Aromatic compounds orarenes areorganic compounds "with a chemistry typified bybenzene" and "cyclically conjugated."[1]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 satisfyingHückel's Rule.Aromatic compounds have the following general properties:
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 onemethine orvinylene (-C= or -CH=CH-) group is replaced by aheteroatom:oxygen,nitrogen, orsulfur.[3] Examples of non-benzene compounds with aromatic properties arefuran, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, andpyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom. Hydrocarbons without an aromatic ring are calledaliphatic. Approximately half of compounds known in 2000 are described as aromatic to some extent.[4]
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-calledaromatic amino acids.
Benzene, C6H6, is the least complex aromatic hydrocarbon, and it was the first one defined as such.[6] Its bonding nature was first recognized independently byJoseph Loschmidt andAugust Kekulé in the 19th century.[6] 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 the ring itself.[5] This represents the equivalent nature of the six carbon-carbon bonds all ofbond order 1.5. This equivalency can also explained byresonance forms.[5] The electrons are visualized as floating above and below the ring, with the electromagnetic fields they generate acting to keep the ring flat.[5]
The circle symbol for aromaticity was introduced bySir Robert Robinson and his student James Armit in 1925 and popularized starting in 1959 by the Morrison & Boyd textbook on organic chemistry.[7] The proper use of the symbol is debated: some publications use it toany cyclic π system, while others use it only for those π systems that obeyHückel'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.[8] In this way the circle symbol for a six-center six-electron bond can be compared to the Y symbol for athree-center two-electron bond.[8]
Benzene derivatives have from one to sixsubstituents attached to the central benzene core.[2] Examples of benzene compounds with just one substituent arephenol, which carries ahydroxyl group, andtoluene with amethyl group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which thearene substitution patternsortho,meta, andpara are devised.[9] When reacting to form more complex benzene derivatives, the substituents on a benzene ring can be described as eitheractivated ordeactivated, which are electron donating and electron withdrawing respectively.[9] Activators are known as ortho-para directors, and deactivators are known as meta directors.[9] Upon reacting, substituents will be added at 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.[9]
For example, threeisomers exist forcresol 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).[10] Given that both the methyl and hydroxyl group are ortho-para directors, the ortho and para isomers are typically favoured.[10]Xylenol has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist.[citation needed]
Arene rings can stabilize charges, as seen in, for example, phenol (C6H5–OH), which isacidic at the hydroxyl (OH), as charge on the oxygen (alkoxide –O−) is partially delocalized into the benzene ring.
Although benzylic arenes are common, non-benzylic compounds are also exceedingly important. Any compound containing a cyclic portion that conforms toHückel's rule and is not a benzene derivative can be considered a non-benzylic aromatic compound.[5]
Ofannulenes larger than benzene, [12]annulene and [14]annulene are weakly aromatic compounds and [18]annulene,Cyclooctadecanonaene, is aromatic, though strain within the structure causes a slight deviation from the precisely planar structure necessary for aromatic categorization.[11] Another example of a non-benzylic monocyclic arene is thecyclopropenyl (cyclopropenium cation), which satisfiesHückel's rule with an n equal to 0.[12] Note, only the cationic form of this cyclic propenyl is aromatic, given that neutrality in this compound would violate either the octet rule orHückel's rule.[12]
Other non-benzylic monocyclic arenes include the aforementioned heteroarenes that can replace carbon atoms with other heteroatoms such as N, O or S.[5] Common examples of these are the five-memberedpyrrole and six-memberedpyridine, both of which have a substituted nitrogen[13]
Polycyclic aromatic hydrocarbons, also known as polynuclear aromatic compounds (PAHs) are aromatic hydrocarbons that consist of fusedaromaticrings and do not containheteroatoms or carrysubstituents.[14]Naphthalene is the simplest example of a PAH. PAHs occur inoil,coal, andtar deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass).[15] As pollutants, they are of concern because some compounds have been identified ascarcinogenic,mutagenic, andteratogenic.[16][17][18][19] PAHs are also found in cooked foods.[15] 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.[15][16] They are also a goodcandidate molecule to act as a basis for the earliest forms of life.[20] Ingraphene the PAH motif is extended to large 2D sheets.[21]
Aromatic ring systems participate in many organic reactions.
In aromaticsubstitution, onesubstituent on the arene ring, usually hydrogen, is replaced by another reagent.[5] The two main types areelectrophilic aromatic substitution, when the active reagent is an electrophile, andnucleophilic aromatic substitution, when the reagent is a nucleophile. Inradical-nucleophilic aromatic substitution, the active reagent is aradical.[22][23]
An example ofelectrophilic aromatic substitution is the nitration ofsalicylic acid, where a nitro group is added para to the hydroxide substituent:
Nucleophilic aromatic substitution involves displacement of aleaving group, such as ahalide, on anaromatic ring. Aromatic rings usually nucleophilic, but in the presence ofelectron-withdrawing groups aromatic compounds undergo nucleophilic substitution. Mechanistically, this reaction differs from a commonSN2 reaction, because it occurs at a trigonal carbon atom (sp2hybridization).[24]
Hydrogenation of arenes create saturated rings. The compound1-naphthol is completely reduced to a mixture ofdecalin-olisomers.[25]
The compoundresorcinol, hydrogenated withRaney nickel in presence of aqueoussodium hydroxide forms anenolate which is alkylated withmethyl iodide to 2-methyl-1,3-cyclohexandione:[26]
Indearomatization reactions the aromaticity of the reactant is lost. In this regard, the dearomatization is related to hydrogenation. A classic approach isBirch reduction. The methodology is used in synthesis.[27]
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