Polynuclear Aromatic Hydrocarbon DAMIETTA UNIVERSITY LECTURE 1 Dr Ali El-Agamey CHEM-323: Polynuclear Aromatic Hydrocarbon

LEARNING OUTCOMES LECTURES 1-2 Understand the principles of biphenyl nomenclature and the different methods used in its synthesis. Understand the various methods involved in the synthesis of the different isolated polynuclear aromatic compounds. Appreciate the effect of phenyl group on the stability of various intermediates. Appreciate the steric effect of phenyl and benzyl groups. Understand the role of phenyl and benzyl groups in electrophilic substitution reactions.

Lecture 1-2: -Nomeclature, synthesis and reactions of biphenyl. -Synthesis and reactions of diphenylmethane, triphenylmethane, triphenylmethyl chloride, triphenylmethanol, trans-stilbene and benzoin. -Nomenclature of fused systems. Lecture 3-6: -Aromatic character of fused systems. -Structure elucidation of naphthalene -Reactions of naphthalene. Lecture 7-11: - Synthesis of naphthalene. - Synthesis and reactions of anthracene and phenanthrene. Lecture 12: -General revision for the whole course. Reading RT Morrison and RN Boyd, Organic Chemistry, Allyn and Bacon, Inc., 1987. Mundy, BP et al, Name reactions and reagents in organic synthesis, Wiley, 2005 H Meislich et al, Organic Chemistry, McGraw-Hill Book Company, 1980. IL Finar, Organic Chemistry, Volume 1 (1985) and Volume 2 (1977).

Polynuclear Aromatic Hydrocarbons Isolated rings (1) Biphenyl Nomenclature تكتب 4

Homework: Give name for the following compounds 2,3'-dibromobiphenyl 2',6'-dichlorobiphenyl-2-carboxylic acid 4,4'-Di(N,N-dimethylamino)biphenyl

(1) Biphenyl Synthesis (a) Fittig’s reaction (b) Gattermann reaction (c) Gomberg reaction Gomberg reaction: Whatever the nature of the substituent in the aromatic compound, o- or p-substitution always occurs predominantly. More examples

(d) Ullmann coupling reaction More examples (e) Benzidine rearrangement Named p84 H3PO2: hypophosphorous acid

(1) Biphenyl Reactions Although –NO2 is m-directing group, p-O2NC6H4 is p-directing!! According to the charge distribution approach, a p-nitro group would have been expected to direct to the m-position in the other ring.

(1) Biphenyl Reactions However, the stabilities of the carbocation intermediates generated following o- and p-substitutions are greater than that generated from m-substitution due to the greater dispersal of charge in the former substitutions. Therefore, o-and p-substitutions are preferred. Halogenation and sulfonation of biphenyl behave similar to nitration reaction.

(2) Diphenylmethane (3) Triphenylmethane (Tritane) Synthesis and reactions (3) Triphenylmethane (Tritane) Synthesis and reactions تكتب Benzyl group is o- and p-directing group. In substitution reactions, the substituent enters mainly the 4-position. Methylene group is active because Ph groups are e-withdrawing groups (see reaction with Br2). 10

Homework: write the product of the following reaction 11

Polynuclear Aromatic Hydrocarbon DAMIETTA UNIVERSITY LECTURE 2 Dr Ali El-Agamey CHEM-323: Polynuclear Aromatic Hydrocarbon 12

(3) Triphenylmethyl chloride (Trityl chloride) Ph3C radical is highly stabilized by resonance. The high stability of Ph3C radical is the driving force for dimer dissociation. Steric effect hindered the formation of hexaphenylethane. 13

Synthesis and reactions (4) Triphenylmethanol (Triphenylcarbinol) Synthesis and reactions (5) trans-Stilbene (trans-1,2-Diphenylethene) Synthesis and reactions (6) Benzoin Synthesis and reactions تكتب 14

Homework: Complete the following equation Homework: Write the mechanism of the following reaction 15

Fused rings Naphthalene Naphthalene, anthracene and phenanthrene are obtained from coal tar, naphthalene being the most abundant (5%) of all constituents of coal tar. Uses: moth balls, which contain naphthalene, are used as insect repellants (naphthalene is very volatile). Many synthetic dyes contain naphthalene moiety. تكتب 16

Nomenclature of naphthalene, anthracene and phenanthrene Fused rings Nomenclature of naphthalene, anthracene and phenanthrene How many isomers for monosubstitution of naphthalene (C8H7X)? How many isomers for monosubstitution of anthracene (C14H9X)? Naphthalene: monosubstitution (C8H7X) = 2 isomers disubstitution (C8H6X2) = 10 isomers disubstitution (C8H6XY) = 14 isomers Anthracene: monosubstitution (C14H9X) = 3 isomers disubstitution (C14H8X2) = 15 isomers Phenanthrene: monosubstitution (C14H9X) = 5 isomers disubstitution (C14H8X2) = 25 isomers Molecular formula: e.g. C8H7X How many isomers for monosubstitution of phenanthrene (C14H9X)? 17

Homework How many isomers for disubstitution of naphthalene (C8H6X2)? How many isomers for disubstitution of naphthalene (C8H6XY)? How many isomers for disubstitution of anthracene (C14H8X2)? How many isomers for disubstitution of phenanthrene (C14H8X2)? Naphthalene: monosubstitution (C8H7X) = 2 isomers disubstitution (C8H6X2) = 10 isomers disubstitution (C8H6XY) = 14 isomers Anthracene: monosubstitution (C14H9X) = 3 isomers disubstitution (C14H8X2) = 15 isomers Phenanthrene: monosubstitution (C14H9X) = 5 isomers disubstitution (C14H8X2) = 25 isomers Molecular formula: e.g. C8H7X In each case write the name of each isomer? 18

Nomenclature of naphthalene, anthracene and phenanthrene 19

Homework: Give names for the following compounds 20

Polynuclear Aromatic Hydrocarbon DAMIETTA UNIVERSITY LECTURE 3 Dr Ali El-Agamey CHEM-323: Polynuclear Aromatic Hydrocarbon 21

Aromatic character of naphthalene, anthracene and phenanthrene Naphthalene, anthracene and phenanthrene are classified as aromatic because of their properties, which resemble those of benzene. (i) Experimental point of view Undergo electrophilic substitution reactions. Like benzene, they are unusually stable as indicated from their low heats of hydrogenation and combustion. 22

Resonance Energy 23

Aromatic character of naphthalene, anthracene and phenanthrene Resonance energy of A = 61 kcal mol-1 Resonance energy of B = 84 kcal mol-1 Resonance energy of C = 92 kcal mol-1 Resonance energy of benzene = 36 kcal mol-1 24

Aromatic character of naphthalene, anthracene and phenanthrene (ii) Theoretical point of view: they have the structure required for aromatic compounds Flat (sp2-hybidized); cyclic p-cloud above and below the plane due to ??? HÜckel`s rule ??? A cyclic ring molecule follows Hückel's rule when the number of its π-electrons equals 4n+2 where n is zero or any positive integer i.e. 4n+2 = No. of π-electrons For n = 0, The No. of π-electrons is 2 π-electrons For n = 1, The No. of π-electrons is 6 π-electrons (e.g. benzene) For n = 2, The No. of π-electrons is 10 π-electrons 25

Resonance structures and bond lengths of naphthalene, anthracene and phenanthrene There are n+1 principal resonance structures for polynuclear aromatic hydrocarbon containing n benzene rings fused together in a linear manner. Double bond character Bond length The different carbon-carbon bond lengths reveal the decreased aromaticity of fused polynuclear aromatic hydrocarbons. Naphthalene is a resonance hybrid of mainly 3 resonance structures. Phenanthrene is an angular polynuclear aromatic hydrocarbon. 26

Fused rings (1) Naphthalene (C10H8) Naphthalene, anthracene and phenanthrene are obtained from coal tar. Naphthalene being the most abundent (5%) of all constituents of coal tar. In moth balls as insect repellants. Many synthetic dyes contain naphthalene moiety. (a) Uses Coal tar is a brown or black liquid of extremely high viscosity, which smells of naphthalene and aromatic hydrocarbons. Coal tar is among the by-products when coal is carbonized to make coke or gasified to make coal gas. Coal tars are complex and variable mixtures of phenols, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic compounds, about 200 substances in all. قطران الفحم من ويكيبيديا، الموسوعة الحرة قطران الفحم هو سائل بني أو أسود عالي الكثافة له رائحة تشبه رائحة النفثالين. وينتج قطران الفحم كناتج ثانوي من تكويك الحصول على فحم الكوك من الفحم الحجري. يتكون هذا القطران من مزيج من الفينولات العالية الوزن الجزيئي ومن الهيدروكربونات العطرية متعددة الحلقات ومن المركبات الحلقية غير المتجانسة [1]. (b) Structure Elucidation 27

Reactivity of naphthalene as compared to benzene In naphthalene, two fused rings share a pair of p–electrons, therefore the aromaticity and the resonance energy per ring is less than that of benzene itself. Resonance energy of naphthalene = 61 kcal mol-1 Resonance energy per ring = 30.5 kcal mol-1 Resonance energy of benzene = 36 kcal mol-1 Reactivity decreases Stability of the compound increases As R.E. increases Resistance to destroy aromatic character increases 28

Reactivity of naphthalene as compared to benzene What happened when aromatic character of one ring of naphthalene is destroyed? Therefore, naphthalene undergoes oxidation or reduction more readily than benzene but only to the stage where a substituted benzene is formed; further oxidation or reduction requires more vigorous conditions. 29

Reactions of naphthalene (1) Oxidation Oxidation by V2O5 is important industrial process Vitamins K1 and K2 are derivatives of 1,4-naphthoquinone. Because of this tendency to form quinones, it is not always feasible to prepare naphthalenecarboxylic acids by oxidation of methyl side chains. 30

Reactions of naphthalene (1) Oxidation Oxidation by V2O5 is important industrial process (naphthalene is available from coal tar). Vitamins K1 and K2 are derivatives of 1,4-naphthoquinone. Because of this tendency to form quinones, it is not always feasible to prepare naphthalenecarboxylic acids by oxidation of methyl side chains. تكتب Oxidation by V2O5 is important industrial process Vitamins K1 and K2 are derivatives of 1,4-naphthoquinone. Because of this tendency to form quinones, it is not always feasible to prepare naphthalenecarboxylic acids by oxidation of methyl side chains. 31

Reactions of naphthalene (2) Reduction Tetralin = 1,2,3,4-Tetrahydronaphthalene Decalin = decahydronaphthalene 32

Reactions of naphthalene (3) Electrophilic susbstitution reactions Fries rule: The most stable arrangement of a polynuclear compound is that form which has the maximum number of rings in the benzenoid condition. 1-Substitution versus 2-substitution Aromatic sextet is retained (benzene has large R. E.) Number of resonance structures Aromatic sextet is preserved in the ring Aromatic sextet is disrupted in the ring Carbocation and T.S. resulting from attack at the 1-position are much more stable than those resulting from attack at the 2-position . Therefore, 1-susbstitution will be preferred. 33

Reactions of naphthalene (i) Nitration 34

Polynuclear Aromatic Hydrocarbon DAMIETTA UNIVERSITY LECTURE 4 Dr Ali El-Agamey CHEM-323: Polynuclear Aromatic Hydrocarbon 35

LEARNING OUTCOMES LECTURE 4 Reactions of naphthalene: Nitration; Halogenation; Chloromethylation; Friedel-Crafts reactions (alkylation and acylation). Sulfonation. Naphthols and naphthylamines. Synthesis of 1-substituted naphthalenes. Synthesis of 2-substituted naphthalenes. Summary of naphthalene reactions. 36

Reactions of naphthalene (i) Nitration 37

Reactions of naphthalene (ii) Halogenation Introduction of NO2 and X groups opens the way to the preparation of a series of 1-substituted naphthalenes. 38

Reactions of naphthalene (iii) Chloromethylation (iv) Friedel-Crafts reactions Friedel-Crafts reactions is carried out under mild conditions. Friedel-Crafts alkylation is of little use because (1) polyalkylation and (2) side reactions. CH2O = paraformaldehyde 1-(chloromethyl)naphthalene Alkylnaphthalenes can be prepared via acylation or ring closure. 39

Reactions of naphthalene (iv) Friedel-Crafts reactions C2H2Cl4 = tetrachloroethane NaOX: sodium hypohalite This is the haloform reaction. The effect of nitrobenzene has been attributed to its forming a complex with the acid chloride and AlCl3 which, because of its bulkiness attacks the roomier 2-position. Acetylation affords access to the preparation of a series of 2-substituted naphthalenes. 40

Reactions of naphthalene (v) Sulfonation 41

Reactions of naphthalene (v) Sulfonation 42

E-2 (E2 + Δ H2) >> E-1 (E1 + Δ H1) k-1 >> k-2 Δ H2 > Δ H1 E-2 (E2 + Δ H2) >> E-1 (E1 + Δ H1) k-1 >> k-2 At low T, 1-S is formed faster than 2-S Therefore, 1-S is called kinetically controlled product However, at high T, which speeds up a reaction, or if the reaction is left for long time, 2-S becomes the main product. 1-S reforms N much faster than does 2-S and hence 2-S accumulates at the expense of 1-S i.e. the equilibrium is established in favor of the formation of 2-S, which is called the thermodynamically controlled product. 43

(v) Sulfonation The lower stability of 1-S is attributed to the steric interaction between the sulfonic group and the hydrogen atom in the 8-position. 44

Reactions of naphthalene 45

Reactions of naphthalene Naphthols and naphthylamines Sodium-2-naphthalene sulfonate Sodium-2-naphthoxide Bucherer reaction 46

Reactions of naphthalene Naphthols and naphthylamines 47

Synthesis of 1-substituted naphthalenes 48

Synthesis of 2-substituted naphthalenes Acetylation and sulfonation affords access to the preparation of a series of 2-substituted naphthalenes.

Summary of naphthalene reactions 50