M. Schlosser and E. Castagnetti
FULL PAPER
reaction mixture was poured onto an excess of freshly crushed dry ice. Once
all the carbon dioxide had evaporated, a known amount of a second
™internal standard∫ (benzoic acid) was added. The organic layer was
extracted with an aqueous solution of sodium hydroxide (1.0m, 3 Â 10 mL),
and washed with brine (2 Â 10 mL), before being examined by gas
chromatography. The peak areas of substrates A and B relative to those
of the neutral standard, before and after treatment with sec-butyllithium,
were listed and were used to calculate the rate ratios (kA/kB). The combined
aqueous layers were acidified (to ꢁpH 1) and extracted with diethyl ether
(3 Â 25 mL). The organic phase was treated with ethereal diazomethane
until the yellow color persisted. The concentrations of the AH- and BH-
derived esters (AE and BE) were determined by gas chromatographic
comparison of their peak areas with that of the methyl ester of the alkali-
soluble ™internal standard∫; unequal detector sensitivities for the various
esters were corrected by calibration factors. In each case it was ascertain
that the consumption of substrates A and B was counterbalanced by the
formation of esters AE and BE in corresponding quantities.
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Conditions of gas chromatographic analysis: As a rule, two columns of
different polarity were used to probe the concentrations of substrates and
products. Different conditions had to be employed for the substrates [30 m,
DB-wax, 308C (5 min) ! 1008C (10 min), heating rate 258CminÀ1; 30 m,
DB-23, 308C (5 min) ! 1008C (10 min), heating rate 258CminÀ1] and the
methyl esters products (30 m, DB-wax, 1508C; 30 m, DB-23, 1508C).
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Acknowledgement
This work was financially supported by the Schweizerische Nationalfonds
zur Fˆrderung der wissenschaftlichen Forschung, Bern (grants 2-293-74,
2-467-75 and 20-49'307-96).
[28] The referees suggested corroborating the p-polarization model by a
molecular orbital treatment and quantum chemical calculations.
Although the validity of this advice is unquestionable, we have to
postpone its execution. So far, the computational approach to
molecular properties of organofluorine compound has proved to be
problematic, regardless of whether attempted on the semiempirical
(e.g. ref. [29]) or the ab initio level (F. Cottet, M. Schlosser,
unpublished results, 1999 2001). The trends are always correctly
reproduced. However, the numerical agreement between theory and
experiment is often poor.
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Received: July 25, 2001 [F3443]
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