α-Fluoroalcohols: Synthesis and Characterization of Perfluorinated Methanol, Ethanol and n-Propanol, and their Oxonium Salts

Article abstract of DOI:10.1002/chem.201804306

The thermally unstable, primary perfluoroalcohols, CF₃OH, C₂F₅OH, and nC₃F₇OH, were conveniently prepared from the corresponding carbonyl compounds in anhydrous HF solution. Experimental values for the reaction enthalpies and entropies were derived from the temperature dependence of the R_fCOF+HF?R_fCF₂OH (R_f=F, CF₃, CF₃CF₂) equilibria by NMR spectroscopy. Electronic structure calculations of the gas-phase and solution reaction energies, gas-phase acidities and heats of formation were carried out at the G3MP2 level, showing that these compounds are strong acids. Protonation of these alcohols in HF/SbF₅ produced the perfluoroalkyl oxonium salts R_fCF₂OH₂⁺SbF₆^?.

Full text of DOI:10.1002/chem.201804306

A Journal of
Accepted Article
Title: α-Fluoroalcohols: Synthesis and Characterization of Perfluoro-
methanol, -ethanol and -n-propanol, and their Oxonium Salts
Authors: Amanda F Baxter, Jonas Schaab, Joachim Hegge, Thomas
Saal, Monica Vasiliu, David A Dixon, Ralf Haiges, and Karl O.
Christe
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To be cited as: Chem. Eur. J. 10.1002/chem.201804306
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α-Fluoroalcohols: Synthesis and Characterization of Perfluoro-methanol,
-ethanol and -n-propanol, and their Oxonium Salts
A. F. Baxter, J. Schaab, J. Hegge, T. Saal, M. Vasiliu, D. A. Dixon, R. Haiges, and K. O. Christe*
Dedicated to Professor H. J. Vollmann on his 85th birthday
equivalent with neat or solutions of acylfluorides and obtained
for COF2, CF3CF2CFO and (CF3)2CFCFO products which they
Abstract: The thermally unstable, primary perfluoroalcohols,
CF3OH, C2F5OH, and n-C3F7OH, can be conveniently
prepared from the corresponding carbonyl compounds in
anhydrous HF solution. Experimental values for the reaction
formulated as adducts of the corresponding perfluoroalcohols
1
with N(CH3)3 and characterized them by their 19F and H NMR
spectra. Because perfluoroalcohols are strong acids and
enthalpies and entropies were derived from the temperature
triethylamine is a strong base, one might expect the products to
dependence of the RfCOF + HF  RfCF2OH (Rf = F, CF3,
be the ionic [HN(CH3)3]+[RfCF2O]- salts instead. Unfortunately,
CF3CF2) equilibria by NMR spectroscopy. Electronic structure
the 19F NMR evidence is not clear cut. CF3OH resonates at -54.5
calculations of the gas-phase and solution reaction energies,
ppm and CF3O- at -20 ppm[17], while Cheburkov’s product
gas-phase acidities and heats of formation were carried out at
shows a signal at -33.18 ppm. The possibility of the existence of
the G3MP2 level, showing that these compounds are strong
an equilibrium between a donor-acceptor and an ionic salt has
acids. Protonation of these alcohols in HF/SbF5 produces
already been suggested,[14] but more work remains to be done to
+
-
perfluoroalkyl oxonium salts, RfCF2OH2 SbF6 .
establish the exact nature of the products. Nevertheless, the
alternative concept of stabilizing the perfluoroalcohols by
formation of their anions and shifting their equilibria with the
acylfluorides to the right by conversion to the corresponding
ethers is interesting. In the present study, we report the synthesis
and properties of perfluoroethanol and perfluoro-n-propanol,
and a re-investigation of the anomalous temperature dependence
reported previously[10] for the equilibrium (2), which could not
be explained satisfactorily.
Under normal conditions, primary perfluorinated alcohols are
unstable due to facile HF elimination.[1] Despite this instability,
the simplest primary perfluorinated alcohol, CF3OH, is
generated in the atmosphere as a by-product of the degradation
of hydrofluorocarbons,[2] and the mechanism of the atmospheric
formation of CF3OH has been established.[3-6]
In the laboratory, perfluorinated alcohols can be generated
by the addition of HF across the C=O double bond of a
perfluorinated carbonyl compound, as demonstrated in 1961 by
Carbonyl fluoride was added to liquid anhydrous HF in
an FEP NMR tube, and the 19F NMR spectra were recorded at
various temperatures after equilibrium had been achieved. The
equilibrium concentrations of carbonyl fluoride and
trifluoromethanol were measured by area integration of the 19F-
Andreades
and
England
who
added
HF
to
perfluorocyclobutanone to give perfluorocyclobutanol [Eq.
(1)].[7] Recently, we reproduced this synthesis and reported the
first crystal structure of an α-fluoroalcohol.[8]
1
NMR signals, using the temperature-sensitive shift of the H-
signal of methanol.[18] for measuring the temperature.
c-C4F6O + HF → c-C4F7OH
(1)
In 1977, trifluoromethanol was synthesized by Seppelt.[9]
He used a low-temperature dichlorine elimination from two
starting materials containing a positively and a negatively
polarized chlorine atom. The reaction was done in three steps
and involves shock-sensitive CF3OCl as an intermediate.
It was found that the molar ratio of CF3OH/COF2
increases with decreasing temperature from 0.37 at 10 °C to
2.52 at -62 °C, and that the previously reported anomaly[10] was
due to the long time required to reach equilibrium at low
temperatures. To ascertain that the system had truly reached
equilibrium, each temperature point was approached from above
and below the desired temperature and, only when the points
converged within an acceptable margin of experimental error,
these data points were accepted. At higher temperatures,
equilibration was fast, and at temperatures ≥ 0 °C less than ten
minutes were required. At temperatures ≤ -50 °C however,
reaching equilibrium required 24 h or longer. Even at the lowest
measured temperatures we still observed signals due to COF2
and it was not possible to shift the equilibrium completely to the
side of the alcohol. Higher conversion to CF3OH could be
achieved by continuously removing the alcohol from the
This synthesis is inefficient and complicated. In 2007, we
published a more convenient access to CF3OH,[10] exploiting the
equilibrium [Eq. 2] between HF and COF2.
COF2 + HF  CF3OH
(2)
Due to facile HF elimination,[11] primary and secondary α-
perfluoroalcohols are thermally unstable at room-temperature.
Only tertiary β-perfluoroalcohols, such as perfluoro-t-butanol,
are significantly more stable due to the absence of α-fluorine
atoms.[12] As a consequence, perfluoroethanol and perfluoro-n-
propanol were until now unknown, and only one paper on a
low-temperature 13C NMR study of perfluoro-iso-propanol has
appeared.[13] There has been one additional paper been
published in 2002 by Cheburkov and Lillquist.[14] Although the
title of their paper, “Perfluoroalcohols”, implies the synthesis of
the alcohols, the evidence for the presence of the free alcohols
was not firmly established. Since perfluoroalcohols are strong
acids, as shown below, they are well-known[15-17] to form stable
RfCF2O- salts. This fact has been cleverly exploited by
Cheburkov and Lillquist. They have reacted a [HN(CH3)3]+F-
+
equilibrium by either conversion into CF3OH2 salts by the
addition of strong Lewis acids or trapping it as an ether by a
Lewis acid-catalyzed reaction with an alkyl fluoride.[10]
Because a very large excess of HF was used in our study,
the change from a 75-fold excess to a 116-fold excess of HF had
little impact on the amount of CF3OH formed in the
equilibrium. Also, solubilities do not influence the equilibrium,
if HF is used in a large enough excess as a solvent. Furthermore,
1 of 7
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no evidence was found in the present study that the previously
suggested[10] increase in the self-association of HF at lower
temperatures plays a significant role in the slow formation of
CF3OH.
The successful synthesis of CF3OH from COF2 and HF
prompted us to extend this approach to the longer chain primary
perfluoroalcohols, pentafluoroethanol and heptafluoro-n-
propanol. Both compounds can be obtained in the same manner
as trifluoromethanol, although with lower yields. As for
CF3OH, the concentrations of C2F5OH and n-C3F7OH increased
with decreasing temperature and decreasing chain length (Figs.
S1 and S2). The maximum observed RfCF2OH/RfCOF molar
ratios were 0.993 and 0.0809 for Rf = CF3 and CF3CF2,
respectively, compared to 2.52 for CF3OH. All three alcohols
were characterized in anhydrous HF-solution by 19F and 13C
NMR spectroscopy. The proton resonances for the hydroxyl
groups of the primary perfluoroalcohols are expected to occur in
the range of 7 to 10 ppm, based upon the value of 8.65 ppm
observed for neat CF3OH.[9] However, the resonance of HF falls
also into this range and the alcohol undergoes a fast exchange
with HF on the NMR timescale, preventing the direct
observation of the hydroxyl proton.
Figure 1. van’t Hoff plots for the equilibria RfCOF + HF  RfCF2OH
(Rf = F, CF3, CF3CF2). Equations for the lines are as follows: R = F: y
= 1798.3x – 11.237 R2 = 0.9913; R = CF3: y = 1634.4x – 11.524 R2 =
0.99284; R = CF3CF2: y = 1087.7x – 11.481 R2 = 0.98735.
Table 1. Summary of the thermodynamic quantities derived from the
van‘t Hoff plots for the RfCOF + HF  RfCF2OH (Rf = F, CF3,
CF3CF2) equilibria. a) enthalpies of reaction (kcal/mol). b) entropies of
reaction (cal/molK) c) Gibbs free energies at 298 K (kcal/mol).
Using the known initial concentrations of RfCOF (Rf = F,
CF3, CF3CF2), the known amounts of HF used and the relative
equilibrium concentrations from the integrated 19F NMR
signals, the equilibrium constants Keq were calculated for each
temperature. These equilibrium constants were then used to
construct van‘t Hoff plots to determine the enthalpies and
entropies of reaction[19] according to equation (3).
Rf = F
-3.6
-22.3
Rf = CF3
-3.2
-22.9
Rf = CF3CF2
-2.2
-22.8
∆ꢋa
∆ꢉb
∆ꢊc (298 K)
3.1
3.6
4.6
The observed ∆ꢋ value of -3.6 kcal/mol for the formation
of CF3OH from COF2 in HF solution is in good agreement with
the most reliable calculated gas-phase values of -5.7 kcal/mol[20]
and -6.5 kcal/mol.[11] These experimental and computational
Δꢅ
ꢆꢇ
ΔS
ꢆ
ꢀꢁꢂꢃꢄ = −
+
(3)
The van‘t Hoff plots for the formation of the three alcohols are
shown in Figure 1, and the thermochemical quantities derived
from these are listed in Table 1. The reaction enthalpies from
these plots are slightly exothermic, and the entropy values are
negative, as expected from the formation of one alcohol
molecule from two molecules of reactants, making the
−ꢈ∆ꢉ term positive and therefore the free energy values, ∆ꢊ =
∆ꢋ − ꢈ∆ꢉ, will pass through zero and become increasingly
more positive with increasing temperature, consistent with the
favored formation of the alcohols at low-temperature and their
decomposition at higher temperatures. The decomposition of
gaseous CF3OH is slow and has a high activation energy barrier
of about 43-45 kcal/mol, but is lowered substantially in the
presence of HF, other CF3OH molecules, or H2O.[2, 11]
values are consistent with an experimental value of -2.8±1.1
1.7
kcal/mol from a photoionization study.[21] The enthalpies of
reaction become less negative with increasing chain length while
the entropy term is very similar in all three cases. Therefore, ∆ꢊ
becomes more positive explaining our observed decrease in
alcohol formation with increasing chain length at all
temperatures.
When the HF solutions of these perfluoroalcohols are
acidified by the addition of strong Lewis acids, such as SbF5, the
alcohols are protonated to the corresponding oxonium salts [Eq.
4 and Tables 2-4]. Since the identification and characterization
RfCF2OH + HF + SbF5 → [RfCF2OH2]+SbF6
(4)
-
of the alcohols and their corresponding oxonium salts relied on
NMR spectroscopy and because the differences in their NMR
parameters are quite small, it is important to measure these
spectra with high accuracy, i.e., on the same instrument under
identical conditions and using the same methods for
referencing.[22] In the present study, we have measured the 19F
and 13C NMR spectra of trifluoromethanol, pentafluoroethanol,
heptafluoro-n-propanol and their corresponding oxonium
cations in anhydrous HF solution, utilizing the substitution
method with 80% CFCl3 in 20% CDCl3 as the external standard.
The results show that protonation of the -OH group influences
the signals of the fluorocarbon backbones only very weakly.
The most pronounced changes are slightly increased shieldings
of the 19F and 13C signals of the α-CFn groups and increases in
The slopes of the van’t Hoff plots depend only on the
temperature dependence of the equilibrium constants. Since the
equilibrium concentrations were measured for each compound
on the same sample, most systematic errors canceled out.
Therefore, we expect the values of ∆ꢋ to be quite accurate. The
∆ꢉ values, obtained from the extrapolated intercepts of the
curves with the ordinate, are much less accurate since any
systematic errors will not necessarily cancel out.
2 of 7
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Table 4: The 19F and 13C-NMR shifts and the corresponding coupling
constants of perfluoro-n-propanol, CF3CF2CF2OH, and its protonated
+, recorded
their 1JCF coupling constants upon protonation. The increases in
the coupling constants upon protonation agree with previous
cation, CF3CF2CF2OH2
respectively .
in HF solution at -55 °C and -50 °C,
+ [10]
experimental and computational results for CF3OH/CF3OH2 .
Observation of the proton chemical shift was complicated by the
alcohols undergoing rapid exchange with the HF solvent and by
the oxonium salts exchanging with the alcohols and HF.
Furthermore, the chemical shift of the proton resonance of HF
depends strongly on the temperature and the acidification with
SbF5. Therefore, no conclusions can be drawn concerning the
proton shifts of these alcohols and their oxonium salts in HF
solution.
*s = singlet, t = triplet, q = quartet,
We have also investigated to what extent the protonation
of CF3OH shifts the COF2 + HF  CF3OH equilibrium to the
right. Although a complete conversion of COF2 to CF3OH2+ at -
60 °C was not observed under our reaction conditions, the
concentration of unreacted COF2 was reduced from 27.6% to
5.7% upon addition of a five-fold molar excess of SbF5 to the
alcohol solution. The difficulty to observe a complete shift of
the equilibrium to the right might be caused by the conversion
of COF2 to CF3OH being a slow process at low temperature,
compared to the fast protonation reaction of the alcohol.
In the protonation reactions of the alcohols, only one set
of signals was always observed. This fact might be explained by
the alcohol and its oxonium salt undergoing a fast exchange on
the NMR timescale. This was verified experimentally by using
only a half molar amount of SbF5 in the protonation reaction. In
the absence of a fast exchange, two sets of signals, one for the
alcohol and one for the oxonium salt, should be observed while,
for a fast exchange, only one set of signals should be seen with
chemical shifts and coupling constants intermediate between
those of the pure alcohol and the pure oxonium salt. The
experimental data, given in the last column of Table 2, show the
latter and are thus proof for rapidly exchanging CF3OH and
The 19F spectra of CF3OH, C2F5OH, and C3F7OH are very
similar to those of CF4, C2F6, and C3F8, respectively,[23]
demonstrating that an OH group behaves as a pseudo-fluorine
atom.[24] Furthermore, the signals of -CF2OH groups do not
occur in the typical CF2 region of 120-130 ppm but closer to the
CF3 region at 80-90 ppm. The 19F-NMR-signals of perfluoro-n-
propanol show interesting through-space coupling[25] between
+
+
CF3CF2CF2OH
-CF2OH -80.64 (q)
CF3CF2CF2OH2
CF3OH2 .
δ 19F obs [ppm]*
δ 13C obs [ppm]
1JCF (1JFC) [Hz]
2JCF [Hz]
-81.82 (q)
-84.51 (t)
-132.78 (s)
115.54 (t of t)
117.40 (q of t)
108.90 (t)
272.8 (275)
284.3 (286)
(264.7)
Table 2: Summary of the 19F-, 13C-, and 1H-NMR shifts and 1JCF coupling
-CF3
-84.43 (t)
+
constants of CF3OH and CF3OH2 .
-CF2-
-132.84 (s)
-CF2OH 115.73 (t of t)
CF3OH
(neat)a
CF3OH (in
HF)b
CF3OH2+ (in
HF)c
CF3OH /
CF3OH2+ (in HF)d
-CF3
-CF2-
117.72 (q of t)
109.22 (t)
δ 19F [ppm]
δ 13C [ppm]
1JCF [Hz]
-54.5 (s)
118 (q)
256
-57.8 (s)
120.92 (q)
254.9
-59.37 (s)
118.40 (q)
270.0
-58.70
119.84 (q)
262.8
-CF2OH 269.4 (270.1)
-CF3
-CF2-
284.5 (286.7)
(263.1)
-CF2OH 30.7
31.3
δ 1H [ppm]
8.65 (s)
(9.39)e
(10.29)f
(10.98)g
-CF3
32.4
40.6
8.0
32.5
39.1
8.0
8.0
-CF2-
-CF2OH
-CF3
-
a) Data from ref 9; b) this study, recorded at -60 °C; c) data for the SbF6 •nSbF5
salt, recorded at -60 °C, using a large excess of SbF5; d) using 0.5 equivalents of
SbF5; e) concentration-dependent exchange of HF and CF3OH; the chemical shift
of pure HF was found to be strongly temperature dependent and was observed at
8.77 ppm and 9.46 ppm at 25 °C and -60 °C, respectively; f) concentration- and
temperature-dependent exchange of HF and CF3OH2+; the addition of SbF5 to
neat HF resulted in a downfield shift of its proton resonance from 8.77 ppm to
4JFF [Hz]
8.0
the terminal CF3- and the CF2OH-groups with 4JF-F = 8.0 Hz. By
analogy with closely related CF3-CF2-CF3,[26] the internal CF2-
group couples only weakly to the fluorine atoms of the terminal
CF3- and CF2OH-groups, and therefore, no additional fine
structure was observed. In C3F8, the JF-F is only 0.7 Hz, while
4JF-F is 7.31 Hz.[26]
The above NMR data show the following general trends.
1) The 19F chemical shifts of the α-perfluoroalcohols mirror
closely those of the corresponding perfluorocarbons,[23,26]
showing the OH group can be treated as a pseudo-fluorine atom.
9.23 ppm and from 9.46 ppm to 10.03 ppm at 25 °C and -60 °C, respectively; g)
concentration-and temperature-dependent exchange of HF,CF3OH and CF3OH2
3
+
.
Table 3: The 19F and 13C-NMR shifts and the corresponding coupling
constants of perfluoroethanol, CF3CF2OH, recorded at -60 °C in HF, and of
+
spectra were recorded
its protonated cation. The CF3CF2OH2
for a 1:3 molar mixture of CF3COF and SbF5 in HF.
at -50 °C
+
CF3CF2OH
+
CF3CF2OH2
2) As expected, the influence of the -OH or -OH2 group is the
δ 19F obs [ppm]*
δ 13C obs [ppm]
1JCF [Hz]
-CF2-
-CF3
-86.00 (q)
-89.89 (t)
-87.50 (q)
-89.49 (t)
strongest on the α-fluorine atoms, however the observed
changes are rather subtle. Protonation of the alcohol increases
1
-CF2-
-CF3
114.39 (t of q)
117.25 (q of t)
114.04 (t of q)
116.63 (q of t)
the one-bond coupling constant, JCF, only by 3-15 Hz. 3) The
proton spectra are not useful for distinguishing the free alcohols
from the oxonium salts due to exchange reactions with each
other and the HF solvent and a strong temperature and
concentration dependence.
In addition to the experimental study described above,
electronic structure computations were carried out to gain a
better understanding of the nature of these perfluoralcohols.
Although numerous theoretical studies have previously been
carried out on free gaseous CF3OH,[2-6,16,17,27-38] very little is
-CF2-
-CF3
268.3
282.7
273.6
283.5
2JCF [Hz]
3JFF [Hz]
-CF2-
-CF3
-CF2-
-CF3
43.0
46.6
2.5
43.8
45.1
2.5
2.5
2.5
*s = singlet, t = triplet, q = quartet,
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known on its condensed phase chemistry and particularly, on
the higher perfluoroalcohols. Therefore, extensive calculations
were carried out at the G3MP2[39] level of theory for all three
fluoroalcohols in the gas phase and in aqueous solutions. We
have calculated the enthalpies and free energies of formation,
reaction and decomposition, the entropies, the decomposition
barriers, and gas-phase acidities. These results together with a
description of the computational methods used have been
compiled in the Supplementary Information Section, and only
the most important findings are summarized below.
chain length and they are all more negative than experiment by
3 – 4 kcal/mol. There is only a small effect of solvation on the
reactions making the free energy values slightly more positive at
298 K. The calculated ΔH values confirm that the reaction
enthalpies for the formation of these alcohols are slightly
exothermic, however at 298 K, the T∆S term dominates, and the
free energies become endothermic. This is in line with the
observations by us and others that these species are unstable at
ambient temperature. Furthermore, the data also show that even
small changes in ∆Gaq are sufficient to significantly shift the
equilibrium.
The calculated gas-phase acidities (Table 5) show that
these perfluoroalcohols are reasonably strong acids in the gas
Table 7: Reaction energies for the reactions in kcal/mol at the G3MP2
Level at 298 K, with solvent effects calculated at
B3LYP/DZVP2/COSMO in water as solvent.
Table 5: Gas-phase acidities (HA → A- + H+) in kcal/mol of the
perfluoroalcohols, calculated at the G3MP2 level, their solvation
energies, and pKa values in H2O at 298 K.
a
Reaction
∆Hg
∆Gg
∆Gsolvation
∆Gaq
∆HHF
CF3OH
329.2
321.9
1.9
CF3CF2OH
CF3CF2CF2OH
-6.6
-7.7
3.5
3.3
2.6
2.9
6.1
6.2
-3.6
-3.2
COF2 + HF → CF3OH
∆Hgas
∆Ggas
325.2
322.9
CF3COF + HF → C2F5OH
-6.7
4.2
2.4
6.6
-2.0
317.9
315.2
C2F5COF + HF → C3F7OH
a) Experimental values for the formation enthalpies in HF solution from Table 1.
2.0
1.5
∆Gaq
The decomposition barriers of the perfluoroalcohols are
strongly influenced by self-association, solvation effects, and
adduct formation. For example, the calculated free energy
barrier for the unimolecular decomposition of gaseous CF3OH
of 45.9 kcal/mol is lowered to 32.1 kcal/mol by one HF and to
28.2 kcal/mol by 2 HF molecules. If H2O and OH are also
present, the decomposition barrier is reduced to 11.0
kcal/mol.[20] Our barrier calculations at the G3MP2 level for
CF3OH, C2F5OH, and C3F7OH confirm that the free energy
barriers of 44-47 kcal/mol for unimolecular decomposition can
be lowered to 27–28 kcal/mol by adduct formation with two
molecules of HF. These barriers are still too high when
compared to the experimentally observed thermal stabilities of
these compounds; and the true barriers in HF solution are
expected to be around 10 kcal/mol. The use of an implicit
solvent model based on aqueous solutions led to a decrease in
the barrier over the gas phase values. Hence, the HF solvent is
playing an additional role in lowering the reaction barrier,
perhaps by serving as a proton transfer agent and further
catalyzing the decomposition reaction.
pKa
1.4
1.5
1.1
phase, with perfluoro-n-propanol having a gas phase acidity
comparable to that of H3PO4.[40] The alcohols become more
acidic in the gas phase with increasing chain length, although
differential solvation effects can change this trend slightly in
aqueous solutions, as observed many years ago for the proton
affinities of the methyl substituted amines. With pKa values
ranging from 1.1 to 1.5 in aqueous solution, they are predicted
to be surprisingly strong acids, stronger than HF (pKa = 3.17),
HN3 (4.72), H3PO4 (2.12), acetic acid (4.76), and HCN (9.4),
although not quite as strong as trifluoroacetic acid (-0.25).
The heats of formation of the free alcohols, calculated at
the G3MP2 level, are summarized in Table 6. The value of -
217.1 kcal/mol for ∆Hf(298K) of CF3OH is in very good
Table 6: Enthalpies of formation in kcal/mol of gaseous
perfluoromethanol, perfluoroethanol, and perfluoro-n-propanol,
calculated at the G3MP2 Level at 0 K and 298 K.
CF3OH
-215.1
-217.1
CF3CF2OH
-315.4
CF3CF2CF2OH
-414.7
The proton affinities of the three perfluoroalcohols
increase slightly from trifluoromethanol to heptafluoro-n-
propanol and all are, as expected, weaker bases than methanol
(Table 8). Again, the agreement between calculated and the
available experimental values[31,41] is very good.
∆fH 0
∆fH 298
-318.0
-417.9
agreement with the experimental values of -220.7 ± 3.2
kcal/mol[31] and ∆Hf(298K) = -217.2 ± 0.9 kcal/mol,[21] and a
calculated value of -217.8 kcal/mol at the CCSD(T)/CBS
level.[11]
Table 8: Proton affinities at 298 K in kcal/mol calculated at the G3MP2
level.
ΔH
ΔG
Expt PA (ΔH)
180.2a
The reaction energies for the formation of the three
alcohols from the corresponding acyl fluoride and HF were
calculated at the G3MP2 level with solvent effects in H2O (H2O
and HF have comparable dielectric coefficients) obtained at the
B3LYP/DZVP2/COSMO level (Table 7) and are in good
agreement with the experimental values derived from the van’t
Hoff plots (Table 1). The order of the gas-phase ΔHrxn values
differs from the experimental values in HF solution as they do
not show a decrease in the reaction enthalpy with increasing
CH3OH
CF3OH
180.5
147.7
148.1
149.1
173.3
140.7
141.3
142.3
151.1 ± 1.7b
CF3CF2OH
CF3CF2CF2OH
a) ref 41; b) ref 31.
4 of 7
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In conclusion, the previously reported convenient access
to CF3OH from carbonyl fluoride and HF was extended to the
syntheses of pentafluoroethanol and heptafluoro-n-propanol.
Although CF3OH had previously been studied in much detail
due to its atmospheric and stratospheric significance, no data
were available in the literature on its higher analogues. It has
now been shown, that the corresponding pentafluoroethanol and
heptafluoro-n-propanol can also be prepared in the same
manner, albeit with lower yields, and exhibit similar
characteristics. They are thermally unstable molecules which
decompose under ambient conditions back to the acyl fluorides
and HF. The equilibria can be shifted towards the alcohol by
protonation to the perfluorinated oxonium salts, RfOH2 SbF6 .
All new compounds were characterized by multinuclear NMR
spectroscopy and electronic structure calculations, and the
previously reported anomalous temperature dependence of the
COF2/HF equilibrium[10] was found to be caused by its slowness
at low temperature. From the temperature dependence of the
equilibria and van’t Hoff plots, the reaction enthalpies were
experimentally determined and agree well with the electronic
structure calculations in the gas phase. The ready access to
perfluoroalcohols could transform them from exotic laboratory
curiosities to useful compounds of significant scientific and
industrial interest.
Program by a subcontract from the Pacific Northwest National
Laboratory (KC0301050-47319). DAD also thanks the Robert
Ramsay Chair Fund of The University of Alabama for support.
AFB thanks NSF for a graduate research fellowship.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: • α-perfluoroalcohols • trifluoromethanol •
pentafluoroethanol • heptafluoro-n-propanol • NMR spectra • correlated
electronic structure calculations
+
-
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6 of 7
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Entry for the Table of Contents
COMMUNICATION
A. F. Baxter, J. Schaab, J.
Hegge, T. Saal, M.
Vasiliu, D. A. Dixon, R.
Haiges, and K. O.
Christe*
The thermally unstable primary perfluorinated
alcohols, CF3OH, C2F5OH, and n-C3F7OH, and
their oxonium salts can conveniently be
prepared from the corresponding carbonyl
compounds in anhydrous HF and superacidic
HF/SbF5 solutions, respectively. Experimental
values for the reaction energies were obtained
from the temperature dependence of the
RfCFO + HF  RfCF2OH (Rf = F, CF3, C2F5)
equilibria by NMR spectroscopy. Theoretical
calculations of the energetics, gas-phase
acidities and heats of formation at the G3MP2
level show that these alcohols are very strong
acids.
Page No. – Page No.
α-Fluoroalcohols:
Synthesis and
Characterization of
Perfluoro-methanol, -
ethanol and -n-propanol,
and their Oxonium Salts
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