in both compounds ensures similar aromatic stability.
• This similarity allows comparisons in their chemical properties and reactions.

Understanding this relationship helps chemists predict how indole and naphthalene might behave in different chemical environments.