Smith et al.
1151
sels was added 1 mL of temperature-equilibrated dry tert-
butyl alcohol and the solution was titrated to determine the
base concentration.
required amount (approximately 20 mg) of the 1-choro-2-
phenyl-1,2-d2 ethane diastereomers was weighed into the re-
action vessel and temperature equilibrated. The reaction was
started by quickly adding the base solution to the reaction
vessel. After ten half-lives (>99.9% reaction) the reaction
was quenched by adding cold water and the organic products
were extracted three times with pentane. After the combined
pentane extracts had been washed with water and dried over
anhyd magnesium sulphate, the pentane was removed at
room temperature in a rotary evaporator. An NMR analysis
of the residue in 1 mL of chloroform-d gave the composition
of the product.
The reactions were started by adding 1 mL of a tempera-
ture equilibrated solution of the substrate (approximately 5 ×
10–5 M, but accurately known) in dry tert-butyl alcohol to
the reaction vessel with a calibrated syringe. At intervals of
several minutes, 1 mL aliquots of the reaction mixture were
withdrawn with a syringe and quenched in 20 mL of the ap-
propriate concentration of hydrochloric acid in distilled 95%
ethanol. The quenched aliquot was diluted to 25 mL with
95% ethanol and the UV spectrum was recorded. Best re-
sults were obtained by measuring against a reference sample
of ethanol or quenching solution, subtracting the absorbance
at a point on the baseline of the product spectrum from the
absorbance at λmax for the product. Ten aliquots were nor-
mally taken from each vessel over two half-lives.
Acknowledgment
The authors gratefully acknowledge the financial support
provided by the Natural Sciences and Engineering Research
Council of Canada (NSERC).
After ten or more half-lives, samples were taken and
quenched to obtain a value for the product absorbance when
the reaction was complete (OD∞). The pseudo first-order rate
constant was obtained from the slope of the plot of ln (OD∞ –
ODt) vs time (t).
References
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Cuvette method
The base solution was prepared by diluting the stock base
solution using the method described above. If required, 18-
crown-6 was first weighed into a 50 mL volumetric flask
and 3 mL of the base solution were pipetted into each of
seven oven dried cuvettes with tightly fitting stoppers. The
cuvettes were left in the thermostated cell holder in the
spectrophotometer for at least 20 min to allow them to reach
thermal equilibrium. One cell was used as the reference. The
reactions were started by adding between 5–10 µL of sub-
strate solution (typically 5 × 10–3 M in tert-butyl alcohol) to
the cuvette using a syringe and inverting the cuvette several
times to ensure mixing.
The computer-controlled spectrophotometer measured ab-
sorbencies for the samples at predetermined intervals over at
least two half lives. Rate constants were obtained from be-
tween 25–40 data points using either the above OD∞ method
or from Guggenheim plots.
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Secondary alpha deuterium kinetic isotope effects
When the water bath method was used to determine the
secondary alpha deuterium kinetic isotope effect, the rate
constants for six reactions were determined simultaneously
in the same constant temperature bath; three reactions used
the deuterated substrate and three used the nondeuterated
substrate. When the cuvette method was used, the rate was
determined for three deuterated and three nondeuterated sub-
strates simultaneously in the thermostated cell holder. The
secondary alpha deuterium kinetic isotope effect was ob-
tained by dividing the average rate constants for the
nondeuterated and deuterated substrates.
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Determination of the elimination stereochemistry
A stock potassium tert-butoxide solution was diluted in a
volumetric flask with dry tert-butyl alcohol, temperature
equilibrated in a constant temperature bath, and filled to the
mark with dry tert-butyl alcohol. This stock solution was at
least twenty times more concentrated than the substrate. The
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