Name: aromatic compounds in plants and fuels
Uploaded: 2023-04-30T10:30:40.000Z
Duration: 1 min 25 s
Channel: JoVE

Les composés aromatiques sont des hydrocarbures avec des liaisons π conjuguées enfermées dans un cycle. Le benzène et ses dérivés sont les exemples les plus connus de composés aromatiques.
À l'origine, les dérivés du benzène étaient isolés des huiles naturelles produites par les plantes et les arbres et étaient appelés « aromatiques » en raison de leur odeur distinctive.
Par exemple, le benzaldéhyde des amandes et des cerises, le toluène du baume de Tolu et l’eugénol du clou de girofle ont des odeurs distinctes.
Au fil du temps, le terme aromatique a continué à décrire tous les dérivés du benzène, qu’ils soient parfumés ou non. Par exemple, l’analgésique aspirine contient de l’acide acétylsalicylique, un composé aromatique inodore.
Structurellement, les composés aromatiques sont des hydrocarbures hautement insaturés et présentent un comportement chimique différent de celui des alcènes, des alcynes et des polyènes à chaîne ouverte.
Les principales sources de composés aromatiques simples sont le goudron de houille et le pétrole.
La distillation fractionnée du goudron de houille produit de nombreux composés aromatiques tels que le benzène, le toluène, le xylène, le naphtalène et autres.
De même, lors du raffinage du pétrole, les alcanes sont convertis en composés aromatiques.

composés aromatiques : aperçu

Aromatic Compounds: Overview

En général, le terme « aromatique » indique une odeur ou un parfum agréable provenant de fleurs fraîches, de café fraîchement préparé, etc. Au début de la chimie organique, de nombreux dérivés du benzène ont été isolés des huiles à odeur agréable des plantes. Par exemple, la vanilline a été isolée de l’huile de vanille, le salicylate de méthyle de l’huile de gaulthérie et le cinnamaldéhyde de l’huile de cannelle. Ils avaient tous une odeur agréable ; d'où le nom aromatique.

En 1825, Faraday isola le benzène du gaz d’éclairage comprimé (le combustible brûlé dans les lampes à gaz) et le nomma « bicarbure d’hydrogène ». Il a également effectué une analyse élémentaire du composé et observé un rapport hydrogène/carbone de 1:1.

Plus tard, Eilhardt Mitscherlich a synthétisé le benzène à partir de l'acide benzoïque (isolé de la gomme benjoin) et de l'oxyde de calcium en 1834. En effectuant des mesures de densité de vapeur, il a montré que la formule moléculaire du benzène était C6H6. Puisque le nouveau composé était dérivé de la gomme de benjoin, il l'a nommé Benzine, maintenant connu sous le nom de benzène. Il est utilisé comme solvant de laboratoire.

<img alt="Figure1" src="/files/ftp_upload/12448/12448_PT_Figure_1.svg" style="text-align: center; height: 125px;">

Le terme « aromatique » est encore utilisé pour désigner tous les dérivés du benzène, parfumés ou non. De nombreux composés naturels et pharmaceutiques sont aromatiques. Par exemple, l'estrone est un stéroïde, l'atorvastatine (Lipitor) est un médicament hypocholestérolémiant, le Zoloft est un antidépresseur et le Serevent utilisé pour traiter l'asthme sont des composés aromatiques.

<img alt="Figure2" src="/files/ftp_upload/12448/12448_PT_Figure_2.svg" style="text-align: center; height: 425px;">

Les composés aromatiques sont principalement obtenus à partir du charbon et du pétrole. Le charbon produit du goudron de houille lorsqu'il est chauffé à 1 000 °C en l'absence d'air. Le goudron de houille, lors d'une distillation fractionnée, produit de nombreux hydrocarbures aromatiques tels que le benzène, le toluène, le diméthylbenzène (xylène), le naphtalène, le biphényle, l'indène, l'anthracène, etc.

Cependant, les composés aromatiques ne peuvent pas être obtenus directement à partir du pétrole. Lors du raffinage du pétrole, les alcanes sont transformés en composés aromatiques en présence d'un catalyseur à 500 °C sous haute pression.

Composés aromatiques : aperçu

En général, le terme « aromatique » indique une odeur ou un parfum agréable provenant de fleurs fraîches, de café fraîchement préparé, etc. Au début de l’histoire...

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JoVE Core

Organic Chemistry

Chimie organique

Unit 1

Unité 1

Aromatic Compounds

Composés aromatiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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, which was used in gaslights. He called it bicarburet of hydrogen and found a 1:1 ratio of hydrogen to carbon. In 1834, Eilhardt Mitscherlich made benzene from benzoic acid, which comes from gum benzoin, and calcium oxide. He used vapor density measurements to show that benzene has the formula C6H6, and he named it Benzin, now called benzene.

Today, the term aromatic includes many benzene derivatives, even when they do not smell pleasant. Benzene itself is used as a laboratory solvent. Many natural and pharmaceutical compounds are aromatic, including estrone, atorvastatin (Lipitor), Zoloft, and Serevent.

Aromatic compounds are mainly obtained from coal and petroleum. When coal is heated to 1000 °C without air, it forms coal tar. Fractional distillation of coal tar gives aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene, biphenyl, indene, and anthracene. Aromatic compounds are not taken directly from petroleum, but petroleum refining can convert alkanes into aromatic compounds with a catalyst at 500 °C under high pressure.

Aromatic Compounds in Plants and Fuels

aromatic compounds: overview

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

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

Explore how aromatic compounds were first found in plant oils, how benzene was identified, and how coal tar and petroleum produce key aromatic hydrocarbons.

Aromatic Compounds in Plants and Fuels | JoVE K12

organic chemistry

JoVE Core Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

Aromatic Compounds in Plants and Fuels

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