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Pyrolysis of Thiols and Sulfides
C H A P T E R
7 Pyrolysis of Thiols and Sulfides
S U B C H A P T E R
7.1
Thiols
GENERAL ASPECTS Thiols contain one or more SH groups in their molecules and can be considered analogs of alcohols, generated by replacing the OH group with SH. The IUPAC name of thiols is made by adding the suffix thiol to the name of the corresponding hydrocarbon (methanethiol for CH3SH, benzenethiol for C6H5-SH, etc.). Other names for thiols are also in use, such as mercaptans (methyl mercaptan for CH3-SH). Thiols are found in nature mainly in more complex combinations. Cysteine, for example is a common amino acid having an SH group. Volatile thiols are known for their strong odor. As an example, 1-butanethiol has an odor threshold of 6 ppb in water and a flavor threshold of 0.004 ppb. There is limited information available on thiols pyrolysis. The stability to higher temperatures of thiols is similar to that of alcohols. Also, similar to some alcohols that eliminate water by pyrolysis, a number of thiols eliminate H2S (see reaction 4.1.4). The elimination of H2S is
Pyrolysis of Organic Molecules https://doi.org/10.1016/B978-0-444-64000-0.00007-X
321
# 2019 Elsevier B.V. All rights reserved.
322
7. PYROLYSIS OF THIOLS AND SULFIDES
shown below for a thiol with hydrogen available to the carbons in α- and β-position to the SH group: ð7:1:1Þ
The temperature of thiol decomposition depends on its structure. As an example, 3methylbutan-1-thiol starts decomposing around 500°C [1]. For thiols, the equivalent of H2 elimination occurring for alcohols (see reaction 4.1.2) does not lead to the formation of thioaldehydes. Thioaldehydes are reactive compounds with a high tendency to form trimers and other condensation products, which decompose easier than thiols. For this reason, some thiols, particularly those for which reaction (7.1.1) is not possible, generate by pyrolysis sulfur, larger molecular weight hydrocarbons, and other condensation products. At higher temperatures, the formation of thiophene from thiols is also noticed [1]. Aromatic thiols are compounds stable to heating. For example, pyrolysis of benzene thiol does not start below 500°C and does not take place following reaction (7.1.1). The pyrolysis typically generates a mixture of compounds, and the formation of sulfides is one of the major decomposition paths. This reaction is shown below: ð7:1:2Þ
The variation in the remaining level of benzene thiol and the formation of phenylthiobenzene and benzene in the range of 600–700°C in a flow experiment with 5.6 s contact time [2] are shown in Fig. 7.1.1. The composition of pyrolyzate at 700°C was found to be 13% undecomposed benzenethiol, 16% benzene, 13% phenylthiobenzene, 2.3% diphenyl disulfide, 0.02% biphenyl, 0.5% diphenyl-trisulfide, 0.3% dibenzo-thiophene, and <0.01% thianthrene. Similar results were obtained in a flash vacuum experiment. FIG. 7.1.1 Decomposition of benzene thiol and formation of benzene and phenylthiobenzene as a function of temperature (5.6 s contact).
80
Yield %
60 PhSH 40
PhSPh C6H6
20 0 550
600
650 700 Temperature ∞C
750
323
GENERAL ASPECTS
Other aromatic thiols behave similarly, as experimentally proven for 4-methylbenzenethiol [2]. The reaction mechanism for the decomposition is very likely radicalic, with the cleavage of the CdS bond and termination reactions between the aryl free radicals. The formation of sulfur bridges and H2S elimination can be used for the synthesis of cyclic strained compounds [3]. One example is the synthesis of the sulflower compound by flash vacuum pyrolysis at 450–530°C and 0.05 Torr. The sequence of reactions to synthesize this compound starts with tetrathiophene, which is treated with lithium diisopropylamide [CH3)2CH]2N-Li+ (LDA) and then with sulfur to generate a thiol, followed by pyrolysis with the formation of new CdSdC bridges. The reactions are shown below. n
n
n
n
n
n n
n
ð7:1:3Þ
References 7.1 [1] W.F. Faragher, J.C. Morrell, S. Comay, Ind. Eng. Chem. 20 (1928) 527. [2] D.E. Johnson, Fuel 66 (1987) 255. [3] K.Y. Chernichenko, V.V. Sumerin, R.V. Shpanchenko, E.S. Balenkova, V.G. Nenajdenko, Angew. Chem. Int. Ed. 45 (2006) 7367.
S U B C H A P T E R
7.2
Sulfides
GENERAL ASPECTS Organic sulfides are compounds with the general formula RdSdR0 , where R and R0 are hydrocarbon radicals. These compounds can be considered as analogs of ethers, generated by
324
7. PYROLYSIS OF THIOLS AND SULFIDES
replacing the oxygen atom with sulfur. The two radicals R and R0 can be identical or different, and they can be aliphatic, aromatic, unsaturated (with the carbon atom connected to the oxygen atom involved in a double bond), and combinations of the three previous possibilities. The common name of sulfides is derived from that of the two groups R and R0 (in alphabetical order) by adding the word sulfide. Thermal decomposition of sulfides is not identical to that of ethers. The typical decomposition reaction for sulfides having hydrogen available on the β-carbon to the S atom takes place as follows:
ð7:2:1Þ
One explanation for this type of reaction is caused by the instability of thioaldehydes and thioketones, which cannot be isolated in pyrolyzates. However, for some sulfides (e.g., diethylsulfide), thiols were found in the pyrolyzate. In the case of sulfides that cannot generate an alkene because no hydrogen is available on the β-carbon, pyrolysis typically leads to mixtures of compounds, the result depending on the molecular structure of the parent molecule. For example, pyrolysis of dibenzyl sulfide leads to the formation of toluene, stilbene, bibenzyl, H2S, and other fragment molecules [1,2]. Diphenyl sulfide is very stable to pyrolysis, and at higher temperatures it generates as the main reaction products H2S, benzene, and diphenylene sulfide, as shown below:
ð7:2:2Þ
Some other sulfides deposit sulfur besides forming H2S during pyrolysis.
References 7.2 [1] C.D. Hurd, The Pyrolysis of Carbon Compounds, A.C.S. Monograph Series No. 50, The Chemical Catalog Co., New York, 1929. [2] W.F. Faragher, J.C. Morrell, S. Comay, Ind. Eng. Chem. 20 (1928) 527.
325
REFERENCES 7.3
S U B C H A P T E R
7.3
Disulfides
GENERAL ASPECTS Disulfides are compounds with the general formula RdSdSdR0 . These compounds are more stable than the corresponding peroxides that contain the OdO group. When the R and R0 are aliphatic radicals with two to five carbons, these compounds decompose in the range of temperatures from 300°C to 500°C. The reaction products include H2S, RdSH (and R0 dSH), sulfur (S8), sulfides, trisulfides, thiophane, thiophene, substituted thiophanes and thiophenes, alkanes, and alkenes [1]. The formation of sulfur can be explained by the reaction: ð7:3:1Þ The formation of substituted thiophanes and thiophenes takes place by the following reaction:
–
–
ð7:3:2Þ
The formation of H2 is not detected in the pyrolyzate as a result of reaction (7.3.2), with the dehydrogenation reaction of thiophane taking place simultaneously with reduction reactions that use the hydrogen to generate, for example, alkanes. Some disulfides decompose at lower temperatures than those containing small hydrocarbon radicals. For example, dibenzyl disulfide decomposes around 200°C to generate sulfur, stilbene, and a mixture of other compounds [2].
References 7.3 [1] C.D. Hurd, The Pyrolysis of Carbon Compounds, A.C.S. Monograph Series No. 50, The Chemical Catalog Co., New York, 1929. [2] W.F. Faragher, J.C. Morrell, S. Comay, Ind. Eng. Chem. 20 (1928) 527.
7 Pyrolysis of Thiols and Sulfides
S U B C H A P T E R
7.1
Thiols
GENERAL ASPECTS Thiols contain one or more SH groups in their molecules and can be considered analogs of alcohols, generated by replacing the OH group with SH. The IUPAC name of thiols is made by adding the suffix thiol to the name of the corresponding hydrocarbon (methanethiol for CH3SH, benzenethiol for C6H5-SH, etc.). Other names for thiols are also in use, such as mercaptans (methyl mercaptan for CH3-SH). Thiols are found in nature mainly in more complex combinations. Cysteine, for example is a common amino acid having an SH group. Volatile thiols are known for their strong odor. As an example, 1-butanethiol has an odor threshold of 6 ppb in water and a flavor threshold of 0.004 ppb. There is limited information available on thiols pyrolysis. The stability to higher temperatures of thiols is similar to that of alcohols. Also, similar to some alcohols that eliminate water by pyrolysis, a number of thiols eliminate H2S (see reaction 4.1.4). The elimination of H2S is
Pyrolysis of Organic Molecules https://doi.org/10.1016/B978-0-444-64000-0.00007-X
321
# 2019 Elsevier B.V. All rights reserved.
322
7. PYROLYSIS OF THIOLS AND SULFIDES
shown below for a thiol with hydrogen available to the carbons in α- and β-position to the SH group: ð7:1:1Þ
The temperature of thiol decomposition depends on its structure. As an example, 3methylbutan-1-thiol starts decomposing around 500°C [1]. For thiols, the equivalent of H2 elimination occurring for alcohols (see reaction 4.1.2) does not lead to the formation of thioaldehydes. Thioaldehydes are reactive compounds with a high tendency to form trimers and other condensation products, which decompose easier than thiols. For this reason, some thiols, particularly those for which reaction (7.1.1) is not possible, generate by pyrolysis sulfur, larger molecular weight hydrocarbons, and other condensation products. At higher temperatures, the formation of thiophene from thiols is also noticed [1]. Aromatic thiols are compounds stable to heating. For example, pyrolysis of benzene thiol does not start below 500°C and does not take place following reaction (7.1.1). The pyrolysis typically generates a mixture of compounds, and the formation of sulfides is one of the major decomposition paths. This reaction is shown below: ð7:1:2Þ
The variation in the remaining level of benzene thiol and the formation of phenylthiobenzene and benzene in the range of 600–700°C in a flow experiment with 5.6 s contact time [2] are shown in Fig. 7.1.1. The composition of pyrolyzate at 700°C was found to be 13% undecomposed benzenethiol, 16% benzene, 13% phenylthiobenzene, 2.3% diphenyl disulfide, 0.02% biphenyl, 0.5% diphenyl-trisulfide, 0.3% dibenzo-thiophene, and <0.01% thianthrene. Similar results were obtained in a flash vacuum experiment. FIG. 7.1.1 Decomposition of benzene thiol and formation of benzene and phenylthiobenzene as a function of temperature (5.6 s contact).
80
Yield %
60 PhSH 40
PhSPh C6H6
20 0 550
600
650 700 Temperature ∞C
750
323
GENERAL ASPECTS
Other aromatic thiols behave similarly, as experimentally proven for 4-methylbenzenethiol [2]. The reaction mechanism for the decomposition is very likely radicalic, with the cleavage of the CdS bond and termination reactions between the aryl free radicals. The formation of sulfur bridges and H2S elimination can be used for the synthesis of cyclic strained compounds [3]. One example is the synthesis of the sulflower compound by flash vacuum pyrolysis at 450–530°C and 0.05 Torr. The sequence of reactions to synthesize this compound starts with tetrathiophene, which is treated with lithium diisopropylamide [CH3)2CH]2N-Li+ (LDA) and then with sulfur to generate a thiol, followed by pyrolysis with the formation of new CdSdC bridges. The reactions are shown below. n
n
n
n
n
n n
n
ð7:1:3Þ
References 7.1 [1] W.F. Faragher, J.C. Morrell, S. Comay, Ind. Eng. Chem. 20 (1928) 527. [2] D.E. Johnson, Fuel 66 (1987) 255. [3] K.Y. Chernichenko, V.V. Sumerin, R.V. Shpanchenko, E.S. Balenkova, V.G. Nenajdenko, Angew. Chem. Int. Ed. 45 (2006) 7367.
S U B C H A P T E R
7.2
Sulfides
GENERAL ASPECTS Organic sulfides are compounds with the general formula RdSdR0 , where R and R0 are hydrocarbon radicals. These compounds can be considered as analogs of ethers, generated by
324
7. PYROLYSIS OF THIOLS AND SULFIDES
replacing the oxygen atom with sulfur. The two radicals R and R0 can be identical or different, and they can be aliphatic, aromatic, unsaturated (with the carbon atom connected to the oxygen atom involved in a double bond), and combinations of the three previous possibilities. The common name of sulfides is derived from that of the two groups R and R0 (in alphabetical order) by adding the word sulfide. Thermal decomposition of sulfides is not identical to that of ethers. The typical decomposition reaction for sulfides having hydrogen available on the β-carbon to the S atom takes place as follows:
ð7:2:1Þ
One explanation for this type of reaction is caused by the instability of thioaldehydes and thioketones, which cannot be isolated in pyrolyzates. However, for some sulfides (e.g., diethylsulfide), thiols were found in the pyrolyzate. In the case of sulfides that cannot generate an alkene because no hydrogen is available on the β-carbon, pyrolysis typically leads to mixtures of compounds, the result depending on the molecular structure of the parent molecule. For example, pyrolysis of dibenzyl sulfide leads to the formation of toluene, stilbene, bibenzyl, H2S, and other fragment molecules [1,2]. Diphenyl sulfide is very stable to pyrolysis, and at higher temperatures it generates as the main reaction products H2S, benzene, and diphenylene sulfide, as shown below:
ð7:2:2Þ
Some other sulfides deposit sulfur besides forming H2S during pyrolysis.
References 7.2 [1] C.D. Hurd, The Pyrolysis of Carbon Compounds, A.C.S. Monograph Series No. 50, The Chemical Catalog Co., New York, 1929. [2] W.F. Faragher, J.C. Morrell, S. Comay, Ind. Eng. Chem. 20 (1928) 527.
325
REFERENCES 7.3
S U B C H A P T E R
7.3
Disulfides
GENERAL ASPECTS Disulfides are compounds with the general formula RdSdSdR0 . These compounds are more stable than the corresponding peroxides that contain the OdO group. When the R and R0 are aliphatic radicals with two to five carbons, these compounds decompose in the range of temperatures from 300°C to 500°C. The reaction products include H2S, RdSH (and R0 dSH), sulfur (S8), sulfides, trisulfides, thiophane, thiophene, substituted thiophanes and thiophenes, alkanes, and alkenes [1]. The formation of sulfur can be explained by the reaction: ð7:3:1Þ The formation of substituted thiophanes and thiophenes takes place by the following reaction:
–
–
ð7:3:2Þ
The formation of H2 is not detected in the pyrolyzate as a result of reaction (7.3.2), with the dehydrogenation reaction of thiophane taking place simultaneously with reduction reactions that use the hydrogen to generate, for example, alkanes. Some disulfides decompose at lower temperatures than those containing small hydrocarbon radicals. For example, dibenzyl disulfide decomposes around 200°C to generate sulfur, stilbene, and a mixture of other compounds [2].
References 7.3 [1] C.D. Hurd, The Pyrolysis of Carbon Compounds, A.C.S. Monograph Series No. 50, The Chemical Catalog Co., New York, 1929. [2] W.F. Faragher, J.C. Morrell, S. Comay, Ind. Eng. Chem. 20 (1928) 527.