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

Нейтральные моноциклические ненасыщенные углеводороды с нечетным числом атомов углерода лишены ароматичности из-за наличия в кольце промежуточного sp3-углерода.
Например, циклопентадиен не является ароматическим, так как имеет всего 4 π электронов и sp3 углерода, который нарушает непрерывное перекрытие p-орбиталей.
Примечательно, что удаление водорода из группы CH2 с обоими, одним или ни одним из связывающих электронов превращает углерод sp3 в sp2, генерируя катион, радикал и анион соответственно.
По сравнению с катионом и радикалом, только анион обладает необходимым числом (4n + 2) π электронов.
Более того, наличие 2p орбитали у sp2 углерода способствует непрерывному перекрытию p орбиталей и делокализации отрицательного заряда по всему кольцу, что делает циклопентадиениланион ароматическим.
Кроме того, π электронов, занимающих связывающие молекулярные орбитали, и пять резонансных структур еще больше подтверждают необычную стабильность ароматического аниона.

Анионы ароматических углеводородов: структурный обзор

Aromatic Hydrocarbon Anions: Structural Overview

Нейтральные углеводороды, такие как циклопентадиен с нечетным числом атомов углерода и одной промежуточной группой CH_2 в кольце, не являются ароматическими. Циклопентадиен с 4 π-электронами не удовлетворяет правилу 4n + 2 π-электронов. Кроме того, промежуточная группа CH_2 гибридизована по sp^3 и не имеет вакантной p-орбитали, тем самым непрерывно прерывая перекрытие p-орбиталей и предотвращая делокализацию π-электронов по всему кольцу.

Из-за отсутствия непрерывного перекрытия р-орбиталей циклопентадиен не соответствует критериям ароматичности. Однако, удаление одного водорода с обоими или одним электронами или без них из группы CH_2 приводит к превращению углерода sp^3 в sp^2, тем самым превращая циклопентадиен в циклопентадиенильный катион, радикал и анион, соответственно, имеющие вакантную p-орбиталь. Среди всех трех разновидностей циклопентадиенильный катион и радикал с четырьмя и пятью π-электронами, соответственно, не соответствуют критериям 4n + 2 π-электронов и не являются ароматическими. С другой стороны, циклопентадиенильный анион с 6 π-электронами, непрерывно перекрывающимся кольцом p-орбиталей и делокализованной π-электронной плотностью становится ароматическим. Карта электростатического потенциала аниона подтверждает делокализацию π-электронов по всему кольцу. Более того, пять резонансных структур циклопентадиенильного аниона и заселенность связывающих молекулярных орбиталей всеми 6 π-электронами на диаграмме Фроста подтверждают необычную стабильность аниона.

Нейтральные углеводороды, такие как циклопентадиен, с нечетным числом атомов углерода и одной промежуточной группой CH2 в кольце, не являются ароматическими. Циклопентадиен...

aromatic-hydrocarbon-anions-structural-overview

Education

JoVE Core

Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Aromatic Compounds

Ароматические соединения

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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Cyclopentadienyl Anion and Aromaticity

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