Aromatic compounds or arenes usually refers to organic compounds "with a chemistry typified by benzene " and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation to their odor. Aromatic compounds are now defined as cyclic compounds satisfying Hückel's Rule . Aromatic compounds have the following general properties:
24-480: Arenes are typically split into two categories - benzoids, that contain a benzene derivative and follow the benzene ring model, and non-benzoids that contain other aromatic cyclic derivatives. Aromatic compounds are commonly used in organic synthesis and are involved in many reaction types, following both additions and removals, as well as saturation and dearomatization. Heteroarenes are aromatic compounds, where at least one methine or vinylene (-C= or -CH=CH-) group
48-421: A leaving group , such as a halide , on an aromatic ring . Aromatic rings usually nucleophilic, but in the presence of electron-withdrawing groups aromatic compounds undergo nucleophilic substitution. Mechanistically, this reaction differs from a common S N 2 reaction , because it occurs at a trigonal carbon atom (sp hybridization ). Hydrogenation of arenes create saturated rings. The compound 1-naphthol
72-454: A carbon atom with four single bonds, where one bond is to a hydrogen atom ( >CH− ). Two or more methine bridges can overlap, forming a chain or ring of carbon atoms connected by alternating single and double bonds, as in piperylene H 2 C=CH−CH=CH−CH 3 , or the compound Every carbon atom in this molecule is a methine carbon atom, except for three; two that are attached to the two nitrogen atoms and not to any hydrogen atoms, and
96-449: A good candidate molecule to act as a basis for the earliest forms of life . In graphene the PAH motif is extended to large 2D sheets. Aromatic ring systems participate in many organic reactions. In aromatic substitution , one substituent on the arene ring, usually hydrogen, is replaced by another reagent. The two main types are electrophilic aromatic substitution , when the active reagent
120-437: Is Birch reduction . The methodology is used in synthesis. Methine group In organic chemistry , a methine group or methine bridge is a trivalent functional group =CH− , derived formally from methane . It consists of a carbon atom bound by two single bonds and one double bond , where one of the single bonds is to a hydrogen . The group is also called methyne or methene , but its IUPAC systematic name
144-429: Is methylylidene or methanylylidene . This group is sometimes called "methylidyne", however that name belongs properly to either the methylidyne group ≡CH (connected to the rest of the molecule by a triple bond) or to the methylidyne radical ⫶ CH (the two atoms as a free molecule with dangling bonds). The name "methine" is also widely used in non-systematic nomenclature for the methanetriyl group (IUPAC):
168-400: Is an electrophile, and nucleophilic aromatic substitution , when the reagent is a nucleophile. In radical-nucleophilic aromatic substitution , the active reagent is a radical . An example of electrophilic aromatic substitution is the nitration of salicylic acid , where a nitro group is added para to the hydroxide substituent: Nucleophilic aromatic substitution involves displacement of
192-427: Is aromatic, though strain within the structure causes a slight deviation from the precisely planar structure necessary for aromatic categorization. Another example of a non-benzylic monocyclic arene is the cyclopropenyl (cyclopropenium cation), which satisfies Hückel's rule with an n equal to 0. Note, only the cationic form of this cyclic propenyl is aromatic, given that neutrality in this compound would violate either
216-415: Is completely reduced to a mixture of decalin -ol isomers . The compound resorcinol , hydrogenated with Raney nickel in presence of aqueous sodium hydroxide forms an enolate which is alkylated with methyl iodide to 2-methyl-1,3-cyclohexandione: In dearomatization reactions the aromaticity of the reactant is lost. In this regard, the dearomatization is related to hydrogenation. A classic approach
240-421: Is replaced by a heteroatom : 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, and pyridine , a heterocyclic compound with a six-membered ring containing one nitrogen atom. Hydrocarbons without an aromatic ring are called aliphatic . Approximately half of compounds known in
264-500: Is the simplest example of a PAH. PAHs occur in oil , coal , and tar deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As pollutants, they are of concern because some compounds have been identified as carcinogenic , mutagenic , and teratogenic . PAHs are also found in cooked foods. Studies have shown that high levels of PAHs are found, for example, in meat cooked at high temperatures such as grilling or barbecuing, and in smoked fish. They are also
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#1732858413989288-453: The Morrison & Boyd textbook on organic chemistry. The proper use of the symbol is debated: some publications use it to any cyclic π system, while others use it only for those π systems that obey Hückel's rule . Some argue that, in order to stay in line with Robinson's originally intended proposal, the use of the circle symbol should be limited to monocyclic 6 π-electron systems. In this way
312-409: The benzene ring. Although benzylic arenes are common, non-benzylic compounds are also exceedingly important. Any compound containing a cyclic portion that conforms to Hückel's rule and is not a benzene derivative can be considered a non-benzylic aromatic compound. Of annulenes larger than benzene, [12]annulene and [14]annulene are weakly aromatic compounds and [18]annulene, Cyclooctadecanonaene ,
336-416: The carbon attached to the nitrogen atom, which is attached to two hydrogen atoms (far right). There is a five-carbon-atom poly-methine chain in the center of this molecule. Chains of alternating single and double bonds often form conjugated systems . When closed, as in benzene (=CH−CH=) 3 , they often give aromatic character to the compound. Xylenol Xylenols are organic compounds with
360-412: The circle symbol for a six-center six-electron bond can be compared to the Y symbol for a three-center two-electron bond . Benzene derivatives have from one to six substituents attached to the central benzene core. Examples of benzene compounds with just one substituent are phenol , which carries a hydroxyl group, and toluene with a methyl group. When there is more than one substituent present on
384-549: The formula (CH 3 ) 2 C 6 H 3 OH. They are volatile colorless solids or oily liquids. They are derivatives of phenol with two methyl groups at various positions relative to the hydroxyl group. Six isomers exist, of which 2,6-xylenol with both methyl groups in an ortho position with respect to the hydroxyl group is the most important. The name xylenol is a portmanteau of the words xylene and phenol . 2,4-Dimethylphenol together with other xylenols and many other compounds are traditionally extracted from coal tar ,
408-411: The methyl and hydroxyl group are ortho-para directors, the ortho and para isomers are typically favoured. Xylenol has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist. Arene rings can stabilize charges, as seen in, for example, phenol (C 6 H 5 –OH), which is acidic at the hydroxyl (OH), as charge on the oxygen (alkoxide –O) is partially delocalized into
432-541: The octet rule or Hückel's rule . Other non-benzylic monocyclic arenes include the aforementioned heteroarenes that can replace carbon atoms with other heteroatoms such as N, O or S. Common examples of these are the six-membered pyrrole and five-membered pyridine , both of which have a substituted nitrogen Polycyclic aromatic hydrocarbons , also known as polynuclear aromatic compounds (PAHs) are aromatic hydrocarbons that consist of fused aromatic rings and do not contain heteroatoms or carry substituents . Naphthalene
456-538: The ortho, para or meta positions, depending on the directivity of the current substituents to make more complex benzene derivatives, often with several isomers. Electron flow leading to re-aromatization is key in ensuring the stability of such products. For example, three isomers exist for cresol because the methyl group and the hydroxyl group (both ortho para directors) can be placed next to each other ( ortho ), one position removed from each other ( meta ), or two positions removed from each other ( para ). Given that both
480-459: The ring itself. This represents the equivalent nature of the six carbon-carbon bonds all of bond order 1.5. This equivalency can also explained by resonance forms . The electrons are visualized as floating above and below the ring, with the electromagnetic fields they generate acting to keep the ring flat. The circle symbol for aromaticity was introduced by Sir Robert Robinson and his student James Armit in 1925 and popularized starting in 1959 by
504-491: The ring, their spatial relationship becomes important for which the arene substitution patterns ortho , meta , and para are devised. When reacting to form more complex benzene derivatives, the substituents on a benzene ring can be described as either activated or deactivated , which are electron donating and electron withdrawing respectively. Activators are known as ortho-para directors, and deactivators are known as meta directors. Upon reacting, substituents will be added at
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#1732858413989528-461: The volatile materials obtained in the production of coke from coal. These residue contains a few percent by weight of xylenols as well as cresols and phenol. The main xylenols in such tar are the 3,5-, 2,4, and 2,3- isomers. 2,6-Xylenol is produced by methylation of phenol using methanol in the presence of metal oxide catalysts: Typically, xylenols also ethylphenols , which have very similar physical properties. The physical properties of
552-413: The year 2000 are described as aromatic to some extent. Aromatic compounds are pervasive in nature and industry. Key industrial aromatic hydrocarbons are benzene, toluene , Xylene called BTX. Many biomolecules have phenyl groups including the so-called aromatic amino acids . Benzene , C 6 H 6 , is the least complex aromatic hydrocarbon, and it was the first one defined as such. Its bonding nature
576-407: Was first recognized independently by Joseph Loschmidt and August Kekulé in the 19th century. Each carbon atom in the hexagonal cycle has four electrons to share. One electron forms a sigma bond with the hydrogen atom, and one is used in covalently bonding to each of the two neighboring carbons. This leaves six electrons, shared equally around the ring in delocalized pi molecular orbitals the size of
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