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Aromatic Compounds: Overview

Aromatic Compounds: Overview

In general, the term ‘aromatic’ indicates a pleasant smell or fragrance from fresh flowers, freshly prepared coffee, etc. In the early history of organic chemistry, many benzene derivatives were isolated from the pleasant odor oils of the plants. For example, vanillin was isolated from the oil of vanilla, methyl salicylate from the oil of wintergreen, and cinnamaldehyde from the oil of cinnamon. They all had a pleasant odor; hence the name aromatic was given.
In 1825, Faraday isolated benzene...

Five-Membered Heterocyclic Aromatic Compounds: Overview

Five-Membered Heterocyclic Aromatic Compounds: Overview

Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom, respectively.

Criteria for Aromaticity and the Hückel 4n + 2 Rule

Criteria for Aromaticity and the Hückel 4n + 2 Rule

Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n + 2 rule.

Frost Circles for Different Conjugated Systems

Frost Circles for Different Conjugated Systems

The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.

Aromatic Hydrocarbon Anions: Structural Overview

Aromatic Hydrocarbon Anions: Structural Overview

Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous overlap of p...

Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products.

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Versatile CO2 Transformations into Complex Products: A One-pot Two-step Strategy

Versatile CO₂ Transformations into Complex Products: A One-pot Two-step Strategy

Published on: November 9, 2019

Engineering discrete stacks of aromatic molecules.

Jeremy K Klosterman¹, Yoshihiro Yamauchi, Makoto Fujita

¹Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

Chemical Society Reviews

|July 10, 2009

Summary

Chemists engineer discrete aromatic molecule stacks using self-assembly. This review covers covalent scaffolds and modern non-covalent strategies for controlled aromatic stacking.

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Area of Science:

Supramolecular Chemistry
Organic Chemistry

Background:

Chemists aim to create precisely controlled stacks of aromatic molecules.
Historically, covalent scaffolds were used, but challenges remain in achieving specific lengths and orientations.
Aromatic molecules tend to form ill-defined, infinite aggregates.

Purpose of the Study:

To review covalent scaffolds for aromatic stacking.
To focus on modern self-assembly strategies for engineering discrete aromatic stacks.
To highlight the use of non-covalent interactions in controlling molecular assembly.

Main Methods:

Review of existing literature on covalent scaffolds for aromatic stacking.
Analysis of modern self-assembly techniques utilizing non-covalent interactions.
Discussion of strategies combining multiple self-assembly methods.

Main Results:

Covalent scaffolds offer a historical approach to aromatic stacking.
Non-covalent interactions (ionic, hydrogen bonds, metal-ligand, aromatic) are key to modern self-assembly.
Combining self-assembly techniques is crucial for achieving desired stack size and conformation.

Conclusions:

Self-assembly methodologies have advanced the engineering of discrete aromatic stacks.
Non-covalent interactions provide versatile tools for controlling molecular assembly.
Further research into combined self-assembly techniques will enable precise control over aromatic supramolecular structures.