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Thermal isomerization of trifluoromethyl trifluorovinyl ether to pentafluoropropionyl fluoride
Journal of Fluorine Chemistry 125 (2004) 199–204
Thermal isomerization of trifluoromethyl trifluorovinyl ether to pentafluoropropionyl fluoride Alberto Zompatori*, V. Tortelli Solvay Solexis s.p.A, via Lombardia 20, 20021 Bollate (MI), Italy Received 11 December 2002; received in revised form 18 July 2003; accepted 28 July 2003
Abstract The thermal rearrangement of trifluoromethyl trifluorovinyl ether (MVE) to pentafluoropropionyl fluoride (PPF) under pressure with and without radical initiators has been studied. The reaction typically gives a mixture of different acyl fluorides. The influence of the reaction parameters (pressure, temperature, concentration, type of initiator and contact time) on the conversion and the selectivity of the process has been carefully examined. In addition, a mechanism which accounts for the formation of all the products has been proposed. # 2003 Elsevier B.V. All rights reserved. Keywords: Isomerization; Perfluoro(vinyl ether); Perfluoroacyl fluoride; Perfluorononanes; Radical initiator
1. Introduction The isomerization of trifluoromethyl trifluorovinyl ether (MVE) to pentafluoropropionyl fluoride (PPF) has been reported previously [1]. The pyrolysis was performed in a platinum tube at 595 8C with a contact time of 1.2 s and gave pentafluoropropionyl fluoride in a total yield of 53% (Fig. 1). Two mechanisms have been proposed for this pyrolytic rearrangement. In the first one, the CF3 radicals, obtained from the thermal cleavage of the CF3–O bond of MVE, propagate a radical chain which gives the PPF through bscission (the coupling of CF3 radicals also explains the formation of C2F6). In the second one, a four-centred intramolecular rearrangement is invoked for the synthesis of the main product. To achieve a deeper understanding of this process, we studied the effect of the variation of the main parameters on conversion and selectivity, particularly focusing on the pressure and the radical initiators.
2. Results and discussion In the following discussion we will refer to the acyl fluorides C6 as dimers and to the acyl fluorides C9 as trimers. *
Corresponding author. Tel.: þ39-02-3835-6373; fax: þ39-02-3835-6355. E-mail address: [email protected] (A. Zompatori). 0022-1139/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2003.07.021
The use of GC–MS analysis and 19 F NMR spectroscopy enabled us to identify two structural isomers of the dimers and several structural isomers of the trimers that are listed in Table 4. Their formation is consistent with a radical chain mechanism such as the following. In the mechanism illustrated in Scheme 1, the kinetic chain is initiated by a perfluoroalkyl radical (originated either by a pyrolysis of MVE or by a decomposition of the radical initiator), which can add either to the C-1 or to the C-2 of the double bond. Thus two different radicals are formed: the secondary radical preferably b-eliminates with the formation of PPF (II), but to some extent it adds to a new molecule of MVE giving the dimer (III); on the contrary, the primary radical adds to another molecule of MVE giving, after another b-elimination, the dimer (IV). A similar radical chain mechanism was described by Karzov et al. [2], who studied the pyrolysis of propyl vinyl ether and other perfluoroalkyl vinyl ethers at 160–205 8C. They concluded that the kinetic chain is sustained only by the secondary radical CF2(OCF3) CF, while the primary radical CF(OCF3) CF2 gives oligomers. Thus, if we accept the hypothesis that the CF3 radical is the propagating species of this radical chain mechanism, we can state that there is a limit to the maximum obtainable yield of PPF, which is determined by the regioselectivity of the attack of the CF3 radical to the double bond of MVE. A good experimental evidence of this mechanism comes from the variation of the ratio between the isomeric dimers (III) and (IV) with temperature. The data are summarised in Table 1.
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Scheme 1. Reaction mechanism leading to the acyl fluorides (II)–(V).
Fig. 1. Reaction scheme of the pyrolysis of MVE.
Table 1 Variation of the ratio between the dimers (III) and (IV) with temperature Temperature (8C)
(IV):(III) (mol/mol)
250 260 290 310
53:47 85:15 93:7 93:7
The molar ratio between the two isomers was obtained by calculating the ratio of the corresponding GC peak areas. These data indicate that the selectivity of isomer (IV) increases with temperature, as a result of the increasing belimination rate of the secondary radical CF3CF2(OCF3) CF.
Fig. 2. Perfluorononanes used as radical initiators.
In our experiments we tested the two following branched perfluorononanes as radical initiators (Fig. 2):1 The synthesis of the perfluorononanes (A) and (B) was performed by fluorination with elemental fluorine at 30 and 105 8C of the trimers of C3F6, respectively, [3]. The pyrolysis of these branched perfluoroalkanes [4,5] initially consists of an equilibrium between the starting reagent and the two perfluoroalkyl radicals deriving from the homolytic cleavage of the most substituted carbon–carbon bond, followed by the rearrangement and the recombination of the 1
All the unmarked bonds are to fluorine.
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201
Fig. 3. Variation of the conversion with temperature, pressure and concentration of radical initiators.
same radicals. Therefore, in the reaction mixture, the perfluorononanes act as a source of CF3, i-C3F7 radicals and other more sterically hindered perfluoroalkyl radicals, which can start the radical chain reaction of isomerization of MVE. As we will discuss later, the most evident effect of the use of perfluorononanes as radical initiators is the strong increase in conversion when compared with the pure thermal reaction. Another important characteristic of this family of perfluoroalkanes is that a wide range of products are available with different kinetic rates of thermal decomposition. This allows to choose the best initiator at the desired reaction temperature. The half-lives of perfluorononanes (A) and (B) at different temperatures are indicated in Table 2. The choice of the right initiator does not depend only upon the temperature. In fact, the decomposition rate of the initiator must be sufficiently low to let it survive for a time almost equal to the contact time of the reaction; therefore, the contact time should be approximately four to five times of the half-life of the initiator. In all of our experiments with radical initiators, the perfluorononane was fed into the reactor as a solute at the desired concentration in the liquid MVE. Table 2 Half-life times of the perfluorononanes (A) and (B) at different temperatures Temperature (8C)
T1/2 of (A) (s)
T1/2 of (B) (s)
250 260 290 310
268 130 18 3
– – 420 34
From the analysis of the experimental data, there is evidence of how the main reaction parameters (temperature, pressure, contact time, concentration and type of radical initiator) affect conversion and selectivity. The data clearly show that conversion increases with pressure: in fact if we compare the reactions run at atmospheric pressure with the ones run at the same temperature and with almost identical contact times but at P ¼ 5:2 atm, the difference is very relevant as shown in Fig. 3. The conversion rises from 2 to 24% at 260 8C and from 27 to 70% at 300 8C. A possible explanation is that by operating under pressure the concentration of MVE is raised, and consequently the rate of isomerization to PPF is increased, according to the kinetic equation:2 d½MVE ¼ k½MVEn dt As expected, conversion increases with temperature. This can be explained by the relationship, expressed by the Arrhenius law, between the kinetic constant k and temperature: k ¼ A exp ðEa =RTÞ. Fig. 3 also shows how conversion varies with the concentration of the radical initiators. One can clearly notice that by doubling the concentration of the initiator perfluorononane (A), the conversion increases approximately by 15%. Also the selectivity in PPF, dimers and trimers depends upon the reaction temperature as outlined in Fig. 4. 2 The value of ‘‘n’’ is not specified since we do not know the exact order of the reaction.
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Fig. 5. Variation of conversion and selectivity with contact time.
Fig. 4. Variation of the selectivity with temperature.
According to the mechanism illustrated in Scheme 1, the increase of the selectivity in PPF and the corresponding decrease in dimers and trimers with temperature can be explained with the increase of the b-scission rate of the secondary radical CF3CF2(OCF3) CF. In particular, the selectivity in PPF abruptly drops at temperatures lower than 260 8C. For example, a reaction run at 250 8C and at P ¼ 5:2 atm resulted in the following: selectivityPPF ¼ 43%, selectivitydimers ¼ 37% and a selectivitytrimers ¼ 20%. In addition, from the GLC of the reaction mixture we calculated the ratio of the two isomers (III) and (IV) of the dimers, which was found to be 47:53. This demonstrates that at 250 8C the b-scission rate of the secondary radical CF3CF2(OCF3) CF to PPF is sufficiently low to let the radical add to a new molecule of MVE, thus giving the isomer (III) of the dimers in greater quantity. However, at temperatures higher than 260 8C the content of this isomer in the reaction mixture is found to be considerably lower (see Table 1). Fig. 5 illustrates the plots of conversion and selectivity versus contact time t. One can clearly see that the conversion increases with an increase of contact time, while the selectivities in PPF, dimers and trimers remain approximately constant and seem to be independent from t. In Fig. 5 the values of conversion and selectivity corresponding to t ¼ 18 and t ¼ 11 min were obtained with the initiator perfluorononane (A), whereas the values corresponding to t ¼ 6 min were obtained with the initiator perfluorononane (B), which has a slower kinetics of thermal decomposition (see the half-lives in Table 2). This property
Table 3 Variation of the selectivity with the radical initiators Percentage of perfluorononane (by weight)
Selectivity of PPF (%)
Selectivity of dimers (%)
Selectivity of trimers (%)
Absent (A) 0.6 (B) 0.6
63 69 68
29 26 24
8 5 8
Temperature ¼ 290 8C, t ¼ 18 min, P ¼ 5:2 atm.
makes perfluorononane (B) the best initiator at temperatures higher than 300 8C. Table 3 shows that the selectivity of PPF seems to be dependent on the presence of the initiator. In fact, when the reaction occurred without radical initiator, the best selectivity achieved was 63%, while in presence of either perfluorononane (A) or perfluorononane (B) the selectivity raised almost to 70%. This trend is quite unexpected since the perfluorononanes were only supposed to raise the conversion by promoting the initiation of new kinetic chains, without affecting the selectivity, which, according to the mechanism shown in Scheme 1, depends on the regioselective attack of the CF3 radical to the double bond of the MVE. 3. Conclusions The results discussed above lead to the following conclusions: 1. The isomerization of trifluoromethyl trifluorovinyl ether to pentafluoropropionyl fluoride is a radical chain reaction where the propagating species is the CF3 radical.
A. Zompatori, V. Tortelli / Journal of Fluorine Chemistry 125 (2004) 199–204
2. The productivity and the conversion of the process may be conveniently raised by operating under pressures greater than 1 atm. 3. An even more pronounced increase of conversion may be reached by operating in the presence of branched perfluorononanes as radical initiators. In particular, the perfluorononane (B), given its thermal decomposition kinetics and its synthetical feasibility, is probably the best candidate. 4. To achieve selectivity values in PPF greater than 60%, it is necessary to operate at temperatures greater than 260 8C in order to favour the b-elimination of the secondary radical CF3CF2(OCF3) CF to PPF. The use of perfluorononanes as radical initiators further increases the selectivity in PPF.
4. Experimental details 4.1. Description of the apparatus Our experiments were performed on a laboratory scale, using a continuous apparatus and operating under pressure (the flow sheet is shown in Fig. 6): The main components of the apparatus are: a 500 ml continuous stirred tank reactor (CSTR) made of AISI 316 stirred at 25 Hz; a thermostated bath made of mechanically stirred fused salts; a stainless steel valve and a pressure transducer to regulate the pressure inside the reactor;
203
two stainless steel valves and a pressure transducer to regulate the feeding pressure. The first valve feeds nitrogen at the top of the cylinder containing the MVE, while the second one compensates the eventual extra pressure of nitrogen; a feeding cylinder placed on a technical balance to verify the flow rate of MVE, which is fed to the reactor through a gas inlet tube connected to a capillary tube of 0.5 mm diameter; a dry-ice cooled trap, which is connected to the reactor with a tube heated at 100 8C to avoid the condensation of high MW acyl fluorides. The procedure followed in each experiment consisted of switching on the heating bath to heat the reactor to a desired temperature. Then the feeding pressure (P1) and the pressure inside the reactor (P2) were set so as to obtain the desired flow rate of MVE, which varies proportionally to the difference between P1 and P2. In the first hour, the system was conditioned without condensing the effluents, which were quenched by bubbling in alcoholic potassium hydroxide. Once the system was conditioned, the gaseous acyl fluorides were condensed in the dry-ice cooled trap containing a known amount of absolute ethanol. At the end of this procedure, the trap was shut down, the feeding of the MVE was stopped and nitrogen was fluxed into the system. The trap was warmed to room temperature and the gas and liquid phases of the reaction mixture, where the acyl fluorides had been converted to the corresponding ethyl esters, were analysed by GLC. The calculation of conversion and selectivity was made possible by using the correction factors of each component of the reaction mixture.
Fig. 6. Flow sheet of the continuous apparatus operating under pressure.
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Table 4 List of the products with their corresponding correction factors Products
b.p.
Correction factor
CF3OCF¼CF2 (I) EtOH CF3CF2COOEt (II) CF3CF2CF(OCF3)CF2COOEt (III) þ CF3CF(OCF3)CF2CF2COOEt (IV) C2F5CF(OCF3)CF2CF(OCF3)CF2COOEt (V) þ C2F5CF(OCF3)CF(OCF3)CF2CF2COOEt (VI) þ other trimers
29 8C at P ¼ 1 atm 78 8C at P ¼ 1 atm 78 8C at P ¼ 1 atm 50 8C at P ¼ 30 mmHg 85 8C at P ¼ 20 mmHg
2.3 1 2.0 2.3 2.5
Table 5 19 F NMR chemical shifts of some of the reaction products Products
a (ppm)
b (ppm)
c (ppm)
d (ppm)
e (ppm)
84.2
122.7
81.2
123.4
141.9
53.7
116.3
m
124.4 AB system (J ¼ 293 Hz)
q (J ¼ 14 Hz)
m
117.7 AB system (J ¼ 273 Hz)
79.0
141.4
54.5
m
sextet (3 J ¼ 14 Hz)
m
122.0 123.1 AB system (J ¼ 293 Hz)
118.1 119.5 AB system (J ¼ 277 Hz)
4.2. Identification and characterisation of the products All the products obtained in our experiments are listed in Table 4 with the corresponding correction factors, which were determined after isolating each component of the reaction mixture by fractional distillation. All the products of the reaction mixture were identified by GC–MS analysis after esterification with absolute ethanol. GLC analyses were performed using an HRGC 5300 CarloErba instrument equipped with thermoconductivity detectors (4.0 m column packed with SE30 as stationary phase). Mass spectra were obtained on a Varian Mat CH7A mass spectrometer equipped with a magnetic analyser. GLC/electron mass spectra were only used to confirm the molecular weight of the products (via the presence of the parent ion peak), since the fragmentation pattern did not clearly identify the structure. 19 F NMR spectra were recorded on a Varian spectrometer operating at 376.2 MHz with CFCl3 as external standard. Table 5 reports the assignments of all the peaks of the compounds (II)–(IV).
Concerning the dimers (III) and (IV), the presence of an AB system and of long-range coupling constants complicates the spectra; however, the 13 C spectroscopy (coupled and decoupled from 19 F) shows a different multiplicity of the CF3– groups (a quartet of triplets for the isomer (III) and a quartet of doublets for the isomer (IV)). This indicates that a different number of fluorine atoms (indicated with letter ‘‘b’’ in Table 5) are attached to the nearest terminal carbon and thus enabled us to discriminate between the two isomers.
References [1] R.N. Haszeldine, A.E. Tipping, J. Chem. Soc. (C) (1968) 398– 405. [2] C.B. Karzov, B.C. Juminov, B.M. Maximov, Zh. Org. Khim. Issue 8 (1992) 1573–1578. [3] S.P. von Halasz, F. Kluge, T. Martini, Chem. Ber. 106 (1973) 2950–2959. [4] V. Tortelli, C. Tonelli, J. Fluorine Chem. 60 (1993) 165–174. [5] C. Tonelli, V. Tortelli, J. Fluorine Chem. 67 (1994) 125–128.
Thermal isomerization of trifluoromethyl trifluorovinyl ether to pentafluoropropionyl fluoride Alberto Zompatori*, V. Tortelli Solvay Solexis s.p.A, via Lombardia 20, 20021 Bollate (MI), Italy Received 11 December 2002; received in revised form 18 July 2003; accepted 28 July 2003
Abstract The thermal rearrangement of trifluoromethyl trifluorovinyl ether (MVE) to pentafluoropropionyl fluoride (PPF) under pressure with and without radical initiators has been studied. The reaction typically gives a mixture of different acyl fluorides. The influence of the reaction parameters (pressure, temperature, concentration, type of initiator and contact time) on the conversion and the selectivity of the process has been carefully examined. In addition, a mechanism which accounts for the formation of all the products has been proposed. # 2003 Elsevier B.V. All rights reserved. Keywords: Isomerization; Perfluoro(vinyl ether); Perfluoroacyl fluoride; Perfluorononanes; Radical initiator
1. Introduction The isomerization of trifluoromethyl trifluorovinyl ether (MVE) to pentafluoropropionyl fluoride (PPF) has been reported previously [1]. The pyrolysis was performed in a platinum tube at 595 8C with a contact time of 1.2 s and gave pentafluoropropionyl fluoride in a total yield of 53% (Fig. 1). Two mechanisms have been proposed for this pyrolytic rearrangement. In the first one, the CF3 radicals, obtained from the thermal cleavage of the CF3–O bond of MVE, propagate a radical chain which gives the PPF through bscission (the coupling of CF3 radicals also explains the formation of C2F6). In the second one, a four-centred intramolecular rearrangement is invoked for the synthesis of the main product. To achieve a deeper understanding of this process, we studied the effect of the variation of the main parameters on conversion and selectivity, particularly focusing on the pressure and the radical initiators.
2. Results and discussion In the following discussion we will refer to the acyl fluorides C6 as dimers and to the acyl fluorides C9 as trimers. *
Corresponding author. Tel.: þ39-02-3835-6373; fax: þ39-02-3835-6355. E-mail address: [email protected] (A. Zompatori). 0022-1139/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2003.07.021
The use of GC–MS analysis and 19 F NMR spectroscopy enabled us to identify two structural isomers of the dimers and several structural isomers of the trimers that are listed in Table 4. Their formation is consistent with a radical chain mechanism such as the following. In the mechanism illustrated in Scheme 1, the kinetic chain is initiated by a perfluoroalkyl radical (originated either by a pyrolysis of MVE or by a decomposition of the radical initiator), which can add either to the C-1 or to the C-2 of the double bond. Thus two different radicals are formed: the secondary radical preferably b-eliminates with the formation of PPF (II), but to some extent it adds to a new molecule of MVE giving the dimer (III); on the contrary, the primary radical adds to another molecule of MVE giving, after another b-elimination, the dimer (IV). A similar radical chain mechanism was described by Karzov et al. [2], who studied the pyrolysis of propyl vinyl ether and other perfluoroalkyl vinyl ethers at 160–205 8C. They concluded that the kinetic chain is sustained only by the secondary radical CF2(OCF3) CF, while the primary radical CF(OCF3) CF2 gives oligomers. Thus, if we accept the hypothesis that the CF3 radical is the propagating species of this radical chain mechanism, we can state that there is a limit to the maximum obtainable yield of PPF, which is determined by the regioselectivity of the attack of the CF3 radical to the double bond of MVE. A good experimental evidence of this mechanism comes from the variation of the ratio between the isomeric dimers (III) and (IV) with temperature. The data are summarised in Table 1.
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Scheme 1. Reaction mechanism leading to the acyl fluorides (II)–(V).
Fig. 1. Reaction scheme of the pyrolysis of MVE.
Table 1 Variation of the ratio between the dimers (III) and (IV) with temperature Temperature (8C)
(IV):(III) (mol/mol)
250 260 290 310
53:47 85:15 93:7 93:7
The molar ratio between the two isomers was obtained by calculating the ratio of the corresponding GC peak areas. These data indicate that the selectivity of isomer (IV) increases with temperature, as a result of the increasing belimination rate of the secondary radical CF3CF2(OCF3) CF.
Fig. 2. Perfluorononanes used as radical initiators.
In our experiments we tested the two following branched perfluorononanes as radical initiators (Fig. 2):1 The synthesis of the perfluorononanes (A) and (B) was performed by fluorination with elemental fluorine at 30 and 105 8C of the trimers of C3F6, respectively, [3]. The pyrolysis of these branched perfluoroalkanes [4,5] initially consists of an equilibrium between the starting reagent and the two perfluoroalkyl radicals deriving from the homolytic cleavage of the most substituted carbon–carbon bond, followed by the rearrangement and the recombination of the 1
All the unmarked bonds are to fluorine.
A. Zompatori, V. Tortelli / Journal of Fluorine Chemistry 125 (2004) 199–204
201
Fig. 3. Variation of the conversion with temperature, pressure and concentration of radical initiators.
same radicals. Therefore, in the reaction mixture, the perfluorononanes act as a source of CF3, i-C3F7 radicals and other more sterically hindered perfluoroalkyl radicals, which can start the radical chain reaction of isomerization of MVE. As we will discuss later, the most evident effect of the use of perfluorononanes as radical initiators is the strong increase in conversion when compared with the pure thermal reaction. Another important characteristic of this family of perfluoroalkanes is that a wide range of products are available with different kinetic rates of thermal decomposition. This allows to choose the best initiator at the desired reaction temperature. The half-lives of perfluorononanes (A) and (B) at different temperatures are indicated in Table 2. The choice of the right initiator does not depend only upon the temperature. In fact, the decomposition rate of the initiator must be sufficiently low to let it survive for a time almost equal to the contact time of the reaction; therefore, the contact time should be approximately four to five times of the half-life of the initiator. In all of our experiments with radical initiators, the perfluorononane was fed into the reactor as a solute at the desired concentration in the liquid MVE. Table 2 Half-life times of the perfluorononanes (A) and (B) at different temperatures Temperature (8C)
T1/2 of (A) (s)
T1/2 of (B) (s)
250 260 290 310
268 130 18 3
– – 420 34
From the analysis of the experimental data, there is evidence of how the main reaction parameters (temperature, pressure, contact time, concentration and type of radical initiator) affect conversion and selectivity. The data clearly show that conversion increases with pressure: in fact if we compare the reactions run at atmospheric pressure with the ones run at the same temperature and with almost identical contact times but at P ¼ 5:2 atm, the difference is very relevant as shown in Fig. 3. The conversion rises from 2 to 24% at 260 8C and from 27 to 70% at 300 8C. A possible explanation is that by operating under pressure the concentration of MVE is raised, and consequently the rate of isomerization to PPF is increased, according to the kinetic equation:2 d½MVE ¼ k½MVEn dt As expected, conversion increases with temperature. This can be explained by the relationship, expressed by the Arrhenius law, between the kinetic constant k and temperature: k ¼ A exp ðEa =RTÞ. Fig. 3 also shows how conversion varies with the concentration of the radical initiators. One can clearly notice that by doubling the concentration of the initiator perfluorononane (A), the conversion increases approximately by 15%. Also the selectivity in PPF, dimers and trimers depends upon the reaction temperature as outlined in Fig. 4. 2 The value of ‘‘n’’ is not specified since we do not know the exact order of the reaction.
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Fig. 5. Variation of conversion and selectivity with contact time.
Fig. 4. Variation of the selectivity with temperature.
According to the mechanism illustrated in Scheme 1, the increase of the selectivity in PPF and the corresponding decrease in dimers and trimers with temperature can be explained with the increase of the b-scission rate of the secondary radical CF3CF2(OCF3) CF. In particular, the selectivity in PPF abruptly drops at temperatures lower than 260 8C. For example, a reaction run at 250 8C and at P ¼ 5:2 atm resulted in the following: selectivityPPF ¼ 43%, selectivitydimers ¼ 37% and a selectivitytrimers ¼ 20%. In addition, from the GLC of the reaction mixture we calculated the ratio of the two isomers (III) and (IV) of the dimers, which was found to be 47:53. This demonstrates that at 250 8C the b-scission rate of the secondary radical CF3CF2(OCF3) CF to PPF is sufficiently low to let the radical add to a new molecule of MVE, thus giving the isomer (III) of the dimers in greater quantity. However, at temperatures higher than 260 8C the content of this isomer in the reaction mixture is found to be considerably lower (see Table 1). Fig. 5 illustrates the plots of conversion and selectivity versus contact time t. One can clearly see that the conversion increases with an increase of contact time, while the selectivities in PPF, dimers and trimers remain approximately constant and seem to be independent from t. In Fig. 5 the values of conversion and selectivity corresponding to t ¼ 18 and t ¼ 11 min were obtained with the initiator perfluorononane (A), whereas the values corresponding to t ¼ 6 min were obtained with the initiator perfluorononane (B), which has a slower kinetics of thermal decomposition (see the half-lives in Table 2). This property
Table 3 Variation of the selectivity with the radical initiators Percentage of perfluorononane (by weight)
Selectivity of PPF (%)
Selectivity of dimers (%)
Selectivity of trimers (%)
Absent (A) 0.6 (B) 0.6
63 69 68
29 26 24
8 5 8
Temperature ¼ 290 8C, t ¼ 18 min, P ¼ 5:2 atm.
makes perfluorononane (B) the best initiator at temperatures higher than 300 8C. Table 3 shows that the selectivity of PPF seems to be dependent on the presence of the initiator. In fact, when the reaction occurred without radical initiator, the best selectivity achieved was 63%, while in presence of either perfluorononane (A) or perfluorononane (B) the selectivity raised almost to 70%. This trend is quite unexpected since the perfluorononanes were only supposed to raise the conversion by promoting the initiation of new kinetic chains, without affecting the selectivity, which, according to the mechanism shown in Scheme 1, depends on the regioselective attack of the CF3 radical to the double bond of the MVE. 3. Conclusions The results discussed above lead to the following conclusions: 1. The isomerization of trifluoromethyl trifluorovinyl ether to pentafluoropropionyl fluoride is a radical chain reaction where the propagating species is the CF3 radical.
A. Zompatori, V. Tortelli / Journal of Fluorine Chemistry 125 (2004) 199–204
2. The productivity and the conversion of the process may be conveniently raised by operating under pressures greater than 1 atm. 3. An even more pronounced increase of conversion may be reached by operating in the presence of branched perfluorononanes as radical initiators. In particular, the perfluorononane (B), given its thermal decomposition kinetics and its synthetical feasibility, is probably the best candidate. 4. To achieve selectivity values in PPF greater than 60%, it is necessary to operate at temperatures greater than 260 8C in order to favour the b-elimination of the secondary radical CF3CF2(OCF3) CF to PPF. The use of perfluorononanes as radical initiators further increases the selectivity in PPF.
4. Experimental details 4.1. Description of the apparatus Our experiments were performed on a laboratory scale, using a continuous apparatus and operating under pressure (the flow sheet is shown in Fig. 6): The main components of the apparatus are: a 500 ml continuous stirred tank reactor (CSTR) made of AISI 316 stirred at 25 Hz; a thermostated bath made of mechanically stirred fused salts; a stainless steel valve and a pressure transducer to regulate the pressure inside the reactor;
203
two stainless steel valves and a pressure transducer to regulate the feeding pressure. The first valve feeds nitrogen at the top of the cylinder containing the MVE, while the second one compensates the eventual extra pressure of nitrogen; a feeding cylinder placed on a technical balance to verify the flow rate of MVE, which is fed to the reactor through a gas inlet tube connected to a capillary tube of 0.5 mm diameter; a dry-ice cooled trap, which is connected to the reactor with a tube heated at 100 8C to avoid the condensation of high MW acyl fluorides. The procedure followed in each experiment consisted of switching on the heating bath to heat the reactor to a desired temperature. Then the feeding pressure (P1) and the pressure inside the reactor (P2) were set so as to obtain the desired flow rate of MVE, which varies proportionally to the difference between P1 and P2. In the first hour, the system was conditioned without condensing the effluents, which were quenched by bubbling in alcoholic potassium hydroxide. Once the system was conditioned, the gaseous acyl fluorides were condensed in the dry-ice cooled trap containing a known amount of absolute ethanol. At the end of this procedure, the trap was shut down, the feeding of the MVE was stopped and nitrogen was fluxed into the system. The trap was warmed to room temperature and the gas and liquid phases of the reaction mixture, where the acyl fluorides had been converted to the corresponding ethyl esters, were analysed by GLC. The calculation of conversion and selectivity was made possible by using the correction factors of each component of the reaction mixture.
Fig. 6. Flow sheet of the continuous apparatus operating under pressure.
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Table 4 List of the products with their corresponding correction factors Products
b.p.
Correction factor
CF3OCF¼CF2 (I) EtOH CF3CF2COOEt (II) CF3CF2CF(OCF3)CF2COOEt (III) þ CF3CF(OCF3)CF2CF2COOEt (IV) C2F5CF(OCF3)CF2CF(OCF3)CF2COOEt (V) þ C2F5CF(OCF3)CF(OCF3)CF2CF2COOEt (VI) þ other trimers
29 8C at P ¼ 1 atm 78 8C at P ¼ 1 atm 78 8C at P ¼ 1 atm 50 8C at P ¼ 30 mmHg 85 8C at P ¼ 20 mmHg
2.3 1 2.0 2.3 2.5
Table 5 19 F NMR chemical shifts of some of the reaction products Products
a (ppm)
b (ppm)
c (ppm)
d (ppm)
e (ppm)
84.2
122.7
81.2
123.4
141.9
53.7
116.3
m
124.4 AB system (J ¼ 293 Hz)
q (J ¼ 14 Hz)
m
117.7 AB system (J ¼ 273 Hz)
79.0
141.4
54.5
m
sextet (3 J ¼ 14 Hz)
m
122.0 123.1 AB system (J ¼ 293 Hz)
118.1 119.5 AB system (J ¼ 277 Hz)
4.2. Identification and characterisation of the products All the products obtained in our experiments are listed in Table 4 with the corresponding correction factors, which were determined after isolating each component of the reaction mixture by fractional distillation. All the products of the reaction mixture were identified by GC–MS analysis after esterification with absolute ethanol. GLC analyses were performed using an HRGC 5300 CarloErba instrument equipped with thermoconductivity detectors (4.0 m column packed with SE30 as stationary phase). Mass spectra were obtained on a Varian Mat CH7A mass spectrometer equipped with a magnetic analyser. GLC/electron mass spectra were only used to confirm the molecular weight of the products (via the presence of the parent ion peak), since the fragmentation pattern did not clearly identify the structure. 19 F NMR spectra were recorded on a Varian spectrometer operating at 376.2 MHz with CFCl3 as external standard. Table 5 reports the assignments of all the peaks of the compounds (II)–(IV).
Concerning the dimers (III) and (IV), the presence of an AB system and of long-range coupling constants complicates the spectra; however, the 13 C spectroscopy (coupled and decoupled from 19 F) shows a different multiplicity of the CF3– groups (a quartet of triplets for the isomer (III) and a quartet of doublets for the isomer (IV)). This indicates that a different number of fluorine atoms (indicated with letter ‘‘b’’ in Table 5) are attached to the nearest terminal carbon and thus enabled us to discriminate between the two isomers.
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