Name: cyclopentadienyl anion and aromaticity
Uploaded: 2023-04-30T10:31:23.000Z
Duration: 1 min 18 s
Channel: JoVE

Neutrale monozyklische ungesättigte Kohlenwasserstoffe mit einer ungeraden Anzahl von Kohlenstoffatomen haben aufgrund des Vorhandenseins eines dazwischenliegenden sp3-Kohlenstoffs im Ring keine Aromatizität.
Zum Beispiel ist Cyclopentadien nicht aromatisch, da es nur 4 π Elektronen und einen sp3-Kohlenstoff hat, der die kontinuierliche Überlappung der p-Orbitale stört.
Bemerkenswert ist, dass das Entfernen eines Wasserstoffs aus der CH2-Gruppe mit beiden, einem oder keinem der Bindungselektronen den sp3-Kohlenstoff in sp2 umwandelt, wodurch ein Kationen, ein Radikal bzw. ein Anion erzeugt werden.
Im Vergleich zum Kation und Radikal hat nur das Anion die erforderliche Anzahl von (4n + 2) π Elektronen.
Darüber hinaus ermöglicht die Verfügbarkeit eines 2p Orbitals am sp2 Kohlenstoff eine kontinuierliche Überlappung der p Orbitale und eine Delokalisierung der negativen Ladung im gesamten Ring, wodurch das Cyclopentadienylanion aromatisch wird.
Darüber hinaus bestätigen die π Elektronen, die die bindenden Molekülorbitale besetzen, und die fünf Resonanzstrukturen die ungewöhnliche Stabilität des aromatischen Anions.

aromatische kohlenwasserstoffanionen: strukturüberblick

Aromatic Hydrocarbon Anions: Structural Overview

Neutrale Kohlenwassersubstanzen wie Cyclopentadien mit einer ungeraden Anzahl von Kohlenstoffatomen und einer dazwischenliegenden CH_2-Gruppe im Ring sind nicht aromatisch. Cyclopentadien mit 4 π-Elektronen erfüllt nicht die 4n + 2 π-Elektronenregel. Darüber hinaus ist die dazwischenliegende CH_2-Gruppe sp^3-hybridisiert und weist kein freies p-Orbital auf, wodurch die Überlappung der p-Orbitale kontinuierlich unterbrochen und die Delokalisierung von π-Elektronen im gesamten Ring verhindert wird.

Aufgrund der fehlenden kontinuierlichen Überlappung der p-Orbitale erfüllt Cyclopentadien nicht die Aromatizitätskriterien. Die Entfernung eines Wasserstoffs mit beiden oder einem oder keinem Elektron aus der CH_2-Gruppe treibt jedoch die Umwandlung von sp^3-Kohlenstoff in sp^2 voran, wodurch Cyclopentadien in ein Cyclopentadienylkation, -radikal bzw. -anion mit einem freien p-Orbital umgewandelt wird. Von allen drei Spezies erfüllen das Cyclopentadienylkation und das Cyclopentadienylradikal mit vier bzw. fünf π-Elektronen nicht die Kriterien von 4n + 2 π-Elektronen und sind nicht aromatisch. Andererseits wird ein Cyclopentadienylanion mit 6 π-Elektronen, einem kontinuierlichen überlappenden Ring aus p-Orbitalen und einer delokalisierten π-Elektronendichte aromatisch. Die Karte des elektrostatischen Potenzials des Anions bestätigt die Delokalisierung der π-Elektronen im gesamten Ring. Darüber hinaus bestätigen die fünf Resonanzstrukturen des Cyclopentadienyl-Anions und die Belegung der Bindungsmolekülorbitale durch alle 6 π-Elektronen im Frostdiagramm die ungewöhnliche Stabilität des Anions.

Aromatische Kohlenwasserstoffanionen: Strukturüberblick

Neutrale Kohlenwasserstoffe wie Cyclopentadien mit einer ungeraden Anzahl von Kohlenstoffatomen und einer dazwischenliegenden CH2-Gruppe im Ring sind nicht aromatisch. Cyclopentadien ...

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 removed from the CH2 group, the carbon changes from sp3 to sp2 hybridization. This creates a vacant p orbital and gives three related species: the cyclopentadienyl cation, radical, and anion.

These three species do not behave the same way. The cyclopentadienyl cation has 4 pi electrons, and the radical has 5 pi electrons. Neither one satisfies the 4n + 2 rule, so neither is aromatic. The cyclopentadienyl anion has 6 pi electrons, a continuous ring of overlapping p orbitals, and delocalized pi electron density. That makes it aromatic and unusually stable.

The electrostatic potential map of the anion supports this electron delocalization across the ring. Its five resonance structures also show how the electrons can be spread out. The frost diagram gives the same result, since all 6 pi electrons fill bonding molecular orbitals. Together, these features explain why the cyclopentadienyl anion is more stable than the neutral ring, the cation, or the radical.

Cyclopentadienyl Anion and 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,...

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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 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

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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

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...

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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

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 why cyclopentadiene is not aromatic and how the cyclopentadienyl anion becomes stable through 6 pi electrons, resonance, and orbital overlap.

Cyclopentadienyl Anion and Aromaticity | JoVE K12

organic chemistry

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Cyclopentadienyl Anion and Aromaticity

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