COMPARISON OF DGꢂ
(GAS PHASE) AND KINETIC ACIDITIES
617
Acid
In conclusion, although the acidities of 9-PhFl and p-
CF3-II are similar, the important proton-transfer step to
form a hydrogen-bonded carbanion and the subsequent
breaking of that weak bond to form a free carbanion in
methanol differ greatly. Studies are continuing to
investigate these reactions.
Kinetic Measurements. Protodetritiation kinetics have
been described previously.6 Protodedeuteriation kinetics
made use of a Hewlett-Packard model 5890 Series II gas
chromatograph with a model 5972 mass-selective
detector using an electron beam energy of 70 eV. Since
there is some separation of the protio- and deuterio-
compounds, the entire GC peak was sampled. Calcula-
tions were made using the parent peak and also the largest
signal which resulted from the loss of a trifluormethyl
group to form the benzylic carbocation. No P-1 peaks
were observed for the fluorinated compounds. The
concentration of sodium methoxide was corrected for
the temperature of each kinetic run.
EXPERIMENTAL
Gas-phase measurements. Gas-phase acidity measure-
ments were performed on an Extrel FTMS 2001 Fourier
transform mass spectrometer. Most of the experimental
techniques used for the measurements of the equilibrium
constants of the reversible proton-transfer reaction (1) are
the same as those described previously.10 Only signifi-
cant changes and additional procedures will be given
here.
Acknowledgements
H.F.K. thanks the donors of the Petroleum Research
Fund, administered by the American Chemical Society,
for support, and M.M. gratefully acknowledges support
by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Science, Sports and Culture,
Japan.
Aꢀ0 AH Aꢀ A0H
ꢃ1
All equilibrium measurements were performed at a 3.0
T magnetic field strength using a cubic (2 Â 2 Â 2 in)
trapped analyzer cell. The typical operating pressures
were 10ꢀ6–10ꢀ7 Torr. The proton-transfer reactions were
initiated by a pulsed electron beam (electron energy ca
0.3 eV, uncorrected, with a pulse width of 5 ms) through
the cell containing ca 10ꢀ7 Torr of methyl nitrite used to
generate methoxide ions. The mass spectra and time plots
were acquired and processed in the Fourier transform
mode. Depending on the pressure of the neutrals, the
proton transfer equilibrium was achieved within 5–10 s
of initiation of the reaction (depending on the pressure of
the neutrals), and the equilibrium constant K for reaction
(1) was evaluated using the expression K = [Aꢀ][A0H]/
[A0ꢀ][AH]. The relative abundances of Aꢀ and A0ꢀ ions
were determined from the relative intensities in the ion
cyclotron resonance (ICR) mass spectra after equilibrium
was attained. The pressures of the neutral reactants were
measured by means of a Bayard–Alpert-type ionization
gauge with appropriate correction factors being applied
to the gauge readings for the different ionization cross-
sections of various compounds.11 The sample inlet
system and the ICR chamber were kept at 50°C.
Literature DG°Acid values for the reference compounds
are9 m-trifluoromethylbenzoic acid 325.3, m-nitrophenol
327.4, malononitrile 328.2, m-cyanophenol 328.9, m-
trifluoromethylphenol 332.5, p-trifluoromethylbenzylni-
trile 332.9, benzoic acid 333.0, p-trifluoromethylaniline
346.1, m-trifluoromethylaniline 349.70 and nitromethane
349.7.
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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 614–617 (1998)