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

According to Huckel&#39;s rule, benzene is stable and aromatic, while cyclobutadiene and cyclooctatetraene are unstable and not aromatic.
The aromaticity of these compounds can be understood from their Frost circle diagrams.
The Frost circle for benzene has six &#960; molecular orbitals. All the low-energy bonding orbitals are fully occupied with six &#960; electrons, while the high-energy antibonding orbitals are empty. Therefore, benzene is stable and aromatic.
The Frost circles for cyclobutadiene and cyclooctatetraene have four and eight &#960; molecular orbitals, respectively.
The bonding orbitals are fully occupied for both of the compounds, while the nonbonding orbitals are singly occupied. The antibonding orbitals are empty.
As is evident, the presence of &#960; electrons in high-energy nonbonding orbitals makes the two compounds unstable and not aromatic.
Overall, a given molecule is considered aromatic when all of the &#960; electrons are paired in bonding molecular orbitals. Compounds with electrons in orbitals other than bonding are not aromatic.

vorstcirkels voor verschillende geconjugeerde systemen

Frost Circles for Different Conjugated Systems

De ingeschreven polygoonmethode is consistent met de 4n + 2-regel van Hückel en helpt om te leren of de gegeven cyclische verbinding aromatisch is of niet. De verbinding is stabiel en aromatisch als elke bindende moleculaire orbitaal (MO) volledig gevuld is met een paar elektronen. Als de niet-bindende of anti-bindende orbitalen echter gevuld zijn met elektronen, is de verbinding onstabiel en niet aromatisch. Neem als bijvoorbeeld de Frost-cirkeldiagrammen voor cycloalkenen die 4 tot 8 koolstofatomen bevatten.


 <img alt="Figure1" src="/files/ftp_upload/12866/12866_Figure_1.svg" style="text-align: center; height: 75px;" />


 Door naar de Frost-cirkels te kijken, wordt waargenomen dat het aantal bindende MO altijd oneven is, en dat het vereiste aantal π-elektronen in de bindende MO 2 of 6 is. Het aantal elektronen voldoet perfect aan de 4n + 2-regel van aromaticiteit. Daarom zijn de verbindingen met volledig gevulde bindende MO aromatisch.


 In het geval van vierledig cyclobutadieen met 4π elektronen en achtledig cyclooctatetraeen met 8π elektronen, zijn de bindende MO volledig gevuld en is elk van de niet-bindende MO afzonderlijk bezet. De aanwezigheid van elektronen in hoogenergetische niet-bindende MO maakt ze onstabiel en niet aromatisch. Het vijfledige cyclopentadienylanion, het zesledige benzeen en het zevenledige cycloheptatrienylkation hebben echter 6π-elektronen en de overeenkomstige bindende MO zijn volledig bezet en stabiel, dus ze zijn aromatisch.


 Over het algemeen worden moleculen met volledig gevulde moleculaire orbitalen als aromatisch beschouwd, terwijl verbindingen met elektronen in andere orbitalen dan binding niet aromatisch zijn.

Vorstcirkels voor verschillende geconjugeerde systemen

The inscribed polygon method is consistent with H&#252;ckel&#8217;s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. ...

frost-circles-for-different-conjugated-systems

Education

JoVE Core

Organic Chemistry

Organische chemie

Unit 1

Onderdeel 1

Aromatic Compounds

Aromatische verbindingen

KEY TERMS AND CONCEPTS

BELANGRIJKE TERMEN EN CONCEPTEN

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

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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,...

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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

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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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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

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Using Frost Circles to Test Aromaticity

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