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Gas phase attack of tritiated phenylium ions on byphenyl
Table 1. Radioactivity found in the products Yield %'
Int..I. AppL Radial. lsoc Vot. 36+ No. 1, pp. 87-88, 1985 Pergamon Press Ltd 1985. Printed in Great Britain. 0020-708X/85 $3.00+ 0.00
Product Relative Absolute o-terphenyl 5.9 2.7 m-terphenyL 82.8 38.0 p-terphenyl 10.2 4.7 triphenylene I. 1 0.5 "Standard deviation of data ca. tO%.
Gas Phase Attack of Tritiated Phenyfimn Ions on Byphenyl G. PEREZ and E. POSSAGNO Istituto di Chimica Nucleate det C.N.R., Area della Ricerca di Roma, C.P. 10, 00016 Monterotondo Stazione, Roma,
Results and Discussion
Italy (Received 21 June 1984) Tritiated phenylium ions generated by spontaneous ~-decay of multitritiated benzene have been allowed to react with gaseous biphenyl molecules. Excited terphenylium ions are formed. The fate of such ions is followed by measuring the radioactivity of the final neutral products. Introduction As it is well known, the spontaneous//-decay of a tritium atom bound to a suitable precursor is the basis of the nuclear decay technique for generating free ions. u-:~ In s~ch a way unsolvated phenylium ions can be conveniently generated by the//-decay of tritiated benzene:
[;] I61
These ions can react with organic substrates to give additional products. ~3~If other tritium atoms are contained in the benzene molecules the fate of the phenilium ions can be followed by measuring the radioactivity of the final neutral products. In the present experiment the gas phase reaction of phenylium ions with biphenyl has been studied at 120°C. According to other gas phase ion molecule reactions, the attack of phenylium ions on biphenyl molecules should lead to the formation of excited protonated terphenylium ions which can undergo isomerization, fragmentation, or collisional stabilization before their neutralization to the final labelled products.
The relative and the absolute yields of the products are reported in Table 1. The yield of biphenyl, a possible fragmentation product, could not be calculated, since it may have been formed by a reaction between phenylium ions and benzene molecules. ~'~ The absolute yields have been obtained by comparing the total activity recovered in the products with the maximum theoretical activity that may be incorporated. This can be calculated from the initial activity and the isotopic composition of tritiated benzene samples (t> 2 tritium atoms for a molecule),~'~ the decay rate of tritium, the abundance of phenylium ions, ~5~and the storage period. It seems likely that the first step of the reaction leading to the products, mentioned in Table I, is the addition of a labelled phenylium ion to a biphenyl molecule to give an intermediate that should not differ from that formed by protonation of a terphenyl molecule. Since the heat of formation of phenylium ions is relatively high (270 kcal tool-I) ~mthe addition reaction is highly exothermic, and the complex, so formed, exists as a free ion with an excess of vibrational energy. Therefore, at low pressure, as in our case, it can undergo many hydrogen and phenyl shifts before its collisional thermalization, in agreement with gas phase protonatio# ~ and alkylation m~experiments. In such a way we can explain the relatively large yield of m-terphenyl reported in Table 1, which is in contrast with biphenyl Friedel-Crafts arylation, when mainly p-terphenyl is formed, but in agreement with isomerization experiments. (gAo~ Furthermore, owing to the relatively high excitation energy of the intermediate, the stericaUy unfavoured oterphenyl can also be formed in our case. Finally the mechanism of formation of triphenylene could be due to a Scholl reaction as observed in the protonation of o-terphenyl." t~ Acknowledgement--The Authors are indebted to Mr E. Lilla for his skilful assistance.
Experimental Multitritiated benzene at a specific activity of 1.21 mCi/mg was kindly furnished by Professor M. Speranza. Its preparation, purification, and analysis have been described elsewhere.~+~ The gaseous samples were prepared, introducing the labelled benzene (ca, 0.5 mCi) into outgassed 500 mL Pyrex vessels containing biphenyl (ca. 40 mg) together with 02 (3 ton'), used as a radical scavenger. The vessels were sealed offand stored at 120°C. After the storage period (6 months) they were opened and their contents diluted with known amounts of inactive carriers of interest. The samples were then subjected to a preparative gas-chromatographic separation until a constant value of the specific radioactivity of each component was reached.
References 1. Cacace F. Kinetics o f Ion Molecule Reactions, (Ed. Ausloos P.) (Plenum Press, New York, 1979). 2. Speranza M. Gazz. Chim. [tal. 113, 87 (1983). 3. Angelini G., Fornarini S. and Speranza M. J. Am. Chem. Soc. 104, 4773 (1982). 4. Cacace F., Speranza M., Wolf A. P. and Ehrenkaufer R. J. Labelled Compd. 19, 905 0982). 5. Carlson T. A. J. Chem. Phys. 32, 1234 (1969). 6. Rosenstock H. R., Larkins J. T. and Walker J. A. Int. J. Mass Spectrom. [on Phys. 11, 309 (1973). 7. Perez G. and Lilla E. Int. J. Appl. Radial. Isot. 35, 541 (1984). 87
88
Technical Note
8. Cacace F. and Possagno E. J. Am. Chem. Soc. 95, 3397 (1973). 9. Muller N. and Pingert F. P. J. Am. Chem. Soc. 76, 4770 (1942).
10. Olah G. A. and Meyer M. W. J. Org. Chem. 27, 3682 (1962). l 1. Allen C. F. H. and Pingert F. P. J. Am. Chem. Soc. 64, 1365 (1942).
Int..I. AppL Radial. lsoc Vot. 36+ No. 1, pp. 87-88, 1985 Pergamon Press Ltd 1985. Printed in Great Britain. 0020-708X/85 $3.00+ 0.00
Product Relative Absolute o-terphenyl 5.9 2.7 m-terphenyL 82.8 38.0 p-terphenyl 10.2 4.7 triphenylene I. 1 0.5 "Standard deviation of data ca. tO%.
Gas Phase Attack of Tritiated Phenyfimn Ions on Byphenyl G. PEREZ and E. POSSAGNO Istituto di Chimica Nucleate det C.N.R., Area della Ricerca di Roma, C.P. 10, 00016 Monterotondo Stazione, Roma,
Results and Discussion
Italy (Received 21 June 1984) Tritiated phenylium ions generated by spontaneous ~-decay of multitritiated benzene have been allowed to react with gaseous biphenyl molecules. Excited terphenylium ions are formed. The fate of such ions is followed by measuring the radioactivity of the final neutral products. Introduction As it is well known, the spontaneous//-decay of a tritium atom bound to a suitable precursor is the basis of the nuclear decay technique for generating free ions. u-:~ In s~ch a way unsolvated phenylium ions can be conveniently generated by the//-decay of tritiated benzene:
[;] I61
These ions can react with organic substrates to give additional products. ~3~If other tritium atoms are contained in the benzene molecules the fate of the phenilium ions can be followed by measuring the radioactivity of the final neutral products. In the present experiment the gas phase reaction of phenylium ions with biphenyl has been studied at 120°C. According to other gas phase ion molecule reactions, the attack of phenylium ions on biphenyl molecules should lead to the formation of excited protonated terphenylium ions which can undergo isomerization, fragmentation, or collisional stabilization before their neutralization to the final labelled products.
The relative and the absolute yields of the products are reported in Table 1. The yield of biphenyl, a possible fragmentation product, could not be calculated, since it may have been formed by a reaction between phenylium ions and benzene molecules. ~'~ The absolute yields have been obtained by comparing the total activity recovered in the products with the maximum theoretical activity that may be incorporated. This can be calculated from the initial activity and the isotopic composition of tritiated benzene samples (t> 2 tritium atoms for a molecule),~'~ the decay rate of tritium, the abundance of phenylium ions, ~5~and the storage period. It seems likely that the first step of the reaction leading to the products, mentioned in Table I, is the addition of a labelled phenylium ion to a biphenyl molecule to give an intermediate that should not differ from that formed by protonation of a terphenyl molecule. Since the heat of formation of phenylium ions is relatively high (270 kcal tool-I) ~mthe addition reaction is highly exothermic, and the complex, so formed, exists as a free ion with an excess of vibrational energy. Therefore, at low pressure, as in our case, it can undergo many hydrogen and phenyl shifts before its collisional thermalization, in agreement with gas phase protonatio# ~ and alkylation m~experiments. In such a way we can explain the relatively large yield of m-terphenyl reported in Table 1, which is in contrast with biphenyl Friedel-Crafts arylation, when mainly p-terphenyl is formed, but in agreement with isomerization experiments. (gAo~ Furthermore, owing to the relatively high excitation energy of the intermediate, the stericaUy unfavoured oterphenyl can also be formed in our case. Finally the mechanism of formation of triphenylene could be due to a Scholl reaction as observed in the protonation of o-terphenyl." t~ Acknowledgement--The Authors are indebted to Mr E. Lilla for his skilful assistance.
Experimental Multitritiated benzene at a specific activity of 1.21 mCi/mg was kindly furnished by Professor M. Speranza. Its preparation, purification, and analysis have been described elsewhere.~+~ The gaseous samples were prepared, introducing the labelled benzene (ca, 0.5 mCi) into outgassed 500 mL Pyrex vessels containing biphenyl (ca. 40 mg) together with 02 (3 ton'), used as a radical scavenger. The vessels were sealed offand stored at 120°C. After the storage period (6 months) they were opened and their contents diluted with known amounts of inactive carriers of interest. The samples were then subjected to a preparative gas-chromatographic separation until a constant value of the specific radioactivity of each component was reached.
References 1. Cacace F. Kinetics o f Ion Molecule Reactions, (Ed. Ausloos P.) (Plenum Press, New York, 1979). 2. Speranza M. Gazz. Chim. [tal. 113, 87 (1983). 3. Angelini G., Fornarini S. and Speranza M. J. Am. Chem. Soc. 104, 4773 (1982). 4. Cacace F., Speranza M., Wolf A. P. and Ehrenkaufer R. J. Labelled Compd. 19, 905 0982). 5. Carlson T. A. J. Chem. Phys. 32, 1234 (1969). 6. Rosenstock H. R., Larkins J. T. and Walker J. A. Int. J. Mass Spectrom. [on Phys. 11, 309 (1973). 7. Perez G. and Lilla E. Int. J. Appl. Radial. Isot. 35, 541 (1984). 87
88
Technical Note
8. Cacace F. and Possagno E. J. Am. Chem. Soc. 95, 3397 (1973). 9. Muller N. and Pingert F. P. J. Am. Chem. Soc. 76, 4770 (1942).
10. Olah G. A. and Meyer M. W. J. Org. Chem. 27, 3682 (1962). l 1. Allen C. F. H. and Pingert F. P. J. Am. Chem. Soc. 64, 1365 (1942).