Reversible protonation of isobutane in liquid superacids in competition with protolytic ionization

Article abstract of DOI:10.1021/ja963848p

The deuterium distribution observed in isobutane recovered after short contact times with the DF-SbF₅ superacid at 0°C shows that a very fast reversible protonation of all C-H bonds occurs before ionization of the alkane, in accord with the Olah σ-basicity concept. Comparison of the amounts of hydrogen with the amount of tert-butyl ions generated during ionization shows that the reaction is purely protolytic in HF containing up to 20 mol % SbF₅, but becomes oxidative at higher concentrations.

Full text of DOI:10.1021/ja963848p

3274
J. Am. Chem. Soc. 1997, 119, 3274-3279
Reversible Protonation of Isobutane in Liquid Superacids in
Competition with Protolytic Ionization
Jean Sommer,* Jozef Bukala, Mohammed Hachoumy, and Roland Jost
Contribution from the Laboratoire de Physico-Chimie des Hydrocarbures, Centre de Recherches
Chimie, UniVersite´ Louis Pasteur, 1, rue Blaise Pascal, 67008 Strasbourg, France
ReceiVed NoVember 6, 1996X
Abstract: The deuterium distribution observed in isobutane recovered after short contact times with the DF-SbF5
superacid at 0 °C shows that a very fast reversible protonation of all C-H bonds occurs before ionization of the
alkane, in accord with the Olah σ-basicity concept. Comparison of the amounts of hydrogen with the amount of
tert-butyl ions generated during ionization shows that the reaction is purely protolytic in HF containing up to 20 mol
% SbF5, but becomes oxidative at higher concentrations.
Introduction
Scheme 1. The Concept of σ-Basicity of Alkanes
Acid-catalyzed hydrocarbon transformations, such as catalytic
cracking, isomerization, and alkylation, are large-scale industrial
processes using solid or liquid strong acids, such as H-zeolites,
chlorinated aluminas, sulfuric acid, and hydrogen fluoride.1 The
high acidity of the catalysts and high temperature compensate
for the well-known inertness of the starting material. It is widely
accepted that the reactions proceed via a classical carbenium
ion mechanism.2 The initial step is, however, often ascribed3
to proton attack on a C-C or a C-H bond, following the
concept of σ-basicity developed by Olah and his group in the
framework of extensive investigations of electrophilic reactions
of single bonds in superacid media4 (Scheme 1).
cracking of hexanes on zeolites at high temperature occurs via
carbonium ion intermediates, semiempirical and ab initio
computations on protonated alkanes in small zeolite clusters
have developed.11 In the frame of our interest in selective
electrophilic alkane activation,12 we now report our results on
the reversible protonation reaction that occurs in competition
with ionization of isobutane in liquid superacid media.
Protonated alkanes or carbonium ions are characterized by a
three-center, two-electron bonded structure having pentacoor-
dinated carbon atoms.5 While these species have a lifetime too
short to allow direct observation in superacid media by NMR,
hydrogen exchange, and cleavage reactions have been de-
scribed.6 Small alkonium ions, such as CH5+ and C2H7+, have
been detected first in high-pressure mass spectrometry experi-
ments7 and later studied on a quantitative basis.8 In 1989, Lee
and co-workers reported the IR spectrum of C2H7+, indicating
the presence of two isomeric ions (C-H or C-C protonated
form).9
Experimental Section
Experimental Procedure. The alkane/CO (1:3 molar ratio) mixture
was bubbled through 1 mL of HF(DF)-SbF5 superacid at 0 °C at a
rate of 4 mL/min. The experiments were generally stopped after 30
min. by freezing the reaction mixture at -78 °C. At the outlet of the
Kel-F reactor, the alkane was condensed continuously during the first
30 min directly in an NMR tube at -78 °C.
The superacid reaction mixture containing the acyl ions was
neutralized by mixing with excess ethanol bicarbonate mixture at -78
°C. The ethanol solution of the resulting esters was used for GC
analysis.
+
+
The structures of CH5 and C2H7 have been investigated
by increasingly refined ab initio computational methods,10 and
2+
even di- and triprotonated methanes (CH6 and CH73+) have
2
NMR Measurements. 1H and H NMR spectra were recorded on
been described as minima on their potential energy surface.10b,c
Following the proposition made by Haag3c a decade ago that
a Bruker AM 400 (400 MHz) spectrometer. To the condensed alkane,
a standard mixture of CDCl3/CHCl3 (1:4 molar ratio) was added. The
sample volume was then adjusted by addition of freon-11 (CFCl3
trichlorofluoromethane). NMR spectra of H(D)F-SbF5 solutions were
taken in quartz tubes at temperatures below ∼-10 °C after addition of
a standard mixture of acetone/acetone-d6.
X Abstract published in AdVance ACS Abstracts, March 1, 1997.
(1) Pines, H. In The Chemistry of Catalytic Hydrocarbon ConVersion;
Academic Press: New York, 1981.
(2) Corma, A. Chem. ReV. 1995, 95, 559.
(3) (a) Corma, A.; Miguel, P. J.; Orchille`s, V. A. J. Catal. 1994, 145,
171. (b) Corma, A.; Planelles, J.; Sandoz-Marin, J.; Thomas, F. J. Catal.
1985, 93, 30. (c) Haag, W. O.; Dessau, R. M. In Proceedings 8th
International Congress on Catalysis; DECHEMA: Franfkurt am Main,
Berlin, 1984; Vol. 2, p 305.
(4) (a) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1973, 12, 173. (b) Olah,
G. A.; Prakash, S. K.; Sommer, J. In Superacids; Wiley: New York, 1985.
(5) Olah, G. A.; Prakash, S. K.; Williams, R. E.; Field, L. D.; Wade, K.
In Hypercarbon Chemistry; Wiley Interscience: New York, 1987.
(6) Olah, G. A.; Klopman, G.; Schlosberg, R. H. J. Am. Chem. Soc. 1969,
91, 3261.
(10) (a) Carneiro, J. W. de M.; Schleyer, P. v. R.; Saunders, M.;
Remington, R.; Schaeffer, H. F., III; Rauk, A.; Soerensen, T. S. J. Am.
Chem. Soc. 1964, 116, 3483 and references therein. (b) Lammertsma, K.;
Olah, G. A.; Barzaghi, M.; Simonetta, M. J. Am. Chem. Soc. 1982, 104,
6851. (c) Olah, G. A., Razul, G. J. Am. Chem. Soc. 1996, 118, 8503.
(11) (a) Kramer, G. J.; Van Santen, R. A.; Emeis, C. A.; Novak, A. K.
Nature 1993, 363. (b) Kazanski, V. B.; Fresh, M. V.; Van Santen, R. A.
Catal. Lett. 1994, 28, 211. (c) Mota, C. J. A.; Esteves, P. M.; de Amorim,
M. B. J. Phys. Chem. 1996, 100, 12418. (d) Blaskowski, S. R.; Nascimento,
M. A. C.; Van Santen, R. A. J. Phys. Chem. 1996, 100, 3463. (e) Collins,
S. J.; Omalley, P. J. Chem. Phys. Lett. 1995, 246, 555. (f) Lercher, J. A.;
Van Santen, R. A.; Vinek, H. Catal. Lett. 1995, 31, 440. (g) Kramer, G. J.;
VanSanten, R. A. J. Am. Chem. Soc. 1995, 117, 1766.
(7) Field, F. H.; Munson, M. S. B. J. Am. Chem. Soc. 1965, 87, 3289.
(8) French, M.; Kebarle´, P. Can. J. Chem. 1975, 53, 2268.
(9) Yeh, L. I.; Price, J. M.; Lee, Y. T. J. Am. Chem. Soc. 1989, 111,
5597.
(12) Sommer, J.; Bukala, J. Acc. Chem. Res. 1993, 26, 370.
S0002-7863(96)03848-6 CCC: $14.00 © 1997 American Chemical Society

ReVersible Protonation of Isobutane in Superacids
Table 1. Product Distribution Obtained from Isobutanea
J. Am. Chem. Soc., Vol. 119, No. 14, 1997 3275
acids (mmol)
gas mixture (mmol) before reaction
products after reaction (mmol)
expt
HF
SbF5
iC4H10
CO
H2
CH4
C2H6
C3H8
Epiv
EiB
tr.
tr.
tr.
tr.
iC4H10*
1
2
3
4
5
6
28.5
28.8
30.9
29.5
37.4
37.4
7.7
7.8
8.4
8.0
9.3
9.3
2.63
2.84
2.65
2.83
2.71
2.63
7.27
7.23
6.80
7.29
8.13
7.89
0.77
0.86
0.80
0.87
0.91
0.86
0.05
0.05
0.05
0.05
0.05
0.05
0.001
0.001
0.002
0.002
0.002
0.003
0.03
0.03
0.03
0.03
0.04
0.02
0.78
0.62
0.85
0.74
0.82
0.76
1.64
1.96
1.76
1.74
1.83
1.68
0.008
0.014
a After 60 min on stream. b Apparently unreacted.
Scheme 2. Protolytic Ionization of Isobutane in Superacids
as follows:16 tertiary C-H > C-C > secondary C-H .
primary C-H.
During our studies on superacid-catalyzed alkane carbony-
lation related with an alternative synthesis for metacrylic ester
from propane,17 we noticed that the reaction system could be
simplified very much by using carbon monoxide as coreactant
in the superacid medium. Since the early attempt of Hogeveen18
to carbonylate methane, reports on direct carbonylation of
alkanes in superacid media include reactions of cycloalkanes,19
C5-C8 alkanes,20 and adamantane.21 Whereas in the classical
Koch-Haaf synthesis22 the intermediate carbocation is obtained
by protonation of an alkene, in the superacid it is generated
directly from the alkane. Moreover, under these conditions,
the unstable carbenium ions generated in the acid are trans-
formed into stable oxocarbenium ions, thereby limiting hydride
transfer reactions.23 These oxocarbenium ions can also be
trapped by quenching the superacid in excess ethanol, and since
the resulting esters can be analyzed by GC, a quantitative study
becomes possible.12 We show here that a satisfactory balance
between the reactant and the products in gas and liquid phase
can be obtained, which is helpful to establish the relative
importance of the initial competitive reaction pathways.
An isobutane/carbon monoxide mixture (molar ratio 1:3) was
bubbled during 1 h at a rate of 220 mL/h through 1.0 mL of
HF-SbF5 solution (molar ratio 4:1) in a Kel-F reactor at -10
°C. The gaseous products as well as the ethyl esters obtained
by quenching the oxocarbenium ions in the liquid phase were
collected and analyzed by GC The reactants and product
distribution measured in a series of experiments are collected
in Table 1, and the various pathways for product formation are
presented in Scheme 4.
Scheme 3. Simultaneous-Occuring Reactions
Results and Discussion
I. Ionization of Isobutane in HF-SbF5 in the Presence
of Carbon Monoxide. The high reactivity of isobutane in
superacidic media was first reported independently by Olah and
Lukas13 and by Hogeveen and co-workers14 (Scheme 2).
The tert-butyl ion is remarkably stable and the NMR spectrum
of the solution is unchanged even after 1 week at room
temperature. Its formation was ascribed to protolytic cleavage
of the methine C-H bond via a carbonium ion intermediate.
The fact that the yield of hydrogen was not stoichiometric with
that of the tert-butyl ion was the subject of a two decade long
controversy on the ability of SbF5 to participate in the ionization
process by directly oxidizing the alkane. We demonstrated this
possibility in 1990, using a proton trap (acetone) as coreactant
in the superacid.15 Taking into account the reactivity of C-C
and C-H bonds, the competitive reactions of an alkane in the
superacid can be summarized with the following reaction
scheme (Scheme 3).
The product distribution shows that the main pathway for
ionization involves protolytic cleavage (path 1) of the tertiary
C-H bond, producing the tert-butyl ion, which is titrated as
ethyl pivalate, and a stoichiometric amount of hydrogen. Some
ethyl pivalate is also formed as a byproduct (path 2b) when the
carbon-carbon bond is cleaved with formation of methane and
propyl cation. The secondary cation, even in the presence of
carbon monoxide will, in part, ionize isobutane by hydride
transfer as shown by the small amount of propane detected in
The simultaneously occurring reactions involve the following
steps: (1) fast reversible protonation (deuteronation unobserved
in the absence of isotopic labeling), (2) protolytic cleavage of
a C-H bond yielding a carbenium ion and hydrogen, (3)
protolytic cleavage of a C-C bond yielding a reactive carbenium
ion and a smaller alkane, (4) hydride transfer reaction between
the various carbenium ions and the various alkanes, and (5)
oxidation of the alkane by SbF5.
(16) Olah, G. A.; Halpern, Y.; Shen, J.; Mo, Y. K. J. Am. Chem. Soc.
1971, 93, 1251.
(17) (a) Delavarenne, S.; Simon, M.; Fauconet, M.; Sommer, J. J. Am.
Chem. Soc. 1989, 111, 383. (b) Delavarenne, S.; Fauconet, M.; Simon, M.;
Sommer, J. Eur. Pat. 270398, October 22, 1987. (c) Delavarenne, S. et al.
Eur. Pat. 272945, October 22, 1987.
(18) Hogeveen, H.; Lukas, J.; Roobeeck, C. F. J. Chem. Soc., Chem.
Commun. 1969, 920-921.
As each alkane obtained by protolytic cleavage or by hydride
transfer can, in turn, undergo protolytic cleavage, etc., it is
understandable that all reports in the past described qualitative
rather than quantitative studies. Olah and his group in the early
1970s classified the relative reactivity of the σ-bonds in alkanes
(19) Paatz, R.; Weisgerber, G. Chem. Ber. 1967, 100, 984.
(20) Yoneda, N.; Takahashi, Y.; Fukura, T.; Suzuki, A. Bull. Chem. Soc.
Jpn. 1986, 57, 2819.
(21) Farouq, O.; Marcelli, M.; Prakash, G. K. S.; Olah, G. A. J. Am.
Chem. Soc. 1988, 110, 864.
(22) (a) Koch, H.; Haaf, W. Org. Synth. 1964, 44, 1. (b) For a review,
see: Bahrmann, H. In Synthesis with Carbon Monoxide; Falbe, J., Ed.;
Springer-Verlag: Berlin, 1980.
(23) Hogeveen, H. In AdVances in Physical Chemistry; Gold, V., Ed.;
1973; Vol. 10, p 29.
(13) Olah, G.; Schlosberg, R. H. J. Am. Chem. Soc. 1968, 90, 2726.
(14) Hogeveen, H.; Bickel, G. F. J. Chem. Soc., Chem. Commun. 1967,
635.
(15) Culmann, J.-C.; Sommer, J. J. Am. Chem. Soc. 1990, 112, 4057.

3276 J. Am. Chem. Soc., Vol. 119, No. 14, 1997
Sommer et al.
Scheme 4. Ionization and Carbonylation of Isobutane in Superacids
the gas phase. In accord with a purely protolytic activation of
isobutane in this superacid medium, no precipitate of SbF3 was
noticed. As ethyl pivalate (Epiv) can be formed via both
pathways 1 and 2b we should find more Epiv than hydrogen.
As this is not the case, we assume that this discrepancy arises
during the neutralization step of the superacid solution, during
which the pivalyl ion loses, in part, CO. The tert-butyl ion
which is thus formed under non-superacidic conditions elimi-
nates a proton producing small quantities of isobutene which
are not recovered under our experimental conditions. On the
basis of product distribution, we assign the relative rates of
pathways 1 and 2 as being on the order of 18:1. This order
confirms all earlier reports on the high reactivity of the tertiary
C-H bond. The relative rates of pathways 2a and 2b which
are here roughly on the order of 1:5 are very much dependent
on the CO/hydrocarbon ratio which controls the competitive
hydride transfer or carbonylation of the propyl cation.
II. Change in Activity of the Superacid Depending on
the SbF5 Concentration. For reasons described above, conver-
sion of isobutane in HF will be dependent on the concentration
of SbF5 which not only governs the acidity of the medium but
also allows for the stabilization of the resultant cations.
In the concentration range of 10-22 mol % SbF5 in HF, the
anions are essentially SbF6- and Sb2F11- in slow equilibrium.24
Figure 1. Isobutane conversion based on ethyl pivalate production
(a) compared with hydrogen production (b) (60 min time on stream).
Scheme 5. Reduction of SbF5 by Isobutane
-
+
SbF5 + 3HF h SbF6 ,H3F2
(1)
(2)
and hydrogen production are roughly parallel, in agreement with
the protolytic ionization. For concentrations higher than 25 mol
% SbF5, however, a change in product distribution is noticed:
notably the production of ethyl pivalate increases whereas the
hydrogen production decreases drastically. The production of
SbF3, observable as a white precipitate indicates that SbF5 is
reduced during isobutane conversion with production of HF as
earlier suggested by Olah4 (Scheme 5).
SbF5 + SbF6 , H3F2+ h Sb2F11-,H3F2
-
+
During the reaction of isobutane, the superacidic and posi-
tively charged species H3F2 is progressively replaced by the
carbenium (or oxocarbenium ion), (HF is generated) and the
acidity decreases accordingly (eq 3).
+
Sb2F11-,H3F2+ + RH h Sb2F11-,R+ + 2HF + H2 (3)
In order to understand why the change of reactivity of the
acid occurred around the concentration of 20-25 mol % SbF5,
we reconsidered the results on the anionic composition of the
HF-SbF5 system we obtained some years ago by 19F NMR.26
When the average conversion of isobutane (based on the
formation of ethyl pivalate during the 1 h experiment), is plotted
as a function of the concentration of SbF5 in HF, a steady and
strong increase of conversion is noted when the concentration
of the Lewis acid increases (Figure 1, curve a).
When the amount of hydrogen is measured (Figure 1, curve
b) it appears that, up to 20-25 mol % SbF5 in HF, conversion
-
We noticed that, in addition to the SbF6-, Sb2F11-, and Sb3F16
anions, a small but increasing amount of uncomplexed SbF5
was present for concentrations higher than 18 mol % SbF5 in
(25) Gillespie, R. J.; Liang, C. J. Am. Chem. Soc. 1988, 110, 6057.
(26) Fauconet, M. Ph.D. Thesis, University Louis Pasteur Strasbourg,
November 1987.
(24) Gillespie, R. J.; Peel, T. E. AdV. Phys. Org. Chem. 1972, 9, 1.

ReVersible Protonation of Isobutane in Superacids
J. Am. Chem. Soc., Vol. 119, No. 14, 1997 3277
Scheme 6. Hammett-Type Indicators Used in Strong
Superacids
Scheme 7. H/D Exchange between Branched Alkanes and
D2SO4
HF. We assume that the change in product distribution is related
to the change in composition of the superacid. At the higher
SbF5 concentrations in HF, SbF5 becomes more available for
participation in the activation process. This observation should
be considered in relation with the limits of superacidity which
we published earlier.27 By measuring the chemical shift
variation of a family of very weak bases, such as 4-methoxy-
benzhydril ions (1-5) (Scheme 6), we found that the weakest
bases 5 and 6 (estimated pK values -22.5 and -23.0,
respectively) could not be protonated even in HF-SbF5 (1:1)
whereas indicator 4 (pK ) -22) was protonated in HF
containing 12% SbF5. This indicated that the acidity of HF-
SbF5 is close to its limit in the 15-20 mol % SbF5 range and
that increasing the concentration of SbF5 beyond 20% does not
increase the acidity and thus should not result in an increase of
the rate of protolysis.
The same regiospecificity in H/D exchange on the positions
vicinal to the branching has recently been reported31 by us for
isoalkanes contacted with D2O-exchanged solid acids, such as
zeolites or sulfated zirconia.
In comparison with other small alkanes, the behavior of
isobutane in HF-SbF5 is rather simple, especially at low SbF5
concentrations. For this reason, we reinvestigated the H/D
exchange process occurring between isobutane and DF-SbF5
in the presence of carbon monoxide.
III. Protium/Deuterium Exchange between DF-SbF5 and
Isobutane. Direct hydron exchange, occurring between alkanes
and superacids, constitutes the simplest example of electrophilic
substitution and was described by Olah in the late 1960s13 and
also by the group at Shell.14 Due to the occurrence of side
reactions (as described on Scheme 3), only a few quantitative
studies have been reported. The rates of dedeuteration of
monodeuteromethane and monodeuteroethane (5 and 6.0 L
mol-1 s-1, respectively) in HF-SbF5 (11:1 molar ratio) were
measured by Hogeveen.28 The relatively small difference in
rate between the two compounds was assigned to a small
difference in basicity of the primary C-H bonds. The time
dependence of dedeuteration of 2-deutero-2-methylpropane at
0 °C in HF containing 4 mol % SbF5 has been also described
by Hogeveen,29 who noticed that the rate of exchange of the
tertiary hydrogen of isobutane was less than 10 times faster than
that of the primary hydrogens of ethane and also that the
exchange was faster than hydride abstraction. Other reports of
H/D exchange between alkanes and superacids are of qualitative
nature. Olah found that in FSO3D/SbF5 or DF/SbF5 at -78 °C
only the methine proton of isobutane was substantially ex-
changed and, for adamantane, the bridgehead hydrons exchanged
preferentially.16
When a mixture of isobutane and carbon monoxide (molar
ratio RH/CO 1:3) is bubbled through 1 mL of DF-SbF5 (molar
1
2
ratio 4:1) during 30 min at a rate of 4 mL/min, the H and H
NMR spectra of the recovered isobutane show that both the
primary and the tertiary hydrogens have been exchanged (7.4
atom % of the hydrogens in the methyl groups and 17.4 atom
% of the methine hydrons were exchanged for deuterons). In
another experiment with a slightly smaller space velocity, the
recovered isobutane showed 9.1 atom % D of the methyl
hydrons and 24.3 atom % D on the methine. These values,
which are average values for 30 min experiments, are in line
with the relative σ-basicity of the tertiary and primary C-H
bonds.3b It is interesting to note that the difference in exchange
rates is rather small, in comparison with the relative energies
needed to ionize these two bonds.
1
2
Analysis of the DF-SbF5 solution by H and H NMR at
-30 °C at the end of the experiment but before neutralization
shows the presence of the pivalyl cation in equilibrium with
the tert-butyl ion formed by slow loss of carbon monoxide. It
is interesting to note that both ions show an important H/D
exchange at the CH3 groups (approximately 8 atom %). (The
quantitative measurement was made by addition of a standard
mixture of acetone-d6 and acetone used as an internal reference
in the superacid.)
These reports, in excellent agreement with formation of
carbonium ion intermediates in superacid media contrast mark-
edly with the H/D exchange observed earlier by Stevenson and
Otvos,30 who found that, in D2SO4, isobutane readily exchanged
all nine primary protons for deuterons but not the methine
proton. In this case, the exchange could be rationalized by a
carbenium ion mechanism, including successive deprotonation
and reprotonation followed by a hydride transfer step (Scheme
7).
In order to check whether the deuteriums were incorporated
in these ions before ionization, we generated the ions, in a
separate experiment, by dissolving their precursors, tert-butyl
chloride and pivalyl chloride, in DF-SbF5. Even after 6 h at
-10 °C, the observable H/D exchange in both cations was on
the order of 1 atom %. This is in good accord with the results
published recently by Olah and his group on studies of the
protosolvation of alkyl cations showing that H/D exchange can
be observed between long-lived ions, such as 2-propyl and tert-
butyl cation and DF-SbF5 (1:1),32 but the exchange rate is
extremely slow. As deprotonation can be excluded under these
conditions, isotopic exchange was explained by protosolvation
(27) Jost, R.; Sommer, J. ReV. Chem. Intermed. 1988, 9, 171.
(28) (a) Hogeveen, H.; Bickel, A. F. Recl. TraV. Chim. Pays-Bas 1969,
88, 371. (b) Hogeveen, H.; Gaasbeck, C. J. Recl. TraV. Chim. Pays-Bas
1968, 97, 319.
(29) Hogeveen, H.; Gaasbeck, C. J.; Bickel, A. F. Recl. TraV. Chim.
Pays-Bas, 1969, 88, 703.
(31) (a) Sommer, J.; Hachoumy, M.; Garin, F.; Barthomeuf, D. J. Am.
Chem. Soc. 1994, 116, 5491. (b) Sommer, J.; Hachoumy, M.; Garin, F.;
Barthomeuf, D.; Vedrine, J. J. Am. Chem. Soc. 1995, 117, 1135.
(30) Otvos, J. W.; Stevenson, D. P.; Wagner, C. D.; Beeck, O. J. Am.
Chem. Soc. 1951, 73, 5741.

3278 J. Am. Chem. Soc., Vol. 119, No. 14, 1997
Scheme 8. Ionization versus Exchange
Sommer et al.
Table 2. Ionization and H/D Exchange Observed in Isobutane at
Various Concentrations of SbF5 in HFa,b
exchange on
SbF5
tertiary primary
total
ionizationd
expt (mol % in HF) C-H
C-H exchangec (mol %) E/Ie
1
2
3
4
5
6
9.7
12.7
17.0
20.0
26.0
33.6
12.5
(0.13) (0.43)
17.5 7.7
(0.18) (0.69)
16.0 10.0
(0.16) (0.90)
9.9 6.5
(0.10) (0.58)
0.8 8.3
(0.01) (0.75)
0.8 1.8
(0.01) (0.16)
4.8
2.3
(0.02)
3.9
(0.04)
14.7
(0.15)
20.7
(0.21)
29
(0.56)
(0.87)
(1.06)
(0.68))
(0.76)
(0.17)
28.0
21.7
7.1
3.2
(0.29)
64
(0.64)
2.6
0.3
a After 30 min of time on stream. b All values between brackets are
normalized to 1 mol of isobutane. c The maximum exchangeable is 10
mol of protons per mole of isobutane. d Based on product analysis.
e Ratio of total H/D exchange versus ionization.
Figure 2. H/D exchange observed on tertiary (a) and primary (b)
protons of isobutane versus SbF5 concentration in HF (30 min on
stream).
depends only on the relative σ-basicity of the proton-accepting
bond and does not reflect the ability of that bond to undergo
protolytic cleavage. The fate of the carbonium ions depends
on the relative energies required for the available reaction
pathways: namely, deprotonation or cleavage to form a trivalent
carbenium ion and a neutral species (in this case HD or
deuteromethane). For this reason, ion I will only exchange
protium for deuterium and ion II will give the stable tert-butyl
ion in competition with H/D exchange. Unfortunately, revers-
ible protonation of C-C bonds (ion III) cannot be detected by
this technique, but in accord with the higher energy needed for
the formation of the 2-propylium ion in comparison with the
tertiary ion, protolytic cleavage of the C-C bond is a pathway
of minor importance as shown by the product distribution.
The relative importance of H/D exchange compared with
ionization depends very much on the acid composition.
of the cation (distonic cations). Thus, our results demonstrate
that, in superacid medium, reversible protonation of the alkane
is much faster than the corresponding ionization reaction. Since
we know that the ionization rate is steadily increasing with SbF5
concentration, it was interesting to monitor the influence of
acidity on the H/D exchange rates. In a series of experiments
in DF containing increasing amounts of SbF5, we collected
isobutane after 30 min on stream for 1H and 2H NMR analysis.
Figure 2 illustrates the results of this variation and Table 2
compares the ionization of isobutane with H/D exchange for
increasing concentration of SbF5 in HF.
The competitive protonation and ionization of isobutane in
DF-SbF5 can be described by Scheme 8.
In Table 2 we have compared the total amount of protons
exchanged for deuteriums per mole of isobutane with the amount
of isobutane ionized during the same time interval (0-30 min).
If we compare the exchange rate of the tertiary C/H with the
ionization rate of isobutane, we notice that at the concentration
of 9.7 mol % SbF5 in HF, H/D exchange on this tertiary bond
alone is 6 times faster than ionization (0.13 versus 0.02). If,
however, we take into account the total amount of protons
Isobutane is reversibly protonated on the various σ-bonds
forming the three isomeric carbonium ions in equilibrium. The
relative concentration of these pentavalent reaction intermediates
(32) (a) Olah, G. A.; Hartz, N.; Rasul, G.; Prakash, G. K. S. J. Am. Chem.
Soc. 1993, 115, 6985. (b) Olah, G. A.; Hartz, N.; Rasul, G.; Prakash, G. K.
S.; Burkhart, M.; Lammertsma, K. J. Am. Chem. Soc. 1994, 116, 3187.
(33) Sommer, J.; Bukala, J.; Rouba, S.; Graff, R.; Ahlberg, P. J. Am.
Chem. Soc. 1992, 114, 5884.

ReVersible Protonation of Isobutane in Superacids
J. Am. Chem. Soc., Vol. 119, No. 14, 1997 3279
exchanged for deuterium per mole of isobutane, we have to
take into account the nine primary protons which multiplies by
nine the value expressed in atom %. This gives a ratio of 28
for E/I (0.56 versus 0.02) in isobutane at this concentration in
SbF5.
(3) At concentrations higher than 25% H/D exchange
becomes negligible in comparison with ionization which occurs
predominantly with simultaneous reduction of SbF5.
Conclusion
We would like to stress here that the reversible protonation
of the alkane is much faster than apparent from isotope
exchange, as the intermediate carbonium ion can also give back
a deuteron. The importance of dedeuteration of the carbonium
ion in comparison with deprotonation depends on the isotope
effect but cannot unfortunately be estimated in these experi-
ments.
The exchange over ionization ratio (E/I) diminishes gradually
with increasing concentration of SbF5 in HF and three areas
can roughly be distinguished.
(1) Below 20 mol % SbF5, the reversible protonation of the
alkane not only on the methine bond but also on the primary
C-H bonds is faster than ionization.
(2) From 20 to 25 mol % SbF5, based on H2 production, we
notice that protolysis becomes more important that H/D
exchange. The amount of deuterium incorporated in isobutane
especially on the reactive tertiary C-H bond decreases drasti-
cally with increasing concentration of SbF5.
The superacid catalyzed H/D exchange of isobutane in DF-
SbF5 takes place via a fast reversible protonation (deuteronation)
of the primary and tertiary C-H bonds by the superacid
following the σ-basicity concept developed by Olah. At
concentrations up to 20 mol % SbF5 in HF, H/D exchange is
much faster than ionization of the isoalkane which occurs
essentially via protolytic cleavage of the tertiary C-H bond.
The superacidity limit is reached. At higher concentration, the
increasing ionization of isobutane is accompanied by simulta-
neous reduction of SbF5 at the expense of hydrogen production.
Supporting Information Available: Experimental details
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