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Preparation of Aniline Derivatives: an Advanced W Undergraduate Laboratory Experiment Exploring Catalytic and Stoichiometric Reaction Methodologies

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Preparation of Aniline Derivatives: an Advanced W Undergraduate Laboratory Experiment Exploring Catalytic and Stoichiometric Reaction Methodologies In the Laboratory Preparation of Aniline Derivatives: An Advanced W Undergraduate Laboratory Experiment Exploring Catalytic and Stoichiometric Reaction Methodologies Joan Colom, Antoni Llobet,* Anna Pla-Quintana, and Anna Roglans** Departament de Química, Universitat de Girona, Campus de Montilivi, E-17071 Girona, Catalonia, Spain; *[email protected], **[email protected] 0 0 This article describes a series of synthetic chemistry Zn /NH4Cl Zn /NH4Cl → experiments, suitable for advanced undergraduate students, PhNO2 PhNO PhNHOH (1) that will allow them to grasp the contrasts between stoichio- Na2Cr2O7 metric and catalytic reactions. The experiments are also aimed It is difficult to control the reaction conditions so that at awakening and stimulating the student’s mind with regard only nitrosobenzene is obtained, because it is easily reduced to the design of cost-effective and environmentally sound to N-phenylhydroxylamine. Therefore we propose to prepare alternative synthetic routes, essential for sustainable global nitrosobenzene from the controlled oxidation of N-phenyl- growth. hydroxylamine with sodium dichromate at 0 °C. This can The use of transition metal complexes to activate oxygen be done either step by step with the isolation of N-phenyl- or hydrogen peroxide for the catalytic oxidation of organic hydroxylamine, or in a one-pot reaction. substrates is a topic of major interest from both a biological Almost all aromatic C-nitroso compounds exist as blue and an industrial point of view (1). In industry, the substi- or green monomers or as colorless bisnitroso dimers (3, 5). tution of expensive, hazardous, and polluting stoichiometric ᎑ This is also the case of nitrosobenzene, which in the solid oxidants such as CrO3 and MnO4 by molecular oxygen or state is a colorless dimer that dissociates readily in solution HOOH would be highly beneficial both economically and (CHCl3) to generate a green monomer (see eq 2). environmentally. In this paper we describe a few experiments that exemplify PhNO Ph(O)NN(O)Ph (2) + green monomer colorless dimer the capability of FeCl2py4 to act as catalyst for the oxidation of aniline using HOOH as the oxidant and yielding their IR spectroscopy is particularly useful in this case because it corresponding derivatives, namely, nitroso-, nitro-, aza-, and permits differentiation between those two structures. For the azoxy-benzene (2). Several system parameters are modified in monomeric species, the N=O stretching vibrational mode ᎑1 order to illustrate the various tools available to control product frequencies appear at 1506 cm , whereas the N–O stretch- ᎑1 distributions and efficiencies. Thus the experiments illustrate ing for the dimeric species shifts to 1397 cm . the customized optimization of a particular catalytic system. In addition, nitro-aromatic derivatives are an important class of compounds synthetically, since they can be easily transformed, PhNO2 through multiple pathways, into their corresponding reduced Zn/NaOH products (3, pp 343–344). In contrast, the oxidative chemistry t-BuOOH D-glucose/NaOH ° of aromatic amines is not as well established, since the reactions 13 h 100 C + 2 h 100 °C FeCl2py4 are more complicated (3, pp 165–169). Depending upon the 10 min r.t. oxidizing agent, different aniline derivatives can be generated. Zn/NH Cl Therefore the catalytic processes described here are oxidation PhNNPhPhNH PhN(O)NPh 4 reactions using aniline as the starting material, whereas the sto- 2 65 °C ichiometric reactions are reduction reactions using nitrobenzene HOOH + as the starting material. FeCl2py4 10 min r.t. PhNHOH Product profiles and product characterization are PhNH2 NaOH achieved through the usual chromatographic (TLC, GC, CH3COOH GC–MS) and spectroscopic (IR, UV–vis, NMR) techniques. 3 h r.t. 24 h r.t. PhNO Results and Discussion Na2Cr2O7 Zn/NH4Cl 65 °C Scheme I offers an integrated graphical summary of the H2SO4 stoichiometric and catalytic reactions described here. 3 min 0 °C Stoichiometric Reactions PhNHOH Nitrosobenzene. Under acidic conditions at 65 °C, nitro- Scheme I benzene is initially reduced by Zn powder to nitrosobenzene solid lines: stoichiometric reactions and subsequently to N-phenylhydroxylamine (4). broken lines: catalytic reactions JChemEd.chem.wisc.edu • Vol. 79 No. 6 June 2002 • Journal of Chemical Education 731 In the Laboratory Azoxybenzene. Azoxybenzene can be prepared by the direct Catalytic Reactions reduction of nitrobenzene in an alkaline medium using D- In this section, we describe how aniline can be glucose as the reducing agent (4). + catalytically oxidized by FeCl2py4 (which is readily obtained D-glucose/NaOH when dissolving [FeCl (H O) ]Clи2H O in pyridine) (2) → 2 2 4 2 2PhNO2 PhN(O)NPh (3) using mainly HOOH as the oxidative reagent. The product The reaction requires 2 hours at 100 °C to be completed. selectivities and efficiencies depend on the catalyst; solvent; Azoxybenzene can also be obtained when nitrosobenzene and temperature; oxidative reagent; relative concentrations of N-phenylhydroxylamine are mixed together in basic solution catalyst, oxidant, and substrate; and reaction times, as (see Scheme I and eq 4). reflected in Table 1. The system described in entry 1 yields almost exclusively NaOH → PhNO after a 10-min reaction time PhNO + PhNHOH PhN(O)NPh + H2O(4) + CH3CN FeCl py →2 4 Azobenzene. Reduction of nitrobenzene in a methanolic PhNH2 + 2HOOH PhNO + 3H2O(7) NaOH solution containing 4 equivalents of zinc powder at with a reaction efficiency of 60.3% relative to the oxidative reflux for 13 h leads to the formation of azobenzene (4). agent HOOH. Thus, HOOH is being used mainly to cata- Zn0/NaOH lytically transform aniline into nitrosobenzene; the remaining → 2PhNO2 PhNNPh (5) equivalents are disproportionated into water and molecular CH3OH oxygen: It is crucial to control the amount of zinc, to avoid further FeCl py + →2 4 reduction to hydrazobenzene, PhNHNHPh. An alternative 2HOOH 2H2O + O2 (8) route to the formation of azobenzene is to react nitroso- After HOOH has been depleted, then azobenzene and azoxy- benzene and aniline in an acetic acid solution (3). benzene are formed catalytically at the expense of PhNO CH3COOH → (entries 2–11 and Fig. 1) (2) PhNO + PhNH2 PhNNPh + H2O(6) FeCl py + →2 4 Azoxybenzene and azobenzene are chromophores with PhNH2 + PhNO PhNNPh + H2O(9) conjugated double bonds that present distinctive features in FeCl py + →2 4 their UV–vis spectra. The azobenzene compound presents two PhNH2 + 3PhNO 2PhN(O)NPh + H2O (10) λ bands in the 210–400-nm region ( max[MeOH] = 228 nm, Entry 12 shows the effect of increasing the ratio of cata- ε ε = 14,200; and 316 nm, = 19,186), whereas the azoxy pre- lyst to oxidizing agent from the original 5/100 to 5/400. This λ ε ε sents three ( max[MeOH] = 231 nm, = 8795; 258 nm, = generates substantial amounts of PhNO and PhN(O)NPh ε 2 7568); and 320 nm, = 14318). at the very beginning of the reaction in addition to PhNO, + Table 1. FeCl2py4 -Catalyzed Transformation of 1 M Aniline in Pyridine System ConditionsW Product Concn/(mM ± 5%) Cat Oxidant Eff Entry Time/ PhN(O)– a Concn/ POhNO PhN PhNNPh (%) min Com- Concn/ 2 NPh mM pound mM 1015H0OO H110 2D9. N b 05.3 03. 60. 2045H0OO H170 3D0. N407. 03. 64. 3065H0OO H150 2D9. N509. 07. 62. 40152 H0OO H100 2D9. N702. 10. 63. 50158 H0OO H150 2D7. N416. 16. 62. 60251 H0OO H130 2D6. N718. 14. 61. 70254 H0OO H110 2D5. N129. 11. 60. 80650 H0OO H120 2D3. N049. 11. 60. 9025,88 H0OO H130 2D1. N251. 23. 59. 100 45,32 H0OO H130 1D8. N753. 29. 54. 101 15H0,08 H0OO 160 1D0. N955. 25. 40. 102 15H0OO H430 483. 95. 28. 115. 42. HOOH 100 103 15 1323.8 27. 01. 52. 71. PhNO 100 104 15t-0BuOOH 1D0 N57D. NDN522. 105 108 H0OO H1D0 NDNDNDN00. 106 158 H0DOO H NDNDNDN00. aEfficiency is calculated with respect to HOOH. It is assumed that the formation of PhNO and PhNNPh from PhNH2 requires 2 molecules of HOOH, whereas b PhNO2 and PhN(O)NPh require 3. ND means not detected. 732 Journal of Chemical Education • Vol. 79 No. 6 June 2002 • JChemEd.chem.wisc.edu In the Laboratory + [FeIII-OOH, pyH ] 40 PhNH2 HOOH 30 A 20 py py + H2O 0 IV OH 0 100 200 Product Conc / mM Fe Time / min 20 FeIII NH Ph 10 IV O H2O B PhNO O H + Fe H H2O N Product Concentration / mM HOOH 0 Ph 0 25 50 75 100 125 150 175 C Time / h Figure 2. Proposed reaction mechanism. + Figure 1. Product profile vs time for the system FeCl2py4 , 5 mM/ HOOH, 100 mM/PhNH2, 1 M/pyridine. The inset shows the first four hours of the reaction (᭜, PhNO; ᭡, PhNNPh; ᭿, PhN(O)NPh). For the stoichiometric reactions in basic solution, the reduction of nitrobenzene leads to the formation of the azo- (PhNNPh) and azoxy-(PhN(O)NPh) derivatives. In contrast, in acidic solution the reduction of nitrobenzene produces N- clearly indicating the presence of alternative reaction pathways phenylhydroxylamine via nitrosobenzene. The former can be favored under this particular reaction condition. A similar oxidized with sodium dichromate to nitrosobenzene in acidic effect is produced using a combination of PhNO and HOOH medium. Under these conditions the formation of azoxy-, as oxidative reagents as outlined in entry 13. Finally, the use azo-, and hydrazobenzene (PhNHNHPh) is inhibited. For of t-BuOOH as the oxidative reagent (entry 14) produces the stoichiometric reactions, PhNO undergoes condensation exclusively PhNO 2 reactions with aniline in acidic solution and with PhNHOH FeCl py + 2 4 in basic solution to generate the corresponding aza and azoxy PhNH + 3t-BuOOH → PhNO + 3t-BuOH + H O (11) 2 2 2 derivatives, respectively. For the catalytic reactions it is illus- + indicating the high selectivity of the system, but with a sig- trated that FeCl2py4 can activate HOOH to directly transform nificant decrease in the reaction efficiency compared to the aniline into nitrosobenzene.
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