Name: cycloheptatrienyl cation and aromaticity
Uploaded: 2023-04-30T11:19:08.000Z
Duration: 1 min 18 s
Channel: JoVE

Cykloheptatrien to kolejny obojętny monocykliczny nienasycony węglowodór o nieparzystej liczbie atomów węgla.
Ma sześć π elektronów, co spełnia regułę Hückla 4n + 2.
Jednak węgiel sp3 zakłóca delokalizację tych π elektronów, dlatego obojętny cykloheptatrien nie jest aromatyczny.
Konwersja węgla sp3 w węgiel sp2 poprzez usunięcie jednego wodoru z obydwoma, jednym lub bez elektronów generuje odpowiednio kation, rodnik i anion.
Jak zaobserwowano, tylko kation cykloheptatrienylowy ma wymaganą liczbę (4n + 2) π elektronów.
Co więcej, generowanie pustego orbitalu 2p ułatwia ciągłe nakładanie się orbitali p. Dlatego kation cykloheptatrienylu jest aromatyczny.
Aromatyczność kationu można uzasadnić na podstawie siedmiu struktur rezonansowych, symetrycznego rozkładu ładunku dodatniego na mapie potencjału elektrostatycznego oraz diagramu Frosta kationu, ze wszystkimi π elektronami w wiążących orbitalach molekularnych.

kationy węglowodorów aromatycznych: przegląd strukturalny

Aromatic Hydrocarbon Cations: Structural Overview

Cykloheptatrien jest obojętnym monocyklicznym nienasyconym węglowodorem, który składa się z nieparzystej liczby atomów węgla i znajdującego się pomiędzy nimi węgla sp^3 w pierścieniu. Trzy wiązania podwójne w pierścieniu odpowiadają 6 elektronom π, co jest liczbą Huckela, a zatem spełnia kryteria 4n + 2 elektronów π. Jednakże znajdujący się pomiędzy nimi węgiel sp^3 zakłóca ciągłe nakładanie się orbitali p. W rezultacie cykloheptatrien nie jest aromatyczny.

Usunięcie jednego wodoru z pośredniej grupy CH_2 z obydwoma elektronami, jednym lub żadnym elektronem generuje odpowiednio kation, rodnik i anion cykloheptatrienylu, przekształcając w ten sposób węgiel sp^3 cykloheptatrienu w sp^2. W rezultacie wygenerowany pusty orbital 2p w kationie cykloheptatrienylu i orbitale p innych węgli w pierścieniu tworzą ciągłą pętlę nakładających się orbitali p. Zatem w porównaniu z rodnikiem i anionem kation cykloheptatrienylowy posiadający 6 elektronów π i ciągłą pętlę nakładających się orbitali p spełnia wszystkie kryteria Huckela i jest aromatyczny.

Kationy węglowodorów aromatycznych: przegląd strukturalny

Cykloheptatrien jest obojętnym monocyklicznym nienasyconym węglowodorem, który składa się z nieparzystej liczby atomów węgla i węgla sp3 znajdującego się pomiędzy nimi w pierścieniu. ...

aromatic-hydrocarbon-cations-structural-overview

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Organic Chemistry

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Aromatic Compounds

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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 cation, radical, and anion, respectively. This change converts the sp3 carbon to sp2 hybridization. In the cation, the empty 2p orbital joins the p orbitals on the other carbons and creates a continuous loop of overlap.

The cycloheptatrienyl cation is the aromatic form in this set. It has 6 π electrons and a full ring of overlapping p orbitals, so it meets Huckel’s criteria, 4n + 2 π electrons, for aromaticity. The radical and anion do not match this same aromatic arrangement as well as the cation does.

Cycloheptatrienyl Cation and 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...

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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.

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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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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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key terms and concepts

See why cycloheptatriene is not aromatic and how the cycloheptatrienyl cation forms a continuous p-orbital loop that meets Huckel’s rule.

Cycloheptatrienyl Cation and Aromaticity | JoVE K12

organic chemistry

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Cycloheptatrienyl Cation and Aromaticity

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