Name: frost circle for pi orbital energies
Uploaded: 2023-04-30T10:31:15.000Z
Duration: 1 min 8 s
Channel: JoVE

La méthode du cercle de givre ou du polygone inscrit est une méthode graphique basée sur la règle de Hückel pour déterminer le nombre et les énergies relatives des orbitales moléculaires π des systèmes planaires, monocycliques et conjugués.
Un cercle de givre est dessiné en inscrivant un polygone, représentant le composé cyclique, à l’intérieur d’un cercle dont l’un des sommets pointe vers le bas.
Les intersections où le cercle touche le polygone sont marquées par de courtes lignes horizontales représentant le nombre et les énergies relatives des orbitales moléculaires de π.
Ensuite, une ligne horizontale pointillée est tracée à travers le centre du cercle, coupant le polygone en deux. Cette droite représente l’énergie des orbitales p participantes.
Toutes les orbitales moléculaires dont les énergies sont inférieures à cette ligne sont liantes, celles au-dessus de la ligne sont antiliantes et celles qui se trouvent sur la ligne sont des orbitales moléculaires non liantes.
Enfin, les électrons π sont ajoutés aux niveaux d'énergie selon la règle de Hund et le principe d'exclusion de Pauli.

diagramme de règle de hückel des π mo : cercle de givre

H&uuml;ckel's Rule Diagram of &pi; MOs: Frost Circle

La méthode du cercle de Frost ou du polygone inscrit est une méthode graphique permettant de déterminer les énergies relatives des orbitales moléculaires (MO) π pour les composés planaires, entièrement conjugués et monocycliques. Cette méthode a été décrite pour la première fois par A. A. Frost et Boris Musulin en 1953.

Un cercle de Frost est construit en dessinant un polygone dont le nombre d'arêtes est égal au nombre de carbones du système cyclique donné, avec l'un des sommets pointant vers le bas. Ensuite, un cercle est dessiné entourant le polygone de manière à ce que tous les sommets du polygone touchent le cercle. Les points où le cercle touche le polygone sont marqués par de courtes lignes horizontales, représentant les niveaux d'énergie des orbitales moléculaires π.

<img alt="Figure1" src="/files/ftp_upload/12454/12454_PT-Figure_1.png" style="text-align: center; height: 250px;">

Ensuite, une ligne horizontale pointillée est tracée passant par le centre du cercle, coupant le polygone en deux. Les niveaux d'énergie en dessous de cette ligne représentent des orbitales moléculaires de liaison, et au-dessus de cette ligne représentent des orbitales moléculaires anti-liantes. Les niveaux d'énergie sur la ligne représentent des orbitales moléculaires non liées, avec une énergie égale aux p orbitales à partir desquelles les orbitales moléculaires sont formées.

Enfin, les électrons π sont ajoutés aux niveaux d’énergie en commençant par le plus bas et en remontant. Les orbitales dégénérées sont d’abord remplies d’un seul électron, puis associées à un autre électron si disponible, conformément au principe d’exclusion de Pauli et aux règles de Hund.

Diagramme de règle de Hückel des π MO : Cercle de givre

La méthode du cercle de givre ou du polygone inscrit est une méthode graphique permettant de déterminer les énergies relatives des orbitales moléculaires (MO) de π ...

Education

Enseignement

JoVE Core

Organic Chemistry

Chimie organique

Unit 1

Unité 1

Aromatic Compounds

Composés aromatiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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. Mark the touch points with short horizontal lines. These lines show the energy levels of the pi molecular orbitals.

Next, draw a dashed horizontal line through the center of the circle. This line divides the energy levels into bonding, non-bonding, and antibonding orbitals. Levels below the line are bonding molecular orbitals. Levels above the line are antibonding molecular orbitals. The levels on the line are non-bonding molecular orbitals, and they have the same energy as the p orbitals that form them.

Finally, place the pi electrons into the energy levels from lowest to highest. If two orbitals have the same energy, they are degenerate orbitals. Fill each degenerate orbital with one electron first, then pair the electrons if more are available. This follows Pauli’s exclusion principle and Hund’s rules.

Frost Circle for Pi Orbital Energies

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

nomenclature-of-aromatic-compounds-with-a-single-substituent

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...

nomenclature of aromatic compounds with multiple substituents

nomenclature-of-aromatic-compounds-with-multiple-substituents

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

structure-of-benzene-molecular-orbital-model

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,...

criteria for aromaticity and the h&uuml;ckel 4n + 2 rule

criteria-for-aromaticity-and-the-hckel-4n-2-rule

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

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-for-different-conjugated-systems

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...

aromatic hydrocarbon anions: structural overview

aromatic-hydrocarbon-anions-structural-overview

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

aromatic-hydrocarbon-cations-structural-overview

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...

five-membered heterocyclic aromatic compounds: overview

five-membered-heterocyclic-aromatic-compounds-overview

Five-Membered Aromatic Rings with Heteroatoms

Five-membered aromatic rings with heteroatoms are cyclic compounds that contain one or more atoms other than carbon in the ring. They are aromatic and can have either five or six atoms in the ring. Common examples include pyrrole, furan, thiophene, and imidazole. These rings differ by the heteroatom present. Pyrrole has one nitrogen atom with one lone pair of electrons. Furan has one oxygen atom, and thiophene has one sulfur atom. Both furan and thiophene have two lone pairs of electrons.

nmr spectroscopy of aromatic compounds

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 the Frost circle maps pi molecular orbital energies in planar conjugated rings, from drawing the polygon to filling bonding, non-bonding, and antibonding levels.

Frost Circle for Pi Orbital Energies | JoVE K12

organic chemistry

JoVE Core Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

Frost Circle for Pi Orbital Energies

Description

Transcript

Tags