Name: kekule’s benzene model and its limits
Uploaded: 2023-04-30T10:30:54.000Z
Duration: 1 min 7 s
Channel: JoVE

1865년에, August Kekulé는 벤젠을 위한 첫번째 명확한 구조를 제안했다.
그는 벤젠이 단일 결합과 이중 결합이 번갈아 가며 6개의 탄소 원자로 구성된 순환 구조를 가지고 있다고 제안했습니다. 각 탄소 원자는 하나의 수소 원자에 부착되어 있습니다.
이 모델은 탄소의 사면적을 충족하여 모든 수소를 동일하게 만들었습니다.
Kekulé의 모델은 이중 결합의 위치가 다른 두 가지 형태의 1,2-dibromobenzene을 예측했습니다. 그러나 1,2-디브로모벤젠의 한 가지 형태만 분리되었습니다.
Kekulé는 현재 Kekulé 구조로 알려진 두 가지 분리 할 수없는 형태의 벤젠 사이의 빠른 평형을 제안함으로써 그것을 옹호했습니다. 그러나 현재 그러한 평형은 존재하지 않는 것으로 알려져 있습니다.
더욱이, 브롬과 치환 반응을 일으키는 벤젠과 부가반응을 겪는 알켄의 특이한 화학적 거동을 설명하지 못했다.

벤젠의 구조: 케쿨레 모델

Structure of Benzene: Kekul&eacute; Model

1865년 아우구스트 케쿨레는 벤젠의 공식은 C_6H_6이고, 벤젠의 모든 수소는 동일하며, 각 탄소는 4가로 인해 4개의 결합을 가져야 한다는 세 가지 주장을 기반으로 유기 화학의 구조 이론에 따라 벤젠의 구조를 제안했습니다.

그는 벤젠이 6개의 탄소 원자가 각각 하나의 수소 원자에 부착되고 3개의 교번 파이 결합을 갖는 고리 구조를 가지고 있다고 제안했습니다.

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

그의 모델에 따르면 그림 2에 표시된 것처럼 1,2-이치환된 벤젠 유도체에는 두 개의 이성질체가 있어야 하며, 한 구조에서는 두 개의 치환된 탄소가 이중 결합으로 연결되어 있지만 다른 구조에서는 단일 결합으로 연결되어 있습니다.

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

1,2-이치환된 벤젠에 대해 단 하나의 구조만 발견되었기 때문에 케쿨레의 이러한 개념은 잘못된 것으로 입증되었습니다. 이를 설명하기 위해 그는 두 가지 형태의 벤젠이 빠른 평형 상태에 있으며 두 화합물을 분리하는 것이 불가능하다고 말했습니다.

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

그러나 그러한 평형은 존재하지 않으며 1,2-이치환된 벤젠 유도체에 대한 구조는 하나만 발견됩니다.

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

벤젠은 4개의 불포화도를 가지며, 1개의 고리와 3개의 파이 결합을 구성합니다. 그러나 벤젠은 알켄이나 알킨과 같은 다른 불포화 탄화수소와 달리 첨가 반응을 일으키지 않습니다. 예를 들어, 에틸렌은 브롬과 첨가 반응을 거쳐 에틸렌 디브로마이드를 형성하지만 벤젠은 그렇지 않습니다.

<img alt="Figure5" src="/files/ftp_upload/12450/12450_PT_Figure5.svg" style="text-align: center; height: 120px;">

<img alt="Figure6" src="/files/ftp_upload/12450/12450_PT_Figure6.svg" style="text-align: center; height: 60px;">

벤젠은 루이스산 브롬화철 존재 하에서 치환 반응을 통해 브롬과 반응하여 브로모벤젠을 형성합니다.

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

케쿨레 모델의 가장 큰 단점은 알켄 및 알킨과 같은 다른 불포화 탄화수소와 비교하여 벤젠의 이러한 비정상적인 반응성을 설명하지 못했다는 것입니다.

1865 년 August Kekule은 세 가지를 기반으로 유기 화학의 구조 이론에 따라 벤젠의 구조를 제안했습니다.

Education

교육

JoVE Core

Organic Chemistry

유기 화학

Unit 1

제1단원

Aromatic Compounds

방향족 화합물

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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 they are linked by a single bond.

That prediction did not match experimental results. Only one structure is found for 1,2-disubstituted benzene derivatives. Kekule tried to explain this by saying the two forms of benzene stay in rapid equilibrium and cannot be separated, but no such equilibrium exists.

Benzene has four degrees of unsaturation, which means one ring and three pi bonds. Even so, it does not undergo addition reactions the way alkenes and alkynes do. Ethylene, for example, adds bromine to form ethylene dibromide, but benzene does not.

Instead, benzene reacts with bromine by substitution when ferric bromide, a Lewis acid, is present. The main weakness of Kekule’s model is that it does not explain this unusual reactivity compared with other unsaturated hydrocarbons.

Kekule’s Benzene Model and Its Limits

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 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-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 how Kekule proposed benzene’s ring structure, why it predicted two isomers, and why the model could not explain benzene’s unusual reactivity.

Kekule’s Benzene Model and Its Limits | 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.

Kekule’s Benzene Model and Its Limits

Description

Transcript