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The influence of structure upon the retention of 82Br in some neutron-irradiated organic compounds
J. Inorg. Nucl. Chem., 1965, Vol. 27, pp. 2445 to 2447. Pergamon Press Ltd. Printed in Northern Ireland
NOTES
The influence of structure upon the retention of 82Br in some neutron-irradiated organic compounds (Receired 12 January 1965)
WHENbromine-containing organic compounds are irradiated with neutrons, they undergo a SZILARDC~fALMEI~Sreaction ~1~in which a fraction of the radio-bromine is found organically bound, the parent compound usually being produced in the greatest proportion. The billiard ball theory (2) explains this fact by postulating a hot-atom reaction in which the recoiling atom loses its high energy by elastic collisions with the atoms of the medium, but recent evidence shows that hot reactions occur to only a small extent, if any, in gaseous methyl iodide (%4) and in liquid bromobenzene. ~5) To explain the phenomena observed in the latter case factors other than kinetic energy of the recoiling atom must be considered, e.g. formation of a charge-transfer complex between radio-bromine and the inactive parent molecule. If this is true for bromobenzene, we would expect structural and electronic factors in the irradiated molecules Io affect the fate of the newly-formed radio-bromine atoms. Experiments with mixtures of aromatic compounds do show the presence of such effects. The irradiated systems were: (a) An equimolar mixture of bromobenzene and o-chlorophenol; (b) an equimolar mixture of chlorobenzene and o-bromophenol. In both cases phS2Br and o-S2BrC~H~OH are formed, as well as other products, by substitution reactions. Owing to the symmetry of the two systems, effects due to the nature of the substituted halogen and to the presence of the aromatic hydroxyl group could be ascertained. Experiments were also performed in the presence of 0.2 molar fraction of aniline, which was chosen because of its strong electron-donor properties. Finally, some experiments were conducted with tetrabromomethane as a source of radio-bromine in mixtures of PhNH2 and PhOH in various mole ratios. Only effects due to ~'~Brwere studied. EXPERIMENTAL
Materials "Puriss" bromobenzene (Fluka AG) and chemically-pure chlorobenzene (E. Merck AG) were further purified by fractional distillation. Chemically-pure phenol (E. Merck AG) and " p u r u m " o-chloro- and o-bromophenol (Fluka AG) were fractionally distilled under reduced pressure. Pro analysi aniline (E. Merck AG) was distilled from zinc dust and subjected to fractional distillation under reduced pressure. Tetrabromomethane (L. Light & Co Ltd) was purified by sublimation before use.
Neutron irradiations These were performed in the swimming pool reactor of The Nuclear Research Centre"Democritus". The mixtures, in sealed pyrex glass ampoules, were placed behind a lead block near the reflector, in order to reduce the gamma field associated with the neutron flux of 1-5 × 10l° n. cm -2 sec -1. The irradiation time was 15 min except in the systems containing CBr4 which were irradiated for 4 hr.
Anal)'sis and counting technique After irradiation each mixture was left for 20 hr at room temperature and then the whole sample ( ~ 2 ml) was extracted with 5 ml aqueous solution containing 5 % Na2S2Oz and 5 % KBr. 1 ml of the (i) L. SZILARD and T. A. CrtALMERS, Nature, Lond. 134, 462 (1934). (21 W. F. LmBv, J. Amer. Chem. Soe. 69, 2523 (1947). (a) G. S. LAURENCEand D. R. STRANKS, Radioisotopes in Physical Sciences and Industry, III I.A.E.A., Vienna 1962, p. 483. ~4~R. J. CROSS and R. L. WOLFGANG, Radioehim. Acta 2, 112 (1964). (5, N. A. KATSANOS,J. Chem. Soe. In press. 2445
2446
Notes
aqueous phase was diluted to 50 ml and 2 ml of this was counted. The organic phase was extracted twice more with the same solution and the activities of the aqueous phases, after suitable corrections for dilution, etc., were added to the first count to give the inorganic 82Br activity. The residual extractable activity was < 0.2 ~ of the total. The extracted organic phase was dried overnight (CaSO4) and the total organic activity counted; and a preweighed amount of it was counted to find the total organic activity; known amounts of suitable carriers were added and the mixture was subjected to gas chromatographic analysis, phS~Br and o-S2BrC6H4OH were collected and counted separately. In the experiments with CBr4 only the total retention and that due to phS~Br were determined. This was done by dissolving the irradiated mixture in 5 ml of benzene, extracting the solution with aqueous Na2S20.~ and KBr as before, and separating the phS2Br activity by careful fractional distillation after the addition of inactive bromobenzene as carrier. Fractional distillation was repeated three times, and the last two distillates had the same specific activity, indicating that the contribution from products other than bromobenzene was negligible. Scintillation counting was used throughout, with a well-type NaI(TI) crystal. Measurements were conducted at least 48 hr after the end of the irradiation to ensure that all 2°Br and a°mBr had decayed. Retentions were calculated in the usual manner, with suitable corrections for decay and dilution. The overall standard error in determining retentions was 3 ~ , including also errors due to counting statistics. RESULTS
AND
DISCUSSION
Table 1 shows that heating the samples at 90°C for 1 hr does not change the retention values significantly, indicating that all post-irradiation reactions ~5) are at equilibrium after 20 hr at room TABLE 1 Composition of the irradiated sample (moles x l0 s) PhBr
o-BrC6H4OH
PhCI
0.63 1 1'
1 1"
PhNH~ 1.03
1 1*
1 1* 1
1
o-CIC~H4OH
Retention ( ~ of total activity)
1 1
0"5 0-5
PhS~Br 0-30 3'93 4.00 9.76 10.33 1.60 3-53
o-S~BrC6H~OH
total
2.61 11.52 12.40 12.10 12.90 4-07 2-25
9.81 62.70 61.20 75.00 76"26 14'87 14'40
* These samples were heated at 90°C (after 20 hr at room temperature) for 1 hr before treatment. temperature. It is also evident that substitution reactions of organically-bound chlorine and bromine by radio-bromine are influenced not only by the nature of the halogen but also by the molecule to which the halogen is attached. This can be seen by observing the retentions of o-8~BrC~HaOH and PhS~Br. The values for the former are essentially the same whether chlorine or bromine is substituted by the recoiling radio-bromine, whilst the amount of phs2Br is higher in the case where it is produced by bromine substitution. This behaviour may be due to electronic factors connected with the rest of the molecule. K~EEERand ANDREWS~ have shown spectrophotometrically that molecular bromine forms a donor-acceptor (charge-transfer) complex with bromobenzene, and that Br2 is probably bound to the bromine atom. WEXLERc7) has observed that atoms often have a large positive charge after neutron capture, owing to internal conversion processes. Since positive bromine ions are much better electron-aeceptors than Br2 molecules, the hypothesis has been advanced ~ that a charge-transfer complex is formed between radio-bromine and the inactive halogen bound to the benzene ring, followed by replacement within the complex. This explains also the negative temperature coefficient of the post-irradiation reaction in bromobenzene35~ Now the observed difference (6) R. M. KEEFER and L. J. ANDREWS,J. Amer. Chem. Soc. 72, 4677 (1950). (7) S. WEXLER and T. H. DAVIES, .1". Chem. Phys. 20, 1688 (1952); S. WEXLER,Phys. Rev. 93, 182 (1954); S. WEXLER and G. R. ANDERSON, J. Chem. Phys. 33, 850 (1960).
Notes
2447
reported here between PhBr and PhC1 towards substitution by radio-bromine is probably due to differences in the stability of the two complexes, the Br-atom in PhBr being a better donor than the CI-atom in PhCI, owingto thelowerpolarizabilityoftheCa~-ClbondcomparedwiththeCar Brbond. A hydroxyl group present in the benzene ring, however, exerts a positive mesomeric effect ( ÷ M), and one would expect both halogens to become more negative, while their difference with respect to polarizability to be less pronounced. This explains both the similarity in the retention of o-~2BrC,;H4OH TABLE 2
Composition of the irradiated sample (molar fraction :< 10~)
Retention (~'~, of total activity)
CBr4
PhNH2
PhOH
phS2Br
Total
1-29 1.40 1.37 1.41 1.30
98.71 88-12 74.52 49.81 41.02
0 10.48 24.10 48.79 57.68
0.42 0.59 0.47 1.01 1.11
3-85 3.68 4.30 4.74 5.81
in the two cases and the higher value of this retention in comparison to that of phs2Br. The difference in the total retention of the two systems (62.7 per cent as compared to 75.0 per cent) also bears out this fact. The hypothesis of complex formation explains another experimental fact, viz. the dramatic fall in retention of all products when aniline is present in the systems. Since aniline is known to be a strong electron-donor, complex formation of radio-bromine on the amino group is favoured to that on the halogen atoms. When complexed in this way, however, radio-bromine can easily abstract hydrogen from the amino group, thus becoming stabilized in inorganic form, and the organic retention is reduced. On the same basis we can understand why in systems containing CBr4 (Table 2) such low retentions are observed, which increase very slowly as the molar fraction of aniline in the system decreases.
Acknowledgement--The authors thank Miss ANNA VASSILAKIfor her assistance in this work. Department of Chemistry Nuclear Research Centre "Democritus" Aghia Paraskevi Attikis, Greece
N. A. KATSANOS A . G . VARV(~.(;LIS
J. lnorg. Nucl. Chem., 1965, Vol. 27, pp. 2447 to 2449. Pergamon Press Ltd. Printed in Northern Ireland
Donor properties of N 1- alkylguanylureas (Received 8 February 1965) IN CONTINUATION of previous work 1 ~ we wish to report here the syntheses of several cobalt (111) complexes containing two Nl-methylguanylurea ligands. Air oxidation of a mixture of aqueous Nl-methylguanylurea and aqueous cobalt chloride in the presence of ammonia provides rose-red crystals of diammine-bis-(Nl-methylguanylurea)cobalt (1II) chloride. The two Nl-methylguanylurea molecules in the univalent complex cation undergo protonation on treatment with dilute acids giving trivalent cations. The two ammonia molecules are believed ~ R. L. DUT'rA and P. RAY, J. Indian Chem. Soc. 36, 499, 567,576 (1959). 121 R. L. DUTTA, Ibid. 37, 499 (1960). ~3) R. L. DUTTA, B. SUR and N. R. SENGUPTA, Ibid. 37, 565, 573 (1960). ~4) R. L. DUTrA and S. LAHmV, lb#L 37, 789 (1960): 38, 689 (1961): 40, 863 (1963).
NOTES
The influence of structure upon the retention of 82Br in some neutron-irradiated organic compounds (Receired 12 January 1965)
WHENbromine-containing organic compounds are irradiated with neutrons, they undergo a SZILARDC~fALMEI~Sreaction ~1~in which a fraction of the radio-bromine is found organically bound, the parent compound usually being produced in the greatest proportion. The billiard ball theory (2) explains this fact by postulating a hot-atom reaction in which the recoiling atom loses its high energy by elastic collisions with the atoms of the medium, but recent evidence shows that hot reactions occur to only a small extent, if any, in gaseous methyl iodide (%4) and in liquid bromobenzene. ~5) To explain the phenomena observed in the latter case factors other than kinetic energy of the recoiling atom must be considered, e.g. formation of a charge-transfer complex between radio-bromine and the inactive parent molecule. If this is true for bromobenzene, we would expect structural and electronic factors in the irradiated molecules Io affect the fate of the newly-formed radio-bromine atoms. Experiments with mixtures of aromatic compounds do show the presence of such effects. The irradiated systems were: (a) An equimolar mixture of bromobenzene and o-chlorophenol; (b) an equimolar mixture of chlorobenzene and o-bromophenol. In both cases phS2Br and o-S2BrC~H~OH are formed, as well as other products, by substitution reactions. Owing to the symmetry of the two systems, effects due to the nature of the substituted halogen and to the presence of the aromatic hydroxyl group could be ascertained. Experiments were also performed in the presence of 0.2 molar fraction of aniline, which was chosen because of its strong electron-donor properties. Finally, some experiments were conducted with tetrabromomethane as a source of radio-bromine in mixtures of PhNH2 and PhOH in various mole ratios. Only effects due to ~'~Brwere studied. EXPERIMENTAL
Materials "Puriss" bromobenzene (Fluka AG) and chemically-pure chlorobenzene (E. Merck AG) were further purified by fractional distillation. Chemically-pure phenol (E. Merck AG) and " p u r u m " o-chloro- and o-bromophenol (Fluka AG) were fractionally distilled under reduced pressure. Pro analysi aniline (E. Merck AG) was distilled from zinc dust and subjected to fractional distillation under reduced pressure. Tetrabromomethane (L. Light & Co Ltd) was purified by sublimation before use.
Neutron irradiations These were performed in the swimming pool reactor of The Nuclear Research Centre"Democritus". The mixtures, in sealed pyrex glass ampoules, were placed behind a lead block near the reflector, in order to reduce the gamma field associated with the neutron flux of 1-5 × 10l° n. cm -2 sec -1. The irradiation time was 15 min except in the systems containing CBr4 which were irradiated for 4 hr.
Anal)'sis and counting technique After irradiation each mixture was left for 20 hr at room temperature and then the whole sample ( ~ 2 ml) was extracted with 5 ml aqueous solution containing 5 % Na2S2Oz and 5 % KBr. 1 ml of the (i) L. SZILARD and T. A. CrtALMERS, Nature, Lond. 134, 462 (1934). (21 W. F. LmBv, J. Amer. Chem. Soe. 69, 2523 (1947). (a) G. S. LAURENCEand D. R. STRANKS, Radioisotopes in Physical Sciences and Industry, III I.A.E.A., Vienna 1962, p. 483. ~4~R. J. CROSS and R. L. WOLFGANG, Radioehim. Acta 2, 112 (1964). (5, N. A. KATSANOS,J. Chem. Soe. In press. 2445
2446
Notes
aqueous phase was diluted to 50 ml and 2 ml of this was counted. The organic phase was extracted twice more with the same solution and the activities of the aqueous phases, after suitable corrections for dilution, etc., were added to the first count to give the inorganic 82Br activity. The residual extractable activity was < 0.2 ~ of the total. The extracted organic phase was dried overnight (CaSO4) and the total organic activity counted; and a preweighed amount of it was counted to find the total organic activity; known amounts of suitable carriers were added and the mixture was subjected to gas chromatographic analysis, phS~Br and o-S2BrC6H4OH were collected and counted separately. In the experiments with CBr4 only the total retention and that due to phS~Br were determined. This was done by dissolving the irradiated mixture in 5 ml of benzene, extracting the solution with aqueous Na2S20.~ and KBr as before, and separating the phS2Br activity by careful fractional distillation after the addition of inactive bromobenzene as carrier. Fractional distillation was repeated three times, and the last two distillates had the same specific activity, indicating that the contribution from products other than bromobenzene was negligible. Scintillation counting was used throughout, with a well-type NaI(TI) crystal. Measurements were conducted at least 48 hr after the end of the irradiation to ensure that all 2°Br and a°mBr had decayed. Retentions were calculated in the usual manner, with suitable corrections for decay and dilution. The overall standard error in determining retentions was 3 ~ , including also errors due to counting statistics. RESULTS
AND
DISCUSSION
Table 1 shows that heating the samples at 90°C for 1 hr does not change the retention values significantly, indicating that all post-irradiation reactions ~5) are at equilibrium after 20 hr at room TABLE 1 Composition of the irradiated sample (moles x l0 s) PhBr
o-BrC6H4OH
PhCI
0.63 1 1'
1 1"
PhNH~ 1.03
1 1*
1 1* 1
1
o-CIC~H4OH
Retention ( ~ of total activity)
1 1
0"5 0-5
PhS~Br 0-30 3'93 4.00 9.76 10.33 1.60 3-53
o-S~BrC6H~OH
total
2.61 11.52 12.40 12.10 12.90 4-07 2-25
9.81 62.70 61.20 75.00 76"26 14'87 14'40
* These samples were heated at 90°C (after 20 hr at room temperature) for 1 hr before treatment. temperature. It is also evident that substitution reactions of organically-bound chlorine and bromine by radio-bromine are influenced not only by the nature of the halogen but also by the molecule to which the halogen is attached. This can be seen by observing the retentions of o-8~BrC~HaOH and PhS~Br. The values for the former are essentially the same whether chlorine or bromine is substituted by the recoiling radio-bromine, whilst the amount of phs2Br is higher in the case where it is produced by bromine substitution. This behaviour may be due to electronic factors connected with the rest of the molecule. K~EEERand ANDREWS~ have shown spectrophotometrically that molecular bromine forms a donor-acceptor (charge-transfer) complex with bromobenzene, and that Br2 is probably bound to the bromine atom. WEXLERc7) has observed that atoms often have a large positive charge after neutron capture, owing to internal conversion processes. Since positive bromine ions are much better electron-aeceptors than Br2 molecules, the hypothesis has been advanced ~ that a charge-transfer complex is formed between radio-bromine and the inactive halogen bound to the benzene ring, followed by replacement within the complex. This explains also the negative temperature coefficient of the post-irradiation reaction in bromobenzene35~ Now the observed difference (6) R. M. KEEFER and L. J. ANDREWS,J. Amer. Chem. Soc. 72, 4677 (1950). (7) S. WEXLER and T. H. DAVIES, .1". Chem. Phys. 20, 1688 (1952); S. WEXLER,Phys. Rev. 93, 182 (1954); S. WEXLER and G. R. ANDERSON, J. Chem. Phys. 33, 850 (1960).
Notes
2447
reported here between PhBr and PhC1 towards substitution by radio-bromine is probably due to differences in the stability of the two complexes, the Br-atom in PhBr being a better donor than the CI-atom in PhCI, owingto thelowerpolarizabilityoftheCa~-ClbondcomparedwiththeCar Brbond. A hydroxyl group present in the benzene ring, however, exerts a positive mesomeric effect ( ÷ M), and one would expect both halogens to become more negative, while their difference with respect to polarizability to be less pronounced. This explains both the similarity in the retention of o-~2BrC,;H4OH TABLE 2
Composition of the irradiated sample (molar fraction :< 10~)
Retention (~'~, of total activity)
CBr4
PhNH2
PhOH
phS2Br
Total
1-29 1.40 1.37 1.41 1.30
98.71 88-12 74.52 49.81 41.02
0 10.48 24.10 48.79 57.68
0.42 0.59 0.47 1.01 1.11
3-85 3.68 4.30 4.74 5.81
in the two cases and the higher value of this retention in comparison to that of phs2Br. The difference in the total retention of the two systems (62.7 per cent as compared to 75.0 per cent) also bears out this fact. The hypothesis of complex formation explains another experimental fact, viz. the dramatic fall in retention of all products when aniline is present in the systems. Since aniline is known to be a strong electron-donor, complex formation of radio-bromine on the amino group is favoured to that on the halogen atoms. When complexed in this way, however, radio-bromine can easily abstract hydrogen from the amino group, thus becoming stabilized in inorganic form, and the organic retention is reduced. On the same basis we can understand why in systems containing CBr4 (Table 2) such low retentions are observed, which increase very slowly as the molar fraction of aniline in the system decreases.
Acknowledgement--The authors thank Miss ANNA VASSILAKIfor her assistance in this work. Department of Chemistry Nuclear Research Centre "Democritus" Aghia Paraskevi Attikis, Greece
N. A. KATSANOS A . G . VARV(~.(;LIS
J. lnorg. Nucl. Chem., 1965, Vol. 27, pp. 2447 to 2449. Pergamon Press Ltd. Printed in Northern Ireland
Donor properties of N 1- alkylguanylureas (Received 8 February 1965) IN CONTINUATION of previous work 1 ~ we wish to report here the syntheses of several cobalt (111) complexes containing two Nl-methylguanylurea ligands. Air oxidation of a mixture of aqueous Nl-methylguanylurea and aqueous cobalt chloride in the presence of ammonia provides rose-red crystals of diammine-bis-(Nl-methylguanylurea)cobalt (1II) chloride. The two Nl-methylguanylurea molecules in the univalent complex cation undergo protonation on treatment with dilute acids giving trivalent cations. The two ammonia molecules are believed ~ R. L. DUT'rA and P. RAY, J. Indian Chem. Soc. 36, 499, 567,576 (1959). 121 R. L. DUTTA, Ibid. 37, 499 (1960). ~3) R. L. DUTTA, B. SUR and N. R. SENGUPTA, Ibid. 37, 565, 573 (1960). ~4) R. L. DUTrA and S. LAHmV, lb#L 37, 789 (1960): 38, 689 (1961): 40, 863 (1963).