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Azulene, Chrysene • Azulene Is an Organic Compound and an Isomer of Naphthalene

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Azulene, Chrysene • Azulene Is an Organic Compound and an Isomer of Naphthalene Azulene, Chrysene • Azulene is an organic compound and an isomer of naphthalene. • • Whereas naphthalene is colourless, azulene is dark blue. • Less stable than naphthalene • Resonance energy of naphthalene is 77 Kcal/mole • Resonance energy of azulene is 49 Kcal/mole • On heating above 350 0C, isomerizes to naphthalene • Azulene has a dipole moment of 1.0 D, while isomeric naphthalene is zero • Absorbs in the range of 230-345 nm indicating the presence of highly conjugated system • Azulene is usually viewed as resulting from fusion of cyclopentadiene and cycloheptatriene rings. • Like naphthalene and cyclodecapentaene, it is a 10 pi electron system. • It exhibits aromatic properties: (i) the peripheral bonds have similar lengths and (ii) it undergoes Friedel-Crafts-like substitutions. • The stability gain from aromaticity is estimated to be half that of naphthalene. • This polarity can be explained by regarding azulene as the fusion of a 6 π-electron cyclopentadienyl anion and a 6 π-electron tropylium cation: one electron from the seven-membered ring is transferred to the five-membered ring to give each ring aromatic stability by Hückel's rule. • Reactivity studies confirm that seven-membered ring is electrophilic and the five-membered ring is nucleophilic. • The ionic structure of azulene (a non benzenoid aromatic compound) is an important contributor to the resonance hybrid The synthesis of azulene is outlined as follows: It can also be prepared from fulvene. Thermolysis of fulvene in the presence of base gives azulene It is a non benzenoid aromatic compound. Aromatic characteristics of azulene are • It does not undergo autoxidation • It does not get polymerized • Under normal conditions it does not act as a diene in Diels Alder reaction • It is capable of undergoing electrophilic substitution reactions in five membered ring. The first substituent enters at position 1 and the second at position-3. • These positions are electron rich in dipolar structures of azulene • Azulene is sensitive to strong acids. It forms azulenium cation with strong acid. Procedure: step 1: cycloheptatriene 2+2 cycloaddition with dichloro ketene step 2: diazomethane insertion reaction step 3: dehydrohalogenation reaction with DMF step 4: Luche reduction to alcohol with sodium borohydride step 5: elimination reaction with Burgess reagent step 6: oxidation with p-chloranil step 7: dehalogenation with polymethylhydrosiloxane, palladium(II) acetate, potassium phosphate and the DPDB ligand Organometallic complexes Polymethylhydrosiloxane (PMHS) is a polymer with the general structure -(CH3(H)Si-O)-. It is used in organicchemistry as a mild and stable reducing agent easily transferring hydrides to metal centers. Mild oxidizing agent Therefore special reagents have to be used for nitration [tetranitromethane pyridine, Copper acetate in Ac2O or HNO3 in AcOH], halogenation (N-halosuccinamide) and sulphonation (dioxane-SO3) • Chrysene is a polycyclic aromatic hydrocarbon (PAH) that consists of four fused benzene rings. • It is a natural constituent of coal tar, from which it was first isolated and characterized. • A colourless solid • M.pt. 251 0C Synthesis of Chrysene 1. By strongly heating 2-(1-naphthyl)-1-phenylethane 2. By Bogert-Cook Synthesis 3. By Pschorr synthesis Ferrocene: synthesis Fe Lab Synthesis Fe + 2 (R3NH)Cl FeCl2 + 2 R3N + H2 FeCl2 + 2 C5H6 + 2 R3N Cp2Fe + 2(R3NH)Cl Cp Fe FeCl2 + 2 NaCp 2 • Most stable member in metallocene series • It sublimes readily and not attacked by air or water but can be oxidized reversibly, electrochemically or by oxidizing agents such as iodine to give the blue ferrocenium cation [Cp2Fe]. Reactions of Ferrocene Ferrocene undergoes electrophilic substitution reactions. Many of its reactions are faster than similar reactions of benzene Necessary requirement: The electrophile should not be oxidizing in nature I Fe 2 Fe I3 p- benzoquinone FeCp2 + HBF4.OEt2 [FeCp2][BF4] Et2O FeCl3 FeCp2 + NH4PF6 [FeCp2][PF6] H2O/Acetone The oxidized Cp2Fe+, ferrocenium cation, will repel the electrophile away. Therefore direct nitration, halogenation and similar reactions cannot be carried out on ferrocene. Acetylation C(O)CH H3C(O)C CH3C(O)Cl C(O)CH3 3 Ac2O/ H3PO4 AlCl3(1:2:2) C(O)CH3 Fe 60 min, 50 °C Fe Fe Fe C(O)CH3 90 % 90 % traces 3.3 x 106 times faster than benzene Chloromercuration (hazardous) Hg(OAc) HgCl Hg(OAc) 2 LiCl Fe Fe Fe Br2/I2 Br, I derivatives 109 times faster than benzene H2 C NR2 HCHO/R2NH Does not happen with benzene; Mannich reaction Fe Fe only with phenols/anilines H3PO4 Li Li N Does not happen with benzene; t-BuLi n-BuLi Lithiation reaction Fe Fe Fe only with bromobenzene TMEDA Li N (3:2 adduct) Lithiation and 1,1’-di-lithiation – access to range of new derivatives + Li CO2/H Cl Si HOOC l 3 Fe SiC 4 Fe Fe B ) 3 uO (B + 1/ S H 8 8 (HO)2B I2 SLi Fe Fe I Fe NaCN CN Fe .
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