Deuteration of hexane by 2HCl in supercritical deuterium oxide

Journal of Supercritical Fluids 15 (1999) 165–172

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 and400°C. The deuteration rate follows pseudo first order kinetics at both temperatures with the methylene reaction ratebeing 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 Kb’s for the methyl groupare 3.5×10−28 and 9.2×10−28, while the methylene groups have Kb’s of 6.0×10−28 and 1.5×10−27 at 380 and 400°C,respectively. No evidence is seen for hydride abstraction, such as formation of carbocation rearrangement species orhydrogen gas evolution as in ‘magic acid’. Hydride abstraction to form carbocations either does not occur or occursat a rate too slow to be observed in the time scale of the experiments reported here. © 1999 Elsevier Science B.V. Allrights reserved.

Keywords: Deuteration; 2HCl; Hexane; Kb; Supercritical deuterium oxide

1. Introduction that a number of organic compounds whose roomtemperature acidity or basicity is negligibly small,

The potential of supercritical water oxidation will undergo reactions with aqueous base or acid(SCWO) for the environmentally benign destruc- at sufficiently rapid rates that C–H exchange withtion of organic wastes has led to recent interest in the solvent is observed in a time scale of minutesthe properties and reactions of a variety of organic [11–13]. The acid/base reaction of hydrocarbonsmolecules in supercritical water [1–10]. Acid/base with deuterated hydroxide in supercritical waterproperties in supercritical water are of fundamental has been shown to be a viable synthetic approachimportance in understanding the physical and to some perdueterated compounds [14,15]. Otherchemical ramifications of SCWO and other reac- studies have shown decreased dissociation oftions in this incompletely characterized medium. hydrogen ions from acidic sites such as HCl andPrevious studies in this laboratory have shown 2-naphthol in supercritical water and have sug-

gested the formation of contact ion pairs betweenH+ and the basic site [16–19]. Thermodynamic

* Corresponding author. Tel.: +1-504-280-6313; studies of aqueous ionization reactions have shownfax: +1-504-280-6860.

dramatic decreases in the acid dissociation con-E-mail address: [email protected] (R.F. Evilia)stant for a wide variety of weak acids in aqueous1Current address: Department of Chemistry, Miami University,

Oxford, OH 45056. solution above 300°C [20]. In this same thermo-

0896-8446/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S0896-8446 ( 99 ) 00003-0

166 Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165–172

dynamic study, the acid dissociation constant forHCl is reported to be approximately 2.5×10−5under the conditions used here [20]. Based uponthe increased tendency to protonate weakly basicsites, these studies suggest that H+ in supercriticalwater is far more reactive than in less severeconditions, perhaps because of reduced hydrationof the ion.

In this report, 2HCl is shown to react withhexane to produce deuterated products involvingdeuterium exchange at both methyl and methylenepositions with ethylene deuteration occurring at arate approximately 1.6 times that of methyl. Thisis in contrast to the reaction in basic supercriticaldeuterium oxide where only methyl deuteration isobserved [11]. In the examples reported here, themechanism is believed to involve the reaction of2H+ with hexane sites to form a five-bonded,positively charged carbon intermediate, analogousto the five-bonded intermediates reported in iso-tope scrambling experiments of alkanes in so called‘magic acid’ media [21–23]. However, contrary tothe behavior in ‘magic acid’, no carbocationrearrangement products or hydrogen evolutionwere observed in the study reported here. Fig. 1. Internal reactor temperature versus time. Oven set to

400°C.

2. Experimentalment was performed on a tube filled with anappropriate amount of water, but did not containReactions were performed in flame sealed quartz

tubes placed in stainless steel tubes containing a a quartz sample tube. Deuteration experimentswere conducted without the thermocouple present.suitable amount of water to balance the internal

pressure developed in the quartz tube when heated. Examination of Fig. 1 shows that approximately30 min heating time is required to reach the setThe stainless steel tube was then sealed with

swadgelok@ plugs. In a typical experiment, temperature of 400°C. Once the set temperaturewas reached, the measured temperature variation0.257 ml of 2.432 M 2HCl and 0.05 ml of hexane

were added to a 0.64 ml volume quartz tube (3 mm was no more than ±1°C for a 5 h test. Thus it ispresumed that the temperature is equal to the setid, 9 cm long). Above the critical temperature

where a single phase exists, the concentration of temperature for an indefinite period after the30 min initial warm-up. Since no measurable2HCl is 0.98 M and the sample density is about

0.46 g ml−1. The 2HCl solution was deaerated by amount of deuterium incorporation was observedin the first 30 min of heating, the relatively longargon for 10 min prior to placing it in the quartz

tube. The internal temperature of the sample tube heat-up time of the experimental arrangement isnot considered to be a source of error in the kineticwas monitored by an internal JK type thermocou-

ple sealed in the stainless steel tube by means of a analysis. Temperatures of 380 and 400°C wereemployed in this study. While it would be desirableTG gland (TG-24-*2, Conax Buffalo) with a

Lava@ sealant. The internal temperature/time pro- to conduct the kinetic study over a wider range oftemperatures, experimental problems preventedfile is shown in Fig. 1. The temperature measure-

167Y. Yang, R.F. Evilia / Journal of Supercritical Fluids 15 (1999) 165–172

our doing so in this case. At temperatures below 3. Results and discussion380°C, excessive scatter was found in the kineticplots. We believe that this scatter is caused by the The first question that naturally arises when

attempting to write a mechanism for the hydrogenpresence of two phases with different reactionrates. Because each time point in a kinetic study exchange that we observed is whether the reaction

involves homolytic or heterolytic C–H bond break-is a new experiment involving a different sealedquartz tube, we are not able to exactly duplicate age. Since hexane is generally considered to be a

non-polar molecule, one would be tempted tothe two phase conditions from one experiment tothe next and, thus, excessive scatter is observed initially assume that the reaction is a homolytic

one involving the production of hydrogen atomsin the kinetic plots. Above 400°C, frequent quartztube failures occurred because of the difficulty in and hexane radicals. Indeed, at high temperatures

and low densities, homolytic free radical reactionsaccurately balancing the internal pressure of thequartz tube with external water pressure [24]. do occur [25]. However, the higher densities and

lower temperatures employed in this study increaseExperiments conducted at higher temperatureswith lower density water have survived heating the possibility that a heterolytic, acid/base type

reaction is occurring in this case.but, because of the lower density involved, repre-sent different reaction conditions. These results Previous reports from this laboratory have

shown selective deuterium incorporation in acidicwill be reported in a future communication.Deuterium incorporation was monitored by and basic media that are qualitatively consistent

with expectations of relative acid/base strengths ofNMR spectroscopy. 13C spectroscopy wasemployed to qualitatively verify deuteration of the the organic substrate. For example, in basic media,

carbon sites adjacent to electron withdrawingcarbon sites through observation of deuteriumcoupling patterns and isotope shifts of the carbon groups deuterated faster than sites several carbons

removed from these groups [11]. Also in thatresonances. While we have previously used 13Cspectra for semi quantitative measurements of earlier work, the preferential deuteration of the

methyl groups of 2-methylpentane in basic mediadeuteration of CH groups, such spectra were notsuitable for quantitative measurements in this case was sited as support for an ionic acid/base reaction

in which a primary carbanion intermediate isbecause of the complex patterns developed bymultiple deuteration and uncertainty about the formed via H+ abstraction by the strong base.

The greater stability of primary carbanions leadsnuclear Overhauser effect for partially deuteratedcases [11,24]. For quantitative measurements, 1H to a preference for methyl deuteration in a basic

solution if an ionic mechanism is involved.NMR spectral integrals were used. Quantitationwas accomplished by extraction of the hexane into Another observation suggesting an ionic mecha-

nism is the decomposition product of isoleucine indeuterochlorform containing a known percentageof protiochloroform. Following extraction, the supercritical HCl at 400°C [13]. The principal

decomposition product formed from isoleucine inconcentration ratio of chloroform to hexane wasmeasured by gas chromatography. The 1H NMR supercritical HCl is methyl-isopropylketone. As

shown later, the formation of this product requiresintegrals for the chloroform and hexane signalswere then evaluated and the percentage of deutera- substantial rearrangement of the original isoleucine

carbon skeleton. Carbon skeleton rearrangementstion of methyl and methylene carbons computedfrom the known chromatographic concentration of this sort are frequently observed when carboc-

ations are formed in the reaction, while free radi-ratio and NMR 1H integral ratio. Spectral overlapprevented accurate, separate integration of the cals tend to produce a wide array of products

formed by joining small fragments together. Olahtwo CH2 signals and, therefore, an average valuefor both CH2’s is reported. In view of the absence et al. has reported the formation of the same

compound, methy-isopropylketone, by carboca-of significant curvature in the CH2 kinetic plots,it is apparent that both CH2’s have very similar tion rearrangement of the pivaloyl dication in

superacid media [26 ]. One can imagine diproto-reaction rates.


nated isoleucine losing water and ammonia to than a factor of 40. To determine a quantitativedifference in deuteration rates, it is necessary toform a dicationic species which could undergo a

related carbocation rearrangement to yield the extend the neutral solution heating until observabledeuteration occurs. Because the concentration ofobserved product if the protonating ability of the

acid is strong enough. It is our contention that 2H+ is approximately 104 times more dilute in theneutral solution, the ionic reaction should requireHCl under supercritical water conditions is a pow-

erful protonating agent similar to a super acid. In about 50 days to produce observable deuteration(~10%). We also note that no deuteration wasview of these qualitative observations, it seems

probable that ionic reaction mechanisms are observed within experimental error (<10%) when2HCl/hexane mixtures were heated for 30 min orinvolved under the conditions of temperature and

pressure employed in this work. less. This indicates that the deuteration occursrapidly after supercritical conditions are reached

CH3CH2CH(CH3)CH(NH2)CO2H but slowly prior to those conditions.Figs. 2 and 3 show that the deuteration reaction

�400°C, 250 Bar

HCl(CH3)2CHCOCH3 follows pseudo first order behavior in 0.98 M

2HCl at 380 and 400°C, respectively. The sampledensity in these experiments is approximatelyAs a test for homolytic hydrogen exchange, a

mixture of hexane and deuterium oxide was main- 0.46 g ml−1 and the maximum pressure is esti-mated to be about 370 bar [27]. The pressure istained at 400°C for 6 h in the sealed tube reactor.

No evidence for incorporation of deuterium or assumed to be equal to that of pure 1H2O at a

density of 0.45 g ml−1 (i.e. the sample densitydecomposition of the hexane was observed by13C NMR spectroscopy following the heating corrected for the atomic weight of deuterium).

Analysis of the data shown in Figs. 2 and 3 yieldsperiod (deuteration <10%). Thus, we concludethat homolytic bond rupture is not a significant pseudo first order deuteration rate constants, kobs,

of (2.3±0.2) · 10−5 and (5.0±0.5)×10−5 s−1 forfactor in the reaction that produces deuteratedproducts at temperatures ≤400°C at the solution CH3 and (3.8±0.3)×10−5 and (8.0±0.5)

×10−5 s−1 for CH2 at 380 and 400°C, respectively.density employed in this study. Since no newproduct peaks were observed in this experiment, it The error estimates correspond to the 95% confi-also seems clear that insignificant amounts ofreactive intermediate species are produced in theneutral aqueous environment at 400°C.

In contrast to the lack of deuterium incorpora-tion in neutral deuterium oxide, extensive hexanedeuteration was observed when the hexane and2HCl solution were subjected to the same temper-ature and pressure conditions. In fact, significantdeuterium incorporation (~20%) was observed by13C NMR spectroscopy after 45 min in a 400°Coven. In view of the heat-up time of approximately30 min, as shown in Fig. 1, this 45 min experimen-tal reaction time corresponds to only about 15 minat 400°C. In comparison with the neutral experi-ment, the deuteration rate in the acid solution isat least 40 times faster. Since no deuteration at allwas seen in the absence of 2HCl, the factor 40 isin reality a lower limit based upon the minimumexperimentally detectable amount of deuteration Fig. 2. Pseudo first order kinetic plot for hexane deuteration

at 380°C.and the true rate difference may be much greater


Thus, the rate of production of deuterated productcan be shown to be equal to half of the forwardrate of Eq. (1), assuming that k−1=k−2. Thislatter assumption is reasonable as both of thesereactions involve the reaction of the same interme-diate with 2H

2O and both should be diffusion

controlled [28].One difficulty in the straightforward utilization

of the kinetic analysis of Eqs. (1) and (2) arisesfrom the fact that the acid, 2HCl, is substantiallyunionized under the reaction conditions employed.At 400°C and the appropriate density for thecurrent reaction conditions, the 1HCl ionizationconstant is approximately 2.5×10−5 [21]. Thus,unlike one would expect in room temperaturewater, the concentration of 2H

3O+ present for

Eq. (1) should be much less than the initial concen-Fig. 3. Pseudo first order kinetic plot for hexane deuteration

tration of 2HCl added to the reaction tube.at 400°C.Because the ionization constant for 2HCl in

supercritical deuterium oxide was not reported, itdence intervals for the slopes. It should be noted

must be estimated for use in this study. Basedthat the first order kinetic plots of Figs. 2 and 3

upon the fact that the ionization constant fordo not extrapolate back to the origin at zero time.

2H2O is nearly one order of magnitude less than

This is caused by the long induction period inthat of 1H

2O over a wide range of temperatures

which the sample temperature varies from roomand pressures [21], we estimate the 2HCl ionization

temperature to the experimental value. Becauseconstant to be 2.5×10−6 in this case. Using this

the temperature varies with time in this intervalestimated value for the ionization constant and

and the degree of deuteration is extremely small,the 0.98 M analytical concentration of 2HCl, the

the data are not interpretable at short heating2H

3O+ concentration is computed to be approxi-

times.mately 1.6×10−3.

Of more fundamental interest than the deutera-From the deuteration rate constant, kobs,tion rate, however, are the respective acid/base

obtained from the pseudo first order plots ofequilibrium constants under the reaction condi-

Figs. 2 and 3, values for k1 at each temperaturetions. One possible way to view the deuteration

are calculated by Eq. (3):reaction sequence is via the two step mechanismshown in Eqs. (1) and (2), below: k

1=2kobs/[2H3

O+ ], (3)

where kobs is the pseudo first order deuterationR1H+2H

3O+P

k−1

k1 R1H2H++2H

2O, (1)

rate constant and [2H3O+ ] is the 2H

3O+ concen-

tration (1.6×10−3 M) in the homogeneous super-critical phase. Substitution of kobs values intoR1H2H++2H

2OP

k2

k−2 R2H+1H2H

2O+ . (2)

Eq. (3) yields values of k1 for methyl of2.9×10−2 and 6.3×10−2 M−1 s−1 at 380 andThe reaction sequence expressed by Eqs. (1) and

(2) can be analyzed by steady-state approximation 400°C, respectively. Methylene deuteration valuesfor k1 of 4.8×10−2 and 1.0×10−1 M−1 s−1 aretechniques since k−2 and k−1 should be much

faster than k1 and k2. Moreover, since the concen- computed at 380 and 400°C, respectively.The equilibrium constant for Eq. (1) at eachtrations of both R2H and 1H2H

2O+ are very small

under the reaction conditions employed, the rate temperature can now be estimated from the experi-mentally determined value for k1 and a value forof the back reaction of R2H can be neglected.


k−1 computed from the theory for diffusion con- mately 7×10−41 for CH3. Because of the substan-tial experimental uncertainty in the Ea value andtrolled reaction rates and the known viscosity of

supercritical water [29–31]. The diffusion con- the small temperature range employed, this roomtemperature value should be considered semi-trolled rate constant, k−1, is calculated by Eq. (4)

[29]: quantitative at best.This study demonstrates the powerful protonat-

k−1=(2RT/3g) [(rA+rB)2/(rArB)], (4) ing tendency of hydrogen ions in supercriticalwater and suggests that one should consider thewhere R is the gas constant, T the absolute temper-formation of protonated intermediates as likelyature, g the solution viscosity and rA and rB thewhen examining the mechanisms of reactions inradii of the reacting substances. Substitution ofacidic solutions under these conditions. Such pro-the viscosity of water for the experimental condi-tonation can lead not only to the exchange oftions [27] and approximating rA and rB from thehydrogen isotopes, but may also result in themolecular volumes of water and hexane [32]formation of carbocations which can rearrange toassuming spherical shapes yields values for k−1 ofunexpected products and hydrogen gas which can2.4×1011 and 2.7×1011 M−1 s−1 at 380 andact as a reducing agent or hazard in the reaction400°C, respectively.sequence.The equilibrium constant for Eq. (1) can now

It is interesting to note that, while the supercriti-be calculated from Keq=k1/k−1 at the two temper-cal conditions employed in this work lead to anatures. This calculation results in values ofincreased reaction between H+ and the substrate1.2×10−13 and 2.3×10−13 for CH3 and(i.e. an increased Kb), the Kw for water is not very2.0×10−13 and 3.7×10−13 for CH2 at 380 anddifferent from that of 25°C liquid water. In fact,400°C, respectively. The base dissociation con-the water ionization is somewhat greater than thestants for the base ionization reaction shown inroom temperature value. This suggests that theEq. (6) can then be calculated from the values ofreaction of H+ with OH− is fundamentallyKeq and Kw by means of Eq. (5):different from other acid/base reactions.

Kb=KeqKw, (5) Examination of the data in Ref. [21] shows that,over a temperature range up to about 300°C, thewhere Kb is the base dissociation constant for thewater ionization constant increases while everyweak base hexane as shown in Eq. (6):other acid examined, except Si(OH)4, decreases.

R1H+2H2OOR1H2H++O2H− . (6) It is only at temperatures above 300°C that Kw

decreases with increasing temperature, but temper-Assuming that Kw is not affected by the presenceatures higher than 400°C and densities less thanof the acid and hexane, the Kw values for 380 and0.5 g ml−1 are necessary to obtain Kw’s much less400°C are calculated to be 3×10−14 andthat 10−14. For example, at 400°C, the Kw values4×10−14, respectively, for 1H

2O [33]. Estimating

for water are 4×10−14 at a density ofthat the Kw for 2H2O is an order of magnitude

0.45 g ml−1 and 5×10−19 at a density ofsmaller, its substitution into Eq. (5) results in Kb 0.2 g ml−1 [33]. Another manifestation of thevalues of 3.5×10−28 and 9.2×10−28 for CH3 anddifference between OH− and other bases is seen in6.0×10−28 and 1.5×10−27 for CH2 at 380 andthe dramatic decrease in the ratio of HCl (and400°C, respectively.other acids) ionization to water ionization as tem-Since rates have been measured at two temper-perature is increased. At 400°C, the Ki/Kw ratioatures, an activation energy for the slow reactionfor HCl/water is more than 10 orders of magnitudecan be estimated. The data given above indicate aless than the room temperature value, indicatingvalue of approximately 140±50 kJ mol−1 for thethat the base strength of Cl− has increased by aactivation energy of the forward reaction (k1). similar 10 orders of magnitude relative to the baseWhen this value of activation energy is used in thestrength of OH− [21]. Thus, it seems that, despiteArrhenius equation and corrections for differencesthe relatively large Kw value, the H+ produced byin temperature and viscosity are made, the room

temperature (25°C ) Kb is found to be approxi- the weak HCl ionization at 400°C is much more


reactive to other bases, in this case hexane, than a very short time period. At the high temperaturesemployed, the forward reactions of even a veryit is to OH− or, probably, water.

Although we did not observe any rearrangement unfavorable equilibrium can occur often enoughto lead to an observable build up of trapped (i.e.compounds or hydrogen gas, which would be

evidence for carbocation formation, we believe deuterated) product. Second, the dielectric con-stant of supercritical water is considerably reducedthat the reaction does involve formation of a five-

bonded carbon intermediate, analogous to the compared with room temperature conditions and,therefore, separation of charge reactions, such asinitial reaction product in ‘magic acid’. We cannot

exclude the possibility that carbocations are the ionization of HCl are disfavored. As shown byRyan et al. [34] and Xiang and Johnston [35], theformed at a much slower rate than hydrogen

scrambling. If hydride abstraction reactions, which low dielectric constant of supercritical water favorsprotonation to produce ions with smaller chargelead to carbocation products from the five-bonded

intermediate, are much slower than k1, then the to volume ratios (i.e. for a fixed +1 charge, thelarger entity). Since hexane is considerably largerrearrangement products would be produced in

amounts too small to be detected in these experi- and less polar than Cl−, this effect would tend tofavor its ionization in competition with the Cl−.ments. Also, the small amount of hydrogen gas

that would be produced in such a reaction would Therefore, hexane is able to compete with Cl−(although the equilibrium still favors Cl− by anot be observed in our experiments even if it was

not consumed in a subsequent redox reaction. In large extent) for the highly reactive, poorly sol-vated proton.support of this analysis, we note that carbocation

formation in ‘magic acid’ media was considerablyslower than the hydrogen scrambling process[22,23]. Because all of the reactions in ‘magic acid’ Acknowledgmentare much faster than the reactions in this study,those earlier workers were able to observe the This work was funded by the National Science

Foundation (grant EHR-9108765). Presented information of rearrangement products despite theirmuch slower (orders of magnitude) formation rate. part at the 215th National ACS Meeting, Dallas,

TX, April 1, 1998.In our case, the several order of magnitude timescale difference expected is so long that it presentsa significant experimental challenge. We are pres-ently performing experiments specifically to look Referencesfor hydrogen gas and rearrangement products thatwould demonstrate hydride abstraction and carbo- [1] T.J. Houser, D.M. Tiffany, Z. Li, M.E. McCarville, M.E.

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