Aromatic compound
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Aromatic compounds are chemical compounds (most commonly organic) that contain one or more rings with pi electrons delocalized all the way around them. In contrast to compounds that exhibit aromaticity, aliphatic compounds lack this delocalization. The term "aromatic" was assigned before the physical mechanism determining aromaticity was discovered, and referred simply to the fact that many such compounds have a sweet or pleasant odor; however, not all aromatic compounds have a sweet odor, and not all compounds with a sweet odor are aromatic compounds.
Contents
General
Aromatic hydrocarbons or arenes, are aromatic organic compounds containing solely carbon and hydrogen atoms. The configuration of six carbon atoms in aromatic compounds is called a "benzene ring", after the simple aromatic compound benzene, or a phenyl group when part of a larger compound.
Not all aromatic compounds are benzene-based; aromaticity can also manifest in heteroarenes, which follow Hückel's rule (for monocyclic rings: when the number of its π electrons equals 4n + 2, where n = 0, 1, 2, 3, ...). In these compounds, at least one carbon atom is replaced by one of the heteroatoms oxygen, nitrogen, or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, thiophene, the sulfur analogue of furan and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.
General recognized properties of aromatic hydrocarbons:
- They display aromaticity
- The carbon–hydrogen ratio is high
- They burn with a strong sooty yellow flame because of the high carbon–hydrogen ratio
- They undergo electrophilic substitution reactions and nucleophilic aromatic substitutions
Inorganic aromatic compounds
Pentazole is a cyclical 5 nitrogen compound with one hydrogen atom which displays aromaticity.
1,2,3,4,5-Pentaphospholanepentayl and diphosphatriazolate anions have also been synthesized.[1]
Although borazine may be described as a π aromatic compound, it is not a globally aromatic species, as the electronic system is not as delocalized as it is in benzene.
General reactions
- Aromatic substitution: In aromatic substitution one substituent on the arene ring, usually hydrogen, is replaced by another substituent. The two main types are electrophilic aromatic substitution when the active reagent is an electrophile and nucleophilic aromatic substitution when the reagent is a nucleophile. In radical-nucleophilic aromatic substitution the active reagent is a radical.
- Coupling reactions: In coupling reactions a metal catalyses a coupling between two formal radical fragments. Common coupling reactions with arenes result in the formation of new carbon–carbon bonds e.g., alkylarenes, vinyl arenes, biraryls, new carbon–nitrogen bonds (anilines) or new carbon–oxygen bonds (aryloxy compounds).
- Hydrogenation: Hydrogenation of arenes create saturated rings. This is usually done with elemental hydrogenation using a platinum group metal catalyst at high pressure.
- Cycloadditions: Cycloaddition reactions are not common. Unusual thermal Diels–Alder reactivity of arenes can be found in the Wagner-Jauregg reaction.
- Dearomatization: In dearomatization reactions the aromaticity of the reactant is permanently lost. Hydrogenation, oxidation of arenes are the most common reactions.
