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Radiation and photochemically induced tritium substitution in aromatic hydrocarbons
tiCAL
PHYSICS
LETTERS
1 (1968)
642-644.
NORTH-HOLLAND
PUBLISHING
COMPANY,
AMSTERDAM
RADIATION AND PHOTOCHEMICALLY INDUCED TRITIUM SUBSTITUTION IN AROMATIC HYDROCARBONS J. KROH and E. HANKIEWICZ D&artment
of Radiation
Chemistry. Lddt, Poland
Received
University,
Technical
13 December
1967
Benzene and toluene. saturated with tritiated water, have been subjected to 6OCo y-radiation and UVlight. The yields of H-H substitution are higher for benzene than for toluene. Naphthalene decreases the substitution yield for benzene. The excited molecule mechanism of substitution is discussed.
he study of radiation and photochemically ind substitution reactions of tritium atoms in latic hydrocarbons would appear to be a val3 method for investigating the behaviour of :a&, ions, and excited molecules in primary esses taking place in the irradiated hydroons. Kroh and PTonka [l] have recently stated tritium substitution yields are higher for benthan for toluene even though toluene displays :her radical yield. Similar results have been ned for deuterium substitutions in the gas e [Z], and indicate that free radicals cannot ’ deciding importance in the substitution reacwhich have been investigated so far. In phoically induced reactions, on the other hand, cannot take part since the energy of the W ition applied (A - 2300 A) is lower than the .ation energy of the compounds being studied. Icreasing amount of work has been published, [3], quoting high radiation yields of excited s in benzene. It is generally considered that ed states play a very important part in eneransfer phenomena, particularly in solutions e one of the components is benzene, e.g. [4]. recently it has been stated [5, S] that the of proton addition to aromatic molecules is I higher for their excited states (singlet or et) than for the ground ones. resent evidence indicates that excited aroc molecules take part in the investigated subtion reactions. The arguments are derived from direct measurements and by discounthe iikelihood that competing reactions (ionic tdical) occur; they follow from experiments lotochemidally initiated .substitutions and on nfluence of oxygen and naphthalene on radiasubstitution yields. iLry 1968
The purification of benzene and toluene, their saturation with tritiated water (with activities of approximately 25 mC/ml), and their preparation for irradiation were carried out as described previous1 [I]. The samples were then irradiated using a 6BCoy-source with a dose rate of the order of 1019 eV/lsec. Certain of the samples were subjected to W radiation (2 300 to 2 700 Ai). The quantum yields were determined using a liquid ethyl-iodide actionometer [7,8]. A number of the samples were distilled over diphenyl (which acts as a carrier for polymeric products) after the tritiated water had been removed by repeated washing with distilled water. The
activity
of the samples
was
measured
by
scintillation technique. The composition of the distillate was determined radiochromatographitally. More exact experimental conditions and the method for identifying the chromatographic Table Radiation
yields
1
per 100 eV of H-H substitution benzene
and toluene
benzene degassed
f naphthaIene
toluene degassed
lo-2M
642
f naphthalene 10-2M
GTOT
0.106
0.024
0.023
0.021
GDIST
0.013
0.003
0.003
0.003
CPCL
0.093
0.021
0.020
0.018
GDIST GTOT
0.12
0.13
0.13
0.14
GPGL GTOT
0.88
0.87
0.87
0.86
in
TRITIUM SUBSTITUTION IN AROMATIC HYDROCARBONS leaks are described elsewhere [l, 91. Some of he obtained results are summarized in table 1. Phe radiation yields GTOT and GDL,ST denote the lumbers of the substituted H atoms per 100 eV .n the undistilled and distilled samples respecively. The tritiated distillate consists mainly of labelled parent molecule (see fig. 1). Cyclohexme and both cyclohexadienes constitute only ca. 7% of the labelled benzene. It is worth noticing hat the direct action of radiation on the water present in the system can not be responsible for :he observed effects. The substitution of all H atoms produced by II20 radiolysis would lead to he yield of ca. 0.003, i.e. to the value lower by two orders of magnitude than the maximum yield listed in table 1. Several conclusions may be drawn by comparing the obtained values in table 1. Firstly radiation yield of H-H substitution are about four times higher for benzene than for toluene even though toluene displays a higher radical yield. The distillates from both benzene and toluene constitute 12% and polymers approximately 2PPfrckmately
CPrn
1300
IX3
88% of all the tritium substituted products, which is in good agreement with the high yields of poLymerit products from irradiated aromatic hydrocarbons. Fig. 2 shows the percentage of substituted H atoms for degassed benzene and toluene. In the aerated undistilled samples oxygen reduces the percentage of substituted H atoms at doses up to 102* eV/i when, since the oxygen is probably all used up the yield of substitution becomes higher than that for degassed samples. The origin of the induction period and consequent enhancement of substitution is’ not clear. In the distilled samples a 4-fold increase in yield for both benzene and toluene is observed in the presence of oxygen. The corresponding substitution yields in the presence of oxygen are also higher for benzene than for toluene. It has been also found in this laboratory that the substitution of tritium for benzene, chlorobenzene and bromobenzene irradiated in the presence of CH30T proceeds with greater yield for benzene than for chlorobenzene and bromoben-. zene though the respective radical yields form the sequence:
2 G(C6H,C1) G(C$L+)
’ ‘(C8Hs)
-
All these results indicate that free radicals formed directly by irradiating aromatic hydrocarbons cannot be of deciding importance in the studied reactions.
4200
4100
nKi
wa
2Do
Fig. 1. Radiochromatogram of the substituted benzene distillate.
Fig. 2. Substitution of H atoms (from tritiated water) in degassed benzene and toluene: @ benzene, TOT A benzene, DIST 0 toluene, TOT a toluene; DIST
4
J. KROH and E. HANKIEWICZ
The results obtained for W-rays give the antum yields of substitution in benzene of the der of 10-4 which are about 8 times higher zn those for tcluene. The energy of radiation sorbed E < 5.6 eV is insufficient for the ionizam of benzene and toluene (the respective ionition potentials are 9.2 and 8.8 eV). On the basis the above results it seems that the role of ionic d radical processes in the substitution is very nited if any. Consequently one must take into account the cited molecules as main intermediates responble for the effects observed by us. It is possible zt excited aromatic molecules reacting with war form as intermediates tritiated cyclohexadiyl radicals C6H6T which then decompose into
T) at.oms and tritiated (normal) benzene. The hancement of substitution for the distilled sam3s in the presence of air (both for y- and UVys) may be caused by an oxygenated product :ilitating the substitution. It is known [12] that aqueous solutions of benzene in the presence air one of the intermediates produced is C6H6CH. Hence instead Of C&T in the de3sed samples it could be also C2C6H6T radicals ich participate in the substitution in the aerated stems. In order to collect more information on the ;sible role of excited molecules in the discus1 reactions benzene and toluene saturated with tiated water were y-irradiated in the presence naphthalene in the concentration 10-2 M. Prom table 1 it can be seen that such addition naphthalene reduces the radiation yield of subtution about 4 times, while not altering the peritage compositions of the distillate or polymers. r toluene, however, the same naphthalene conltration does not affect the substitution yield. 10m2M solutions of naphthalene, the yields IT, DIST and POL are the same for both benie and toluene. This may be due to a certain, iignificant contribution of another mechanism substitution or to inefficiency of naphthaiene scavenge all the excited states involved in the ,estigated I+rocess. Naphthalone (energy of triplet state 61 kcal/mol)
can act as an effective scavenger of benzene triplet state (84 kcal/mol) [3]. In the case of eiectron or H-atom capture the naphthalene-ions or naphthalene-H-radicals should be formed respectively. However, neither of them have been observed by pulse radiolysis technique whereas naphthalene triplets are tasily detectable [4]. The evidence given above would appear to be the most direct argument that these are excited molecules which take main part in the substitution reactions studied, and could also be used to explain the higher substitution yields in benzene than in toluene. It is possible that higher excited states or even several excited molecules (excited complex) participate in single substitution act. Further experiments on the LET-effects and on the influence of some other scavengers should elucidate this problem. The authors wish to thank Dr. A. Sokofowska and Mr. Rojek of the Radiochemistry Department, Nuclear Research Institute, Warsaw, for assistance in the radiochromatographic analyses.
References [I] .J.Kroh and A.M.Flonka, Buil.Acad.Folon.Sci. Ser. Sci. Chim. 14 (1966) 351. [2] J. Kroh and E.Hankiewicz, Buil.Acad. Folon. Sci. Ser. Sci. Chim. 14 (1966) 243. [3] F-Fischer. H.B. Lehmann and G-Stein. J.Chem. Fhys. 45 (1966) 3905. [4]F.S.Dainton. T-J-Kemp, G.A.SalmocandJ.P. Keene. Nature 203 (1964) 1050. [5] M.G.Kuzmin, B.M.?Einow. G.Sentberdi and J.W. Bierezin, Zh.Fiz.Kiim.41 (1967) 446. [S] M.G.Kuzmin, B.M.UZinow, G.Sentberdi and J.W. .Bierezin, Zh.Fiz.Khim.41 (1967) 769. [fl B.M.Norton. J.Am.Chem.Soc.56 (1934) 2294. 181R. Hentz and M. Burton, J-Am. Chem. Sot. 73 (1951) 532. [S] A.Haipern and _\.Sokofowska. J.Inorg.Nuc.Chem. 27 (1965) 1893. [lo] W.A.Noyes, W.A.MulacandD.A.Harter, J.Chem. Phys. 44 (1966) 2100. [ll] E.BurzyrQska. private communication. [12] L.M.Dorfman, J.A.TaubandR.E.Biihier, J.Chem. Fhys. 36 (1962) 3051.
PHYSICS
LETTERS
1 (1968)
642-644.
NORTH-HOLLAND
PUBLISHING
COMPANY,
AMSTERDAM
RADIATION AND PHOTOCHEMICALLY INDUCED TRITIUM SUBSTITUTION IN AROMATIC HYDROCARBONS J. KROH and E. HANKIEWICZ D&artment
of Radiation
Chemistry. Lddt, Poland
Received
University,
Technical
13 December
1967
Benzene and toluene. saturated with tritiated water, have been subjected to 6OCo y-radiation and UVlight. The yields of H-H substitution are higher for benzene than for toluene. Naphthalene decreases the substitution yield for benzene. The excited molecule mechanism of substitution is discussed.
he study of radiation and photochemically ind substitution reactions of tritium atoms in latic hydrocarbons would appear to be a val3 method for investigating the behaviour of :a&, ions, and excited molecules in primary esses taking place in the irradiated hydroons. Kroh and PTonka [l] have recently stated tritium substitution yields are higher for benthan for toluene even though toluene displays :her radical yield. Similar results have been ned for deuterium substitutions in the gas e [Z], and indicate that free radicals cannot ’ deciding importance in the substitution reacwhich have been investigated so far. In phoically induced reactions, on the other hand, cannot take part since the energy of the W ition applied (A - 2300 A) is lower than the .ation energy of the compounds being studied. Icreasing amount of work has been published, [3], quoting high radiation yields of excited s in benzene. It is generally considered that ed states play a very important part in eneransfer phenomena, particularly in solutions e one of the components is benzene, e.g. [4]. recently it has been stated [5, S] that the of proton addition to aromatic molecules is I higher for their excited states (singlet or et) than for the ground ones. resent evidence indicates that excited aroc molecules take part in the investigated subtion reactions. The arguments are derived from direct measurements and by discounthe iikelihood that competing reactions (ionic tdical) occur; they follow from experiments lotochemidally initiated .substitutions and on nfluence of oxygen and naphthalene on radiasubstitution yields. iLry 1968
The purification of benzene and toluene, their saturation with tritiated water (with activities of approximately 25 mC/ml), and their preparation for irradiation were carried out as described previous1 [I]. The samples were then irradiated using a 6BCoy-source with a dose rate of the order of 1019 eV/lsec. Certain of the samples were subjected to W radiation (2 300 to 2 700 Ai). The quantum yields were determined using a liquid ethyl-iodide actionometer [7,8]. A number of the samples were distilled over diphenyl (which acts as a carrier for polymeric products) after the tritiated water had been removed by repeated washing with distilled water. The
activity
of the samples
was
measured
by
scintillation technique. The composition of the distillate was determined radiochromatographitally. More exact experimental conditions and the method for identifying the chromatographic Table Radiation
yields
1
per 100 eV of H-H substitution benzene
and toluene
benzene degassed
f naphthaIene
toluene degassed
lo-2M
642
f naphthalene 10-2M
GTOT
0.106
0.024
0.023
0.021
GDIST
0.013
0.003
0.003
0.003
CPCL
0.093
0.021
0.020
0.018
GDIST GTOT
0.12
0.13
0.13
0.14
GPGL GTOT
0.88
0.87
0.87
0.86
in
TRITIUM SUBSTITUTION IN AROMATIC HYDROCARBONS leaks are described elsewhere [l, 91. Some of he obtained results are summarized in table 1. Phe radiation yields GTOT and GDL,ST denote the lumbers of the substituted H atoms per 100 eV .n the undistilled and distilled samples respecively. The tritiated distillate consists mainly of labelled parent molecule (see fig. 1). Cyclohexme and both cyclohexadienes constitute only ca. 7% of the labelled benzene. It is worth noticing hat the direct action of radiation on the water present in the system can not be responsible for :he observed effects. The substitution of all H atoms produced by II20 radiolysis would lead to he yield of ca. 0.003, i.e. to the value lower by two orders of magnitude than the maximum yield listed in table 1. Several conclusions may be drawn by comparing the obtained values in table 1. Firstly radiation yield of H-H substitution are about four times higher for benzene than for toluene even though toluene displays a higher radical yield. The distillates from both benzene and toluene constitute 12% and polymers approximately 2PPfrckmately
CPrn
1300
IX3
88% of all the tritium substituted products, which is in good agreement with the high yields of poLymerit products from irradiated aromatic hydrocarbons. Fig. 2 shows the percentage of substituted H atoms for degassed benzene and toluene. In the aerated undistilled samples oxygen reduces the percentage of substituted H atoms at doses up to 102* eV/i when, since the oxygen is probably all used up the yield of substitution becomes higher than that for degassed samples. The origin of the induction period and consequent enhancement of substitution is’ not clear. In the distilled samples a 4-fold increase in yield for both benzene and toluene is observed in the presence of oxygen. The corresponding substitution yields in the presence of oxygen are also higher for benzene than for toluene. It has been also found in this laboratory that the substitution of tritium for benzene, chlorobenzene and bromobenzene irradiated in the presence of CH30T proceeds with greater yield for benzene than for chlorobenzene and bromoben-. zene though the respective radical yields form the sequence:
2 G(C6H,C1) G(C$L+)
’ ‘(C8Hs)
-
All these results indicate that free radicals formed directly by irradiating aromatic hydrocarbons cannot be of deciding importance in the studied reactions.
4200
4100
nKi
wa
2Do
Fig. 1. Radiochromatogram of the substituted benzene distillate.
Fig. 2. Substitution of H atoms (from tritiated water) in degassed benzene and toluene: @ benzene, TOT A benzene, DIST 0 toluene, TOT a toluene; DIST
4
J. KROH and E. HANKIEWICZ
The results obtained for W-rays give the antum yields of substitution in benzene of the der of 10-4 which are about 8 times higher zn those for tcluene. The energy of radiation sorbed E < 5.6 eV is insufficient for the ionizam of benzene and toluene (the respective ionition potentials are 9.2 and 8.8 eV). On the basis the above results it seems that the role of ionic d radical processes in the substitution is very nited if any. Consequently one must take into account the cited molecules as main intermediates responble for the effects observed by us. It is possible zt excited aromatic molecules reacting with war form as intermediates tritiated cyclohexadiyl radicals C6H6T which then decompose into
T) at.oms and tritiated (normal) benzene. The hancement of substitution for the distilled sam3s in the presence of air (both for y- and UVys) may be caused by an oxygenated product :ilitating the substitution. It is known [12] that aqueous solutions of benzene in the presence air one of the intermediates produced is C6H6CH. Hence instead Of C&T in the de3sed samples it could be also C2C6H6T radicals ich participate in the substitution in the aerated stems. In order to collect more information on the ;sible role of excited molecules in the discus1 reactions benzene and toluene saturated with tiated water were y-irradiated in the presence naphthalene in the concentration 10-2 M. Prom table 1 it can be seen that such addition naphthalene reduces the radiation yield of subtution about 4 times, while not altering the peritage compositions of the distillate or polymers. r toluene, however, the same naphthalene conltration does not affect the substitution yield. 10m2M solutions of naphthalene, the yields IT, DIST and POL are the same for both benie and toluene. This may be due to a certain, iignificant contribution of another mechanism substitution or to inefficiency of naphthaiene scavenge all the excited states involved in the ,estigated I+rocess. Naphthalone (energy of triplet state 61 kcal/mol)
can act as an effective scavenger of benzene triplet state (84 kcal/mol) [3]. In the case of eiectron or H-atom capture the naphthalene-ions or naphthalene-H-radicals should be formed respectively. However, neither of them have been observed by pulse radiolysis technique whereas naphthalene triplets are tasily detectable [4]. The evidence given above would appear to be the most direct argument that these are excited molecules which take main part in the substitution reactions studied, and could also be used to explain the higher substitution yields in benzene than in toluene. It is possible that higher excited states or even several excited molecules (excited complex) participate in single substitution act. Further experiments on the LET-effects and on the influence of some other scavengers should elucidate this problem. The authors wish to thank Dr. A. Sokofowska and Mr. Rojek of the Radiochemistry Department, Nuclear Research Institute, Warsaw, for assistance in the radiochromatographic analyses.
References [I] .J.Kroh and A.M.Flonka, Buil.Acad.Folon.Sci. Ser. Sci. Chim. 14 (1966) 351. [2] J. Kroh and E.Hankiewicz, Buil.Acad. Folon. Sci. Ser. Sci. Chim. 14 (1966) 243. [3] F-Fischer. H.B. Lehmann and G-Stein. J.Chem. Fhys. 45 (1966) 3905. [4]F.S.Dainton. T-J-Kemp, G.A.SalmocandJ.P. Keene. Nature 203 (1964) 1050. [5] M.G.Kuzmin, B.M.?Einow. G.Sentberdi and J.W. Bierezin, Zh.Fiz.Kiim.41 (1967) 446. [S] M.G.Kuzmin, B.M.UZinow, G.Sentberdi and J.W. .Bierezin, Zh.Fiz.Khim.41 (1967) 769. [fl B.M.Norton. J.Am.Chem.Soc.56 (1934) 2294. 181R. Hentz and M. Burton, J-Am. Chem. Sot. 73 (1951) 532. [S] A.Haipern and _\.Sokofowska. J.Inorg.Nuc.Chem. 27 (1965) 1893. [lo] W.A.Noyes, W.A.MulacandD.A.Harter, J.Chem. Phys. 44 (1966) 2100. [ll] E.BurzyrQska. private communication. [12] L.M.Dorfman, J.A.TaubandR.E.Biihier, J.Chem. Fhys. 36 (1962) 3051.