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

홀수의 탄소 원자를 갖는 중성 단환 불포화 탄화수소는 고리에 개입하는 sp3 탄소의 존재로 인해 방향성이 부족합니다.
예를 들어, 사이클로펜타디엔은 4개의 π 전자와 p 오비탈의 연속적인 중첩을 방해하는 sp3 탄소만 가지고 있기 때문에 방향족이 아닙니다.
특히, 결합 전자가 둘 다, 하나 또는 전혀 없는CH2 그룹에서 수소를 제거하면 sp3 탄소가 sp2 로 변환되어 각각 양이온, 라디칼 및 음이온이 생성됩니다.
양이온 및 라디칼에 비해 음이온 만이 필요한 수의 (4n + 2) π 전자를 가지고 있습니다.
더욱이, sp2 탄소에서 2p 오비탈의 가용성은 p 오비탈의 연속적인 중첩과 고리 전체의 음전하의 비편소화를 용이하게 하여 시클로펜타디에닐 음이온을 방향족으로 만듭니다.
또한, 결합 분자 궤도를 차지하는 π개의 전자와 5개의 공명 구조는 방향족 음이온의 특이한 안정성을 더욱 확증합니다.

방향족 탄화수소 음이온: 구조적 개요

Aromatic Hydrocarbon Anions: Structural Overview

홀수의 탄소 원자와 고리에 하나의 CH_2 그룹이 개입된 사이클로펜타디엔과 같은 중성 탄화수소는 방향족이 아닙니다. 4개의 π 전자를 가진 사이클로펜타디엔은 4n + 2 π 전자 규칙을 충족하지 않습니다. 추가적으로, 중간에 있는 CH_2 그룹은 sp^3 혼성화되어 있고 빈 p 오비탈이 없기 때문에 p 오비탈의 연속적인 중첩을 방해하고 고리 전체에서 π 전자의 비편재화를 방지합니다.

p 오비탈이 연속적으로 겹치지 않기 때문에 사이클로펜타디엔은 방향족 기준을 준수하지 않습니다. 그러나 CH_2 그룹에서 전자 둘 다 또는 하나 또는 전자가 없는 수소 하나를 제거하면 sp^3 탄소가 sp^2로 변환되어 사이클로펜타디엔이 각각 빈 p 궤도를 갖는 사이클로펜타디에닐 양이온, 라디칼 및 음이온으로 변환됩니다. 세 종 모두 중에서 사이클로펜타디에닐 양이온과 각각 4개 및 5개의 π 전자를 갖는 라디칼은 4n + 2 π 전자 기준을 충족하지 않으며 방향족이 아닙니다. 반면, 6개의 π 전자, p 오비탈의 연속적인 중첩 고리 및 비편재화된 π 전자 밀도를 갖는 사이클로펜타디에닐 음이온은 방향족이 됩니다. 음이온의 정전기 전위 맵은 고리 전체에 걸쳐 π 전자의 비편재화를 확증합니다. 더욱이, 사이클로펜타디에닐 음이온의 5가지 공명 구조와 프로스트 다이어그램에서 모든 6개의 π 전자에 의한 결합 분자 궤도의 점유는 음이온의 특이한 안정성을 입증합니다.

홀수의 탄소 원자와 고리에 개입하는 하나의 CH2 그룹을 가진 사이클로펜타디엔과 같은 중성 탄화수소는 방향족이 아닙니다. 사이클로펜타디엔 ...

aromatic-hydrocarbon-anions-structural-overview

Education

교육

JoVE Core

Organic Chemistry

유기 화학

Unit 1

제1단원

Aromatic Compounds

방향족 화합물

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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,...

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

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 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

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.

Cyclopentadienyl Anion and Aromaticity

Description

Transcript

Tags