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Lecture

Aromaticity: Energetic Evaluations

In course

CH-431: Physical and computational organic chemistry

This course introduces modern computational electronic structure methods and their broad applications to organic chemistry. It also discusses physical organic concepts to illustrate the stability and reactivity of organic molecules. ...

Description

This lecture delves into the concept of aromaticity, exploring the energy profiles of various compounds and the resonance energies involved. It discusses the stability of aromatic compounds, the resonance energy versus aromatic stabilization energy, and the methods used to measure aromaticity, such as Nucleus-Independent Chemical Shifts (NICS) and magnetic criteria. The presentation also covers the multifaceted definition of aromaticity, quantifying antiaromaticity, and identifying aromatic molecules.

Instructor

Anne-Clémence Corminboeuf

Official source

Related concepts (32)

Aromatic ring current

An aromatic ring current is an effect observed in aromatic molecules such as benzene and naphthalene. If a magnetic field is directed perpendicular to the plane of the aromatic system, a ring current is induced in the delocalized π electrons of the aromatic ring. This is a direct consequence of Ampère's law; since the electrons involved are free to circulate, rather than being localized in bonds as they would be in most non-aromatic molecules, they respond much more strongly to the magnetic field.

Aromaticity

In chemistry, aromaticity means the molecule has cyclic (ring-shaped) structures with pi bonds in resonance (those containing delocalized electrons). Aromatic rings give increased stability compared to saturated compounds having single bonds, and other geometric or connective non-cyclic arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have enhanced stability.

Möbius aromaticity

In organic chemistry, Möbius aromaticity is a special type of aromaticity believed to exist in a number of organic molecules. In terms of molecular orbital theory these compounds have in common a monocyclic array of molecular orbitals in which there is an odd number of out-of-phase overlaps, the opposite pattern compared to the aromatic character to Hückel systems. The nodal plane of the orbitals, viewed as a ribbon, is a Möbius strip, rather than a cylinder, hence the name.

Antiaromaticity

Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i.e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule ([4n+2] π electrons) and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions.

Conjugated system

In theoretical chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability. It is conventionally represented as having alternating single and multiple bonds. Lone pairs, radicals or carbenium ions may be part of the system, which may be cyclic, acyclic, linear or mixed. The term "conjugated" was coined in 1899 by the German chemist Johannes Thiele.

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Molecular Orbitals in Conjugated Systems

Explores molecular orbitals in conjugated systems, covering bond lengths, energy barriers, electronic interactions, and aromaticity.

Reaction Mechanisms: Aromaticity Insights

Explores reaction mechanisms with a focus on aromaticity, NICS method, ring strain, and potential energy surfaces.

Advanced General Chemistry: Organic Compounds and Molecular Properties

Explores organic compounds, molecular properties, and colligative properties of solutions in advanced general chemistry.

Chemical Shifts: Basics and Applications

Explores the basics of chemical shifts in nuclear magnetic resonance and their diverse applications in various compounds and environments.

Organic Chemistry: Delocalization and Resonance Structures