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

De acordo com a regra de Huckel, o benzeno é estável e aromático, enquanto o ciclobutadieno e o ciclooctatetraeno são instáveis e não aromáticos.
A aromaticidade desses compostos pode ser entendida a partir de seus diagramas de círculo de Frost.
O círculo de Frost para benzeno tem seis orbitais moleculares π. Todos os orbitais de ligação de baixa energia estão totalmente ocupados com seis elétrons π, enquanto os orbitais antiligantes de alta energia estão vazios. Portanto, o benzeno é estável e aromático.
Os círculos de Frost para ciclobutadieno e ciclooctatetraeno têm quatro e oito orbitais moleculares π, respectivamente.
Os orbitais de ligação estão totalmente ocupados para ambos os compostos, enquanto os orbitais não ligantes estão ocupados isoladamente. Os orbitais antiligantes estão vazios.
Como é evidente, a presença de elétrons π em orbitais não ligantes de alta energia torna os dois compostos instáveis e não aromáticos.
No geral, uma determinada molécula é considerada aromática quando todos os π elétrons estão emparelhados em orbitais moleculares de ligação. Compostos com elétrons em orbitais diferentes da ligação não são aromáticos.

círculos de frost para diferentes sistemas conjugados

Frost Circles for Different Conjugated Systems

O método do polígono inscrito é consistente com a regra 4n + 2 de Hückel e ajuda a saber se um determinado composto cíclico é aromático ou não. O composto é estável e aromático se cada orbital molecular (MO) de ligação estiver completamente preenchido com um par de elétrons. No entanto, se os orbitais não ligantes ou antiligantes estiverem preenchidos com elétrons, o composto é instável e não aromático. Considere os diagramas do círculo de Frost para cicloalcenos contendo 4 a 8 carbonos.

<img alt="Figure1" src="/files/ftp_upload/12866/12866_Figure_1.svg" style="text-align: center; height: 75px;">

Ao observar os círculos de Frost, nota-se que o número de MO de ligação é sempre ímpar, e o número necessário de elétrons π nos MO de ligação é 2 ou 6. O número de elétrons satisfaz perfeitamente a regra 4n + 2 de aromaticidade. Consequentemente, os compostos com MO de ligação completamente preenchido são aromáticos.

No caso do ciclobutadieno de quatro membros com 4 elétrons π e do ciclooctatetraeno de oito membros com 8 elétrons π, os MO ligantes são completamente preenchidos, e cada um dos MO não ligantes é ocupado individualmente. A presença de elétrons nos MO não ligantes de alta energia os torna instáveis ​​e não aromáticos. No entanto, o ânion ciclopentadienila de cinco membros, o benzeno de seis membros e o cátion cicloheptatrienila de sete membros têm 6 elétrons π, e o MO de ligação correspondente está totalmente ocupado e estável, portanto são aromáticos.

Em resumo, moléculas com orbitais moleculares de ligação completamente preenchidos são consideradas aromáticas, enquanto compostos com elétrons em orbitais que não são de ligação não são aromáticos.

Círculos de Frost para Diferentes Sistemas Conjugados

O método do polígono inscrito é consistente com a regra 4n + 2 de Hückel e ajuda a aprender se o composto cíclico dado é aromático ou não.

frost-circles-for-different-conjugated-systems

Education

JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Aromatic Compounds

Compostos Aromáticos

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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,...

structure of benzene: kekul&eacute; model

Kekule’s Benzene Model and Its Limits

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...

structure of benzene: molecular orbital model

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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...

aromatic hydrocarbon anions: structural overview

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Cyclopentadienyl Anion and Aromaticity

Cyclopentadiene is a neutral hydrocarbon that is not aromatic. It contains an odd number of carbon atoms and one CH2 group in the ring. With only 4 pi electrons, it does not follow the 4n + 2 pi electron rule. The CH2 carbon is sp3 hybridized, so it has no vacant p orbital to keep p orbitals overlapped around the ring. That break in overlap stops pi electrons from spreading across the entire ring. As a result, cyclopentadiene does not meet the usual aromaticity criteria. If one hydrogen is...

aromatic hydrocarbon cations: structural overview

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Cycloheptatrienyl Cation and Aromaticity

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon with seven carbon atoms. One carbon in the ring is sp3 hybridized, which breaks the continuous overlap of p orbitals. Even though the ring has three double bonds and 6 π electrons, cycloheptatriene is not aromatic because the p orbitals do not form one uninterrupted loop. Removing one hydrogen from the CH2 group changes the ring. If that hydrogen is removed with both, one, or no electrons, the products are the cycloheptatrienyl...

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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

JoVE Core - Química Orgânica explica conceitos básicos de estereoquímica, reações e mecanismos usando vídeos de animação concisos e fáceis de entender. Além disso, os vídeos de Cientistas em ação demonstram a aplicação de conceitos relacionados em experimentos de pesquisa clássicos e originais realizados nos laboratórios atuais em todo o mundo.

Using Frost Circles to Test Aromaticity

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