high lipophilicity and exchanges anions across the liquid-liquid interface (Dehmlow and Dehmlow, 1983).
Even without PTC, there is a significant conversion of benzyl chloride as it is seen in the effect of catalyst loading. Therefore, the reaction proceeds through both the uncatalyzed and catalyzed pathway. Based on the above facts, the catalytic pathway is pictorially represented by Scheme 4.2. The reaction that proceeds through the uncatalyzed pathway is similar to that of catalytic pathway except that the anions (S2− and HS−) in the form of ammonium sulfide and ammonium hydrosulfide are directly transferred to organic phase from aqueous phase instead of transferring via the formation of active catalyst as in the case of catalytic pathway. The ammonium sulfide and ammonium hydrosulfide then react with benzyl chloride present in the organic phase to produce DBS and BM, respectively, as shown previously in the Scheme 4.1.
the reaction rate without significantly affecting the selectivity. The process involves a complex mechanism. The existence of an ionic equilibrium among NH3, H2S, and water, producing sulfide and hydrosulfide ions in the aqueous phase was established. The two active ion pairs (Q+S−2Q+ and Q+SH−) formed in the aqueous phase are first transferred to the organic phase and then react with benzyl chloride to produce DBS and BM, respectively. DBS is also formed by the reaction of BM and benzyl chloride.
Table 4.1. Effect of temperature on selectivity of DBSa
Selectivity of DBS (%) at a conversion of benzyl chloride (%) of Temperature, K
10 20 30
303 7.6 17.4 28.2 318 7.9 18.1 29.5 333 8.2 16.7 26.2 343 9.0 18.4 28.2
a Volume of organic phase = 6.5×10-5 m3; concentration of benzyl chloride = 2.0 kmol/m3; volume of aqueous phase = 5.0×10-5 m3; concentration of sulfide = 1.06 kmol/m3; NH3/H2S mole ratio = 5.3; concentration of TBAB = 8.92×10-2 kmol/m3; stirring speed = 1500 rev/min.
0 50 100 150 200 250 0
10 20 30 40 50 60 70
Conversion of benzyl chloride (%)
Reaction time (min)
1000 rev/min 1500 rev/min 2000 rev/min
Fig. 4.1. Effect of stirring speed on conversion of benzyl chloride. Volume of organic phase = 5.0×10−5 m3; concentration of benzyl chloride = 2.0 kmol/m3; volume of aqueous phase = 5.0×10−5 m3; concentration of TBAB = 8.83×10−2 kmol/m3 of organic phase; temperature = 333 K;
concentration of sulfide = 1.32 kmol/m3; NH3:H2S mole ratio = 4.2.
0 50 100 150 200 250 300 350 400 450 0
10 20 30 40 50 60 70 80
Conversion of benzyl chloride (%)
Reaction time (min) 303 K
318 K 333 K 343 K
Fig. 4.2. Effect of temperature on conversion of benzyl chloride. All conditions are same as in Table 4.1.
2.9x10-3 3.0x10-3 3.1x10-3 3.2x10-3 3.3x10-3 -10.0
-9.5 -9.0 -8.5 -8.0
-7.5 Experimental
Linear fit (R2=0.94)
ln (initial rate, kmol/m3 s)
1/T (K-1)
Fig. 4.3. Arrhenius plot of ln (initial rate) versus 1/T.
0 40 80 120 160 200 240 280 0
20 40 60 80 100
NH3:H2S 2.45 4.21 5.10
Conversion of benzyl chloride (%)
Reaction time (min)
Fig. 4.4. Effect of ammonia concentration on the conversion of benzyl chloride. Volume of organic phase = 5.0×10−5 m3; concentration of benzyl chloride = 2.6 kmol/m3; volume of aqueous phase = 5.0×10−5 m3; concentration of sulfide = 1.6 kmol/m3; concentration of TBAB = 0.11 kmol/m3 of organic phase; temperature = 333 K; stirring speed = 1500 rev/min.
0 10 20 30 40 50 60 70 80 90 100 0
10 20 30 40 50 60 70
NH3: H2S 2.45 4.21 5.10
Selectivity of DBS (%)
Conversion of benzyl chloride (%)
Fig. 4.5. Effect of ammonia concentration on selectivity of DBS. All conditions are same as in Fig. 4.4.
0 40 80 120 160 200 240 280 0
10 20 30 40 50 60 70 80 90 100
NH3:H2S 2.46 4.25 5.27
Conversion of benzyl chloride (%)
Reaction time (min)
Fig. 4.6. Effect of sulfide concentration on conversion of benzyl chloride.
Volume of organic phase = 5.0×10−5 m3; concentration of benzyl chloride
= 2.0 kmol/m3; volume of aqueous phase = 5.0×10−5 m3; concentration of NH3 = 5.62 kmol/m3; concentration of TBAB = 8.83×10−2 kmol/m3 of organic phase; temperature = 333 K; stirring speed = 1500 rev/min.
0 20 40 60 80 100 0
10 20 30 40 50 60 70
80 NH3:H2S 2.46 4.25 5.27
Selectivity of DBS (%)
Conversion of benzyl chloride (%)
Fig. 4.7. Effect of sulfide concentration on selectivity of DBS. All conditions are same as in Fig. 4.6.
0 50 100 150 200 250 300 350 400 450 0
10 20 30 40 50 60 70 80 90 100
TBAB concn.x102(kmol/m3 of org. phase) 0.0 8.93
5.2 14.27
Conversion of benzyl chloride (%)
Reaction time (min)
Fig. 4.8. Effect of catalyst loading on conversion of benzyl chloride. Volume of organic phase = 5.0×10−5 m3; concentration of benzyl chloride = 1.44 kmol/m3; volume of aqueous phase = 5.0×10−5 m3; concentration of sulfide = 1.06 kmol/m3; NH3:H2S mole ratio = 5.27; temperature = 333 K; stirring speed = 1500 rev/min.
0 10 20 30 40 50 60 70 80 90 100 0
10 20 30 40 50 60 70
TBAB concn.x102 (kmol/m3 of org. phase) 0.0 8.93
5.2 14.27
Selectivity of DBS (%)
Conversion of benzyl chloride (%)
Fig. 4.9. Effect of catalyst loading on selectivity of DBS. All conditions are same as in Fig.4.8.
0 50 100 150 200 250 300 350 400 450 0
10 20 30 40 50 60 70 80 90 100
benzyl chloride 0.78 kmol/m3 1.44 kmol/m3 2.0 kmol/m3
Conversion of benzyl chloride (%)
Reaction time (min)
Fig. 4.10. Effect of benzyl chloride concentration on conversion of benzyl chloride. Volume of toluene = 5.0×10−5 m3; TBAB = 5.8×10−3 mol;
volume of aqueous phase = 5.0×10−5 m3; concentration of sulfide = 1.06 kmol/m3; NH3:H2S mole ratio = 5.27; temperature = 333 K; stirring speed = 1500 rev/min.
0 10 20 30 40 50 60 70 80 90 100 0
10 20 30 40 50 60 70 80 90 100
benzyl chloride 0.78 kmol/m3 1.44 kmol/m3 2.0 kmol/m3
Selectivity of DBS (%)
Conversion of benzyl chloride (%)
Fig. 4.11. Effect of benzyl chloride concentration on selectivity of DBS. All conditions are same as in Fig.4.10.
0 50 100 150 200 250 300 350 400 450 0
10 20 30 40 50 60 70 80 90 100
Volume of aqueous phase 25 mL
50 mL 75 mL
Conversion of benzyl chloride (%)
Reaction time (min)
Fig. 4.12. Effect of volume of aqueous phase on conversion of benzyl chloride.
Volume of organic phase = 5.0×10−5 m3; concentration of benzyl chloride
= 1.44 kmol/m3; concentration of H2S = 1.06 kmol/m3; NH3:H2S mole ratio = 5.27; concentration of TBAB = 8.92×10−2 kmol/m3; temperature = 333 K; stirring speed = 1500 rev/min.
0 10 20 30 40 50 60 70 80 90 100 0
10 20 30 40 50 60 70 80 90 100
Volume of aqueous phase 25 mL
50 mL 75 mL
Selectivity of DBS (%)
Conversion of benzyl chloride (%)
Fig. 4.13. Effect of volume of aqueous phase on selectivity of DBS. All conditions are same as in Fig.4.12.
0 100 200 300 400
0.0 0.5 1.0 1.5 2.0
Concentration (kmol/m3 )
Reaction time (min)
Benzyl Chloride BM
DBS Sulfide
Fig. 4.14. Concentration profile for a typical run. Volume of toluene = 5.0×10−5 m3; concentration of benzyl chloride = 2.0 kmol/m3; TBAB =5.8×10−3 mol; volume of aqueous phase = 5.0×10−5 m3; concentration of sulfide = 1.06 kmol/m3; NH3:H2S mole ratio = 5.27; temperature = 333 K; stirring speed = 1500 rev/min.
REDUCTION OF NITROARENES BY AQUEOUS AMMONIUM SULFIDE UNDER LIQUID-LIQUID PHASE TRANSFER CATALYSIS
NO2
R
NH2
R 1. Aqueous (NH4)2S
2. PTC (TBAB)
Nitroarenes Aryl amines
+
(NH4)2S2O3+
S0Ammonium Thiosulfate Elemental Sulfur where, R=CH3 (Nitrotoluenes) or Cl (Chloronitrobenzenes)
5.1 INTRODUCTION
The reduction reaction of nitroarenes by negative divalent sulfur (sulfide, hydrosulfide, and polysulfides) is called Zinin reduction (Dauben, 1973).
Sodium sulfide, sodium disulfide, and ammonium sulfide are most commonly used for this purpose. In aqueous ammonium sulfide, the sulfide ions (S2−) and the hydrosulfide ions (HS−) remain in equilibrium, as represented by Scheme 5.1 (Rumpf et al., 1999; Beutier et al., 1978). This property of ammonium sulfide makes it different from the other reducing agents like sodium sulfide or disulfide.
NH3 + H2O ⇌ NH4+ + HO− H2S ⇌ H+ + HS−
HS− ⇌ H+ + S2−
H2O ⇌ H++HO− Scheme 5.1
The overall stoichiometry of the Zinin original reduction of nitrobenzene by aqueous ammonium sulfide under two-phase conditions is given by Eq. 5.1 (Dauben, 1973). This stoichiometry is also applicable to the reduction of nitroarenes with sodium sulfide (Bhave and Sharma, 1981; Pradhan and Sharma, 1992a; Pradhan, 2000; Yadav et al., 2003a, 2003b).
4ArNO2 + 6S2− + 7H2O → 4ArNH2 + 3S2O32− + 6HO− (5.1)
For the preparation of p-aminophenylacetic acid from p-nitrophenylacetic acid using aqueous ammonium sulfide, Gilman (1941) reported that the sulfide ions were oxidized to elemental sulfur instead of to thiosulfate, following the stoichiometry of Eq. 5.2. Lucas and Scudder (1928) also reported similar observations for the reduction of 2-bromo-4-nitrotoluene to 2-bromo-4- aminotoluene by an alcoholic solution of ammonium sulfide. The formation of elemental sulfur was reported for the preparation of 3-amino-5-nitrobenzyl alcohol using ammonium sulfide prepared from ammonium chloride and crystalline sodium sulfide dissolved in methanol (Meindl et al., 1984).
ArNO2 + 3HS− + H2O → ArNH2 + 3S+ 3HO− (5.2)
The overall stoichiometry of the reduction reaction using disulfide as the reducing agent is as follows (Hojo et al., 1960; Bhave and Sharma, 1981):
ArNO2 + S22− + H2O → ArNH2 + S2O32− (5.3)
Therefore, the two different reactions leading to the formation of either elemental sulfur or thiosulfate may be operative for the reduction of nitroarenes with ammonium sulfide. A detailed study of such reactions is, therefore, not only commercially important, but also academically interesting.
In the present work, the effects ammonia (NH3) concentration and elemental sulfur loading on the conversion of nitroarenes (nitrotoluenes and chloronitrobenzenes) were studied to establish the mechanism of the reaction.
The effect of stirring speed and temperature on the reaction was studied to determine the mass transfer effects on the reaction, and to calculate the activation energy respectively. In addition, the effects of various parameters, such as catalyst (tetrabutylammonium bromide, TBAB) concentration, sulfide concentration, and concentration of nitroarenes (nitrotoluenes and chloronitrobenzenes), on the reactions of nitroarenes with aqueous ammonium sulfide were studied to determine the dependencies of the reaction rates on the concentrations of various species present in the reaction system. Such
knowledge is essential for proposing a kinetic rate expression for such reaction systems. Furthermore, a generalized empirical kinetic model based on experimental observations was developed to correlate the experimentally obtained conversion versus time data.