Name: using frost circles to test aromaticity
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 18 s
Channel: JoVE

Según la regla de Huckel, el benceno es estable y aromático, mientras que el ciclobutadieno y el ciclooctatetraeno son inestables y no aromáticos.
La aromaticidad de estos compuestos se puede entender a partir de sus diagramas de círculos de Frost.
El círculo de Frost para el benceno tiene seis orbitales moleculares π. Todos los orbitales de enlace de baja energía están completamente ocupados con seis electrones de π, mientras que los orbitales de enlace de alta energía están vacíos. Por lo tanto, el benceno es estable y aromático.
Los círculos de Frost para el ciclobutadieno y el ciclooctatetraeno tienen cuatro y ocho orbitales moleculares π, respectivamente.
Los orbitales de enlace están completamente ocupados para ambos compuestos, mientras que los orbitales de no enlace están ocupados individualmente. Los orbitales antienlazantes están vacíos.
Como es evidente, la presencia de electrones π en orbitales no enlazantes de alta energía hace que los dos compuestos sean inestables y no aromáticos.
En general, una molécula dada se considera aromática cuando todos los electrones π están emparejados en orbitales moleculares de enlace. Los compuestos con electrones en orbitales distintos de los enlaces no son aromáticos.

círculos de escarcha para diferentes sistemas conjugados

Frost Circles for Different Conjugated Systems

El método del polígono inscrito es consistente con la regla 4n + 2 de Hückel y ayuda a saber si el compuesto cíclico dado es aromático o no. El compuesto es estable y aromático si cada orbital molecular (MO) de enlace está completamente lleno con un par de electrones. Sin embargo, si los orbitales no enlazantes o antienlazantes están llenos de electrones, el compuesto es inestable y no aromático. Considere los diagramas del círculo de Frost para cicloalquenos que contienen de 4 a 8 carbonos.

<img alt="Figure1" src="/files/ftp_upload/12866/12866_Figure_1.svg" style="text-align: center; height: 75px;">

 Al observar los círculos de Frost, se observa que el número de MO enlazantes siempre es impar, y el número requerido de electrones π en el MO enlazante es 2 o 6. El número de electrones satisface perfectamente la regla de aromaticidad 4n + 2. . Por lo tanto, los compuestos con MO de enlace completamente lleno son aromáticos.


 En el caso del ciclobutadieno de cuatro miembros con 4π electrones y del ciclooctatetraeno de ocho miembros con 8π electrones, los MO enlazantes están completamente llenos y cada uno de los MO no enlazantes está ocupado individualmente. La presencia de electrones en los MO no enlazantes de alta energía los hace inestables y no aromáticos. Sin embargo, el anión ciclopentadienilo de cinco miembros, el benceno de seis miembros y el catión cicloheptatrienilo de siete miembros tienen electrones 6π, y los MO de enlace correspondientes están completamente ocupados y son estables, por lo que son aromáticos.


 En general, las moléculas que tienen orbitales moleculares de enlace completamente llenos se consideran aromáticas, mientras que los compuestos con electrones en orbitales distintos de los de enlace no son aromáticos.

Círculos de escarcha para diferentes sistemas conjugados

El método del polígono inscrito es consistente con la regla 4n + 2 de Hückel y ayuda a aprender si el compuesto cíclico dado es aromático o no.

frost-circles-for-different-conjugated-systems

Education

Educación

JoVE Core

Organic Chemistry

Química Orgánica

Unit 1

Unidad 1

Aromatic Compounds

Compuestos aromáticos

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

using frost circles to test aromaticity

Frost circles help test whether a cyclic compound is aromatic by using the inscribed polygon method and Hückel’s 4n + 2 rule. A compound is stable and aromatic when every bonding molecular orbital (MO) is completely filled with a pair of electrons. If electrons occupy non-bonding or antibonding orbitals, the compound is unstable and not aromatic.

Frost circle diagrams for cycloalkenes with 4 to 8 carbons show a clear pattern. The number of bonding MOs is always odd. The electrons needed to fill only the bonding MOs are usually 2 or 6, and that matches the 4n + 2 rule for aromaticity. When those bonding orbitals are completely filled, the compound is aromatic.

Some examples do not fit this stable pattern. Cyclobutadiene has 4π electrons, and cyclooctatetraene has 8π electrons. In both cases, the bonding MOs are filled, but each non-bonding MO is singly occupied. Electrons in these higher-energy non-bonding orbitals make the compounds unstable and not aromatic.

Other examples do satisfy the aromatic pattern. The cyclopentadienyl anion, benzene, and the cycloheptatrienyl cation each have 6π electrons. Their bonding MOs are fully occupied, so they are stable and aromatic. In general, molecules with completely filled bonding molecular orbitals are aromatic, while molecules with electrons in orbitals other than bonding are not.

Using Frost Circles to Test Aromaticity

aromatic compounds: overview

Aromatic Compounds in Plants and Fuels

Aromatic compounds are linked to both pleasant smells and important chemical structures. The word aromatic first came from fragrant natural sources such as fresh flowers and spices. Early organic chemists isolated benzene derivatives from oils of plants, including vanillin from vanilla, methyl salicylate from wintergreen, and cinnamaldehyde from cinnamon. The history of benzene also helped shape the idea of aromatic compounds. In 1825, Faraday isolated benzene from compressed illuminating gas,...

nomenclature of aromatic compounds with a single substituent

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Naming Benzene Derivatives with One Group

Benzene derivatives with one substituent are named by attaching the substituent name to benzene. Benzene is the simplest aromatic hydrocarbon, also called an arene. For simple monosubstituted benzene compounds, IUPAC naming uses the group name as a prefix. Halogen-substituted benzene compounds follow this pattern. The halogen prefix can be fluoro, chloro, bromo, or iodo. When an alkyl group is attached to the benzene ring, the name depends on the chain length of that group. If the alkyl chain...

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Naming Benzene Rings with Multiple Groups

Naming benzene rings with multiple groups depends on how many substituents are attached. For benzene derivatives with two groups, three constitutional isomers are possible. The position of each group is described with IUPAC naming rules. For disubstituted benzene compounds, the relative positions can be shown with ortho, meta, and para. These terms are also written as o, m, and p. They can also be written with numbers, such as 1,2- for ortho, 1,3- for meta, and 1,4- for para. Dimethyl benzene,...

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Kekule’s benzene model describes benzene as a six-carbon ring with alternating pi bonds. August Kekule proposed this structure in 1865 from the formula C6H6, the idea that all benzene hydrogens are equivalent, and the rule that each carbon has four bonds because of tetravalency. In this model, each carbon is attached to one hydrogen atom. It also predicts two isomers for 1,2-disubstituted benzene derivatives. In one form, the substituted carbons are linked by a double bond, and in the other...

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Benzene π Orbitals and Electron Delocalization

Benzene has a planar ring structure with delocalized π electrons. Its six carbon atoms form a regular hexagon, and each carbon is sp2 hybridized. The C–C–C and H–C–C bond angles are 120°, which matches the flat shape of the ring. Each carbon is bonded to three other atoms. The C–H bond length is 109 pm, and the C–C bond length is 139 pm. That C–C distance is midway between the single bond length of sp3 hybridized carbons, 154 pm, and the sp2 value, 133 pm. Beneath and above the ring plane,...

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H&uuml;ckel Rule and Aromatic Ring Criteria

Aromatic rings follow H&amp;uuml;ckel’s rule, also called the 4n + 2 rule. This rule helps identify which unsaturated cyclic compounds are aromatic and why some ring systems are unstable instead. Benzene is the classic example, while cyclobutadiene and cyclooctatetraene show different behavior and are not aromatic. Eric H&amp;uuml;ckel, a German chemical physicist, first described the structural features needed for aromaticity. An aromatic compound must be cyclic, planar, and continuously...

h&uuml;ckel's rule diagram of &pi; mos: frost circle

Frost Circle for Pi Orbital Energies

The Frost circle is a graphical method for showing the relative energies of pi molecular orbitals in planar, fully conjugated, monocyclic compounds. It is also called the inscribed polygon method. A. A. Frost and Boris Musulin first described this method in 1953. To build a Frost circle, draw a polygon with the same number of edges as the number of carbons in the cyclic system. Place one vertex pointing downward. Then draw a circle around the polygon so that every vertex touches the circle.

frost circles for different conjugated systems

Frost circles help test whether a cyclic compound is aromatic by using the inscribed polygon method and Hückel’s 4n + 2 rule. A compound is stable and aromatic when every bonding molecular orbital (MO) is completely filled with a pair of electrons. If electrons occupy non-bonding or antibonding orbitals, the compound is unstable and not aromatic. Frost circle diagrams for cycloalkenes with 4 to 8 carbons show a clear pattern. The number of bonding MOs is always odd. The electrons needed to...

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Cyclopentadienyl Anion and Aromaticity

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Cycloheptatrienyl Cation and Aromaticity

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Five-Membered Aromatic Rings with Heteroatoms

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Aromatic Ring Shifts in NMR Spectra

Aromatic ring shifts in NMR spectra help identify aromatic compounds using proton NMR and carbon-13 NMR. Aromatic hydrogens are often strongly deshielded by the aromatic ring current, so they usually appear in the 6.5–8.0 ppm range in proton NMR. In benzene, the hydrogens directly bonded to the ring absorb at 7.3 ppm. Larger aromatic rings can place hydrogens outside the usual range. In [18]annulene, the 12 outer hydrogens are highly deshielded by the ring current and absorb downfield at 9.3...

key terms and concepts

Learn how Frost circles and the inscribed polygon method use Hückel’s 4n + 2 rule to identify aromatic cyclic compounds and unstable non-aromatic cases.

Using Frost Circles to Test Aromaticity | JoVE K12

organic chemistry

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Using Frost Circles to Test Aromaticity

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