1. Theme: Mononuclear arenes. Polynuclear arenes with condensed and isolated cycles. LECTURE № 3 associate. prof. Ye. B. Dmukhalska, assistant. I.I. Medvid

2. Outline Outline The structure of benzene ring The structure of benzene ring The methods of arenes extraction The methods of arenes extraction Physical properties of arenes Physical properties of arenes Chemical properties of arenes Chemical properties of arenes The orientation in benzoic ring The orientation in benzoic ring Polycyclic arenes Polycyclic arenes Unbenzenoid aromatic systems Unbenzenoid aromatic systems

3. 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.

5. 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.

7. The nomenclature and isomers 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.

8. Table 1. Names of some frequently Encountered derivatives of benzene

9. 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.

10. 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,

11. 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.

12. 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:

13. When the benzene ring is named as a substituent, the word “phenyl” stands for C 6 H 5 −. Similarly, an arene named as a substituent is called an aryl group. A benzyl group is C 6 H 5 CH 2 −. Biphenyl is the accepted IUPAC name for the compound in which two benzene rings are connected by a single bond.

14. The methods of arens preparation 1. Extraction from oil (oil contains cyclohexane). 2. Cyclotrimerisation of alkynes 3HC≡CH →

15. 3. Physical properties of arenes 3. Vurts-Fittih reaction 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.

16. CHEMICAL PROPERTIES OF ARENES I. The reactions of substitution 1. Nitration 2. Sulphation

17. 3. Halogenation 4. Alkylation after Fridel-Krafts Cl 2

18. II. The reactions of accession 1. The reaction with chlorine 2. The reaction with hydrogen

19. III. The reactions of oxidation 1. The oxidation of benzene 2. The oxidation of benzene homologs

20. 3. Ozonation 4. Burning 2C 6 H 6 + 15O 2 → 6H 2 O + 12 CO 2 + Q

21. 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: the first group of orientators: −Cl −Br −I −OH −NH 2 −CH 3 and other alkyl radicals These orientators orient other substituents to ortho- and para-locations in benzoic ring.

22. 2. the second group of orientators: These orientators orient other substituents to meta-locations in benzoic ring.

23. 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>NR 2 >NHR>NH 2 >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>SO 3 H>NO 2 >CHO>COCH 3 >CN

24. Chemical properties of arenes The chemical properties of arenes depend on different functional groups are present in the molecule. The hydrocarbon group from benzene (C 6 H 5 −) is called a phenyl group. The phenyl functional group has the next chemical properties:

25. 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.

26. 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.

27. 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.

28. 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

29. 5. Dehydration 6. Dehydrohalogenation

30. 7. Hydrogenation 8. Halogenation

31. 9. Hydrohalogenation

32. Polycyclic arens 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.

33. The structural formulas of naphthalene, anthracene, and phenanthrene are shown along with the numbering system used to name their substituted derivatives:

34. 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:

35. 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.

36. 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.

37. 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.

38. 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

39. Biphenyl, diphenylmethane, triphenylmethane are polycyclic benzenoid aromatic hydrocarbons with isolated benzoic rings. In their molecules all rings have common bond, alkyl- or other radicals.

40. Naphthalene, also known as naphthalin, or antimite and not to be confused with naphtha, is a crystalline, aromatic, white, solid hydrocarbon with formula C 10 H 8 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.

41. 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 iron 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 sulphuric or phosphoric acid as the catalyst.

42. Anthracene Anthracene has the ability to photodimerize with irradiation by UV light. This results in considerable changes in the physical properties of the material.

43. Phenanthrene isinsoluble 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.

44. 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.

45. Diphenylmethane Diphenylmethane is an organic compound with the formula (C 6 H 5 ) 2 CH 2. 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: Diphenylmethane is an organic compound with the formula (C 6 H 5 ) 2 CH 2. 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: C 6 H 5 CH 2 Cl + C 6 H 6 → (C 6 H 5 ) 2 CH 2 + HCl

46. Triphenylmethane Triphenylmethane, or triphenyl methane, is the hydrocarbon with the formula (C 6 H 5 ) 3 CH. 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.

47. Preparation of triphenylmethane Triphenylmethane can be synthesized by Friedel- Crafts reaction from benzene and chloroform with aluminium chloride catalyst: 3 C 6 H 6 + CHCl 3 → Ph 3 CH + 3 HCl Alternatively, benzene may react with carbon tetrachloride using the same catalyst to obtain the tritely chloride-aluminium chloride adduct, which is hydrolyzed with dilute acid: 3 C 6 H 6 + CCl 4 + AlCl 3 → Ph 3 CCl · AlCl 3 Ph 3 CCl · AlCl 3 + HCl → Ph 3 CH Synthesis from benzylidene chloride, prepared from benzaldehyde and phosphorus pentachloride, is used as well.

48. Triarylmethane dyes Examples of triarylmethane dyes are bromocresol green: or malachite green:

49. Unbenzenoid aromatic systems Except of benzene and its derivatives there are some unbenzenoid aromatic compounds which have cycles with double bonds in their molecules.

50. 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.

51. 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.

52. 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.

53. 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.

54. Thank you for attention!