Name: five-membered aromatic rings with heteroatoms
Uploaded: 2023-04-30T10:31:30.000Z
Duration: 1 min 13 s
Channel: JoVE

Les composés aromatiques cycliques ayant un ou plusieurs hétéroatomes sont appelés composés aromatiques hétérocycliques.
Les composés aromatiques hétérocycliques à cinq chaînons comprennent le pyrrole, le furane, le thiophène et l’imidazole, ayant de l’azote, de l’oxygène et du soufre comme hétéroatomes.
Dans ces composés planaires, tous les atomes sont hybridés sp2, y compris l’hétéroatome.
Comme le montre la structure de liaison π du pyrrole, un électron π de chaque carbone et un doublet non liant d’électrons de l’hétéroatome occupent les orbitales 2p non hybridées.
Ces cinq orbitales se chevauchent, formant une boucle fermée de 6 π électrons, ce qui rend le pyrrole aromatique.
Dans le furane et le thiophène, les doublets non liants supplémentaires d’électrons occupent une orbitale hybride sp2 et ne font pas partie du système π.
L’imidazole a deux atomes d’azote. Le couple non liant sur l’un des azotes fait partie du sextuor aromatique. Le doublet non liant du deuxième azote occupe une orbitale hybride sp2 et n’est pas impliqué dans le système π.

composés aromatiques hétérocycliques à cinq membres : aperçu

Five-Membered Heterocyclic Aromatic Compounds: Overview

Les composés aromatiques hétérocycliques sont des composés cycliques aromatiques et possédant un ou plusieurs hétéroatomes (atomes autres que le carbone) dans le cycle. Selon le nombre d'atomes présents dans le cycle, ils peuvent avoir cinq ou six chaînons. Des exemples de composés aromatiques hétérocycliques à cinq chaînons comprennent le pyrrole, le furane, le thiophène et l'imidazole. Le pyrrole est constitué d'un atome d'azote possédant une paire d'électrons libres. Le furane et le thiophène ont respectivement un hétéroatome d’oxygène et un hétéroatome de soufre. Les deux possèdent deux paires d’électrons libres. L'imidazole diffère des trois autres en ce qu'il possède deux azotes comme hétéroatomes avec une paire libre d'électrons sur chaque atome d'azote. Dans les quatre composés hétérocycliques, tous les atomes sont hybridés sp2, y compris les hétéroatomes.

Par conséquent, tous les atomes ont une orbitale p non hybridée perpendiculaire au plan de l’anneau. Dans chaque cas, tous les atomes de carbone contribuent à un électron π et les hétéroatomes contribuent à une seule paire d’électrons, occupant l’orbitale p non hybridée. Ceci complète le sextet aromatique et forme une boucle fermée de 6 électrons π .

La paire libre supplémentaire d'électrons d'hétéroatome d'oxygène et de soufre dans le furane et le thiophène, respectivement, occupent l'orbitale hybride sp2, qui est perpendiculaire à l'orbitale p, et ne participent pas au système π. Dans le cas de l'imidazole, la paire d'électrons libres sur le deuxième atome d'azote occupe l'orbitale hybride sp2 et ne participe pas au système π aromatique. Par conséquent, la paire d’électrons libres d’un seul atome d’azote est impliquée dans le sextet aromatique.

Composés aromatiques hétérocycliques à cinq membres : aperçu

Les composés aromatiques hétérocycliques sont des composés cycliques qui sont aromatiques et qui ont un ou plusieurs hétéroatomes (des atomes autres que le carbone) dans le cycle. ...

five-membered-heterocyclic-aromatic-compounds-overview

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JoVE Core

Organic Chemistry

Chimie organique

Unit 1

Unité 1

Aromatic Compounds

Composés aromatiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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. Imidazole has two nitrogen atoms, and each nitrogen has one lone pair of electrons.

In these compounds, the ring atoms are sp2 hybridized, including the heteroatoms. That means each atom has one unhybridized p orbital standing perpendicular to the ring plane. The carbon atoms each contribute one pi electron. The heteroatoms contribute one lone pair from the p orbital, which helps complete the aromatic sextet and forms a closed loop of 6 pi electrons.

Not every lone pair takes part in aromaticity. In furan and thiophene, the extra lone pair on oxygen or sulfur stays in an sp2 hybrid orbital and does not join the pi system. In imidazole, the lone pair on the second nitrogen also stays in an sp2 hybrid orbital. As a result, only one nitrogen lone pair is part of the aromatic sextet.

Five-Membered Aromatic Rings with Heteroatoms

five-membered-aromatic-rings-with-heteroatoms

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

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-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 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 pyrrole, furan, thiophene, and imidazole build aromatic 5-membered rings with heteroatoms, lone pairs, and 6 pi electrons.

Five-Membered Aromatic Rings with Heteroatoms | JoVE K12

organic chemistry

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Five-Membered Aromatic Rings with Heteroatoms

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