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Pyrolysis of Peroxy Compounds
C H A P T E R
6 Pyrolysis of Peroxy Compounds
S U B C H A P T E R
6.1
Hydroperoxides
GENERAL ASPECTS Organic hydroperoxides are compounds with the formula RdOdOH, where R is an alkyl or aryl organic radical. The R group can consist also of an acyl group. In this case, the resulting compounds are peroxy acids. Peroxy acids (peracids) are discussed in Subchapter 6.4. The OdO bond in a hydroperoxide functional group can break easily, generating free radicals. Hydroperoxides are used for this reason as radical initiators in polymerization reactions. Among the more common compounds used for this purpose is methyl ethyl ketone peroxide 2,2’-peroxydi(butane-2-peroxol). The hydroperoxide of isopropyl benzene (cumene) obtained by the oxidation of isopropyl benzene is an intermediate compound in an important industrial process for the synthesis of phenol and acetone. Typically, hydroperoxides decompose at low temperatures. For this reason, thermal decomposition of some hydroperoxides is not a true pyrolysis (because the decomposition may take place below 300°C).
Pyrolysis of Organic Molecules https://doi.org/10.1016/B978-0-444-64000-0.00006-8
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# 2019 Elsevier B.V. All rights reserved.
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6. PYROLYSIS OF PEROXY COMPOUNDS
METHYL HYDROPEROXIDE The decomposition of methyl hydroperoxide generates free radicals by the reaction: H3 C OOH ð+MÞ ! CH3 O + OH ð+MÞ ΔG ¼ 44:2 0:034T kcal=mol
(6.1.1)
As suggested in reaction (6.1.1), the process may take place with the participation of an inert molecule M. The reaction has been studied in solution as well as in the gas phase [1]. The decomposition in solution (dimethyl phthalate solvent) takes place very rapidly, with a reaction rate (first order) of about 3 105 s1 at 112°C and 2 104 s1 at 153°C. Similar to the decomposition of H2O2, the concentration of M plays a role in reaction kinetics [2]. In the gas phase, the reaction kinetic was found to be different (and slower). In the presence of toluene vapors, the decomposition of methyl hydroperoxide leads to the formation of dibenzyl as a result of the reaction of CH3O• and OH• with toluene molecules and to the formation of benzyl radicals followed by a termination reaction between benzyl radicals. The process was studied in the temperature range of 565–650°C. For a methyl hydroperoxide with partial pressure around 25 Torr and a heating time around 0.9 s, the decomposition of methyl hydroperoxide increased from about 16% (at 565°C) to about 71% (at 651°C). In the gas phase, the Arrhenius equation for methyl hydroperoxide was found to have the expression [1]: (6.1.2) k ¼ 10 +112 exp ð32 5Þ 10 +3 =RT =s The molecules formed from methyl hydroperoxide in the presence of an H donor (such as toluene) are CH3OH and H2O.
OTHER HYDROPEROXIDES Some hydroperoxides are explosive materials, such as methyl ethyl ketone peroxide (2,2-dihydroperoxybutane). Thermal decomposition of this compound has been studied in solution starting at 32°C and cannot be considered a pyrolysis [3]. Among the studies of thermal decomposition of hydroperoxides, the decomposition of isopropylbenzene hydroperoxide (2-hydroperoxy-2-phenylpropane, cumene hydroperoxide) received special attention. The compound is industrially prepared by the oxidation of cumene (see e.g., [4]) and is used to prepare phenol and acetone by hydrolysis in a slightly acidic medium. The reaction takes place as follows:
ð6:1:3Þ
GENERAL ASPECTS
313
The reactions do not involve the formation of free radicals, but because it is an exothermic process, it may lead to the thermal decomposition of cumene hydroperoxide by a radicalic mechanism and a runaway or explosive reaction. Also, cumene hydroperoxide is used to prepare dicumyl peroxide in a reaction with α-cumyl alcohol. Dicumyl peroxide is used as an initiator in polymerization reactions. The thermal decomposition of cumene hydroperoxide was studied in cumene solutions at concentrations between 20% and 80%. The reaction order was determined to be 0.5, and Arrhenius parameters were determined to have the values Ea ¼ 122.0 3.0 kJ/mol and the values for log(A) ¼ 20.0 1.2 (min1 mol0.5) [5–7]. The decomposition in other solvents such as chlorobenzene [4] and the autocatalytic effects during decomposition also were evaluated [8,9]. The main reaction products are α-cumyl alcohol, acetophenone, di-α-cumylperoxide, and α-methylstyrene. A hydroperoxide used as a free radical initiator or for the synthesis of tert-butyl alcohol is tert-butyl hydroperoxide. Its thermal decomposition is reported in the literature [10,11].
References 6.1 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
A.D. Kirk, Can. J. Chem. 43 (1965) 2236. A. Tessier, W. Frost, Can. J. Chem. 52 (1974) 794. M.-H. Yuan, C.-M. Shu, A.A. Kossoy, Thermochim. Acta 430 (2005) 67. J.A. Howard, J.E. Bennett, G. Brunton, Can. J. Chem. 59 (1981) 2253. Y.-S. Duh, C.-S. Kao, H.-H. Hwang, W.W.-L. Lee, Process Saf. Environ. Protect. 76 (1998) 271. Y.-S. Duh, C.-S. Kao, H.-H. Hwang, W.W.-L. Lee, Trans. IChemE 76 (B) (1998) 271. H.-Y. Hou, C.-M. Shu, Y.-S. Duh, AIChE 47 (2001) 1893. T.-K. Miao, C.-M. Shu, D.-J. Peng, M.-L. Shyu, S.-C. Chen, http://www.cs.fiu.edu/chens/PDF/DSC_TAM.pdf. C.-C. Chen, C.-M. Shu, C.-A. Yeh, http://www.cs.fiu.edu/chens/PDF/autocatalytic.pdf. J.R. Sanderson, T.E. Marquis, J.F. Knofton, US Patent 4922036, 1990. C.F. Cullis, J.A. Garcia Dominguez, D. Kiraly, D.L. Trimm, Proc. R. Soc. Lond. Ser. A 291 (1966) 235.
S U B C H A P T E R
6.2
Peroxides
GENERAL ASPECTS Organic peroxides are compounds with the formula RdOdOdR0 , where R and R0 are alkyl or aryl organic radicals. When R (or R0 ) is an acyl group, the compound is a peroxy ester, and pyrolysis of these compounds is discussed further in Subchapter 6.4. Similar to the case of
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6. PYROLYSIS OF PEROXY COMPOUNDS
hydroperoxides, the OdO bond can break easily, generating free radicals. Peroxides are typically used as radical initiators in the polymer industry. Some peroxides are used as explosives, such as hexamethylenetriperoxide-diamine (HMTD). Some peroxides are found in nature (e.g., artemisinin). Thermal decomposition of peroxides also takes place at low temperatures, and their thermal decomposition is not truly pyrolysis.
DIMETHYL PEROXIDE The decomposition of dimethyl peroxide (CH3dOdOdCH3) has been studied in the range of temperatures from 110°C to 140°C [1] and in a similar range (118–159°C) [2,3]. The initial decomposition reaction leads to the formation of free radicals by the cleavage of the OdO bond: ð6:2:1Þ The Arrhenius equation for this reaction was reported to have the expression: log k s1 ¼ ð15:7 0:5Þ ð37:1 0:9Þ=2:3RT
(6.2.2)
Following the initiation reaction with the formation of free radicals, the propagation takes place with the interaction of methoxy radicals with molecules of dimethyl peroxide, as shown below: ð6:2:3Þ H2 C ¼ O + H3 C O ! CH3 OH + HC ¼ O
(6.2.4)
HC ¼ O + H3 C O ! CO + CH3 OH
(6.2.5)
The result of this process is the formation of CO and methanol. The overall result can be written as follows: 2 CH3 O O CH3 ! 3 CH3 OH + CO
(6.2.6)
In the range of evaluated temperature, reaction (6.2.6) was found to have a reaction rate of k 5 10+4 mol/s. The reaction was also studied in the presence of NO2 and of O2 [2].
OTHER PEROXIDES Thermal decomposition of several other peroxides is reported in the literature. These include acetone peroxide dimer (3,3,6,6-tetramethyl-1,2,4,5-tetraoxane) and trimer, di-tert-butyl peroxide [4–9], di-cumyl peroxide [10], di-trifluoromethyl peroxide [11,12], and organosilicon peroxides [13]. Some of these compounds are explosive materials, and their thermal decomposition starts at low temperatures.
315
REFERENCES 6.2
The decomposition of di-tert-butyl peroxide in the temperature range from 110°C to 180°C takes place similarly to the decomposition of dimethyl peroxide. The initiation reaction occurs as follows: ð6:2:7Þ
The formation of free radicals continues with the following propagation reactions: ðCH3 Þ3 CO ! CH3 ðCOÞCH3 + CH3
CH3 + CH3 ðCOÞCH3 ! CH4 + CH3 ðCOÞCH2
(6.2.8)
(6.2.9)
Termination reactions lead to the formation of ethane, ethyl methyl ketone, and biacetonyl (hexane-2,5-dione). Also, trace amounts of tert-butyl alcohol, tert-butyl methyl ether, etc., were formed. The main reaction products remain acetone and ethane, with the overall reaction:
ð6:2:10Þ
The Arrhenius parameters for this reaction (of the first order) were estimated as log A (s1) ¼ 15.80 0.03 and Ea (kJ/mol) ¼ 158.07 0.25 (for the temperature range from 90°C to 350°C) [5]. Another peroxide evaluated regarding its decomposition parameters is 3,3,6,6tetramethyl-1,2,4,5-tetraoxane (acetone cyclic diperoxide), which decomposes mainly with the formation of acetone [14]. Peroxides with additional functional groups such as peroxyamines also are known, and their thermal decomposition has been reported [15].
References 6.2 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
L. Batt, R.D. McCulloch, In. J. Chem. Kinet. 8 (1976) 491. J.R. Barker, S.W. Benson, D.M. Golden, In. J. Chem. Kinet. 9 (1977) 31. Y. Takezaki, T. Miyazaki, N. Nakahara, J. Chem. Phys. 25 (1956) 536. E.R. Bell, F.F. Rust, W.E. Vaughan, J. Am. Chem. Soc. 72 (1950) 337. D.K. Lewis, Can. J. Chem. 54 (1976) 581. L. Batt, S.W. Benson, J. Chem. Phys. 36 (1962) 895. M. Flowers, L. Batt, S.W. Benson, J. Chem. Phys. 37 (1962) 2662. P.J. Skrdla, In. J. Chem. Kinet. 36 (2004) 386. Y. Iizuka, M. Surianarayanan, Ind. Eng. Chem. Res. 42 (2003) 2987. E. Marco, S. Cuartielles, J.A. Pen˜a, J. Santamaria, Thermochim. Acta 362 (2000) 49. L. Batt, R. Walsh, In. J. Chem. Kinet. 15 (1983) 605. W. Reints, D.A. Pratt, H.-G. Korth, P. Mulder, J. Phys. Chem. A 104 (2000) 10713. N.D. Kagramanov, I.O. Bragilevskii, V.A. Yablokov, A.V. Tomadze, A.K. Malьtsev, Russ. Chem. Bull. 36 (1987) 1573. [14] L.F.R. Cafferata, J.D. Lombardo, In. J. Chem. Kinet. 26 (1993) 503. [15] E.G.E. Hawkins, Angew. Chem. Int. Ed. 12 (1973) 783.
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S U B C H A P T E R
6.3
Ozonides
GENERAL ASPECTS Organic ozonides are typically formed in a reaction between ozone and an alkene. The compounds have a trioxolane structure, and their formation can be written as shown below:
ð6:3:1Þ
Ozonides are not stable compounds, and they decompose at temperatures as low as 70–80° C. Because the process takes place at low temperatures, the ozonide decomposition was used for synthetic purposes with the preservation of specific structural characteristics of the molecule. One example of such a reaction is the preparation of 1,8,8-trimethyl-2-oxabicyclo[3.2.1] octan-3-one starting with an ozonide generated from camphor and formaldehyde oxide [1]. The reactions are shown below:
ð6:3:2Þ
Other ozonides decompose thermally with the formation of an unsaturated carboxylic acid [1].
Reference 6.3 [1] R. Lapalme, H.-J. Borschberg, P. Soucy, P. Deslongchamps, Can. J. Chem. 57 (1979) 3272.
GENERAL ASPECTS
317
S U B C H A P T E R
6.4
Acyl Peroxides
GENERAL ASPECTS Several types of compounds contain in their molecules the peroxy group OdO connected to one or two acyl substituents. These groups of compounds include peracids, which have the general formula RC(]O)OdOH; peroxyesters, which have the general formula R1C(]O) OdOR2; diacyl peroxides with the formula R1C(]O)OdOC(]O)R2; esters of monoperoxycarbonic acid with the formula R1OC(]O)OdOR2; carbamoyl peroxides with the formula R1NHC(]O)OdOC(]O)R2; and peroxy lactones. These compounds can be considered as derivatives of carboxylic acids RC(]O)OH. However, the peroxy group OdO plays the main role in the thermal decomposition of these compounds, and this makes thermal properties of acyl peroxides similar to those of hydroperoxides and peroxides (discussed in Subchapters 6.1 and 6.2, respectively). Thermal decomposition of acyl peroxides takes place at low temperatures and cannot be classified as pyrolysis. In many instances, the thermal decomposition process takes place in a solvent. The decomposition of these compounds occurs with the formation of free radicals. Free radical formation at a low temperature explains the utilization of these compounds (similar to that of other peroxides) as radical initiators in polymerization reactions. The activation energies in various reactions with the cleavage of the OdO bond are around 35 kcal/mol, depending on the attached acyl radicals [1,2]. For each peroxide, secondary reactions take place after the first step of OdO cleavage. Peroxy acids generate by thermal decomposition chiefly CO2 and an alcohol, following the reaction indicated below:
ð6:4:1Þ
Peroxy esters are common polymerization initiators [3]. These compounds decompose with the formation of two free radicals, and the main reaction is the formation of an ether, as shown below:
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6. PYROLYSIS OF PEROXY COMPOUNDS
ð6:4:2Þ
Various other reactions take place between the generated free radicals, with the result of the decomposition being a mixture of ethers, esters, and hydrocarbons. Various studies were performed on thermal decomposition of diacyl peroxides, such as acetyl peroxide [4,5], acetyl propionyl peroxide [6], benzoyl peroxide [7–9], substituted benzoyl peroxides [10], lauroyl peroxide, decanoyl peroxide, octanoyl peroxide [2], and 1apocamphoryl benzoyl peroxide [11]. For diacyl peroxides, the initial reaction is the cleavage of the OdO bond similar to the other peroxides.
ð6:4:3Þ
The elimination of CO2 from the radicals generated in reaction (6.4.3) also leads to the formation of free radicals R1• and R2•. Further interactions between all those free radicals lead to a mixture of reaction products, including acetyl peroxide generates CO2, ethane, other alkanes, alkenes, and methyl acetate. Benzoyl peroxide generates benzoic acid, phenyl benzoate, biphenyl, benzene, CO2, terphenyls, etc. When the reaction is performed in a solvent, reaction products involving the solvent molecules also are generated [12]. For some peroxides, such as δ-phenylvaleryl peroxide, a carboxy inversion also seems to be involved in part in the decomposition process [5]. This reaction takes place as shown below:
ð6:4:4Þ
Various other studies have been performed on diacyl peroxide thermal decomposition, typically related to their use as polymerization initiators [13]. Also, kinetic data for various decompositions are provided in the literature [2]. More complex acyl peroxides have been studied as well [14].
References 6.4 [1] G.K. Williams, T.B. Brill, Appl. Spectrosc. 51 (1997) 423. [2] J.E. Guillet, J.C. Gilmer, Can. J. Chem. 47 (1969) 4405. [3] www.luperox.com.
REFERENCES 6.4
319
[4] H. Levy, M. Steinberg, M. Szwark, J. Am. Chem. Soc. 76 (1954) 5979. [5] T. Kashiwagi, S. Kozuka, S. Oae, Tetrahedron 26 (1970) 3619. [6] E.D. Skakovskii, A.I. Stankevich, L.Y. Tychinskaya, O.V. Shirokii, Y.P. Choban, V.L. Murashko, S.V. Rykov, Russ. J. Gen. Chem. 74 (2004) 1719. [7] A. Mozˇe, T. Malavasˇicˇ, I. Vizovisˇek, S. Lapanje, Angew. Makromol. Chem. 46 (1975) 89. [8] X.-R. Li, H. Koseki, J. Loss Prev. Process Ind. 18 (2005) 460. [9] G.R. Chalfont, D.H. Hey, K.S.Y. Liang, M.J. Perkins, Chem. Commun. (Lond.) (1967) 369. [10] K.H. Pausacker, Aust. J. Chem. 10 (1957) 49. [11] S. Oae, K. Fujimori, S. Kozuka, Tetrahedron 28 (1972) 5327. [12] D.F. DeTar, C. Weis, J. Am. Chem. Soc. 79 (1957) 3045. [13] N.S. Tsvetkov, Y.P. Kovalskii, React. Kinet. Catal. Lett. 21 (1982) 335. [14] R. Okazaki, O. Simamura, Bull. Chem. Soc. Jpn. 47 (1974) 1981.
6 Pyrolysis of Peroxy Compounds
S U B C H A P T E R
6.1
Hydroperoxides
GENERAL ASPECTS Organic hydroperoxides are compounds with the formula RdOdOH, where R is an alkyl or aryl organic radical. The R group can consist also of an acyl group. In this case, the resulting compounds are peroxy acids. Peroxy acids (peracids) are discussed in Subchapter 6.4. The OdO bond in a hydroperoxide functional group can break easily, generating free radicals. Hydroperoxides are used for this reason as radical initiators in polymerization reactions. Among the more common compounds used for this purpose is methyl ethyl ketone peroxide 2,2’-peroxydi(butane-2-peroxol). The hydroperoxide of isopropyl benzene (cumene) obtained by the oxidation of isopropyl benzene is an intermediate compound in an important industrial process for the synthesis of phenol and acetone. Typically, hydroperoxides decompose at low temperatures. For this reason, thermal decomposition of some hydroperoxides is not a true pyrolysis (because the decomposition may take place below 300°C).
Pyrolysis of Organic Molecules https://doi.org/10.1016/B978-0-444-64000-0.00006-8
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# 2019 Elsevier B.V. All rights reserved.
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6. PYROLYSIS OF PEROXY COMPOUNDS
METHYL HYDROPEROXIDE The decomposition of methyl hydroperoxide generates free radicals by the reaction: H3 C OOH ð+MÞ ! CH3 O + OH ð+MÞ ΔG ¼ 44:2 0:034T kcal=mol
(6.1.1)
As suggested in reaction (6.1.1), the process may take place with the participation of an inert molecule M. The reaction has been studied in solution as well as in the gas phase [1]. The decomposition in solution (dimethyl phthalate solvent) takes place very rapidly, with a reaction rate (first order) of about 3 105 s1 at 112°C and 2 104 s1 at 153°C. Similar to the decomposition of H2O2, the concentration of M plays a role in reaction kinetics [2]. In the gas phase, the reaction kinetic was found to be different (and slower). In the presence of toluene vapors, the decomposition of methyl hydroperoxide leads to the formation of dibenzyl as a result of the reaction of CH3O• and OH• with toluene molecules and to the formation of benzyl radicals followed by a termination reaction between benzyl radicals. The process was studied in the temperature range of 565–650°C. For a methyl hydroperoxide with partial pressure around 25 Torr and a heating time around 0.9 s, the decomposition of methyl hydroperoxide increased from about 16% (at 565°C) to about 71% (at 651°C). In the gas phase, the Arrhenius equation for methyl hydroperoxide was found to have the expression [1]: (6.1.2) k ¼ 10 +112 exp ð32 5Þ 10 +3 =RT =s The molecules formed from methyl hydroperoxide in the presence of an H donor (such as toluene) are CH3OH and H2O.
OTHER HYDROPEROXIDES Some hydroperoxides are explosive materials, such as methyl ethyl ketone peroxide (2,2-dihydroperoxybutane). Thermal decomposition of this compound has been studied in solution starting at 32°C and cannot be considered a pyrolysis [3]. Among the studies of thermal decomposition of hydroperoxides, the decomposition of isopropylbenzene hydroperoxide (2-hydroperoxy-2-phenylpropane, cumene hydroperoxide) received special attention. The compound is industrially prepared by the oxidation of cumene (see e.g., [4]) and is used to prepare phenol and acetone by hydrolysis in a slightly acidic medium. The reaction takes place as follows:
ð6:1:3Þ
GENERAL ASPECTS
313
The reactions do not involve the formation of free radicals, but because it is an exothermic process, it may lead to the thermal decomposition of cumene hydroperoxide by a radicalic mechanism and a runaway or explosive reaction. Also, cumene hydroperoxide is used to prepare dicumyl peroxide in a reaction with α-cumyl alcohol. Dicumyl peroxide is used as an initiator in polymerization reactions. The thermal decomposition of cumene hydroperoxide was studied in cumene solutions at concentrations between 20% and 80%. The reaction order was determined to be 0.5, and Arrhenius parameters were determined to have the values Ea ¼ 122.0 3.0 kJ/mol and the values for log(A) ¼ 20.0 1.2 (min1 mol0.5) [5–7]. The decomposition in other solvents such as chlorobenzene [4] and the autocatalytic effects during decomposition also were evaluated [8,9]. The main reaction products are α-cumyl alcohol, acetophenone, di-α-cumylperoxide, and α-methylstyrene. A hydroperoxide used as a free radical initiator or for the synthesis of tert-butyl alcohol is tert-butyl hydroperoxide. Its thermal decomposition is reported in the literature [10,11].
References 6.1 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
A.D. Kirk, Can. J. Chem. 43 (1965) 2236. A. Tessier, W. Frost, Can. J. Chem. 52 (1974) 794. M.-H. Yuan, C.-M. Shu, A.A. Kossoy, Thermochim. Acta 430 (2005) 67. J.A. Howard, J.E. Bennett, G. Brunton, Can. J. Chem. 59 (1981) 2253. Y.-S. Duh, C.-S. Kao, H.-H. Hwang, W.W.-L. Lee, Process Saf. Environ. Protect. 76 (1998) 271. Y.-S. Duh, C.-S. Kao, H.-H. Hwang, W.W.-L. Lee, Trans. IChemE 76 (B) (1998) 271. H.-Y. Hou, C.-M. Shu, Y.-S. Duh, AIChE 47 (2001) 1893. T.-K. Miao, C.-M. Shu, D.-J. Peng, M.-L. Shyu, S.-C. Chen, http://www.cs.fiu.edu/chens/PDF/DSC_TAM.pdf. C.-C. Chen, C.-M. Shu, C.-A. Yeh, http://www.cs.fiu.edu/chens/PDF/autocatalytic.pdf. J.R. Sanderson, T.E. Marquis, J.F. Knofton, US Patent 4922036, 1990. C.F. Cullis, J.A. Garcia Dominguez, D. Kiraly, D.L. Trimm, Proc. R. Soc. Lond. Ser. A 291 (1966) 235.
S U B C H A P T E R
6.2
Peroxides
GENERAL ASPECTS Organic peroxides are compounds with the formula RdOdOdR0 , where R and R0 are alkyl or aryl organic radicals. When R (or R0 ) is an acyl group, the compound is a peroxy ester, and pyrolysis of these compounds is discussed further in Subchapter 6.4. Similar to the case of
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6. PYROLYSIS OF PEROXY COMPOUNDS
hydroperoxides, the OdO bond can break easily, generating free radicals. Peroxides are typically used as radical initiators in the polymer industry. Some peroxides are used as explosives, such as hexamethylenetriperoxide-diamine (HMTD). Some peroxides are found in nature (e.g., artemisinin). Thermal decomposition of peroxides also takes place at low temperatures, and their thermal decomposition is not truly pyrolysis.
DIMETHYL PEROXIDE The decomposition of dimethyl peroxide (CH3dOdOdCH3) has been studied in the range of temperatures from 110°C to 140°C [1] and in a similar range (118–159°C) [2,3]. The initial decomposition reaction leads to the formation of free radicals by the cleavage of the OdO bond: ð6:2:1Þ The Arrhenius equation for this reaction was reported to have the expression: log k s1 ¼ ð15:7 0:5Þ ð37:1 0:9Þ=2:3RT
(6.2.2)
Following the initiation reaction with the formation of free radicals, the propagation takes place with the interaction of methoxy radicals with molecules of dimethyl peroxide, as shown below: ð6:2:3Þ H2 C ¼ O + H3 C O ! CH3 OH + HC ¼ O
(6.2.4)
HC ¼ O + H3 C O ! CO + CH3 OH
(6.2.5)
The result of this process is the formation of CO and methanol. The overall result can be written as follows: 2 CH3 O O CH3 ! 3 CH3 OH + CO
(6.2.6)
In the range of evaluated temperature, reaction (6.2.6) was found to have a reaction rate of k 5 10+4 mol/s. The reaction was also studied in the presence of NO2 and of O2 [2].
OTHER PEROXIDES Thermal decomposition of several other peroxides is reported in the literature. These include acetone peroxide dimer (3,3,6,6-tetramethyl-1,2,4,5-tetraoxane) and trimer, di-tert-butyl peroxide [4–9], di-cumyl peroxide [10], di-trifluoromethyl peroxide [11,12], and organosilicon peroxides [13]. Some of these compounds are explosive materials, and their thermal decomposition starts at low temperatures.
315
REFERENCES 6.2
The decomposition of di-tert-butyl peroxide in the temperature range from 110°C to 180°C takes place similarly to the decomposition of dimethyl peroxide. The initiation reaction occurs as follows: ð6:2:7Þ
The formation of free radicals continues with the following propagation reactions: ðCH3 Þ3 CO ! CH3 ðCOÞCH3 + CH3
CH3 + CH3 ðCOÞCH3 ! CH4 + CH3 ðCOÞCH2
(6.2.8)
(6.2.9)
Termination reactions lead to the formation of ethane, ethyl methyl ketone, and biacetonyl (hexane-2,5-dione). Also, trace amounts of tert-butyl alcohol, tert-butyl methyl ether, etc., were formed. The main reaction products remain acetone and ethane, with the overall reaction:
ð6:2:10Þ
The Arrhenius parameters for this reaction (of the first order) were estimated as log A (s1) ¼ 15.80 0.03 and Ea (kJ/mol) ¼ 158.07 0.25 (for the temperature range from 90°C to 350°C) [5]. Another peroxide evaluated regarding its decomposition parameters is 3,3,6,6tetramethyl-1,2,4,5-tetraoxane (acetone cyclic diperoxide), which decomposes mainly with the formation of acetone [14]. Peroxides with additional functional groups such as peroxyamines also are known, and their thermal decomposition has been reported [15].
References 6.2 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
L. Batt, R.D. McCulloch, In. J. Chem. Kinet. 8 (1976) 491. J.R. Barker, S.W. Benson, D.M. Golden, In. J. Chem. Kinet. 9 (1977) 31. Y. Takezaki, T. Miyazaki, N. Nakahara, J. Chem. Phys. 25 (1956) 536. E.R. Bell, F.F. Rust, W.E. Vaughan, J. Am. Chem. Soc. 72 (1950) 337. D.K. Lewis, Can. J. Chem. 54 (1976) 581. L. Batt, S.W. Benson, J. Chem. Phys. 36 (1962) 895. M. Flowers, L. Batt, S.W. Benson, J. Chem. Phys. 37 (1962) 2662. P.J. Skrdla, In. J. Chem. Kinet. 36 (2004) 386. Y. Iizuka, M. Surianarayanan, Ind. Eng. Chem. Res. 42 (2003) 2987. E. Marco, S. Cuartielles, J.A. Pen˜a, J. Santamaria, Thermochim. Acta 362 (2000) 49. L. Batt, R. Walsh, In. J. Chem. Kinet. 15 (1983) 605. W. Reints, D.A. Pratt, H.-G. Korth, P. Mulder, J. Phys. Chem. A 104 (2000) 10713. N.D. Kagramanov, I.O. Bragilevskii, V.A. Yablokov, A.V. Tomadze, A.K. Malьtsev, Russ. Chem. Bull. 36 (1987) 1573. [14] L.F.R. Cafferata, J.D. Lombardo, In. J. Chem. Kinet. 26 (1993) 503. [15] E.G.E. Hawkins, Angew. Chem. Int. Ed. 12 (1973) 783.
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6. PYROLYSIS OF PEROXY COMPOUNDS
S U B C H A P T E R
6.3
Ozonides
GENERAL ASPECTS Organic ozonides are typically formed in a reaction between ozone and an alkene. The compounds have a trioxolane structure, and their formation can be written as shown below:
ð6:3:1Þ
Ozonides are not stable compounds, and they decompose at temperatures as low as 70–80° C. Because the process takes place at low temperatures, the ozonide decomposition was used for synthetic purposes with the preservation of specific structural characteristics of the molecule. One example of such a reaction is the preparation of 1,8,8-trimethyl-2-oxabicyclo[3.2.1] octan-3-one starting with an ozonide generated from camphor and formaldehyde oxide [1]. The reactions are shown below:
ð6:3:2Þ
Other ozonides decompose thermally with the formation of an unsaturated carboxylic acid [1].
Reference 6.3 [1] R. Lapalme, H.-J. Borschberg, P. Soucy, P. Deslongchamps, Can. J. Chem. 57 (1979) 3272.
GENERAL ASPECTS
317
S U B C H A P T E R
6.4
Acyl Peroxides
GENERAL ASPECTS Several types of compounds contain in their molecules the peroxy group OdO connected to one or two acyl substituents. These groups of compounds include peracids, which have the general formula RC(]O)OdOH; peroxyesters, which have the general formula R1C(]O) OdOR2; diacyl peroxides with the formula R1C(]O)OdOC(]O)R2; esters of monoperoxycarbonic acid with the formula R1OC(]O)OdOR2; carbamoyl peroxides with the formula R1NHC(]O)OdOC(]O)R2; and peroxy lactones. These compounds can be considered as derivatives of carboxylic acids RC(]O)OH. However, the peroxy group OdO plays the main role in the thermal decomposition of these compounds, and this makes thermal properties of acyl peroxides similar to those of hydroperoxides and peroxides (discussed in Subchapters 6.1 and 6.2, respectively). Thermal decomposition of acyl peroxides takes place at low temperatures and cannot be classified as pyrolysis. In many instances, the thermal decomposition process takes place in a solvent. The decomposition of these compounds occurs with the formation of free radicals. Free radical formation at a low temperature explains the utilization of these compounds (similar to that of other peroxides) as radical initiators in polymerization reactions. The activation energies in various reactions with the cleavage of the OdO bond are around 35 kcal/mol, depending on the attached acyl radicals [1,2]. For each peroxide, secondary reactions take place after the first step of OdO cleavage. Peroxy acids generate by thermal decomposition chiefly CO2 and an alcohol, following the reaction indicated below:
ð6:4:1Þ
Peroxy esters are common polymerization initiators [3]. These compounds decompose with the formation of two free radicals, and the main reaction is the formation of an ether, as shown below:
318
6. PYROLYSIS OF PEROXY COMPOUNDS
ð6:4:2Þ
Various other reactions take place between the generated free radicals, with the result of the decomposition being a mixture of ethers, esters, and hydrocarbons. Various studies were performed on thermal decomposition of diacyl peroxides, such as acetyl peroxide [4,5], acetyl propionyl peroxide [6], benzoyl peroxide [7–9], substituted benzoyl peroxides [10], lauroyl peroxide, decanoyl peroxide, octanoyl peroxide [2], and 1apocamphoryl benzoyl peroxide [11]. For diacyl peroxides, the initial reaction is the cleavage of the OdO bond similar to the other peroxides.
ð6:4:3Þ
The elimination of CO2 from the radicals generated in reaction (6.4.3) also leads to the formation of free radicals R1• and R2•. Further interactions between all those free radicals lead to a mixture of reaction products, including acetyl peroxide generates CO2, ethane, other alkanes, alkenes, and methyl acetate. Benzoyl peroxide generates benzoic acid, phenyl benzoate, biphenyl, benzene, CO2, terphenyls, etc. When the reaction is performed in a solvent, reaction products involving the solvent molecules also are generated [12]. For some peroxides, such as δ-phenylvaleryl peroxide, a carboxy inversion also seems to be involved in part in the decomposition process [5]. This reaction takes place as shown below:
ð6:4:4Þ
Various other studies have been performed on diacyl peroxide thermal decomposition, typically related to their use as polymerization initiators [13]. Also, kinetic data for various decompositions are provided in the literature [2]. More complex acyl peroxides have been studied as well [14].
References 6.4 [1] G.K. Williams, T.B. Brill, Appl. Spectrosc. 51 (1997) 423. [2] J.E. Guillet, J.C. Gilmer, Can. J. Chem. 47 (1969) 4405. [3] www.luperox.com.
REFERENCES 6.4
319
[4] H. Levy, M. Steinberg, M. Szwark, J. Am. Chem. Soc. 76 (1954) 5979. [5] T. Kashiwagi, S. Kozuka, S. Oae, Tetrahedron 26 (1970) 3619. [6] E.D. Skakovskii, A.I. Stankevich, L.Y. Tychinskaya, O.V. Shirokii, Y.P. Choban, V.L. Murashko, S.V. Rykov, Russ. J. Gen. Chem. 74 (2004) 1719. [7] A. Mozˇe, T. Malavasˇicˇ, I. Vizovisˇek, S. Lapanje, Angew. Makromol. Chem. 46 (1975) 89. [8] X.-R. Li, H. Koseki, J. Loss Prev. Process Ind. 18 (2005) 460. [9] G.R. Chalfont, D.H. Hey, K.S.Y. Liang, M.J. Perkins, Chem. Commun. (Lond.) (1967) 369. [10] K.H. Pausacker, Aust. J. Chem. 10 (1957) 49. [11] S. Oae, K. Fujimori, S. Kozuka, Tetrahedron 28 (1972) 5327. [12] D.F. DeTar, C. Weis, J. Am. Chem. Soc. 79 (1957) 3045. [13] N.S. Tsvetkov, Y.P. Kovalskii, React. Kinet. Catal. Lett. 21 (1982) 335. [14] R. Okazaki, O. Simamura, Bull. Chem. Soc. Jpn. 47 (1974) 1981.