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Aromatic Hydrocarbon Cations: Structural Overview

Aromatic Hydrocarbon Cations: Structural Overview

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group with both...

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

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

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.

Nucleophilic Aromatic Substitution: Elimination–Addition

Nucleophilic Aromatic Substitution: Elimination–Addition

Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is confirmed through isotopic...

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...

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Capturing the elusive aromaticity of bicalicene.

W P Oziminski¹, M Palusiak, J Dominikowska

¹National Medicines Institute Laboratory of Theoretical Methods and Computation, 30/34 Chelmska, 00-725 Warsaw, Poland. wojozim@gmail.com

Physical Chemistry Chemical Physics : PCCP

|January 30, 2013

Summary

Bicalicene exhibits aromaticity due to strong local circulations in its pentagonal rings, not typical conjugated circuits. This arises from ionic contributions, challenging simple Hückel models.

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Area of Science:

Computational Chemistry
Theoretical Chemistry
Organic Chemistry

Background:

Aromaticity is a key concept in chemistry, influencing molecular stability and reactivity.
Bicalicene, a non-benzenoid hydrocarbon, presents a unique case for studying aromaticity due to its structure.
Understanding the electronic properties of bicalicene is crucial for its potential applications.

Purpose of the Study:

To investigate the origin of ring-current aromaticity in the bicalicene molecule.
To analyze the electronic circulation patterns and their contribution to aromaticity.
To explain the limitations of traditional models in describing bicalicene's aromaticity.

Main Methods:

Computation of current-density maps using ipsocentric RHF/6-311G** and DFT/6-311G** levels of theory.
Analysis of conjugated-circuit models and valence-bond theory.
Canonical molecular-orbital analysis, focusing on frontier-orbital contributions.

Main Results:

Strong local diatropic circulations on the pentagonal rings were identified as the source of bicalicene's aromaticity.
Conjugated-circuit models failed to capture the observed circulation pattern due to significant 'ionic' contributions.
Cancellation of paratropic and diatropic frontier-orbital contributions was observed, explaining difficulties with Hückel-based models.

Conclusions:

Bicalicene's aromaticity is primarily driven by local circulations and ionic contributions, deviating from simple conjugated models.
The study highlights the inadequacy of Hückel-based models for accurately predicting current-density maps in such systems.
Alternative measures of aromaticity confirm the dominant 'tetraionic' contribution to bicalicene's aromatic character.