Name: hückel rule and aromatic ring criteria
Uploaded: 2023-04-30T10:31:08.000Z
Duration: 1 min 20 s
Channel: JoVE

벤젠(Benzene), 시클로부타디엔(cyclobutadiene) 및 시클로옥타테트라엔(cyclooctatetraene)은 두 가지 동등한 구조의 하이브리드로 표시됩니다.
그러나 벤젠과 달리 시클로부타디엔과 시클로옥타테트라엔은 불안정하고 방향족이 없습니다.
이러한 차이를 설명하기 위해 Erich Hückel은 Hückel의 규칙 또는 4n + 2 규칙으로 알려진 방향성의 기저에 있는 일련의 구조적 특징을 설명했습니다.
Hückel의 법칙에 따르면 방향족 화합물은 고리형이고 평면적이어야 하며 각 탄소에 혼성화되지 않은 p 오비탈이 있어야 합니다.
p 오비탈은 (4n + 2) π 전자의 연속 루프를 형성하기 위해 겹쳐야 하며, 여기서 n은 양의 정수입니다.
벤젠은 Hückel의 법칙에 대한 모든 기준을 충족하여 방향성을 확립합니다.
대조적으로, cyclooctatetraene은 인접한 p 안와탈 사이에 겹치지 않는 비평면 튜브 형태를 가정합니다. 게다가, 그것은 (4n + 2) π 전자가 결여되어 그러므로 방향족이 아니다.
유사하게, 시클로부타디엔은 고리형이고 평면적이며 p 궤도가 연속적으로 겹칩니다. 그러나 (4n + 2) π 전자가 부족하고 방향성을 잃습니다.

방향족 및 휘켈 4n + 2 규칙에 대한 기준

Criteria for Aromaticity and the H&uuml;ckel 4n + 2 Rule

벤젠과 마찬가지로 사이클로부타디엔과 사이클로옥타테트라엔은 단일 결합과 이중 결합이 번갈아 나타나는 고리형 화합물입니다. 그러나 화학적 거동은 불안정하고 방향족이 아니므로 벤젠과 구별됩니다. 그렇다면 방향족으로 분류되는 불포화 화합물의 구조적 특징은 무엇일까요?

처음으로 독일의 화학 물리학자인 에릭 휘겔은 방향족으로 분류되는 화합물의 구조적 특징 세트를 도출했습니다. 이는 휘겔법 또는 4n + 2 법칙으로 알려져 있습니다. 이 규칙에 따르면 방향족 화합물은 홀수 개의 π 전자 또는 4n + 2 π 전자 쌍을 갖는 고리형, 평면형, 연속 공액이어야 합니다. 여기서 n은 정수(n = 0, 1, 2…)입니다. 따라서 방향족 화합물은 2(n = 0), 6(n = 1), 10(n = 2), 14(n = 3) 등의 π 전자 수 또는 1, 3, 5, 등의 π 전자 쌍을 가져야 합니다.

예를 들어, 1,3,5-헥사트리엔과 벤젠은 6개의 π 전자를 가지고 있지만, 두 말단 탄소 원자의 p 오비탈이 겹치지 않아 결합으로 탄소가 연결되지 않기 때문에 1,3,5-헥사트리엔은 방향족이 아닙니다. 그러므로 화합물이 방향족이 되려면 고리형이어야 합니다.

<img alt="Figure1" src="/files/ftp_upload/12453/12453_PT-Figure_1.svg" style="text-align: center; height: 150px;">

사이클로옥타테트라엔은 연속된 4개의 π 결합을 갖는 팔원자 고리형 화합물이지만 불안정하고 방향족이 아닙니다. 이는 사이클로옥타테트라엔이 비평면이며, 이러한 형태가 욕조 형태를 취하기 때문입니다. 결과적으로, π 전자 구름의 폐쇄 루프를 형성하기 위한 p 오비탈의 효과적인 중첩은 실현 가능하지 않습니다. 특히, 벤젠과 달리 부가 반응을 겪습니다.

화합물이 연속적으로 결합되려면 각 탄소에 반쯤 채워져 있는 혼성화되지 않은 p 오비탈과 함께 모든 sp^2 혼성 탄소가 있어야 합니다. 예를 들어, 1,3-사이클로헥사디엔과 1,3,5-사이클로헵타트리엔은 혼성화되지 않은 p 오비탈이 없는 sp^3 혼성 탄소를 가지므로 연속적인 중첩이 불가능합니다. 따라서 둘 다 방향성은 없습니다.

<img alt="Figure2" src="/files/ftp_upload/12453/12453_PT-Figure_2.svg" style="text-align: center; height: 85px;">

반면, 연속적인 공액을 갖는 고리형 평면 화합물인 사이클로부타디엔은 불안정하고 방향족이 아닙니다. 2쌍의 π 전자 또는 4개의 π 전자를 갖고 있어 4n + 2 π 전자를 갖는다는 기준을 충족하지 않습니다. 그러므로 방향족이 아닙니다.

벤젠과 마찬가지로 시클로부타디엔과 시클로옥타테트라엔은 단일 및 이중 결합이 번갈아 가며 있는 고리형 화합물입니다. 그러나 그들의 화학적 거동은 다릅니다 ...

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

Education

교육

JoVE Core

Organic Chemistry

유기 화학

Unit 1

제1단원

Aromatic Compounds

방향족 화합물

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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 conjugated. It must also contain an odd number of pairs of pi electrons, which means it has 4n + 2 pi electrons, where n is an integer.

That electron pattern gives possible counts of 2, 6, 10, 14, and so on pi electrons. These are the same as 1, 3, 5, 7, and so on pairs of pi electrons. Benzene fits this pattern, and it is aromatic. Cyclobutadiene does not, even though it is cyclic and planar with continuous conjugation. It has 4 pi electrons, or 2 pairs of pi electrons, so it fails the 4n + 2 rule and is not aromatic.

A ring must also stay closed to count as aromatic. 1,3,5-Hexatriene has 6 pi electrons, like benzene, but it is not aromatic because it is not cyclic. The p orbitals on the two terminal carbon atoms do not overlap, since no bond connects those carbons.

Planarity is another key requirement. Cyclooctatetraene has four pi bonds in an eight-membered ring, but it is nonplanar and adopts a tub shape. Because of that shape, its p orbitals cannot overlap well enough to form a closed loop of pi electrons. It is unstable, not aromatic, and unlike benzene it undergoes addition reactions.

Continuous conjugation also matters. Every carbon in the ring must be sp2 hybridized and have a half-filled unhybridized p orbital. In 1,3-cyclohexadiene and 1,3,5-cycloheptatriene, some carbons are sp3 hybridized, so they do not have the needed p orbital. As a result, continuous overlap cannot happen, and both compounds are not aromatic.

H&uuml;ckel Rule and Aromatic Ring Criteria

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

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-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 the H&uuml;ckel 4n + 2 rule and the key features of aromatic rings, including cyclic shape, planarity, continuous conjugation, and pi electrons.

H&uuml;ckel Rule and Aromatic Ring Criteria | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Hückel Rule and Aromatic Ring Criteria

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