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The ultrasonically induced reaction of benzoyl chloride with nitrobenzene: an unexpected sonochemical effect and a possible mechanism
Ultrasonics Sonochemistry 9 (2002) 245–249 www.elsevier.com/locate/ultsonch
The ultrasonically induced reaction of benzoyl chloride with nitrobenzene: an unexpected sonochemical effect and a possible mechanism M. Vinatoru a
a,*
, R. Stavrescu a, A.B. Milcoveanu b, M. Toma a, T.J. Mason
c
Romanian Academy, ‘‘Costin D. Nenitzescu’’ Institute of Organic Chemistry, 71141 Bucharest, P.O. Box 15-254, Romania b Research and Development Institute for Mechanical Equipment, Sos. Oltenitei 116-118, 75651 Bucharest, Romania c School of Science and the Environment, Coventry University, Priory Street, Coventry CV1 5FB, United Kingdom Received 7 November 2001; accepted 22 January 2002
Abstract A mixture of benzoyl chloride and nitrobenzene is not known to be chemically reactive, indeed the mixture is chemically inert when subjected individually to either ultrasonic irradiation or heating. However, if this system is initially subjected to ultrasound and then heated for several hours at 200 °C, a reaction does occur and the products are benzoic anhydride, hydrochloric acid, nitrous and nitric acid together with some minor products. To the best of our knowledge, this is the first example of a reaction where the effect of ultrasound does not appear to be the consequence of the direct action of acoustic cavitation bubbles. A possible explanation of this behaviour is advanced which involves an electron transfer reaction in which nitrobenzene is first activated by ultrasound and then acts as oxidant in the thermal stage of reaction. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Sonochemistry; Pre-sonication; Benzoyl chloride; Nitrobenzene
1. Introduction The chemical effects of ultrasound have been studied for over 50 years [1–3]. In spite of these intense studies, the understanding of the chemical effects of power ultrasonic irradiation on chemical systems is still limited. It is generally accepted [2,3] that sonochemistry in solution is primarily the result of acoustic cavitation i.e. the energy associated with the creation, expansion, and implosive collapse of bubbles generated by ultrasound. For continuously sonicated liquids one can say that cavitation bubbles are responsible for promoting reaction. In the case of sonicated systems that do not reveal detectable chemical transformation, but are able to give products when subsequently subjected to another energy source, it is not easy to ascribe this to the collapsing bubbles. An illustration of the latter type of activation is the reaction of benzoyl chloride (BC) with nitrobenzene (NB) which yields no products under ultrasound alone,
*
Corresponding author. Tel.: +40-1-637-5948; fax:+40-1-312-1601. E-mail address: [email protected] (M. Vinatoru).
but does afford products after heating the sonicated reaction mixture. It is known that under powerful ultrasonic irradiation nitrobenzene is slowly decomposed by a mechanism which is reported to involve the homolytic cleavage of the carbon–nitrogen bond leading to the generation of a polymeric like compound [4,5]. The authors have shown that during sonication the NB changes color from light yellow to dark brown. As far as we are aware there are no reports on any reaction between nitrobenzene and benzoyl chloride under either sonication or heating. This makes the chemical effects that are observed after pre-sonication followed by heating remarkable.
2. Experimental details 2.1. Reagents Nitrobenzene, technical grade (Chimopar, Romania), was purified by steam distillation; after separation, nitrobenzene was dried on sodium sulfate, and distilled at
1350-4177/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 2 ) 0 0 0 8 2 - 2
246
M. Vinatoru et al. / Ultrasonics Sonochemistry 9 (2002) 245–249
normal pressure, using a column with 10 theoretical plates, filled with stainless steel small springs. The purity was checked by both GC-MS and conductometric measurements. The final purity was >99.9% and the conductivity was 0.001 lS, the color was pale yellow. Benzoyl chloride, analytical grade (Merck, art. 801804) was purified by distillation (using the same column as for NB) and analyzed by GC-MS, showing a purity of >99.9%. 2.2. Experimental A flat bottom flask (250 ml), equipped with side arms for gas and sample addition, was filled with a mixture of 28.1 g (34 ml, 0.2 moles) benzoyl chloride and 49.2 g (59.2 ml, 0.4 moles) nitrobenzene. The flask was immersed in a cleaning bath (Langford Sonomatic Ultrasonic T175 cleaning bath, the power of which did not exceed 0.5 wcm2 ) and ultrasonically irradiated for 5 h under argon which provided both an inert atmosphere and a gas carrier. The ultrasonic source was removed and the mixture was heated for a further 40 h at 200 °C. Any gases evolved during either procedure were trapped in distilled water. After 35 h of heating, evolution of nitrous vapors (a red–brown gas) throughout the condenser was observed. The evolution of gases ceased within 4 h. The reaction was followed by a GC-MS analysis of aliquots from the liquid phase. After three separate sonication/heating experiments, the combined water solution containing the trapped gases from all three was analyzed titrimetrically which revealed the presence of hydrochloric acid, nitrous and nitric acids together with small amounts of benzoic acid. The gases trapped in the water were analyzed first with sodium hydroxide for total acidity and afterwards with silver nitrate to determine hydrochloric acid. The nitrous acid was determined iodometrically. Nitric acid was determined by difference. Benzoic acid was determined by concentration of neutralized solution, acidification of the concentrate and subsequent diethyl ether extraction of the acidic solution. Treatment of the aqueous solution, containing all acids, with p-nitroaniline followed by coupling with a-naphthol gave a red dye, identical with a diazo derivative [6]. The reaction mixture was worked up by distillation under reduced pressure (5 mm Hg) and the following fractions were collected: The first fraction (52 g, bp 78–80 °C) contained unreacted benzoyl chloride and nitrobenzene; second fraction (13.4 g, bp 185 °C) contained benzoic anhydride identified by comparison of its IR, 1 H and 13 C NMR spectra, and mass spectrum with those of an authentic sample. 1 The residue in the distillation flask (8 g) yielded
a black solid melting over 450 °C, that burnt without ash and which did not show any e.p.r. signal. 2 2.3. Thermal reactions A mixture of 28.1 g (34 ml, 0.2 moles) benzoyl chloride and 49.2 g (59.2 ml, 0.4 moles) nitrobenzene was heated together, under argon, for 60 h at 200 °C; the reaction was followed by GC-MS (Carlo Erba Instruments QMD 1000). No chemical products were found. Neat benzoyl chloride and nitrobenzene were heated separately, each at their boiling points, for 25 h and no reaction products were found. Neat nitrobenzene was sonicated for 5 h and reaction followed by GC-MS. No reaction products were detected. However, the color of nitrobenzene changed from yellow to red–brown. A sample of nitrobenzene was then heated for 60 h following 5 h of sonication. No further change was observed. Ultrasonic treatment of benzoyl chloride over a period of 30 h generated some products and produced color changes. Chlorine was evolved and chlorinated biphenyls were produced. This suggests that benzoyl chloride undergoes a reaction during this treatment. The results of neat benzoyl chloride sonication will be the subject for a forthcoming publication. Sonication of neat benzoyl chloride (5 h) and after adding nitrobenzene followed by heating to reflux for 40 h, did not generate any products, in spite of some chlorine formation during the sonication of benzoyl chloride. Sonication of neat nitrobenzene (5 h) followed by the addition of benzoyl chloride and heating under reflux resulted in a violent reaction suggesting that nitrobenzene sonication had generated a very reactive species. This reaction afforded a similar mixture of compounds to those shown in Table 1 together with polymeric material. Due to the violent character of the reaction it proved impossible to produce a mass balance for the reaction. The composition of the organic liquid phase of the main reaction, determined by GS-MS analysis, is presented in the Table 1. The amounts of inorganic acids trapped in the water at the end of reaction are given in the Table 2. It can be seen that the yield of benzoic anhydride is around 60% with respect to benzoyl chloride (one mole of benzoyl chloride should afford 0.5 moles benzoic anhydride). On the other hand the total moles of hydrochloric acid generated can be entirely ascribed to the decomposition of benzoyl chloride. The nitrogen 2
1
An authentic sample was obtained from benzoyl chloride and sodium benzoate in pyridine.
Experiments with neat nitrobenzene and mixture of benzoyl chloride and nitrobenzene, using a vibracell probe system afford solid that exhibits ESR signal as those described in Ref. [5].
M. Vinatoru et al. / Ultrasonics Sonochemistry 9 (2002) 245–249 Table 1 Analytic results of reaction mixture Compound
3. Discussion
Weight percent
Moles in reaction mixture
18.59
0.0593
1.39
0.0088
0.24
0.0011
0.69
0.0025
0.30
0.0011
0.06
0.0002
0.03
0.0001
0.29
0.0011
0.55
0.0021
0.16
0.0006
54.10 23.60
0.3170 0.1214
1
5 6
7
8
9
10
11
The effect of ultrasound upon NB was followed by conductometric measurements over a period of 1 h, using a Metrohm device and conductometric cell having the constant k ¼ 0:675 cm1 . To avoid the destruction of black platinum from the cell electrodes, the measurements were made with the ultrasound switched off. The conductivity rose from 0.001 to 16 lS and at the end of this period the ultrasound was switched off permanently and monitoring was continued for a further hour (Fig. 1). After the sonication had been stopped the conductivity decreased until it reached a residual value that was higher than that obtained for the original NB. This behavior suggests that during sonication nitrobenzene somehow produces ionic species that are responsible for the increase of conductivity and that after switching off the ultrasound, these species disappear, possibly by recombination to neutral species. The color change observed on sonication of NB has been described elsewhere [5], although in our case the source of ultrasound was a low power cleaning bath. Attempts to record e.p.r. spectra for the sonicated NB failed to give any readings however a solution of DPPH in NB was decolorized when sonicated for 10 min. This experiment confirms that during NB sonication radical species were formed. It has been reported that during sonication the electrical conductivity of many liquids change as do their dielectric properties [7–13]. It has also been demonstrated that certain liquids, when submitted to a rapidly alternating electrical field, generate fluctuations in volume, which result in the production of cavitation [14,15]. These phenomena could be likened to the piezoelectricity or electrostriction of liquids. Assuming
12
13 Nitrobenzene Benzoyl chloride
247
Table 2 Analysis of the compounds in the water trap Compound
Total moles
Hydrochloric acid, 2 Nitrous acid, 3 Nitric acid, 4
0.0660 0.0098 0.0098
containing acids must come from the denitration process of nitrobenzene that occurs during the heating of the reaction mixture.
Fig. 1. Conductometric values for sonicated nitrobenzene.
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M. Vinatoru et al. / Ultrasonics Sonochemistry 9 (2002) 245–249
that during sonication the liquids could adopt, for a very short time, a solid like structure, it is possible to consider such liquids to be behaving as charged electrical condensers. Chanon suggested that sonochemical effects might derive from the generation of short lives cathodes and anodes within the molecular structure [16]. With this in mind it therefore seems logical to propose that ultrasonic activation might promote electron transfer reactions to the detriment of ionic ones [17,18], and that this may be due to the production of a sonochemically generated short lived charged condenser. A possible explanation for the observation that NB conductivity first rose in the presence of ultrasound and then diminished when it was switched off is the generation of radical ion intermediates. These species could be formed by an electron transfer reaction between two molecules of NB (Scheme 1). If such radical ions are generated sonochemically it is hard to envisage that they would then have long enough lifetimes to be available in the next (thermal) stage of the reaction. Indeed this does not conform to the accepted theories of ultrasonic activation where it is the in situ (and concurrent) collapse of cavitation bubbles within a system which generate local high temperatures and pressures during the course of the reaction. In our case it would appear that the sonication produced some form of stored reactivity within NB, so that reaction with BC can take place after a period of heating after sonication has been stopped. Clearly this is something different from normal thermal activation because a mixture of these two components was not at all reactive under heating alone. Together with the main reaction product (benzoic anhydride) small quantities of compounds have been identified which may have been the result of radical type
reactions. Sonication tends to promote electron transfer reactions and BC itself decomposes under sonication. Therefore it seems reasonable to suggest that during ultrasonic treatment, benzoyl chloride also undergoes an electron transfer reaction, leading to an ion radical pair (Scheme 2). Both radical ions could then split leading to relatively stable fragments (Scheme 3). During the sonication and heating steps of the NB/ BC mixture, NB may oxidize benzoyl chloride leading to a pair of radical ions (Scheme 4). The radical cation 18 could split into chlorine atom and benzoyl cation that would then react with the radical anion of NB 16, leading to the radical 24 (Scheme 5). The radical 24 is similar to those observed in the spin trapping reaction of benzoyloxy radical 25, with nitrozobenzene 26, and therefore it is reasonable to assume that radical 24 could split, thermally, into these reagents (Scheme 6). The radical 25 could then be reduced by a nitrobenzene radical anion to the benzoate anion 27, and its
Scheme 4. Radical ions of BC and NB reaction.
Scheme 5. Formation of radical precursor 24.
Scheme 6. Splitting products of radical 24. Scheme 1. Suggested ionic species produced by sonication of NB.
Scheme 2. Electron transfer between two molecules of benzoyl chloride.
Scheme 3. Splitting products of radical ions of benzoyl chloride.
Scheme 7. Benzoic anhydride production.
Scheme 8. Denitration mechanism of NB.
M. Vinatoru et al. / Ultrasonics Sonochemistry 9 (2002) 245–249
249
Scheme 9. Mechanism of biphenyls formation.
subsequent reaction with the cation 20, leads to benzoic anhydride 28 (Scheme 7). The benzoyloxy radical 25 is a strong oxidizing reagent and it is therefore capable of oxidizing in its turn, both benzoyl chloride and nitrobenzene. The most important feature is probably the oxidation of NB, leading to NB radical cation, which is unstable, and breaks into a phenyl cation and nitrogen dioxide, explaining the denitration process that occurs during the heating period (Scheme 8). When neat NB is sonicated for 100 h, water extraction reveals the presence of nitrous acid. This shows that, most probably, the radical anion 15 is also formed during ultrasonic irradiation. The phenyl cation resulting from the denitration reaction of NB, could react with the radical anion of NB, leading to nitro-biphenyls (Scheme 9). In addition to the reactions described above the benzoyloxy radical 25 can undergo normal decarboxylation, which leads to phenyl radical and carbon dioxide. The phenyl radical could react with a chlorine atom leading to chlorophenyl or dimerize yielding biphenyls that could also suffer chlorination giving chloro-biphenyls. An experiment in which benzoyl peroxide was employed to generate radicals in the reaction mixture demonstrated that this reagent induces reaction, but the products were not the same as those derived from the neat reagents alone. This aspect of the research is still under investigation and will be the subject for a future publication. Two significant observations result from the experiments reported here: the formation of benzoic anhydride and the denitration of nitrobenzene. The former requires an oxygen abstraction from nitrobenzene while the latter is probably the consequence of the fate of an unstable reactive intermediate generated from nitrobenzene.
4. Conclusion The results presented in this paper demonstrate for the first time that ultrasound can induce thermal reactivity in a chemical system, which without sonication is
chemically inert. However, the precise reasons why the studied system acquires such reactivity and how this reactivity is transmitted from the sonication to the heating stage of reaction remains to be discovered.
Acknowledgements The authors wish to thank the Ministry of Research and Technology, Romania, for financial support (grant no. 921/396-1997), and the Royal Society (UK), for a short stay grant at the Sonochemistry Centre in Coventry University.
References [1] I.E. Elpiner, Ultrasound: Physical, Chemical, and Biological Effects, Consultant Bureau, New York, 1964. [2] K.S. Suslick, Modern Synthetic Methods, Springer Verlag, New York, 1986; K.S. Suslick (Ed.), Ultrasound: Its Chemical, Physical and Biological Effects, VCH Publishers, New York, 1986. [3] T.J. Mason (Ed.), Advances in Sonochemistry, vol. 1, JAI Press, London, 1990. [4] D.J. Donaldson, M.D. Farrington, P. Kruus, J. Phys. Chem. 83 (1979) 3130. [5] G.K. Diedrich, P. Kruus, L.M. Rachlis, Can. J. Chem. 50 (1972) 1743. [6] D. Davidson, Paint, Oil Chem. Rev. 86 (1) (1928) 18D; Chem. Abstr. 22 (1928) 4252. [7] F. Seidel, Z. Physik. 116 (1940) 359. [8] N. Berkowitz, Can. J. Chem. 37 (1959) 13. [9] I. Hrazdira, Naturwissenschaften 50 (1963) 398. [10] E. Constantin, A. Ciuppe, D. Auslander, Stud. Univ. BabesBolyai, Ser. Math. Phys. 1 (1967) 113. [11] E. Constantin, D. Auslander, I. Dabala, C. Serban, Stud. Univ. Babes-Bolyai, Ser. Math. Phys. 13 (1968) 101. [12] E. Constantin, D. Auslander, A. Ciuppe, Stud. Univ. BabesBolyai, Ser. Math. Phys. 14 (1968) 103; N. Tuchel, Gr.N. Popescu, Farmacia (Bucharest) 15 (1967) 279. [13] Vinatoru, M., Ph.D. Thesis, Bucharest 1996. [14] E. Gerdes, Wiss. Z. Univ. Rostock, Sonderh. 10 (1961) 15. [15] I. Isaev, D. Mitev, Pharmazie 27 (1972) 236. [16] M. Chanon, Bul. Soc. Chim. France 2 (1985) 209–238. [17] J.L. Luche, C. Einhorn, J. Einhorn, J.V. Sinisterra-Gago, Tetrahedron Lett. (1990) 4125. [18] C. Einhorn, J. Einhorn, M.J. Dickens, J.L. Luche, Tetrahedron Lett. (1990) 4129.
The ultrasonically induced reaction of benzoyl chloride with nitrobenzene: an unexpected sonochemical effect and a possible mechanism M. Vinatoru a
a,*
, R. Stavrescu a, A.B. Milcoveanu b, M. Toma a, T.J. Mason
c
Romanian Academy, ‘‘Costin D. Nenitzescu’’ Institute of Organic Chemistry, 71141 Bucharest, P.O. Box 15-254, Romania b Research and Development Institute for Mechanical Equipment, Sos. Oltenitei 116-118, 75651 Bucharest, Romania c School of Science and the Environment, Coventry University, Priory Street, Coventry CV1 5FB, United Kingdom Received 7 November 2001; accepted 22 January 2002
Abstract A mixture of benzoyl chloride and nitrobenzene is not known to be chemically reactive, indeed the mixture is chemically inert when subjected individually to either ultrasonic irradiation or heating. However, if this system is initially subjected to ultrasound and then heated for several hours at 200 °C, a reaction does occur and the products are benzoic anhydride, hydrochloric acid, nitrous and nitric acid together with some minor products. To the best of our knowledge, this is the first example of a reaction where the effect of ultrasound does not appear to be the consequence of the direct action of acoustic cavitation bubbles. A possible explanation of this behaviour is advanced which involves an electron transfer reaction in which nitrobenzene is first activated by ultrasound and then acts as oxidant in the thermal stage of reaction. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Sonochemistry; Pre-sonication; Benzoyl chloride; Nitrobenzene
1. Introduction The chemical effects of ultrasound have been studied for over 50 years [1–3]. In spite of these intense studies, the understanding of the chemical effects of power ultrasonic irradiation on chemical systems is still limited. It is generally accepted [2,3] that sonochemistry in solution is primarily the result of acoustic cavitation i.e. the energy associated with the creation, expansion, and implosive collapse of bubbles generated by ultrasound. For continuously sonicated liquids one can say that cavitation bubbles are responsible for promoting reaction. In the case of sonicated systems that do not reveal detectable chemical transformation, but are able to give products when subsequently subjected to another energy source, it is not easy to ascribe this to the collapsing bubbles. An illustration of the latter type of activation is the reaction of benzoyl chloride (BC) with nitrobenzene (NB) which yields no products under ultrasound alone,
*
Corresponding author. Tel.: +40-1-637-5948; fax:+40-1-312-1601. E-mail address: [email protected] (M. Vinatoru).
but does afford products after heating the sonicated reaction mixture. It is known that under powerful ultrasonic irradiation nitrobenzene is slowly decomposed by a mechanism which is reported to involve the homolytic cleavage of the carbon–nitrogen bond leading to the generation of a polymeric like compound [4,5]. The authors have shown that during sonication the NB changes color from light yellow to dark brown. As far as we are aware there are no reports on any reaction between nitrobenzene and benzoyl chloride under either sonication or heating. This makes the chemical effects that are observed after pre-sonication followed by heating remarkable.
2. Experimental details 2.1. Reagents Nitrobenzene, technical grade (Chimopar, Romania), was purified by steam distillation; after separation, nitrobenzene was dried on sodium sulfate, and distilled at
1350-4177/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 2 ) 0 0 0 8 2 - 2
246
M. Vinatoru et al. / Ultrasonics Sonochemistry 9 (2002) 245–249
normal pressure, using a column with 10 theoretical plates, filled with stainless steel small springs. The purity was checked by both GC-MS and conductometric measurements. The final purity was >99.9% and the conductivity was 0.001 lS, the color was pale yellow. Benzoyl chloride, analytical grade (Merck, art. 801804) was purified by distillation (using the same column as for NB) and analyzed by GC-MS, showing a purity of >99.9%. 2.2. Experimental A flat bottom flask (250 ml), equipped with side arms for gas and sample addition, was filled with a mixture of 28.1 g (34 ml, 0.2 moles) benzoyl chloride and 49.2 g (59.2 ml, 0.4 moles) nitrobenzene. The flask was immersed in a cleaning bath (Langford Sonomatic Ultrasonic T175 cleaning bath, the power of which did not exceed 0.5 wcm2 ) and ultrasonically irradiated for 5 h under argon which provided both an inert atmosphere and a gas carrier. The ultrasonic source was removed and the mixture was heated for a further 40 h at 200 °C. Any gases evolved during either procedure were trapped in distilled water. After 35 h of heating, evolution of nitrous vapors (a red–brown gas) throughout the condenser was observed. The evolution of gases ceased within 4 h. The reaction was followed by a GC-MS analysis of aliquots from the liquid phase. After three separate sonication/heating experiments, the combined water solution containing the trapped gases from all three was analyzed titrimetrically which revealed the presence of hydrochloric acid, nitrous and nitric acids together with small amounts of benzoic acid. The gases trapped in the water were analyzed first with sodium hydroxide for total acidity and afterwards with silver nitrate to determine hydrochloric acid. The nitrous acid was determined iodometrically. Nitric acid was determined by difference. Benzoic acid was determined by concentration of neutralized solution, acidification of the concentrate and subsequent diethyl ether extraction of the acidic solution. Treatment of the aqueous solution, containing all acids, with p-nitroaniline followed by coupling with a-naphthol gave a red dye, identical with a diazo derivative [6]. The reaction mixture was worked up by distillation under reduced pressure (5 mm Hg) and the following fractions were collected: The first fraction (52 g, bp 78–80 °C) contained unreacted benzoyl chloride and nitrobenzene; second fraction (13.4 g, bp 185 °C) contained benzoic anhydride identified by comparison of its IR, 1 H and 13 C NMR spectra, and mass spectrum with those of an authentic sample. 1 The residue in the distillation flask (8 g) yielded
a black solid melting over 450 °C, that burnt without ash and which did not show any e.p.r. signal. 2 2.3. Thermal reactions A mixture of 28.1 g (34 ml, 0.2 moles) benzoyl chloride and 49.2 g (59.2 ml, 0.4 moles) nitrobenzene was heated together, under argon, for 60 h at 200 °C; the reaction was followed by GC-MS (Carlo Erba Instruments QMD 1000). No chemical products were found. Neat benzoyl chloride and nitrobenzene were heated separately, each at their boiling points, for 25 h and no reaction products were found. Neat nitrobenzene was sonicated for 5 h and reaction followed by GC-MS. No reaction products were detected. However, the color of nitrobenzene changed from yellow to red–brown. A sample of nitrobenzene was then heated for 60 h following 5 h of sonication. No further change was observed. Ultrasonic treatment of benzoyl chloride over a period of 30 h generated some products and produced color changes. Chlorine was evolved and chlorinated biphenyls were produced. This suggests that benzoyl chloride undergoes a reaction during this treatment. The results of neat benzoyl chloride sonication will be the subject for a forthcoming publication. Sonication of neat benzoyl chloride (5 h) and after adding nitrobenzene followed by heating to reflux for 40 h, did not generate any products, in spite of some chlorine formation during the sonication of benzoyl chloride. Sonication of neat nitrobenzene (5 h) followed by the addition of benzoyl chloride and heating under reflux resulted in a violent reaction suggesting that nitrobenzene sonication had generated a very reactive species. This reaction afforded a similar mixture of compounds to those shown in Table 1 together with polymeric material. Due to the violent character of the reaction it proved impossible to produce a mass balance for the reaction. The composition of the organic liquid phase of the main reaction, determined by GS-MS analysis, is presented in the Table 1. The amounts of inorganic acids trapped in the water at the end of reaction are given in the Table 2. It can be seen that the yield of benzoic anhydride is around 60% with respect to benzoyl chloride (one mole of benzoyl chloride should afford 0.5 moles benzoic anhydride). On the other hand the total moles of hydrochloric acid generated can be entirely ascribed to the decomposition of benzoyl chloride. The nitrogen 2
1
An authentic sample was obtained from benzoyl chloride and sodium benzoate in pyridine.
Experiments with neat nitrobenzene and mixture of benzoyl chloride and nitrobenzene, using a vibracell probe system afford solid that exhibits ESR signal as those described in Ref. [5].
M. Vinatoru et al. / Ultrasonics Sonochemistry 9 (2002) 245–249 Table 1 Analytic results of reaction mixture Compound
3. Discussion
Weight percent
Moles in reaction mixture
18.59
0.0593
1.39
0.0088
0.24
0.0011
0.69
0.0025
0.30
0.0011
0.06
0.0002
0.03
0.0001
0.29
0.0011
0.55
0.0021
0.16
0.0006
54.10 23.60
0.3170 0.1214
1
5 6
7
8
9
10
11
The effect of ultrasound upon NB was followed by conductometric measurements over a period of 1 h, using a Metrohm device and conductometric cell having the constant k ¼ 0:675 cm1 . To avoid the destruction of black platinum from the cell electrodes, the measurements were made with the ultrasound switched off. The conductivity rose from 0.001 to 16 lS and at the end of this period the ultrasound was switched off permanently and monitoring was continued for a further hour (Fig. 1). After the sonication had been stopped the conductivity decreased until it reached a residual value that was higher than that obtained for the original NB. This behavior suggests that during sonication nitrobenzene somehow produces ionic species that are responsible for the increase of conductivity and that after switching off the ultrasound, these species disappear, possibly by recombination to neutral species. The color change observed on sonication of NB has been described elsewhere [5], although in our case the source of ultrasound was a low power cleaning bath. Attempts to record e.p.r. spectra for the sonicated NB failed to give any readings however a solution of DPPH in NB was decolorized when sonicated for 10 min. This experiment confirms that during NB sonication radical species were formed. It has been reported that during sonication the electrical conductivity of many liquids change as do their dielectric properties [7–13]. It has also been demonstrated that certain liquids, when submitted to a rapidly alternating electrical field, generate fluctuations in volume, which result in the production of cavitation [14,15]. These phenomena could be likened to the piezoelectricity or electrostriction of liquids. Assuming
12
13 Nitrobenzene Benzoyl chloride
247
Table 2 Analysis of the compounds in the water trap Compound
Total moles
Hydrochloric acid, 2 Nitrous acid, 3 Nitric acid, 4
0.0660 0.0098 0.0098
containing acids must come from the denitration process of nitrobenzene that occurs during the heating of the reaction mixture.
Fig. 1. Conductometric values for sonicated nitrobenzene.
248
M. Vinatoru et al. / Ultrasonics Sonochemistry 9 (2002) 245–249
that during sonication the liquids could adopt, for a very short time, a solid like structure, it is possible to consider such liquids to be behaving as charged electrical condensers. Chanon suggested that sonochemical effects might derive from the generation of short lives cathodes and anodes within the molecular structure [16]. With this in mind it therefore seems logical to propose that ultrasonic activation might promote electron transfer reactions to the detriment of ionic ones [17,18], and that this may be due to the production of a sonochemically generated short lived charged condenser. A possible explanation for the observation that NB conductivity first rose in the presence of ultrasound and then diminished when it was switched off is the generation of radical ion intermediates. These species could be formed by an electron transfer reaction between two molecules of NB (Scheme 1). If such radical ions are generated sonochemically it is hard to envisage that they would then have long enough lifetimes to be available in the next (thermal) stage of the reaction. Indeed this does not conform to the accepted theories of ultrasonic activation where it is the in situ (and concurrent) collapse of cavitation bubbles within a system which generate local high temperatures and pressures during the course of the reaction. In our case it would appear that the sonication produced some form of stored reactivity within NB, so that reaction with BC can take place after a period of heating after sonication has been stopped. Clearly this is something different from normal thermal activation because a mixture of these two components was not at all reactive under heating alone. Together with the main reaction product (benzoic anhydride) small quantities of compounds have been identified which may have been the result of radical type
reactions. Sonication tends to promote electron transfer reactions and BC itself decomposes under sonication. Therefore it seems reasonable to suggest that during ultrasonic treatment, benzoyl chloride also undergoes an electron transfer reaction, leading to an ion radical pair (Scheme 2). Both radical ions could then split leading to relatively stable fragments (Scheme 3). During the sonication and heating steps of the NB/ BC mixture, NB may oxidize benzoyl chloride leading to a pair of radical ions (Scheme 4). The radical cation 18 could split into chlorine atom and benzoyl cation that would then react with the radical anion of NB 16, leading to the radical 24 (Scheme 5). The radical 24 is similar to those observed in the spin trapping reaction of benzoyloxy radical 25, with nitrozobenzene 26, and therefore it is reasonable to assume that radical 24 could split, thermally, into these reagents (Scheme 6). The radical 25 could then be reduced by a nitrobenzene radical anion to the benzoate anion 27, and its
Scheme 4. Radical ions of BC and NB reaction.
Scheme 5. Formation of radical precursor 24.
Scheme 6. Splitting products of radical 24. Scheme 1. Suggested ionic species produced by sonication of NB.
Scheme 2. Electron transfer between two molecules of benzoyl chloride.
Scheme 3. Splitting products of radical ions of benzoyl chloride.
Scheme 7. Benzoic anhydride production.
Scheme 8. Denitration mechanism of NB.
M. Vinatoru et al. / Ultrasonics Sonochemistry 9 (2002) 245–249
249
Scheme 9. Mechanism of biphenyls formation.
subsequent reaction with the cation 20, leads to benzoic anhydride 28 (Scheme 7). The benzoyloxy radical 25 is a strong oxidizing reagent and it is therefore capable of oxidizing in its turn, both benzoyl chloride and nitrobenzene. The most important feature is probably the oxidation of NB, leading to NB radical cation, which is unstable, and breaks into a phenyl cation and nitrogen dioxide, explaining the denitration process that occurs during the heating period (Scheme 8). When neat NB is sonicated for 100 h, water extraction reveals the presence of nitrous acid. This shows that, most probably, the radical anion 15 is also formed during ultrasonic irradiation. The phenyl cation resulting from the denitration reaction of NB, could react with the radical anion of NB, leading to nitro-biphenyls (Scheme 9). In addition to the reactions described above the benzoyloxy radical 25 can undergo normal decarboxylation, which leads to phenyl radical and carbon dioxide. The phenyl radical could react with a chlorine atom leading to chlorophenyl or dimerize yielding biphenyls that could also suffer chlorination giving chloro-biphenyls. An experiment in which benzoyl peroxide was employed to generate radicals in the reaction mixture demonstrated that this reagent induces reaction, but the products were not the same as those derived from the neat reagents alone. This aspect of the research is still under investigation and will be the subject for a future publication. Two significant observations result from the experiments reported here: the formation of benzoic anhydride and the denitration of nitrobenzene. The former requires an oxygen abstraction from nitrobenzene while the latter is probably the consequence of the fate of an unstable reactive intermediate generated from nitrobenzene.
4. Conclusion The results presented in this paper demonstrate for the first time that ultrasound can induce thermal reactivity in a chemical system, which without sonication is
chemically inert. However, the precise reasons why the studied system acquires such reactivity and how this reactivity is transmitted from the sonication to the heating stage of reaction remains to be discovered.
Acknowledgements The authors wish to thank the Ministry of Research and Technology, Romania, for financial support (grant no. 921/396-1997), and the Royal Society (UK), for a short stay grant at the Sonochemistry Centre in Coventry University.
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