Journal of Supercritical Fluids 15 (1999) 165172

Deuteration of hexane by 2HCl in supercritical deuterium oxide

Ying Yang 1, Ronald F. Evilia *
Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA
Received 23 July 1998; received in revised form 26 November 1998; accepted 21 December 1998

Abstract
Hexane is shown to undergo isotopic hydrogen exchange with 2HCl in supercritical deuterium oxide at 380 and
400C. The deuteration rate follows pseudo first order kinetics at both temperatures with the methylene reaction rate
being about 1.6 times that of methyl. The isotopic exchange reaction is analyzed as a two step acid/base mechanism,
with hexane acting as a base analogous to its behavior in magic acid solution. Measured K s for the methyl group
b
are 3.51028 and 9.21028, while the methylene groups have K s of 6.01028 and 1.51027 at 380 and 400C,
b
respectively. No evidence is seen for hydride abstraction, such as formation of carbocation rearrangement species or
hydrogen gas evolution as in magic acid. Hydride abstraction to form carbocations either does not occur or occurs
at a rate too slow to be observed in the time scale of the experiments reported here. 1999 Elsevier Science B.V. All
rights reserved.
Keywords: Deuteration; 2HCl; Hexane; K ; Supercritical deuterium oxide
b

1. Introduction
The potential of supercritical water oxidation
(SCWO) for the environmentally benign destruction of organic wastes has led to recent interest in
the properties and reactions of a variety of organic
molecules in supercritical water [110]. Acid/base
properties in supercritical water are of fundamental
importance in understanding the physical and
chemical ramifications of SCWO and other reactions in this incompletely characterized medium.
Previous studies in this laboratory have shown
* Corresponding author. Tel.: +1-504-280-6313;
fax: +1-504-280-6860.
E-mail address: revilia@uno.edu (R.F. Evilia)
1Current address: Department of Chemistry, Miami University,
Oxford, OH 45056.

that a number of organic compounds whose room

temperature acidity or basicity is negligibly small,
will undergo reactions with aqueous base or acid
at sufficiently rapid rates that CH exchange with
the solvent is observed in a time scale of minutes
[1113]. The acid/base reaction of hydrocarbons
with deuterated hydroxide in supercritical water
has been shown to be a viable synthetic approach
to some perdueterated compounds [14,15]. Other
studies have shown decreased dissociation of
hydrogen ions from acidic sites such as HCl and
2-naphthol in supercritical water and have suggested the formation of contact ion pairs between
H+ and the basic site [1619]. Thermodynamic
studies of aqueous ionization reactions have shown
dramatic decreases in the acid dissociation constant for a wide variety of weak acids in aqueous
solution above 300C [20]. In this same thermo-

0896-8446/99/$ see front matter 1999 Elsevier Science B.V. All rights reserved.
PII: S0 8 9 6 -8 4 4 6 ( 9 9 ) 0 0 00 3 - 0

166

Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165172

dynamic study, the acid dissociation constant for

HCl is reported to be approximately 2.5105
under the conditions used here [20]. Based upon
the increased tendency to protonate weakly basic
sites, these studies suggest that H+ in supercritical
water is far more reactive than in less severe
conditions, perhaps because of reduced hydration
of the ion.
In this report, 2HCl is shown to react with
hexane to produce deuterated products involving
deuterium exchange at both methyl and methylene
positions with ethylene deuteration occurring at a
rate approximately 1.6 times that of methyl. This
is in contrast to the reaction in basic supercritical
deuterium oxide where only methyl deuteration is
observed [11]. In the examples reported here, the
mechanism is believed to involve the reaction of
2H+ with hexane sites to form a five-bonded,
positively charged carbon intermediate, analogous
to the five-bonded intermediates reported in isotope scrambling experiments of alkanes in so called
magic acid media [2123]. However, contrary to
the behavior in magic acid, no carbocation
rearrangement products or hydrogen evolution
were observed in the study reported here.

Fig. 1. Internal reactor temperature versus time. Oven set to

400C.

2. Experimental
Reactions were performed in flame sealed quartz
tubes placed in stainless steel tubes containing a
suitable amount of water to balance the internal
pressure developed in the quartz tube when heated.
The stainless steel tube was then sealed with
swadgelok@ plugs. In a typical experiment,
0.257 ml of 2.432 M 2HCl and 0.05 ml of hexane
were added to a 0.64 ml volume quartz tube (3 mm
id, 9 cm long). Above the critical temperature
where a single phase exists, the concentration of
2HCl is 0.98 M and the sample density is about
0.46 g ml1. The 2HCl solution was deaerated by
argon for 10 min prior to placing it in the quartz
tube. The internal temperature of the sample tube
was monitored by an internal JK type thermocouple sealed in the stainless steel tube by means of a
TG gland ( TG-24-*2, Conax Buffalo) with a
Lava@ sealant. The internal temperature/time profile is shown in Fig. 1. The temperature measure-

ment was performed on a tube filled with an

appropriate amount of water, but did not contain
a quartz sample tube. Deuteration experiments
were conducted without the thermocouple present.
Examination of Fig. 1 shows that approximately
30 min heating time is required to reach the set
temperature of 400C. Once the set temperature
was reached, the measured temperature variation
was no more than 1C for a 5 h test. Thus it is
presumed that the temperature is equal to the set
temperature for an indefinite period after the
30 min initial warm-up. Since no measurable
amount of deuterium incorporation was observed
in the first 30 min of heating, the relatively long
heat-up time of the experimental arrangement is
not considered to be a source of error in the kinetic
analysis. Temperatures of 380 and 400C were
employed in this study. While it would be desirable
to conduct the kinetic study over a wider range of
temperatures, experimental problems prevented

Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165172

our doing so in this case. At temperatures below

380C, excessive scatter was found in the kinetic
plots. We believe that this scatter is caused by the
presence of two phases with different reaction
rates. Because each time point in a kinetic study
is a new experiment involving a different sealed
quartz tube, we are not able to exactly duplicate
the two phase conditions from one experiment to
the next and, thus, excessive scatter is observed
in the kinetic plots. Above 400C, frequent quartz
tube failures occurred because of the difficulty in
accurately balancing the internal pressure of the
quartz tube with external water pressure [24].
Experiments conducted at higher temperatures
with lower density water have survived heating
but, because of the lower density involved, represent different reaction conditions. These results
will be reported in a future communication.
Deuterium incorporation was monitored by
NMR spectroscopy. 13C spectroscopy was
employed to qualitatively verify deuteration of the
carbon sites through observation of deuterium
coupling patterns and isotope shifts of the carbon
resonances. While we have previously used 13C
spectra for semi quantitative measurements of
deuteration of CH groups, such spectra were not
suitable for quantitative measurements in this case
because of the complex patterns developed by
multiple deuteration and uncertainty about the
nuclear Overhauser effect for partially deuterated
cases [11,24]. For quantitative measurements, 1H
NMR spectral integrals were used. Quantitation
was accomplished by extraction of the hexane into
deuterochlorform containing a known percentage
of protiochloroform. Following extraction, the
concentration ratio of chloroform to hexane was
measured by gas chromatography. The 1H NMR
integrals for the chloroform and hexane signals
were then evaluated and the percentage of deuteration of methyl and methylene carbons computed
from the known chromatographic concentration
ratio and NMR 1H integral ratio. Spectral overlap
prevented accurate, separate integration of the
two CH signals and, therefore, an average value
2
for both CH s is reported. In view of the absence
2
of significant curvature in the CH kinetic plots,
2
it is apparent that both CH s have very similar
2
reaction rates.

167

3. Results and discussion

The first question that naturally arises when
attempting to write a mechanism for the hydrogen
exchange that we observed is whether the reaction
involves homolytic or heterolytic CH bond breakage. Since hexane is generally considered to be a
non-polar molecule, one would be tempted to
initially assume that the reaction is a homolytic
one involving the production of hydrogen atoms
and hexane radicals. Indeed, at high temperatures
and low densities, homolytic free radical reactions
do occur [25]. However, the higher densities and
lower temperatures employed in this study increase
the possibility that a heterolytic, acid/base type
reaction is occurring in this case.
Previous reports from this laboratory have
shown selective deuterium incorporation in acidic
and basic media that are qualitatively consistent
with expectations of relative acid/base strengths of
the organic substrate. For example, in basic media,
carbon sites adjacent to electron withdrawing
groups deuterated faster than sites several carbons
removed from these groups [11]. Also in that
earlier work, the preferential deuteration of the
methyl groups of 2-methylpentane in basic media
was sited as support for an ionic acid/base reaction
in which a primary carbanion intermediate is
formed via H+ abstraction by the strong base.
The greater stability of primary carbanions leads
to a preference for methyl deuteration in a basic
solution if an ionic mechanism is involved.
Another observation suggesting an ionic mechanism is the decomposition product of isoleucine in
supercritical HCl at 400C [13]. The principal
decomposition product formed from isoleucine in
supercritical HCl is methyl-isopropylketone. As
shown later, the formation of this product requires
substantial rearrangement of the original isoleucine
carbon skeleton. Carbon skeleton rearrangements
of this sort are frequently observed when carbocations are formed in the reaction, while free radicals tend to produce a wide array of products
formed by joining small fragments together. Olah
et al. has reported the formation of the same
compound, methy-isopropylketone, by carbocation rearrangement of the pivaloyl dication in
superacid media [26 ]. One can imagine diproto-

168

Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165172

nated isoleucine losing water and ammonia to

form a dicationic species which could undergo a
related carbocation rearrangement to yield the
observed product if the protonating ability of the
acid is strong enough. It is our contention that
HCl under supercritical water conditions is a powerful protonating agent similar to a super acid. In
view of these qualitative observations, it seems
probable that ionic reaction mechanisms are
involved under the conditions of temperature and
pressure employed in this work.
CH CH CH(CH )CH(NH )CO H
3
2
3
2
2
HCl
(CH ) CHCOCH

32
3
400C, 250 Bar
As a test for homolytic hydrogen exchange, a
mixture of hexane and deuterium oxide was maintained at 400C for 6 h in the sealed tube reactor.
No evidence for incorporation of deuterium or
decomposition of the hexane was observed by
13C NMR spectroscopy following the heating
period (deuteration <10%). Thus, we conclude
that homolytic bond rupture is not a significant
factor in the reaction that produces deuterated
products at temperatures 400C at the solution
density employed in this study. Since no new
product peaks were observed in this experiment, it
also seems clear that insignificant amounts of
reactive intermediate species are produced in the
neutral aqueous environment at 400C.
In contrast to the lack of deuterium incorporation in neutral deuterium oxide, extensive hexane
deuteration was observed when the hexane and
2HCl solution were subjected to the same temperature and pressure conditions. In fact, significant
deuterium incorporation (~20%) was observed by
13C NMR spectroscopy after 45 min in a 400C
oven. In view of the heat-up time of approximately
30 min, as shown in Fig. 1, this 45 min experimental reaction time corresponds to only about 15 min
at 400C. In comparison with the neutral experiment, the deuteration rate in the acid solution is
at least 40 times faster. Since no deuteration at all
was seen in the absence of 2HCl, the factor 40 is
in reality a lower limit based upon the minimum
experimentally detectable amount of deuteration
and the true rate difference may be much greater

than a factor of 40. To determine a quantitative

difference in deuteration rates, it is necessary to
extend the neutral solution heating until observable
deuteration occurs. Because the concentration of
2H+ is approximately 104 times more dilute in the
neutral solution, the ionic reaction should require
about 50 days to produce observable deuteration
(~10%). We also note that no deuteration was
observed within experimental error (<10%) when
2HCl/hexane mixtures were heated for 30 min or
less. This indicates that the deuteration occurs
rapidly after supercritical conditions are reached
but slowly prior to those conditions.
Figs. 2 and 3 show that the deuteration reaction
follows pseudo first order behavior in 0.98 M
2HCl at 380 and 400C, respectively. The sample
density in these experiments is approximately
0.46 g ml1 and the maximum pressure is estimated to be about 370 bar [27]. The pressure is
assumed to be equal to that of pure 1H O at a
2
density of 0.45 g ml1 (i.e. the sample density
corrected for the atomic weight of deuterium).
Analysis of the data shown in Figs. 2 and 3 yields
pseudo first order deuteration rate constants, k ,
obs
of (2.30.2) 105 and (5.00.5)105 s1 for
CH
and (3.80.3)105 and (8.00.5)
3
105 s1 for CH at 380 and 400C, respectively.
2
The error estimates correspond to the 95% confi-

Fig. 2. Pseudo first order kinetic plot for hexane deuteration

at 380C.

Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165172

Fig. 3. Pseudo first order kinetic plot for hexane deuteration

at 400C.

dence intervals for the slopes. It should be noted

that the first order kinetic plots of Figs. 2 and 3
do not extrapolate back to the origin at zero time.
This is caused by the long induction period in
which the sample temperature varies from room
temperature to the experimental value. Because
the temperature varies with time in this interval
and the degree of deuteration is extremely small,
the data are not interpretable at short heating
times.
Of more fundamental interest than the deuteration rate, however, are the respective acid/base
equilibrium constants under the reaction conditions. One possible way to view the deuteration
reaction sequence is via the two step mechanism
shown in Eqs. (1) and (2), below:
k1
R1H+2H O+ P
R1H2H++2H O,
(1)
3
2
k1
k2
R2H+1H2H O+.
(2)
R1H2H++2H O P
2
2
k2
The reaction sequence expressed by Eqs. (1) and
(2) can be analyzed by steady-state approximation
techniques since k
and k
should be much
2
1
faster than k and k . Moreover, since the concen1
2
trations of both R2H and 1H2H O+ are very small
2
under the reaction conditions employed, the rate
of the back reaction of R2H can be neglected.

169

Thus, the rate of production of deuterated product

can be shown to be equal to half of the forward
rate of Eq. (1), assuming that k =k . This
1
2
latter assumption is reasonable as both of these
reactions involve the reaction of the same intermediate with 2H O and both should be diffusion
2
controlled [28].
One difficulty in the straightforward utilization
of the kinetic analysis of Eqs. (1) and (2) arises
from the fact that the acid, 2HCl, is substantially
unionized under the reaction conditions employed.
At 400C and the appropriate density for the
current reaction conditions, the 1HCl ionization
constant is approximately 2.5105 [21]. Thus,
unlike one would expect in room temperature
water, the concentration of 2H O+ present for
3
Eq. (1) should be much less than the initial concentration of 2HCl added to the reaction tube.
Because the ionization constant for 2HCl in
supercritical deuterium oxide was not reported, it
must be estimated for use in this study. Based
upon the fact that the ionization constant for
2H O is nearly one order of magnitude less than
2
that of 1H O over a wide range of temperatures
2
and pressures [21], we estimate the 2HCl ionization
constant to be 2.5106 in this case. Using this
estimated value for the ionization constant and
the 0.98 M analytical concentration of 2HCl, the
2H O+ concentration is computed to be approxi3
mately 1.6103.
From the deuteration rate constant, k ,
obs
obtained from the pseudo first order plots of
Figs. 2 and 3, values for k at each temperature
1
are calculated by Eq. (3):
k =2k /[2H O+],
(3)
1
obs
3
where k
is the pseudo first order deuteration
obs
rate constant and [2H O+] is the 2H O+ concen3
3
tration (1.6103 M ) in the homogeneous supercritical phase. Substitution of k
values into
obs
Eq. (3) yields values of k for methyl of
1
2.9102 and 6.3102 M1 s1 at 380 and
400C, respectively. Methylene deuteration values
for k of 4.8102 and 1.0101 M1 s1 are
1
computed at 380 and 400C, respectively.
The equilibrium constant for Eq. (1) at each
temperature can now be estimated from the experimentally determined value for k and a value for
1

170

Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165172

k computed from the theory for diffusion con1

trolled reaction rates and the known viscosity of
supercritical water [2931]. The diffusion controlled rate constant, k , is calculated by Eq. (4)
1
[29]:
k =(2RT/3g) [(r +r )2/(r r )],
(4)
1
A B
A B
where R is the gas constant, T the absolute temperature, g the solution viscosity and r and r the
A
B
radii of the reacting substances. Substitution of
the viscosity of water for the experimental conditions [27] and approximating r and r from the
A
B
molecular volumes of water and hexane [32]
assuming spherical shapes yields values for k of
1
2.41011 and 2.71011 M1 s1 at 380 and
400C, respectively.
The equilibrium constant for Eq. (1) can now
be calculated from K =k /k at the two tempereq
1 1
atures. This calculation results in values of
1.21013 and 2.31013 for CH
and
3
2.01013 and 3.71013 for CH at 380 and
2
400C, respectively. The base dissociation constants for the base ionization reaction shown in
Eq. (6) can then be calculated from the values of
K and K by means of Eq. (5):
eq
w
K =K K ,
(5)
b
eq w
where K is the base dissociation constant for the
b
weak base hexane as shown in Eq. (6):
R1H+2H OOR1H2H++O2H.
(6)
2
Assuming that K is not affected by the presence
w
of the acid and hexane, the K values for 380 and
w
400C are calculated to be 31014 and
41014, respectively, for 1H O [33]. Estimating
2
that the K for 2H O is an order of magnitude
w
2
smaller, its substitution into Eq. (5) results in K
b
values of 3.51028 and 9.21028 for CH and
3
6.01028 and 1.51027 for CH at 380 and
2
400C, respectively.
Since rates have been measured at two temperatures, an activation energy for the slow reaction
can be estimated. The data given above indicate a
value of approximately 14050 kJ mol1 for the
activation energy of the forward reaction (k ).
1
When this value of activation energy is used in the
Arrhenius equation and corrections for differences
in temperature and viscosity are made, the room
temperature (25C ) K is found to be approxib

mately 71041 for CH . Because of the substan3

tial experimental uncertainty in the E value and
a
the small temperature range employed, this room
temperature value should be considered semiquantitative at best.
This study demonstrates the powerful protonating tendency of hydrogen ions in supercritical
water and suggests that one should consider the
formation of protonated intermediates as likely
when examining the mechanisms of reactions in
acidic solutions under these conditions. Such protonation can lead not only to the exchange of
hydrogen isotopes, but may also result in the
formation of carbocations which can rearrange to
unexpected products and hydrogen gas which can
act as a reducing agent or hazard in the reaction
sequence.
It is interesting to note that, while the supercritical conditions employed in this work lead to an
increased reaction between H+ and the substrate
(i.e. an increased K ), the K for water is not very
b
w
different from that of 25C liquid water. In fact,
the water ionization is somewhat greater than the
room temperature value. This suggests that the
reaction of H+ with OH is fundamentally
different from other acid/base reactions.
Examination of the data in Ref. [21] shows that,
over a temperature range up to about 300C, the
water ionization constant increases while every
other acid examined, except Si(OH ) , decreases.
4
It is only at temperatures above 300C that K
w
decreases with increasing temperature, but temperatures higher than 400C and densities less than
0.5 g ml1 are necessary to obtain K s much less
w
that 1014. For example, at 400C, the K values
w
for water are 41014 at a density of
0.45 g ml1 and 51019 at a density of
0.2 g ml1 [33]. Another manifestation of the
difference between OH and other bases is seen in
the dramatic decrease in the ratio of HCl (and
other acids) ionization to water ionization as temperature is increased. At 400C, the K /K ratio
i w
for HCl/water is more than 10 orders of magnitude
less than the room temperature value, indicating
that the base strength of Cl has increased by a
similar 10 orders of magnitude relative to the base
strength of OH [21]. Thus, it seems that, despite
the relatively large K value, the H+ produced by
w
the weak HCl ionization at 400C is much more

Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165172

reactive to other bases, in this case hexane, than

it is to OH or, probably, water.
Although we did not observe any rearrangement
compounds or hydrogen gas, which would be
evidence for carbocation formation, we believe
that the reaction does involve formation of a fivebonded carbon intermediate, analogous to the
initial reaction product in magic acid. We cannot
exclude the possibility that carbocations are
formed at a much slower rate than hydrogen
scrambling. If hydride abstraction reactions, which
lead to carbocation products from the five-bonded
intermediate, are much slower than k , then the
1
rearrangement products would be produced in
amounts too small to be detected in these experiments. Also, the small amount of hydrogen gas
that would be produced in such a reaction would
not be observed in our experiments even if it was
not consumed in a subsequent redox reaction. In
support of this analysis, we note that carbocation
formation in magic acid media was considerably
slower than the hydrogen scrambling process
[22,23]. Because all of the reactions in magic acid
are much faster than the reactions in this study,
those earlier workers were able to observe the
formation of rearrangement products despite their
much slower (orders of magnitude) formation rate.
In our case, the several order of magnitude time
scale difference expected is so long that it presents
a significant experimental challenge. We are presently performing experiments specifically to look
for hydrogen gas and rearrangement products that
would demonstrate hydride abstraction and carbocation formation that would add additional evidence for the analogy with magic acid chemistry.
The fact that hexane can be protonated by HCl
under conditions where the HCl is only partially
ionized itself is rather surprising considering the
virtually non-polar nature of this and other
alkanes. It might also be argued, however, that
the well documented protonation of Cl under
these experimental conditions is also unexpected
in view of the very weak basisity of Cl consistent
with the strong acid behavior of room temperature
HCl. There are at least two possible contributing
factors that lead to the observed behavior. First,
our experimental procedure produces product
species from intermediates that may exist for only

171

a very short time period. At the high temperatures

employed, the forward reactions of even a very
unfavorable equilibrium can occur often enough
to lead to an observable build up of trapped (i.e.
deuterated) product. Second, the dielectric constant of supercritical water is considerably reduced
compared with room temperature conditions and,
therefore, separation of charge reactions, such as
the ionization of HCl are disfavored. As shown by
Ryan et al. [34] and Xiang and Johnston [35], the
low dielectric constant of supercritical water favors
protonation to produce ions with smaller charge
to volume ratios (i.e. for a fixed +1 charge, the
larger entity). Since hexane is considerably larger
and less polar than Cl, this effect would tend to
favor its ionization in competition with the Cl.
Therefore, hexane is able to compete with Cl
(although the equilibrium still favors Cl by a
large extent) for the highly reactive, poorly solvated proton.

Acknowledgment
This work was funded by the National Science
Foundation (grant EHR-9108765). Presented in
part at the 215th National ACS Meeting, Dallas,
TX, April 1, 1998.

References
[1] T.J. Houser, D.M. Tiffany, Z. Li, M.E. McCarville, M.E.
Houghton, Reactivity of some organic compounds with
supercritical water, Fuel 65 (1985) 827832.
[2] L.A. Torry, R. Kaninsky, M.T. Klein, M.R. Klotz, The
effect of salts on hydrolysis in supercritical and near-critical
water: reactivity and availability, J. Supercrit. Fluids 5
(1992) 163168.
[3] S.H. Townsend, M.T. Klein, Dibenzyl ether as a probe
into the supercritical fluid solvent extraction of volatiles
from coal with water, Fuel 64 (1985) 635638.
[4] T.D. Thornton, P.E. Savage, Phenol oxidation pathways
in supercritical water, Ind. Engng Chem. Res. 31 (1992)
24512456.
[5] R.K. Helling, J.W. Tester, Oxidation of simple compounds
and mixtures in supercritical water: carbon monoxide,
ammonia and ethanol, Environ. Sci. Technol. 22 (1988)
13191324.
[6 ] T.D. Thornton, LaDueD.E., III, P.E. Savage, Phenol oxidation in supercritical water: formation of dibenzofuran,

172

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16 ]

[17]

[18]

[19]

[20]

[21]

Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165172

dibenzo-p-dioxin and related compounds, Environ. Sci.
Technol. 25 (1991) 15071510.
M.R. Hibbs, J. Yao, R.F. Evilia, Reactivity of activated
phenyl groups in supercritical water, High Temp. Mater.
Sci. 36 (1996) 914.
A.R. Katritzky, R.A. Barcock, M. Balasubramanian, J.V.
Greenhill, M. Siskin, W.N. Olmestead, Aqueous high-temperature chemistry of carbo- and heterocycles. 20.
Reactions of some benzenoid hydrocarbons and oxygencontaining derivatives in supercritical water at 460C,
Energy Fuels 8 (1994) 498506.
A.R. Katriztzky, R.A. Barcock, M. Balasubramanian, J.V.
Greenhill, M. Siskin, W.N. Olmestead, Aqueous high-temperature chemistry of carbo- and heterocycles. 21.
Reactions of sulfur-containing compounds in supercritical
water at 460C, Energy Fuels 8 (1994) 487497.
A.R. Katritzky, P.A. Shipkova, S.M. Allin, R.A. Barcock,
M. Siskin, W.N. Olmstead, Aqueous high-temperature
chemistry. 24. Nitrogen-containing heterocycles in supercritical water at 460C, Energy Fuels 9 (1995) 580589.
J. Yao, R.F. Evilia, Deuteration of extremely weak organic
acids by enhanced acidbase reactivity in supercritical
deuteroxide solution, J. Am. Chem. Soc. 116 (1994)
1122911233.
Y. Yang, R.F. Evilia, Deuteration of amino acids in basic
deuterium oxide solution at supercritical temperatures,
J. Supercrit. Fluids 9 (1996) 113118.
Y. Yang, R.F. Evilia, Deuteration of amino acids in acidic
deuterium oxide solution at supercritical temperatures,
J. Supercrit. Fluids 9 (1996) 161167.
T. Junk, W.J. Catallo, J. Elguero, Syntheses in superheated
aqueous media: preparation of fully deuterated pyrazoles
and quinoxaline, Tetrahedron Lett. 38 (1997) 63096312.
T. Junk, W.J. Catallo, Preparative supercritical deuterium
exchange in arenes and heteroarenes, Tetrahedron Lett. 37
(1996) 34453448.
P.B. Balbuena, K.P. Johnston, P.J. Rossky, Molecular
dynamics simulation of electrolyte solutions in ambient
and supercritical water. 1. Ion solvation, J. Phys. Chem.
100 (1996) 27062715.
P.B. Balbuena, K.P. Johnston, P.J. Rossky, Molecular
dynamics simulation of electrolyte solutions in ambient
and supercritical water. 2. Relative acidity of HCl, J. Phys.
Chem. 100 (1996) 27162722.
E.T. Ryan, T. Xiang, K.P. Johnston, M.A. Fox, Excitedstate proton transfer reactions in subcritical and supercritical water, J. Phys. Chem. 100 (1996) 93959402.
S. Green, T. Xiang, K.P. Johnston, M.A. Fox, Excitedstate deprotonation of b-naphthol in supercritical water,
J. Phys. Chem. 99 (1995) 1378713795.
R.E. Mesmer, W.L. Marshall, D.A. Palmer, J.M.
Simonson, H.F. Holmes, Thermodynamics of aqueous
association and ionization reactions at high temperatures
and pressures, J. Solution Chem. 17 (1988) 699718.
G.A. Olah, J. Lukas, Stable carbonium ions. XXXIX.
Formation of alkylcarbonium ions via hydride ion abstrac-

[22]

[23]

[24]

[25]

[26 ]

[27]

[28]
[29]

[30]
[31]

[32]
[33]

[34]

[35]

tion from alkanes in fluorosulfonic acidantimony pentafluoride solution. Isolation of some crystalline
alkylcarbonium ion salts, J. Am. Chem. Soc. 89 (1967)
22272228.
G.A. Olah, G. Klopman, R.H. Schlosberg, Chemistry in
super acids. III. Protonation of alkanes and the intermediacy of alkanonium ions, pentacoordinated carbon cations
of the CH+ type. Hydrogen exchange, protolytic cleavage,
5
hydrogen abstraction and polycondensation of methane,
ethane,
2,2-dimethylpropane
(neopentane)
and
2,2,3,3-tetramethylbutane in FSO H-SbF (magic acid)
3
5
solution, J. Am. Chem. Soc. 91 (1969) 32613268.
H. Hogeveen, A.F. Bickel, Chemistry and spectroscopy in
strongly acidic solutions: electrophilic substitution at
alkane carbon by protons, Chem. Commun. (1967)
635636.
Y. Yang, Studies of Acid/Base Reactivity in Superheated
and Supercritical Water, PhD dissertation, University of
New Orleans, 1997.
S. Gopalan, P.E. Savage, Phenol oxidation in supercritical
water: from global kinetics and product identities to an
elementary reaction model, in: K.W. Hutchenson, N.R.
Foster ( Eds.), Innovations in Supercritical Fluids,
American Chemical Society, Washington, DC, 1995,
pp. 217231.
G.A. Olah, A. Burrichter, G. Rasul, G.K.S. Prakash, M.
Hachoumy, J. Sommer, Protioacyl dications: hydrogen/
deuterium exchange, rearrangements and theoretical
studies, J. Am. Chem. Soc. 118 (1996) 1042310428.
C.W. Burnham, J.R. Holloway, N.F. Davis, N.F.
Thermodynamic Properties of Water to 1000C and
10 000 bar, Special paper no. 132, The Geological Society
of America, New York, 1969, p. 19.
J.R. Jones, The Ionisation of Carbon Acids, Academic
Press, New York, 1973, pp. 124149.
M.V. Smoluchowski, Mathematical theory of the kinetics
of the coagulation of colloidal solutions, Z. Phys. Chem.
92 (1917) 129168.
W.C. Gardiener, Jr., Rates and Mechanisms of Chemical
Reactions, Benjamin, New York, 1969, pp. 165169.
J.H. Keenan, F.G. Keyes, P.G. Hill, J.G. Moore, Steam
Tables, Thermodynamic Properties of Water Including
Vapor, Liquid and Solid Phases, Wiley, New York, 1969,
p. 114.
A. Bondi, van der Waals volumes and radii, J. Phys. Chem.
68 (1964) 441451.
W.L. Marshall, E.U. Franck, Ion product of water substance, 01000C, 110 000 bar. New international formulation and its background, J. Phys. Chem. Ref. Data 10
(1981) 295304.
E.T. Ryan, T. Xiang, K.P. Johnston, M.A. Fox,
Absorption and fluorescence studies of acridine in subcritical and supercritical water, J. Phys. Chem. 101 (1997)
18271835.
T. Xiang, K.P. Johnston, Acidbase behavior of organic
compounds in supercritical water, J. Phys. Chem. 98 (1994)
79157922.