Presentation on theme: "Polynuclear Aromatic Hydrocarbon"— Presentation transcript:
1 Polynuclear Aromatic Hydrocarbon DAMIETTAUNIVERSITYLECTURE 1Dr Ali El-AgameyCHEM-323:Polynuclear Aromatic Hydrocarbon
2 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.
3 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.ReadingRT Morrison and RN Boyd, Organic Chemistry, Allyn and Bacon, Inc., 1987.Mundy, BP et al, Name reactions and reagents in organic synthesis, Wiley, 2005H Meislich et al, Organic Chemistry, McGraw-Hill Book Company, 1980.IL Finar, Organic Chemistry, Volume 1 (1985) and Volume 2 (1977).
8 (1) BiphenylReactionsAlthough –NO2 is m-directing group, p-O2NC6H4 is p-directing!!According to the charge distribution approach, a p-nitro groupwould have been expected to direct to the m-position in the other ring.
9 (1) BiphenylReactionsHowever, the stabilities of the carbocation intermediates generatedfollowing o- and p-substitutions are greater than that generated fromm-substitution due to the greater dispersal of charge in the formersubstitutions. Therefore, o-and p-substitutions are preferred.Halogenation and sulfonation of biphenyl behave similar to nitration reaction.
10 (2) Diphenylmethane (3) Triphenylmethane (Tritane) Synthesis and reactions(3) Triphenylmethane (Tritane)Synthesis and reactionsتكتبBenzyl 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
11 Homework: write the product of the following reaction 11
12 Polynuclear Aromatic Hydrocarbon DAMIETTAUNIVERSITYLECTURE 2Dr Ali El-AgameyCHEM-323:Polynuclear Aromatic Hydrocarbon12
13 (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
14 Synthesis and reactions (4) Triphenylmethanol(Triphenylcarbinol)Synthesis and reactions(5) trans-Stilbene(trans-1,2-Diphenylethene)Synthesis and reactions(6) BenzoinSynthesis and reactionsتكتب14
15 Homework: Complete the following equation Homework: Write the mechanism of the following reaction15
16 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.تكتب16
17 Nomenclature of naphthalene, anthracene and phenanthrene Fused ringsNomenclature of naphthalene, anthracene and phenanthreneHow many isomers for monosubstitution of naphthalene (C8H7X)?How many isomers for monosubstitution of anthracene (C14H9X)?Naphthalene: monosubstitution (C8H7X) = 2 isomersdisubstitution (C8H6X2) = 10 isomersdisubstitution (C8H6XY) = 14 isomersAnthracene: monosubstitution (C14H9X) = 3 isomersdisubstitution (C14H8X2) = 15 isomersPhenanthrene: monosubstitution (C14H9X) = 5 isomersdisubstitution (C14H8X2) = 25 isomersMolecular formula: e.g. C8H7XHow many isomers for monosubstitution of phenanthrene (C14H9X)?17
18 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 isomersdisubstitution (C8H6X2) = 10 isomersdisubstitution (C8H6XY) = 14 isomersAnthracene: monosubstitution (C14H9X) = 3 isomersdisubstitution (C14H8X2) = 15 isomersPhenanthrene: monosubstitution (C14H9X) = 5 isomersdisubstitution (C14H8X2) = 25 isomersMolecular formula: e.g. C8H7XIn each case write the name of each isomer?18
19 Nomenclature of naphthalene, anthracene and phenanthrene 19
20 Homework: Give names for the following compounds 20
21 Polynuclear Aromatic Hydrocarbon DAMIETTAUNIVERSITYLECTURE 3Dr Ali El-AgameyCHEM-323:Polynuclear Aromatic Hydrocarbon21
22 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 viewUndergo electrophilic substitution reactions.Like benzene, they are unusually stable as indicated from their low heats of hydrogenation and combustion.22
24 Aromatic character of naphthalene, anthracene and phenanthrene Resonance energy of A = 61 kcal mol-1Resonance energy of B = 84 kcal mol-1Resonance energy of C = 92 kcal mol-1Resonance energy of benzene = 36 kcal mol-124
25 Aromatic character of naphthalene, anthracene and phenanthrene (ii) Theoretical point of view: they have the structure required foraromatic compoundsFlat (sp2-hybidized); cyclicp-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 integeri.e. 4n+2 = No. of π-electronsFor n = 0, The No. of π-electrons is 2 π-electronsFor 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-electrons25
26 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 characterBond lengthThe different carbon-carbon bond lengths reveal thedecreased aromaticity of fused polynuclear aromatichydrocarbons.Naphthalene is a resonance hybrid of mainly 3 resonance structures.Phenanthrene is an angular polynuclear aromatichydrocarbon.26
27 Fused rings (1) Naphthalene (C10H8) Naphthalene, anthracene and phenanthrene are obtained from coal tar.Naphthalene being the most abundent (5%) of all constituentsof coal tar.In moth balls as insect repellants.Many synthetic dyes contain naphthalene moiety.(a) UsesCoal 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 الفحممن ويكيبيديا، الموسوعة الحرةقطران 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 .(b) Structure Elucidation27
28 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-1Resonance energy per ring = 30.5 kcal mol-1Resonance energy of benzene = 36 kcal mol-1Reactivity decreasesStability of the compound increasesAs R.E. increasesResistance to destroy aromatic character increases28
29 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
30 Reactions of naphthalene (1) OxidationOxidation by V2O5 is important industrial processVitamins 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
31 Reactions of naphthalene (1) OxidationOxidation 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.تكتبOxidation by V2O5 is important industrial processVitamins 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
32 Reactions of naphthalene (2) ReductionTetralin = 1,2,3,4-TetrahydronaphthaleneDecalin = decahydronaphthalene32
33 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-substitutionAromatic sextet is retained(benzene has large R. E.)Number of resonance structuresAromatic sextet is preserved in the ringAromatic sextet is disrupted in the ringCarbocation 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
35 Polynuclear Aromatic Hydrocarbon DAMIETTAUNIVERSITYLECTURE 4Dr Ali El-AgameyCHEM-323:Polynuclear Aromatic Hydrocarbon35
36 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
38 Reactions of naphthalene (ii) HalogenationIntroduction of NO2 and X groups opens the way to the preparation of a series of 1-substituted naphthalenes.38
39 Reactions of naphthalene (iii) Chloromethylation(iv) Friedel-Crafts reactionsFriedel-Crafts reactions is carried out under mild conditions.Friedel-Crafts alkylation is of little use because (1) polyalkylation and (2) side reactions.CH2O = paraformaldehyde1-(chloromethyl)naphthaleneAlkylnaphthalenes can be prepared via acylation or ring closure.39
40 Reactions of naphthalene (iv) Friedel-Crafts reactions C2H2Cl4 = tetrachloroethaneNaOX: 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
43 E-2 (E2 + Δ H2) >> E-1 (E1 + Δ H1) k-1 >> k-2 Δ H2 > Δ H1E-2 (E2 + Δ H2) >> E-1 (E1 + Δ H1)k-1 >> k-2At low T, 1-S is formed faster than 2-STherefore, 1-S is called kinetically controlled productHowever, at high T, which speeds up a reaction, or if the reaction is left forlong time, 2-S becomes the main product.1-S reforms N much faster than does 2-S and hence 2-S accumulates at theexpense 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
44 (v) SulfonationThe lower stability of 1-S is attributed to the steric interaction between thesulfonic group and the hydrogen atom in the 8-position.44
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