The Journal of Organic Chemistry
Article
greater s character in the vinyl CH bond makes this bond more
acidic.
Despite the qualitative character of the ranking of CH
ACKNOWLEDGMENTS
■
As reflected in coauthorships of prior papers, many Oberlin
College students participated in the synthetic work reported
here. Their contributions are gratefully acknowledged. Con-
tributors to previously unpublished work were A. Sonan
Osmani, Robin A. Holmes and Yuhua Lu. N.C.C. was
supported by a Dreyfus Senior Scholar Mentor grant and by
Oberlin College. The Beowulf computer cluster at Oberlin
College is supported by NSF Grant No. 0420717. Allocation of
time on the Ohio Supercomputer is gratefully acknowledged.
acidities in Table 1, it is possible to bracket the pK ’s by
a
reference to known species. All of the weakly acidic CH bonds
in Table 1 are significantly more acidic than ethylene (pK =
a
1
1
4
4) and less acidic than acetylene (pK = 24). Acetylene
a
formed in the exchanges of 1-bromoethylene and 1-fluoro-
ethylene exchanged quickly. In addition, Miller and Lee found
5
34 for the pKa of cis-1,2-dibromoethylene in methanol.
Although the pK in methanol should be larger than in water,
a
the value in water will not differ greatly. All considered, the
REFERENCES
species ranked in Table 1 should have pK ’s in the range of 40
■
a
to 30.
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EXPERIMENTAL SECTION
■
Details of the conditions for the various exchange reactions can be
found in the references given in Table 1. Here some general
descriptions will suffice. All reactions were carried out in flame-sealed
tubes. Of course, the attack of base on the glass lowers the base
concentration over time. To slow the rate of attack of base on the
reaction vessel in long exchanges, quartz tubes (with graded
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temperature to remove the water that would degrade the isotopic
(
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nitrogen temperature. Careful freezing of the water prevented breaking
the reaction vessel through formation of a solid plug of ice. The
2
exchanged material was typically distilled from the D O solution at
2
(
room temperature and then dried by distillation through a column
containing phosphorus pentoxide. Repeated exchanges were used in
most cases to obtain a high isotopic purity. The extent of exchange was
assessed by gas-phase infrared spectra. Exchanges were done with
isomeric mixtures unless otherwise indicated.
(
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to be planar even though planarity was not enforced. For the
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ASSOCIATED CONTENT
■
(
25) Craig, N. C.; Oertel, C. M.; Oertel, D. C.; Tubergen, M. J.;
*
S
Supporting Information
Lavrich, R. J.; Chaka, A. M. J. Phys. Chem. A 2002, 106, 4230−4235.
Energies computed with the three QC models, MP2/cc-pVTZ,
MP2/cc-pVQZ, and MP2/aug-cc-pVQZ, and energy differ-
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.:
Robb, M. A.; Cheeseman, J. R.; Montgomery Jr., J. A.; Vreven, T.;
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09, revision C.01; Gaussian, Inc., Wallingford, CT, 2004, 2011.
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AUTHOR INFORMATION
■
*
(28) Craig, N. C.; Piper, L. G.; Wheeler, V. L. J. Phys. Chem. 1971,
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(
29) Wiberg, K. B.; Rablen, P. R. J. Comput. Chem. 1993, 14, 1504−
1
518.
30) The percent of s and p character for the bonds of the anion were
Notes
(
The authors declare no competing financial interest.
computed at the MP2/cc-pVQZ level. G09 Rev C.01 failed to give this
1
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dx.doi.org/10.1021/jo402703v | J. Org. Chem. 2014, 79, 1729−1735