Mononuclear arenMononuclear areneess..

Polynuclear Polynuclear arenes with arenes with condensed and isolated condensed and isolated

cycles.cycles.

Ass. Medvid I.I., ass. Burmas N.I.Ass. Medvid I.I., ass. Burmas N.I.

OutlineOutline

• The structure of benzene ring

• The methods of extraction of arenes

• Physical properties of arenes

• Chemical properties of arenes

• The orientation in benzoic ring

• Polycyclic arenes

• Unbenzenoid aromatic systems

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 the University 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 carbon nor 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 ora 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 common names of long standing that have been retained in the IUPAC system. Table below lists some of the most important ones.

Table 1. Names of some frequently Encounteredderivatives of benzene

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. For example,

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. When no 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 benzyl group 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

3H2t=300, Pt

+

3HC≡CH →

19. Physical properties of arenes

3. Vurts-Fittih reaction

Br

2Na Br C2H5

C2H5

2NaBr+ + +

In normal 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.

4. Chemical properties of arenesI. The reactions of substitution 1. Nitration

2. Sulphation

HNO3

NO2

H2OH2SO4, t=60

+ +

H2SO4

SO3H

H2O+ +

3. Halogenation

4. Alkylation after Fridel-Krafts

H2SO4FeCl3

Cl

HCl+ +

C2H5ClAlCl3

C2H5

HCl+ +

II. The reactions of joining 1. The reaction with chlorine

2. The reaction with hydrogen

3Cl2

Cl

Cl

Cl

Cl Cl

Cl

H2O+ +

3H2Ni

+

III. The reactions of oxidation 1. The oxidation of benzene

2. The oxidation of benzene homologs

O2,V2O5CH

CH

C

CO

O

O

2CO2 2H2O++

CH3

CH2CH3

[O]COOH

COOH

H2O CO2+ +

3. Ozonation

4. Burning

3O3

CH

CH

O O

O O

CHO

CH

O

CH

CH

O

O

O

3H2OC C

H H

O O

3H2O2 C CHO OH

O O3H2O+ +3 3 +

benzene triozonide

oxalic acid

2C6H6 + 15O2 → 6H2O + 12 CO2 + Q

5 The orientation in benzoic ringIf 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:

•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

O

OH

C

O

H

S

O

OH

N

O

O

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

6. 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 use fulness beginning in the 1940s.

In other reactions the phenyl radical is very stable and only substituents take place in reactions

Alkyl-substituted arenes give 1,4-cyclohexadienes in which the alkyl group is a substituent on the double bond.

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

7. Polycyclic arenesMembers 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.

It is interesting. The 1996 Nobel Prize in chemistry was awarded to Professors Harold W. Kroto (University of Sussex), Robert F. Curl, and Richard E. Smalley (both of Rice University) for groundbreaking work involving elemental carbon that opened up a whole new area of chemistry. The work began when Kroto wondered whether polyacetylenes of the type HC≡C−(C≡C)n−C≡CH might be present in interstellar space and discussed experiments to test this idea while visiting Curl and Smalley at Rice in the spring of 1984. Smalley had developed a method for the laser-induced evaporation of metals at very low pressure and was able to measure the molecular weights of the various clusters of atoms produced.

graphite

Kroto, Curl, and Smalley felt that by applying this technique to graphite the vaporized carbon produced might be similar to that produced by a carbon-rich star.

When the experiment was carried out in the fall of 1985, Kroto, Curl, and Smalley found that under certain conditions a species with a molecular formula of C60 was present in amounts

much greater than any other. On speculating about what C60 might be, they concluded that its

most likely structure is the spherical cluster of carbon atoms shown below and suggested it be called buckminsterfullerene because of its similarity to the geodesic domes popularized by the American architect and inventor R. Buckminster Fuller. (It is also often referred to as a “buckyball.”)

Other carbon clusters, some larger than C60 and

some smaller, were also formed in the experiment, and the general term fullerene refers to such carbon clusters.

Buckminsterfullerene is one of the most elegant ring structures. It consists solely of 60 carbon atoms in rings that curve back on themselves to form a football-shaped cage. All of the carbon atoms in buckminsterfullerene are equivalent and are sp2-hybridized; each one simultaneously belongs to one five-membered ring and two benzene-like six-membered rings. The strain caused by distortion of the rings from coplanarity is equally distributed among all of the carbons.

Confirmation of the structure proposed for C60 required isolation of enough material to allow the arsenal of modern techniques of structure determination to be applied. A quantum leap in fullerene research came in 1990 when a team led by Wolfgang Krätschmer of the Max Planck Institute for Nuclear Physics in Heidelberg and Donald Huffman of the University of Arizona successfully prepared buckminsterfullerene in amounts sufficient for its isolation, purification and detailed study. Not only was the buckminsterfullerene structure shown to be correct, but academic and industrial scientists around the world seized the opportunity afforded by the availability of C60 in quantity to study its properties.

Speculation about the stability of C60 centered on the extent to which the aromaticity associated with its 20 benzene rings is degraded by their non planarity and the accompanying angle strain. It is now clear that C60 is a relatively reactive substance, reacting with many substances toward which benzene itself is inert. Many of these reactions are characterized by the addition of nucleophilic substances to buckminsterfullerene, converting sp2-hybridized carbons to sp3-hybridized ones and reducing the overall strain.

The field of fullerene chemistry expanded in an unexpected direction in 1991 when Sumio Ijima of the NEC Fundamental Research Laboratories in Japan discovered fibrous carbon clusters in one of his fullerene preparations. This led, within a short time, to substances of the type portrayed below called single-walled nanotubes. The best way to think about this material is as a “stretched” fullerene. Take a molecule of C60, cut it in half, and place a cylindrical tube of fused six-membered carbon rings between the two halves.

Thus far, the importance of carbon cluster chemistry has been in the discovery of new knowledge. Many scientists feel that the earliest industrial applications of the fullerenes will be based on their novel electrical properties. Buckminsterfullerene is an insulator, but has a high electron affinity and is a superconductor in its reduced form. Nanotubes have aroused a great deal of interest for their electrical properties and as potential sources of carbon fibers of great strength.

Although the question that began the fullerene story, the possibility that carbon clusters are formed in stars, still remains unanswered, the attempt to answer that question has opened the door to novel structures and materials.

Hydrocarbon frameworks rarely consist of single rings or chains, but are often branched. Rings, chains, and branches are all combined in structures like that of polystyrene, a polymer made of six-membered rings dangling from linear carbon chains, or of b-carotene, the compound that makes carrots orange.

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.

8.NaphthaleneNaphthalene, also known as naphthalin, or antimite and not to be confused with naphtha, is a crystalline, aromatic, white, solid hydrocarbon with formula C10H8 and the structure of two fused benzene rings. It is best known as the traditional, primary ingredient 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 of naphthalene• 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 in naphthalene 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 in naphthalene 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 of naphthalene

• 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.

9.Anthracene 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 properties 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).

Anthracene derivatives having a hydroxyl group are 1-hydroxyanthracene and 2-hydroxyanthracene, homologous to phenol and naphthols, and hydroxyanthracene is also called anthrol, and anthracenol. Hydroxyanthracene derivatives are pharmacologically active, and are contained in aloe for example (specifically in the aloe latex - the references cited for this are inaccurate as they describe the aloe latex, which is typically not a part of the plant used in commerce or is removed during processing [ IASC, 2009]. Anthracene is an organic semiconductor. It is used as a scintillator for detectors of high energy photons, electrons and alpha particles. Plastics such as polyvinyltoluene can be doped with anthracene to produce a plastic scintillator that is approximately water equivalent for use in radiation therapy dosimetry. Anthracene's emission spectrum peaks at between 400 nm and 440 nm.

10.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.

11.Biphenyl Biphenyl occurs naturally in coal tar, crude oil, and natural gas and can be produced from these sources by distillation. Biphenyl is insoluble in water, but soluble in typical organic solvents. The biphenyl molecule consists of two connected phenyl rings. Lacking functionalization, it is not very reactive.

• 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.

12.Diphenylmethane

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

13.TriphenylmethaneTriphenylmethane, 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 of triphenylmethaneTriphenylmethane can be synthesized by Friedel-Crafts reaction from benzene and chloroform with aluminium chloride catalyst:

3 C6H6 + CHCl3 → Ph3CH + 3 HClAlternatively, 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·AlCl3Ph3CCl·AlCl3 + HCl → Ph3CH

Synthesis from benzylidene chloride, prepared from benzaldehyde and phosphorus pentachloride, is used as well.

Triarylmethane dyes

Examples of triarylmethane dyes are bromocresol green:

or malachite green:

14. Unbenzenoid aromatic systemsExcept of benzene and its derivatives there are

some unbenzenoid aromatic compounds which have cycles with double bonds in their molecules.

In 1911 Richard Willstätter prepared cyclooctatetraene by a lengthy degradation of pseudopelletierine, a natural product obtained from the bark of the pomegranate tree. Nowadays, cyclooctatetraene is prepared from acetylene in a reaction catalyzed by nickel cyanide.

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.

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