Brookhaven National Laboratory Annual Report: 1957 Page: 47
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47
Table 3
Toluene + 2-Methylpyrazine
Radiochemical Specific
yield, activity, Replaceable
% m C/mg C hydrogens
Toluene 1.99 0.133 -
o-Xylene 0.96 0.274* 2
m-Xylene 1.35 0.220* 2
p-Xylene 0.46 0.135* 1
Ethylbenzene 0.85 0.122* 3
*Carrier material.
hydrogens available for replacement. A marked
deviation from a statistical result also obtains in
ethylbenzene. The high yield of m-xylene (the
thermodynamically most stable xylene) may sup-
port the hypothesis that the intermediate in the
labeling process has a sufficient lifetime to allow
establishment of partial equilibrium between the
isomers before collapse to the final product occurs.
The low yield in replacement of methyl hydrogens
indicates a clear preference for aromatic hydrogen
substitution over side chain aliphatic hydrogen
substitution.
Degradation of the re-entry product, toluene,
and the synthesis products, o-xylene and ethylben-
zene, gave the results indicated in Table 4. This
supports the evidence obtained in the acetamide
system and presented in an earlier report. In re-
entry products (e.g., labeled toluene from toluene,
and labeled acetamide from acetamide) whererandom replacement would be expected on statis-
tical grounds, such randomness is not observed.
Indeed the activity in the methyl group is 23%
lower than expected for random replacement. In
the synthesis products, o-xylene and ethylbenzene,
where specific labeling might be expected, some
smearing of the label is observed. It is, however,
of interest to note that in this system more of the
activity is found in the expected position than was
found in the expected position in propionamide
obtained from acetamide (CH,-CH2-COOH;
52%, 24%, 24%, respectively, after hydrolysis).
The significance of these degradations in rela-
tion to the use of the recoil method for labeling
purposes is clear. The user of these compounds for
trace purposes must be aware, insofar as one can
generalize from available evidence, that neither
random labeling nor specific labeling is found in
those cases where such labeling might be expected.
The deviation from a statistical distribution again
emphasizes the contention that chemical mecha-
nisms are operative in deciding the positional fate
of the carbon-14. A purely statistical picture for
atom replacement is not tenable.
These results closely parallel those reported for
the synthesis product, toluene, obtained from ben-
zene. A possible mechanism in this case is the re-
versible formation of the cycloheptatrienylium
(tropylium) ion. Sufficient energy for such a re-
action is probably available, since theoretical con-
siderations indicate an energy of about 50 ev
associated with the region in which the motion
of the carbon- 14 becomes diffusive. Another pos-
sible mechanism would be the formation of aTable 4
Activity Distribution in Products from Toluene Irradiation
Toluene o-Xylene Ethylbenzene
CH3
CH, CH2- CH,
CH3
% Calculated 85.7 14.3* 0 100 0 0 100
% Found 89 11 15 85 7 13 80
*If completely random.
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Brookhaven National Laboratory. Brookhaven National Laboratory Annual Report: 1957, report, December 1957; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc67224/m1/89/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.