Mononuclear arenes.
Polynuclear arenes with condensed and isolated cycles.
1. The structure of benzene ring
Arenes are hydrocarbons based on the benzene ring as a structural unit. Benzene, toluene, and naphthalene, for example, are arenes.
One factor that makes conjugation in arenes special is its cyclic nature. A conjugated system that closes upon itself can have properties that are much different from those of open-chain polyenes. Arenes are also referred to as aromatic hydrocarbons.
Used in this sense, the word “aromatic” has nothing to do with odor but means instead that arenes are much more stable than we expect them to be based on their formulation as conjugated trienes. The benzene ring is a fundamental structural unit in the molecules of arenes and effects as a substituent.
In 1825, Michael Faraday isolated a new hydrocarbon from illuminating gas, which he called “bicarburet of hydrogen.” Nine years later Eilhardt Mitscherlich of theUniversity of Berlin prepared the same substance by heating benzoic acid with lime and found it to be a hydrocarbon having the empirical formula CnHn.
Eventually, because of its relationship to benzoic acid, this hydrocarbon came to be named benzin, then later benzene, the name by which it is known today.
Arenes, are cyclic unsaturated compounds that have such strikingly different chemical properties from conjugated alkenes (polyenes) that it is convenient to consider them as a separate class of hydrocarbon. The simplest member is benzene, C6H6, which frequently is represented as a cyclic conjugated molecule of three single and three double carbon-carbon bonds. Actually, all the carbon-carbon bonds are equivalent but it is convenient to represent the structure in the manner shown:
The atoms of carbon are sp2-hybridizated in benzene ring. Each of the six π-electrons, therefore, is localized neither on a single carboor in a bond between two carbons (as in an alkene). Instead, each π-electron is shared by all six carbons. The six π-electrons are delocalized—they roam freely within the doughnutshaped clouds that lie over and under the ring of carbon atoms. Consequently, benzene can be represented by a hexagon containing either dashed lines or a circle, to symbolize the six delocalized electrons.
This type of representation makes it clear that there are no double bonds in benzene. The actual structure of benzene is a Kekulé structure with delocalized electrons.
2. The nomenclature and isomery
All compounds that contain a benzene ring are aromatic, and substituted derivatives of benzene make up the largest class of aromatic compounds. Many such compounds are named by attaching the name of the substituent as a prefix to benzene.
Many simple monosubstituted derivatives of benzene have commoames of long standing that have been retained in the IUPAC system. Table below lists some of the most important ones.
Dimethyl derivatives of benzene are called xylenes. There are three xylene isomers, the ortho (o)-, meta (m)-, and para ( p)– substituted derivatives. This is their isomery.
The prefix ortho signifies a 1,2-disubstituted benzene ring, meta signifies 1,3-disubstitution, and para signifies 1,4-disubstitution. The prefixes o, m, and p can be used when a substance is named as a benzene derivative or when a specific base name (such as acetophenone) is used.
The prefixes o, m, and p are not used when three or more substituents are present on benzene; numerical locants must be used instead.
In these examples the base name of the benzene derivative determines the carbon at which numbering begins: anisole has its methoxy group at C-1, toluene its methyl group at C-1, and aniline its amino group at C-1. The direction of numbering is chosen to give the next substituted position the lowest number irrespective of what substituent it bears.
The order of appearance of substituents in the name is alphabetical. Wheo simple base name other than benzene is appropriate, positions are numbered so as to give the lowest locant at the first point of difference. Thus, each of the following examples is named as a 1,2,4-trisubstituted derivative of benzene rather than as a 1,3,4-derivative:
When the benzene ring is named as a substituent, the word “phenyl” stands for C6H5−. Similarly, an arene named as a substituent is called an aryl group. A benzylgroup is C6H5CH2−.
Biphenyl is the accepted IUPAC name for the compound in which two benzene rings are connected by a single bond.
3. The methods of extraction of arenes
1. Extraction from oil (oil contains cyclohexane).
2. Cyclotrimerisation of alkynes
3HC≡CH →
3. Vurts-Fittih reaction
Physical properties of arenes
Iormal conditions benzene and other members of homological row are liquids. They are not dissoluble in water but are dissoluble in different organic solvents. A lot of arenes are good solvents too. They have specific smell. Benzene and toluene are poisonous.
Chemical properties of arenes
I. The reactions of substitution
1. Nitration
2. Sulphation
3. Halogenation
4. Alkylation after Fridel-Chrafts
II. The reactions of joining
1. The reaction with chlorine
2. The reaction with hydrogen
III. The reactions of oxidation
1. The oxidation of benzene
2. The oxidation of benzene homologs
3. Ozonation
4. Burning
2C6H6 + 15O2 → 6H2O + 12 CO2 + Q
The orientation in benzoic ring
If there are one substituent in benzoic ring the second substituent has certain location relatively the first one. All substituents are divided into 2 groups by their orientational action:
1. the first group of orientators: −Cl
−Br
−I
−OH
−NH2
−CH3 and other alkyl radicals
These orientators orient other substituents to ortho- and para-locations in benzoic ring.
2. the second group of orientators:
−C≡N
These orientators orient other substituents to meta-locations in benzoic ring.
If two substituents are orientators of the first group the location of the third substituent is determined by the stronger orientator from row below:
O>NR2>NHR>NH2>OH>OR>NHCOR>OCOR>Alk>F>Cl>Br>I
If two substituents are orientators of the second group the location of the third substituent is determined by the stronger orientator from row below:
COOH>SO3H>NO2>CHO>COCH3>CN
Polycyclic arenes
Members of a class of arenes called polycyclic benzenoid aromatic hydrocarbons possess substantial resonance energies because each is a collection of benzene rings fused together. Naphthalene, anthracene, and phenanthrene are the three simplest members of this class. They are all present in coal tar, a mixture of organic substances formed when coal is converted to coke by heating at high temperatures (about 1000°C) in the absence of air. Naphthalene is bicyclic (has two rings), and its two benzene rings share a common side. Anthracene and phenanthrene are both tricyclic aromatic hydrocarbons. Anthracene has three rings fused in a “linear” fashion, and “angular” fusion characterizes phenanthrene.
The structural formulas of naphthalene, anthracene, and phenanthrene are shown along with the numbering system used to name their substituted derivatives:
In general, the most stable resonance structure for a polycyclic aromatic hydrocarbon is the one which has the greatest number of rings that correspond to Kekulé formulations of benzene. Naphthalene provides a fairly typical example:
A large number of polycyclic benzenoid aromatic hydrocarbons are known. Many have been synthesized in the laboratory, and several of the others are products of combustion. Benzo[a]pyrene, for example, is present in tobacco smoke, contaminates food cooked on barbecue grills, and collects in the soot of chimneys. Benzo[a]pyrene is a carcinogen (a cancer-causing substance). It is converted in the liver to an epoxy diol that can induce mutations leading to the uncontrolled growth of certain cells.
There are polycyclic benzenoid aromatic hydrocarbons with condensed and isolated benzoic rings.
Naphthalene, anthracene, phenanthrene, tetracene, chrysene are polycyclic benzenoid aromatic hydrocarbons with condensed benzoic rings. In their molecules all rings have common atoms of carbon.
chrysene
Biphenyl, diphenylmethane, triphenylmethane are polycyclic benzenoid aromatic hydrocarbons with isolated benzoic rings. In their molecules all rings have common bond, alkyl- or other radicals.
Physical properties of arenes
In general, arenes resemble other hydrocarbons in their physical properties. They are nonpolar, insoluble in water, and less dense than water. In the absence of polar substituents, intermolecular forces are weak and limited to van der Waals attractions of the induced-dipole/induced-dipole type.
At one time, benzene was widely used as a solvent. This use virtually disappeared when statistical studies revealed an increased incidence of leukemia among workers exposed to atmospheric levels of benzene as low as 1 ppm. Toluene has replaced benzene as an inexpensive organic solvent, because it has similar solvent properties but has not been determined to be carcinogenic in the cell systems and at the dose levels that benzene is.
Chemical properties of arenes
The chemical properties of arenes depend on different functional groups are present in the molecule. The hydrocarbon group from benzene (C6H5−) is called a phenyl group. The phenyl functional group has the next chemical properties:
1. Benzene with sodium and methanol or ethanol in liquid ammonia converts to 1,4-cyclohexadiene. Metal–ammonia–alcohol reductions of aromatic rings are known as Birch reductions, after the Australian chemist Arthur J. Birch, who demonstrated their usefulness beginning in the 1940s.
Alkyl-substituted arenes give 1,4-cyclohexadienes in which the alkyl group is a substituent on the double bond.
In other reactions the phenyl radical is very stable and only substituents take place in reactions.
2. Halogenation
3. Chromic acid, for example, prepared by adding sulfuric acid to aqueous sodium dichromate, is a strong oxidizing agent but does not react either with benzene or with alkanes.
On the other hand, an alkyl side chain on a benzene ring is oxidized on being heated with chromic acid. The product is benzoic acid or a substituted derivative of benzoic acid.
4. Dehydrogenation
5. Dehydration
6. Dehydrohalogenation
7. Hydrogenation
8. Halogenation
9. Hydrohalogenation
10. Unbenzenoid aromatic systems
Except of benzene and its derivatives there are some unbenzenoid aromatic compounds which have cycles with double bonds in their molecules.
Structural studies confirm the absence of appreciable π-electron delocalization in cyclooctatetraene. Its structure is as pictured below — a nonplanar hydrocarbon with four short carbon–carbon bond distances and four long carbon–carbon bond distances. Cyclooctatetraene is satisfactorily represented by a single Lewis structure having alternating single and double bonds in a tub-shaped eight-membered ring. All the evidence indicates that cyclooctatetraene lacks the “special stability” of benzene, and is more appropriately considered as a conjugated polyene than as an aromatic hydrocarbon.
Cyclobutadiene escaped chemical characterization for more than 100 years. Despite numerous attempts, all synthetic efforts met with failure. It became apparent not only that cyclobutadiene was not aromatic but that it was exceedingly unstable. Beginning in the 1950s, a variety of novel techniques succeeded in generating cyclobutadiene as a transient, reactive intermediate. Thus cyclobutadiene, like cyclooctatetraene, is not aromatic. Cyclic conjugation, although necessary for aromaticity, is not sufficient for it. Some other factor or factors must contribute to the special stability of benzene and its derivatives. To understand these factors, it is necessary to return to the molecular orbital description of benzene.
The general term annulene has been coined to apply to completely conjugated monocyclic hydrocarbons. A numerical prefix specifies the number of carbon atoms. Cyclobutadiene is [4]-annulene, benzene is [6]-annulene, and cyclooctatetraene is [8]-annulene.
Naphthalene, also known as naphthalin, or antimite and not to be confused with naphtha, is a crystalline, aromatic, white, solid hydrocarbon with formula C10H8 the structure of two fused benzene rings. It is best known as the traditional, primary ingredient and of mothballs. It is volatile, forming an inflammable vapor, and readily sublimes at room temperature, producing a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass.
Structure and reactivity
A naphthalene molecule is derived by the fusion of a pair of benzene rings. (In organic chemistry, rings are fused if they share two or more atoms.) Accordingly, naphthalene is classified as a benzenoid polycyclic aromatic hydrocarbon (PAH). There are two sets of equivalent hydrogen atoms: the alpha positions are positions 1, 4, 5, and 8 on the drawing below, and the beta positions are positions 2, 3, 6, and 7.
Unlike benzene, the carbon-carbon bonds iaphthalene are not of the same length. The bonds C1–C2, C3–C4, C5–C6 and C7–C8 are about 1.36 Å (136 pm) in length, whereas the other carbon-carbon bonds are about 1.42 Å (142 pm) long. This difference, which was established by x-ray diffraction, is consistent with the valence bond model of bonding iaphthalene that involves three resonance structures (as shown below); whereas the bonds C1–C2, C3–C4, C5–C6 and C7–C8 are double in two of the three structures, the others are double in only one.
Like benzene, naphthalene can undergo electrophilic aromatic substitution. For many electrophilic aromatic substitution reactions, naphthalene reacts under milder conditions than does benzene. For example, whereas both benzene and naphthalene react with chlorine in the presence of a ferric chloride or aluminium chloride catalyst, naphthalene and chlorine can react to form 1-chloronaphthalene even without a catalyst. Similarly, whereas both benzene and naphthalene can be alkylated using Friedel-Crafts reactions, naphthalene can also be alkylated by reaction with alkenes or alcohols, with sulfuric or phosphoric acid as the catalyst.
Substituted derivatives
Two isomers are possible for mono-substituted naphthalenes, corresponding to substitution at an alpha or beta position. Usually, electrophiles attack at the alpha position. The selectivity for alpha over beta substitution can be rationalized in terms of the resonance structures of the intermediate: for the alpha substitution intermediate, seven resonance structures can be drawn, of which four preserve an aromatic ring. For beta substitution, the intermediate has only six resonance structures, and only two of these are aromatic. Sulfonation, however, gives a mixture of the “alpha” product 1-naphthalenesulfonic acid and the “beta” product 2-naphthalenesulfonic acid, with the ratio dependent on reaction conditions. The 1-isomer forms predominantly at 25 °C, and the 2-isomer at 160 °C.
Naphthalene can be hydrogenated under high pressure in the presence metal catalysts to give 1,2,3,4-tetrahydronaphthalene or tetralin (C10H12). Further hydrogenation yields decahydronaphthalene or decalin (C10H18). Oxidation with chromate or permanganate, or catalytic oxidation with O2 and a vanadium catalyst, gives phthalic acid.
Anthracene has the ability to photodimerize with irradiation by UV light. This results in considerable changes in the physical properties of the material.
The dimer is connected by two covalent bonds resulting from the [4+4] cycloaddition. The dimer reverts to anthracene thermally or with UV irradiation below 300 nm. The reversible bonding and photochromic property of anthracenes is the basis of many potential applications using poly and monosubstituted anthracene derivatives. The reaction is sensitive to oxygen. Reduction of anthracene generally yields 9,10-dihydroanthracene (destroying the aromaticity of the center ring) rather than 1,4-dihydroanthracene (which would destroy the aromaticity of one of the terminal rings). This preference for reduction at the 9 and 10 positions is explained by the fact that aromatic stabilization energy is directly correlated with the number of conjugated pi bonds in an aromatic system. Since 9,10-dihydroanthracene essentially preserves two “benzene” rings (a total of 6 conjugated pi bonds), whereas the 1,4-isomer only preserves one and a half such rings (a total of 5 pi bonds), the latter is not the thermodynamically favorable product. Similarly, electrophilic substitution occurs at the “9” and “10” positions of the center ring; oxidation occurs readily, giving anthraquinone, C14H8O2 (below).
Phenanthrene is insoluble in water but is soluble in most organic solvents such as toluene, carbon tetrachloride, ether, chloroform, acetic acid and benzene. A classical phenanthrene synthesis is the Bardhan-Sengupta Phenanthrene Synthesis (1932). In the second step of this reaction 9,10-dihydrophenanthrene is oxidized with elemental selenium.
Phenanthrene can also be obtained photochemically from certain diarylethenes.
Reactions of phenanthrene typically occur at the 9 and 10 positions, including:
hrough tobacco smoke and ingestion of food contaminated with combustion products.
Biphenyl (or diphenyl or phenyl benzene or 1,1′-biphenyl or lemonene) is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula (C6H5)2. It is notable as a starting material for the production of polychlorinated biphenyls (PCBs), which were once widely used as dielectric fluids and heat transfer agents. Biphenyl is also an intermediate for the production of a host of other organic compounds such as emulsifiers, optical brighteners, crop protection products, and plastics.
Substituted biphenyls can be prepared synthetically by various coupling reactions including the Suzuki reaction and the Ullmann reaction and have many uses. Polychlorinated biphenyls were once used as cooling and insulating fluids and polybrominated biphenyls are flame retardants. The biphenyl motif also appears in drugs such as Valsartan and Telmisartan. The abbreviation E7 stands for a liquid crystal mixture consisting of several cyanobiphenyls with long aliphatic tails used commercially in liquid crystal displays. A variety of benzidine derivatives are used in dyes and polymers. Research into Biphenyl liquid crystal candidates mainly focuses on molecules with highly polar heads (for example cyano or halide groups) and aliphatic tails.
Diphenylmethane is an organic compound with the formula (C6H5)2CH2. The compound consists of methane wherein two hydrogen atoms are replaced by two phenyl groups. Diphenylmethane forms a common skeleton in organic chemistry; the diphenylmethyl group is also known as benzhydryl. It is prepared by the reaction of benzyl chloride with benzene in the presence of a Lewis acid such as aluminium trichloride:
C6H5CH2Cl + C6H6 → (C6H5)2CH2 + HCl
Triphenylmethane, or triphenyl methane, is the hydrocarbon with the formula (C6H5)3CH. This colorless solid is soluble in nonpolar organic solvents and not in water. Triphenylmethane has the basic skeleton of many synthetic dyes called triarylmethane dyes, many of them are pH indicators, and some display fluorescence. A trityl group in organic chemistry is a triphenylmethyl group Ph3C, e.g. triphenylmethyl chloride — trityl chloride.
Preparation
Triphenylmethane can be synthesized by Friedel-Crafts reaction from benzene and chloroform with aluminium chloride catalyst:
3 C6H6 + CHCl3 → Ph3CH + 3 HCl
Alternatively, benzene may react with carbon tetrachloride using the same catalyst to obtain the trityl chloride-aluminium chloride adduct, which is hydrolyzed with dilute acid:
3 C6H6 + CCl4 + AlCl3 → Ph3CCl·AlCl3
Ph3CCl·AlCl3 + HCl → Ph3CH
Synthesis from benzylidene chloride, prepared from benzaldehyde and phosphorus pentachloride, is used as well.
