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

Benzeno, ciclobutadieno e ciclooctatetraeno são representados como um híbrido de duas estruturas equivalentes.
No entanto, ao contrário do benzeno, o ciclobutadieno e o ciclooctatetraeno são instáveis e não aromáticos.
Para explicar essas diferenças, Erich Hückel delineou um conjunto de características estruturais subjacentes à aromaticidade, conhecidas como regra de Hückel ou regra 4n + 2.
A regra de Hückel afirma que um composto aromático deve ser cíclico e planar, com um orbital p não hibridizado em cada carbono.
Os orbitais p devem se sobrepor para formar um loop contínuo de (4n + 2) π elétrons, onde n é um inteiro positivo.
O benzeno preenche todos os critérios da regra de Hückel, estabelecendo sua aromaticidade.
Em contraste, o ciclooctatetraeno assume uma conformação de banheira não planar sem sobreposição entre os orbitais p adjacentes. Além disso, carece de (4n + 2) π elétrons e, portanto, não é aromático.
Da mesma forma, o ciclobutadieno é cíclico, planar e tem uma sobreposição contínua de orbitais p. No entanto, falta (4n + 2) π elétrons e perde aromaticidade.

critérios para aromaticidade e a regra hückel 4n + 2

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

Assim como o benzeno, o ciclobutadieno e o ciclooctatetraeno são compostos cíclicos com ligações simples e duplas alternadas. Porém, seu comportamento químico difere do benzeno, pois são instáveis ​​e não aromáticos. Então, quais são as características estruturais dos compostos insaturados classificados como aromáticos?

Pela primeira vez, Eric Hückel, um físico químico alemão, derivou um conjunto de características estruturais para um composto ser classificado como aromático. Isso agora é conhecido como regra de Hückel ou regra 4n + 2. Segundo essa regra, um composto aromático deve ser cíclico, planar, continuamente conjugado, possuindo um número ímpar de pares de elétrons π ou 4n + 2 elétrons π, onde n é um número inteiro (n = 0, 1, 2…). Portanto, um composto aromático deve ter 2 (n = 0), 6 (n = 1), 10 (n = 2), 14 (n = 3), etc., número de elétrons π ou 1, 3, 5, 7 , etc., pares de elétrons π.

Por exemplo, 1,3,5-hexatrieno e benzeno têm 6 elétrons π; mesmo assim, o 1,3,5-hexatrieno não é aromático, pois os orbitais p nos dois átomos de carbono terminais não se sobrepõem, pois uma ligação não conecta esses carbonos. Portanto, os compostos devem ser cíclicos para serem aromáticos.

<img alt="Figure1" src="/files/ftp_upload/12453/12453_PT-Figure_1.svg" style="text-align: center; height: 150px;">

Embora seja um composto cíclico de oito membros com quatro ligações π contínuas, o ciclooctatetraeno é instável e não aromático. Isso ocorre porque ele não é plano; ele adota uma conformação de banheira. Consequentemente, uma sobreposição efetiva de orbitais p para formar um circuito fechado da nuvem de elétrons π não é viável. Notavelmente, ao contrário do benzeno, ele sofre reações de adição.

Para que um composto seja continuamente conjugado, todos os carbonos devem ter hibridização sp^2 com um orbital p não hibridizado meio preenchido em cada um. Por exemplo, 1,3-ciclohexadieno e 1,3,5-cicloheptatrieno têm carbonos hibridizados sp^3 que não possuem um orbital p não hibridizado, então a sobreposição contínua é inatingível. Portanto, ambos não são aromáticos.

<img alt="Figure2" src="/files/ftp_upload/12453/12453_PT-Figure_2.svg" style="text-align: center; height: 85px;">

Por outro lado, o ciclobutadieno, um composto cíclico e planar com conjugação contínua, é instável e não aromático. Ele tem 2 pares de elétrons π ou 4 elétrons π, o que não satisfaz o critério de ter 4n + 2 elétrons π. Portanto, não é aromático.

Critérios para Aromaticidade e a Regra Hückel 4n + 2

Como o benzeno, o ciclobutadieno e o ciclooctatetraeno são compostos cíclicos com ligações simples e duplas alternadas. No entanto, seu comportamento químico difere ...

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

Education

JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Aromatic Compounds

Compostos Aromáticos

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

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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,...

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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...

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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,...

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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...

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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.

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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

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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...

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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

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Hückel Rule and Aromatic Ring Criteria

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