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

1つ以上のヘテロ原子を有する環状芳香族化合物は、複素環式芳香族化合物と呼ばれます。
五員環芳香族化合物には、ピロール、フラン、チオフェン、イミダゾールが含まれ、ヘテロ原子として窒素、酸素、硫黄を有する。
これらの平面化合物では、ヘテロ原子を含むすべての原子がsp2混成されています。
ピロールのπ結合構造から明らかなように、各炭素からの1つのπ電子とヘテロ原子からの1つの孤立電子対が、非ハイブリダイゼーションされた2p軌道を占めます。
これらの5つの軌道が重なり合い、6つのπ電子の閉ループを形成し、ピロールを芳香族にします。
フランとチオフェンでは、追加の孤立電子対の電子はsp2混成軌道を占め、π系の一部ではありません。
イミダゾールには2つの窒素原子があります。窒素の1つ上の唯一のペアは、芳香族六重奏の一部です。2番目の窒素上の孤立電子対はsp2ハイブリッド軌道を占め、π系に関与していません。

5員複素環芳香族化合物: 概要

Five-Membered Heterocyclic Aromatic Compounds: Overview

複素環芳香族化合物は、芳香族であり、環内に 1 つ以上のヘテロ原子 (炭素以外の原子) を有する環状化合物です。 環内に存在する原子の数に応じて、それらは 5 員または 6 員になります。 ５員複素環芳香族化合物の例としては、ピロール、フラン、チオフェン、イミダゾールなどが挙げられる。 ピロールは、1 つの孤立電子対を持つ 1 つの窒素原子から構成されます。 フランとチオフェンは、それぞれ酸素と硫黄のヘテロ原子を 1 つずつ持っています。 どちらも 2 つの孤立電子対を持っています。 イミダゾールは、各窒素原子上に 1 つの孤立電子対を持つヘテロ原子として 2 つの窒素を有する点で他の 3 つとは異なります。 4 つの複素環式化合物すべてにおいて、ヘテロ原子を含むすべての原子が sp^2 混成軌道されています。

その結果、すべての原子は、環の平面に垂直な、混成されていない 1 つの p 軌道を持ちます。 それぞれの場合において、すべての炭素原子は 1 つの π 電子に寄与し、ヘテロ原子は 1 つの孤立電子対に寄与し、混成されていない p 軌道を占めます。 これで芳香族六重奏が完成し、6 個の π 電子の閉ループが形成されます。

フランとチオフェンにそれぞれ含まれる酸素と硫黄のヘテロ原子の追加の孤立電子対は、p 軌道に垂直な sp^2 混成軌道を占有し、π 系には関与しません。 イミダゾールの場合、2 番目の窒素原子上の孤立電子対は sp^2 混成軌道を占有し、芳香族 π 系には参加しません。 したがって、1 つの窒素原子のみの孤立電子対が芳香族六重項に関与します。

複素環芳香族化合物は、芳香族化合物であり、環内に1つ以上のヘテロ原子(炭素以外の原子)を持つ環状化合物です。...

five-membered-heterocyclic-aromatic-compounds-overview

Education

JoVE Core

Organic Chemistry

有機化学

Unit 1

ユニット1

Aromatic Compounds

芳香族化合物

KEY TERMS AND CONCEPTS

主要用語と概念

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

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