A aromatic hydrocarbon or arene (or sometimes aryl hydrocarbon ) is a hydrocarbon with sigma bonds and the electron pi is delocalized between the carbon atoms in a circle. In contrast, aliphatic hydrocarbons do not have this delocalisation. The term "aromatic" is assigned before the physical mechanisms that determine aromaticity are found; the term was created as such just because many compounds have a sweet or pleasant smell. The configuration of six carbon atoms in aromatic compounds is known as the benzene ring, after the simplest benzene hydrocarbons. Aromatic hydrocarbons can be monocyclic (MAH) or polycyclic (PAH).
Some non-benzene compounds called heteroarenes , which follow the HÃÆ'¼ckel rule (for the monocyclic ring: when the number of electrons is equal to 4 n Ã, Ã,2, where n Ã, = Ã, 0, 1, 2, 3,...), also called aromatic compounds. In this compound, at least one carbon atom is replaced by one of the oxygen heteroatoms, nitrogen, or sulfur. An example of a non-benzene compound with an aromatic property is a furan, a heterocyclic compound with a five-membered ring comprising a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.
Video Aromatic hydrocarbon
model cincin Benzene
Benzene, C 6 H 6 , is the simplest aromatic hydrocarbon, and it is the first named as such. The nature of the bond was first recognized by Kekulà © August in the 19th century. Each carbon atom in a hexagonal cycle has four electrons to share. One goes to a hydrogen atom, and each to two adjacent carbon. This leaves an electron to share with one of two adjacent carbon atoms, thus creating a double bond with one carbon and leaving a single bond with the other, which is why the benzene molecule is pulled by a single and double bond alternately around the hexagon.
This structure is illustrated as a circle around the inside of the ring to show the six electrons floating within the delocalized molecular orbitals of the size of the ring itself. This representation represents the equivalent properties of six carbon-carbon bonds of all bond sequences 1.5; equality is explained by the resonance form. Electrons are visualized as floating above and below the ring with the electromagnetic field they produce acting to keep the ring flat.
General properties of aromatic hydrocarbons:
- They display aromatisitas
- High carbon-hydrogen ratio
- They burn with a very yellow yellow flame due to the high carbon-hydrogen ratio
- They undergo electrophilic substitution reactions and nucleophilic aromatic substitutions
The circle symbol for aromaticity was introduced by Sir Robert Robinson and his disciple James Armit in 1925 and was popularized beginning in 1959 by Morrison & Boyd's textbook on organic chemistry. The proper use of the symbol is debatable; is it used to describe every cyclic? system in some publications, or just that? systems that comply with HÃÆ'¼ckel rules on others. Jensen argues that, in line with Robinson's original proposal, the use of the circle symbol should be limited to a 6-electron monocyclic system. In this way the circular symbol for a six-center six-electron bond can be compared with the Y symbol for a two-electron three-center bond.
Maps Aromatic hydrocarbon
Arene Synthesis
The reactions that make up arene compounds from unsaturated or partially unsaturated cyclic precursors are referred to as aromatization . Many laboratory methods exist for the organic synthesis of arenes from non-arene precursors. Many methods depend on the cycloaddition reaction. Alkyne trimerization describes the cyclization [2 2 2] of the three alkynes, in the reaction DÃÆ'¶tz alkyne, carbon monoxide and the chromium carbene complex are reactants. Diels-Alder reaction of alkynes with pyrone or cyclopentadienone by expulsion of carbon dioxide or carbon monoxide also forms the aren compound. In Bergman the reactant cyclization is enyne plus a hydrogen donor.
Another set of methods is the aromatization of cyclohexane butterflies and other aliphatic rings: reagents are the catalysts used in hydrogenation such as platinum, palladium and nickel (reverse hydrogenation), quinones and sulfur and selenium elements.
Reactions arene
Arenes are reactants in many organic reactions.
Aromatic substitution
In an aromatic substitution a substituent in an arene ring, usually hydrogen, is replaced by another substituent. The two main types are electrophilic aromatic substitutions when the active reagent is an electrophile and aromatic nucleophilic substitution when the reagent is a nucleophile. In the aromatic substitution of the nucleophiles, the active reagents are radical. An example of electrophilic aromatic substitution is salicylic acid nitration:
Coupling reactions
In coupling, a metal catalyzes coupling between two formal radical fragments. The general merging reaction with arenes results in the formation of new carbon-carbon bonds eg, alkylene, vinyl arena, biraries, new carbon-nitrogen bonds (aniline) or new carbon-oxygen bonds (aryloxy compounds). An example is direct arylation of perfluorobenzenes
Hydrogenation
Palm hydrogenation creates a saturated ring. The 1-naphthol compound is completely reduced to the mixture of the desalin-ol isomer.
The resorcinol compounds, hydrogenated with Raney nickel in the presence of aqueous sodium hydroxide, form an alkylated enolate with methyl iodide to 2-methyl-1,3-cyclohexandione:
Cycloadditions
The cycloaddition reaction is not common. The unusual thermal diels-alder reactivity of arenes can be found in the Wagner-Jauregg reaction. Other photochemical cycloaddition reactions with alkene occur through excimers.
Dearomatization
In the beloved reaction, the aromaticity of the reactant is lost permanently.
Benzene and derivatives of benzene
Benzene derivatives have one to six substituents attached to the central benzene core. An example of a benzene compound with only one substituent is phenol, which carries a hydroxyl group, and toluene with a methyl group. When more than one substituent is present in the ring, spatial relationships are important for substitution patterns aren ortho , meta , and para designed. For example, three isomers exist for cresol because the methyl groups and hydroxyl groups can be placed adjacent ( ortho ), one position is removed from each other ( meta ) , or two positions removed from each other ( para ). Xylenol has two methyl groups other than the hydroxyl group, and, for this structure, 6 isomers exist. Representative of arene compound
The arene ring has the ability to stabilize costs. This is seen in, for example, phenol (C 6 H 5 -OH), which is acidic in hydroxyl (OH), since the charge on this oxygen (alkoxide) -O - ) partially delocalized into the benzene ring.
Other monocyclic aromatic hydrocarbons
Other monocyclic aromatic hydrocarbons include Cyclotetradecaheptaene or Cyclooctadecanonaene.
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are aromatic hydrocarbons comprising a uniformly melted ring and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of PAH. PAHs occur in deposits of oil, coal, and tar, and are produced as a by-product of fuel combustion (whether fossil fuels or biomass). As a pollutant, they are of concern because several compounds have been identified as carcinogenic, mutagenic, and teratogenic. PAH is also found in cooked foods. Studies have shown that high levels of PAH are found, for example, in meat cooked at high temperatures such as grilling or roasting, and in smoked fish.
They are also found in interstellar mediums, in comets, and in meteorites and are candidate molecules to act as the basis for the earliest life forms. In graphene, PAH motifs are extended to large 2D sheets.
See also
- Aromatic Substituents: Aryl, Aryloxy and Arenediyl
- Asphaltene
- Hydrodealkylation
- Simple aromatic ring
- Rhodium-platinum oxide, a catalyst used to hydrogenate aromatic compounds.
References
External links
Media related to Aromatic Hydrocarbons in Wikimedia Commons
Source of the article : Wikipedia
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