Chemical Reactions of Organic Molecules Adsorbed at Ice: 2. Chloride

A new reaction is reported on the ice surface, namely the conversion of adsorbed 2-methyl-2-propanol [(CH3)3COH] to adsorbed 2-chloro-2-methylpropane ...
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Langmuir 2000, 16, 3244-3248

Chemical Reactions of Organic Molecules Adsorbed at Ice: 2. Chloride Substitution in 2-Methyl-2-propanol James D. Graham and Jeffrey T. Roberts* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431 Received May 19, 1999. In Final Form: November 16, 1999 A new reaction is reported on the ice surface, namely the conversion of adsorbed 2-methyl-2-propanol [(CH3)3COH] to adsorbed 2-chloro-2-methylpropane [(CH3)3CCl] in the presence of absorbed HCl. The reaction was studied using temperature-programmed desorption mass spectrometry on ultrathin ice films under ultrahigh vacuum. Formation of (CH3)3CCl occurs at two temperatures: at or below 155 K and at 185 K. The lower-temperature process occurs at the surface of the film, but the origin of product at 185 K is unclear. The yield of the surface reaction product is low, approximately 0.04 monolayers. Importantly, adsorbed HCl is unreactive toward 2-methyl-2-propanol; reactive chlorine is derived from a dissociatively ionized state in the very near surface region of the film. The lower-temperature pathway for chloride substitution is observed only at the surface of an amorphous film, an observation that may have implications regarding the nature of surface reaction sites. Possible mechanisms of the surface reaction are discussed.

Introduction Chemical reactions that are promoted by the surface or near-surface regions of ice particles in type II polar stratospheric clouds (PSCs) play a role in the sequence of events that ultimately opens the Antarctic ozone “hole.”1 The most significant processes occurring over ice are believed to be:2-5 ice

HCl + ClONO2 98 Cl2 + HNO3 ice

HCl + HOCl 98 Cl2 + H2O ice

N2O5 + H2O 98 2HNO3

(1) (2) (3)

Most laboratory studies of heterogeneous atmospheric chemistry have been restricted to surrogates for cloud and aerosol particles at atmospherically relevant temperatures and pressures.2-8 The approach has been successful with regard to the determination of reaction kinetics and product distributions, but an understanding of the mechanisms of these reactions is still lacking. There is disagreement, for instance, over the nature of reactive HCl, specifically whether HCl is bound to the ice surface as part of a molecularly9,10 or dissociatively adsorbed phase,4,11 or whether it is confined to the near-surface region, perhaps in a liquidlike layer.11 * To whom correspondence should be addressed. Tel.: (612) 6252363; fax: (612) 626-7541; e-mail: [email protected]. (1) Solomon, S. Nature 1990, 347, 347-354. (2) Molina, M. J.; Tso, T.-L.; Molina, L. T.; Wang, F. C.-Y. Science 1987, 238, 1253-1257. (3) Tolbert, M. A.; Rossi, M. J.; Golden, D. M. Science 1988, 240, 1018-1021. (4) Hanson, D. R.; Ravishankara, A. R. J. Phys. Chem. 1992, 96, 2682-2691. (5) Abbatt, J. P. D.; Molina, M. J. Geophys. Res. Lett. 1992, 19, 461464. (6) Hanson, D. R.; Ravishankara, A. R. J. Phys. Chem. 1994, 98, 5728-5735. (7) Leu, M.-T. Geophys. Res. Lett. 1988, 15, 851-854. (8) Marti, J.; Mauersberger, K. Geophys. Res. Lett. 1993, 20, 359362. (9) Kroes, G.-J.; Clary, D. C. Geophys. Res. Lett. 1992, 19, 13551358. (10) Kroes, G.-J.; Clary, D. C. J. Phys. Chem. 1992, 96, 7079-7088.

One difficulty with extracting mechanistic information under atmospherically mimetic conditions is that the relevant reactions are rapid and multistep. One way to circumvent this obstacle is to work at lower temperatures, where the overall reaction rates are lower, and the elementary steps involved in a surface-mediated transformation could be isolated and studied individually.12 A second difficulty is that very few heterogeneous reactions have actually been identified as important in the stratosphere. Because detailed information is available on only a small number of reactions, the relations between reactant structure and reactivity are not well established. If such relations were established, a deeper understanding of mechanism might come about. Here we report the observation of a new heterogeneous reaction, the conversion of 2-methyl-2-propanol [(CH3)3COH] to 2-chloro-2-methylpropane [(CH3)3CCl] over the icy solid HCl‚6H2O. The reaction was identified in ultrahigh vacuum under conditions far removed from those of the troposphere or stratosphere. The formation of (CH3)3CCl from (CH3)3COH is not important in the atmosphere, because gas-phase (CH3)3COH is essentially nonexistent. Nonetheless, the findings reported in this paper, which is the second in a series of papers on organic reactivity at the ice surface,13 are relevant to the subject of heterogeneous atmospheric processing because they suggest possible mechanisms for reactions on the surfaces of ice and related materials. Experimental Section Experiments were conducted in two stainless steel vacuum chambers of base pressure ∼10-8 Pa that are described elsewhere.12,14,15 The chambers were equipped with sample manipulators capable of rotation around one axis, as well as translation along three mutually orthogonal axes. The ice samples were deposited onto single-crystal substrates, either Pt(111) or W(100), that were spot-welded to the sample manipulators. The substrates (11) Abbatt, J. P. D.; Beyer, K. D.; Fucaloro, A. F.; McMahon, J. R.; Wooldridge, P. J.; Zhang, R.; Molina, M. J. J. Geophys. Res. 1992, 97, 15819-15826. (12) Graham, J. D.; Roberts, J. T. J. Phys. Chem. 1994, 98, 59745983. (13) Graham, J. D.; Roberts, J. T. J. Phys. Chem. B 2000, 104, 978982. (14) Blanchard, J. L.; Roberts, J. T. Langmuir 1994, 10, 3303-3310. (15) Schaff, J. E.; Roberts, J. T. J. Phys. Chem. 1994, 98, 6900-6902.

10.1021/la9906166 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/17/2000

Reactions of Organic Molecules Adsorbed at Ice could be cooled to 100 K via thermal conduction from a liquid nitrogen-filled reservoir, and they could be heated using a filament, either radiatively or by electron bombardment. Sample temperatures were measured using thermocouple junctions, W-5%Re/W-26%Re for the tungsten substrate and chromel/ alumel for the platinum substrate. Electronic ice points substituted for reference junctions. Amorphous, ultrathin ice films were prepared by condensing water vapor onto the cold (ca. 120 K) metal substrates.12,14 Ice film thicknesses, which were inferred from the water desorption yields, are given in units of monolayer equivalents (ML), where 1 ML is defined as the exposure necessary to form a saturated water adlayer on the metal substrate. Details regarding the growth and characterization of ultrathin ice films are given elsewhere; here we summarize what is most pertinent to this work. First, ice films thicker than 6 ML are free of pinholes and related defects that if present would expose the underlying metal substrate to vacuum.14 Second, the substrates exert no measurable influence on the chemical properties of films more than 10 ML thick. Because the temperature-programmed desorption (TPD) data presented herein were obtained on films 12 ML or more thick, they characterize adsorption on ice. Finally, adsorbateaccessible surface areas of the films are independent of thickness and roughly equivalent to that of the underlying single-crystal surface. Gases were adsorbed on ice using a directed dosing scheme. The substrate was positioned approximately 5 mm in front of a doser designed to deliver a uniform flux across the surface. The pressure at the ice surface during exposure to a gas was greater than that recorded by the ionization gauge by a factor of 70 ( 10. Exposures are given in units of Pa‚s, and represent the product of the exposure time and the true pressure at the ice surface. Water was deionized and triply distilled, and it was degassed via several freeze-pump-thaw cycles before use each day. Methanol (Fisher), 2-methyl-2-propanol (Aldrich, 99.5%), and 2-chloro-2methylpropane (Eastman Chemicals) were degassed daily but otherwise used as received. Hydrogen chloride (Aldrich, 99+%) was purchased in lecture bottles and used without further purification.

Results The uptake of HCl by ice under ultrahigh vacuum is described elsewhere.12,16,17 Here we briefly summarize those aspects of the HCl + ice system that are relevant to the reaction described below. Hydrogen chloride is initially absorbed by amorphous ice at 100 K to form the amorphous solid hexahydrate phase, HCl‚6H2O(amorph). Upon completion of the bulk phase, HCl is adsorbed to the hexahydrate surface. There is some evidence that, for HCl adsorbed on HCl‚6H2O, the H-Cl bond is intact. Heating leads to desorption of adsorbed HCl at ∼150 K, to crystallization of the hexahydrate film to HCl‚6H2O(cryst) near 150 K, and to sublimation of the hexahydrate phase near 180 K. Infrared spectra of the amorphous and crystalline hexahydrate phases are reproduced in Figure 1. Justification for the assignment of the desorption states and infrared spectra, as well as a detailed description of the thickness and exposure dependences, appear elsewhere.12,17 The HCl‚6H2O structure, which was determined by X-ray diffraction, is characterized by a unit cell consisting of H9O4+Cl-‚2H2O in which the H3O+ cation sits at the center of H9O4+.18 The compound 2-methyl-2-propanol [(CH3)3COH] reacts when adsorbed on amorphous HCl‚6H2O to form the substitution product 2-chloro-2-methylpropane [(CH3)3CCl]. Figure 2 shows a representative set of TPD spectra, resulting from exposure of a 36-ML amorphous hexahy(16) Graham, J. D.; Roberts, J. T. Geophys. Res. Lett. 1995, 22, 251254. (17) Graham, J. D.; Roberts, J. T. Chemom. Intell. Lab. Syst. 1997, 37, 139-148. (18) Ritzhaupt, G.; Devlin, J. P. J. Phys. Chem. 1991, 95, 91-95.

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Figure 1. Single-reflection infrared spectra of (a) amorphous HCl‚6H2O and (b) crystalline HCl‚6H2O on Pt(111). The films were ∼12-ML thick. The amorphous hexahydrate was prepared by exposing an amorphous ice film to HCl (4 × 10-7 Pa‚s) at 120 K. It converted to crystalline HCl‚6H2O upon heating to 170 K.

drate film at 100 K to (CH3)3COH at 100 K. For this experiment, the (CH3)3COH exposure was 8 × 10-7 Pa‚s, which is approximately what is required to form one saturated adlayer on the underlying metal substrate. Also, the hexahydrate film was prepared so that only absorbed HCl was present. (This can be achieved by using a suitably low HCl exposure, because the absorbed and adsorbed states of HCl fill sequentially.12) Four desorption products were detected by mass spectrometry during TPD of (CH3)3COH from HCl‚6H2O(amorph): (CH3)3CCl, H2O, HCl, and (CH3)3COH. The compounds (CH3)3CCl and (CH3)3COH were usually detected as the fragment ions m/e 77 and 59, respectively, because of the low abundances of their molecular ions. Assignment of the reaction product to (CH3)3CCl rather than to another isomer of stoichiometry C4H9Cl was made possible by comparing the mass spectrometric fragmentation pattern of the TPD product to that of an authentic (CH3)3CCl sample leaked into the vacuum chamber. Water was always detected as m/e 16, a very weak component of the total H2O-derived signal, to preserve the dynamic range of the detection electronics for the other, much less abundant desorption products. Water and HCl evolution in Figure 2 result from the sublimation of the hexahydrate phase of hydrogen chloride. The spectra are essentially identical to those observed from the sublimation of pure HCl‚6H2O. We infer that significant quantities of (CH3)3COH are not incorporated into the bulk, either as a dissolved state or as a stoichiometrically fixed (CH3)3COH + H2O + HCl phase. Assignment of the (CH3)3COH desorption spectrum is more difficult. Desorption occurs near 185 K, well after the onset of HCl‚6H2O sublimation, which implies that the state is not associated with simple thermal desorption from the hexahydrate surface. The compound (CH3)3COH desorbs from Pt(111) near 200 K, suggesting that alcohol evolution

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Figure 2. Temperature-programmed desorption (TPD) spectra resulting from the interaction of (CH3)3COH (8 × 10-7 Pa‚s) with a 36-ML thick amorphous HCl‚6H2O film deposited on Pt(111). The amorphous hexahydrate was prepared by exposing an amorphous ice film to HCl (1.2 × 10-5 Pa‚s) at 125 K, a procedure that results in a surface free of adsorbed HCl. The four products, H2O, HCl, (CH3)3COH, and (CH3)3CCl, were detected as m/e 16 (O+), m/e 36 (H35Cl+), C3H7O+ (m/e 59), and m/e 77 (C3H635Cl+), respectively. The heating rate was 5 K‚s-1. Multiplication factors are not corrected for fragmentation or ion transmission through the mass spectrometer.

Figure 3. Formation of (CH3)3CCl during TPD of (CH3)3COH on (a) HCl‚6H2O(amorph) without adsorbed HCl, (b) HCl‚ 6H2O(amorph) with adsorbed HCl, and (c) HCl‚6H2O(cryst) without adsorbed HCl. The (CH3)3COH exposures were 8 × 10-7 Pa‚s. The films were deposited on W(100) and were approximately 12 ML thick. They were prepared by (a) exposure of an amorphous ice film to HCl (1.6 × 10-5 Pa‚s) at 120 K, (b) exposure of an amorphous ice film to HCl (1.2 × 10-5 Pa‚s) at 120 K, and (c) exposure of an amorphous ice film to HCl (1.6 × 10-5 Pa‚s) at 120 K followed by heating to 160 K. The heating rates were 6 K‚s-1, and (CH3)3CCl was detected as C3H635Cl+ (m/e 77).

in Figure 2 may be associated with desorption from the metal substrate. Because the hexahydrate films are dense and free of microscopic pinholes,12 adsorption must have originally occurred on the HCl‚6H2O surface. During sublimation of the HCl‚6H2O film, the adsorbed alcohol may be transferred from the hexahydrate surface to the Pt(111) substrate, from which it subsequently desorbs. The two (CH3)3CCl states, at 155 and 185 K, result from the reaction of (CH3)COH with an amorphous HCl‚6H2O film. (The very weak feature near 135 K in Figure 2 cannot be routinely reproduced and is not considered to be experimentally significant.) The low-temperature state evolves before any measurable sublimation of the film has occurred. Furthermore, the product yield at 155 K is essentially independent of film thickness over the range investigated, from 10 to 40 ML. For these reasons, (CH3)3CCl at 155 K is assigned to originate from a reaction at the hexahydrate surface. Because (CH3)3CCl evolution at 185 K is nearly coincident with sublimation of the hexahydrate, it might be imagined that (CH3)3CCl in this temperature range results from a reaction within the thin film bulk. However, thickness-dependent measurements show that the product yield at 185 K is essentially independent of thickness, which argues for a surfacemediated reaction. Nevertheless, (CH3)3CCl at 185 K cannot be unambiguously attributed to a reaction at the surface for reasons that will be considered in the Discussion. The presence of adsorbed HCl on the surface of the amorphous hexahydrate leads to a dramatic decrease in

the (CH3)3CCl yield near 155 K. Similarly, the lowtemperature state is nearly absent when (CH3)3COH is thermally desorbed from the crystalline hexahydrate surface. In Figure 3, we show three (CH3)3CCl TPD spectra, recorded after adsorbing (CH3)3COH on (a) HCl‚6H2O(amorph) with no adsorbed HCl present, (b) HCl‚6H2O(amorph) with adsorbed HCl present, and (c) HCl‚6H2O(cryst) with no adsorbed HCl present. The (CH3)3COH exposures used to generate the TPD spectra were roughly equivalent, as were the film thicknesses. Note that while the product yield near 185 K is essentially independent of the state of the original hexahydrate film, formation of (CH3)3CCl near 155 K occurs only when (CH3)3COH is adsorbed on an amorphous hexahydrate surface that is free of adsorbed HCl. Adsorbed HCl apparently blocks the low-temperature pathway for formation of (CH3)3CCl, and it is apparently unreactive toward (CH3)3COH. No reaction products other than (CH3)3CCl are produced during TPD of a coadsorbed (CH3)3COH/HCl mixture on HCl‚6H2O(amorph). Furthermore, the desorption spectrum of adsorbed HCl is essentially unperturbed by the presence of small amounts of coadsorbed (CH3)3COH. The adsorption of (CH3)3CCl on ice was briefly examined. Exposure-dependent TPD spectra from 18-ML amorphous ice films deposited on W(100) are shown in Figure 4. Three desorption states, which fill roughly sequentially from high to low temperature, are observed at 140, 145, and 155 K. The lowest-temperature state does not saturate with increasing exposure, and is derived from the sublimation of a (CH3)3CCl multilayer. The two states at 145

Reactions of Organic Molecules Adsorbed at Ice

Figure 4. TPD of (CH3)3CCl from 18-ML amorphous ice films deposited on W(100). The exposures were (a) 8.0 × 10-7 Pa‚s, (b) 1.2 × 10-6 Pa‚s, and (c) 1.6 × 10-6 Pa‚s. The heating rates were 4 K‚s-1, and (CH3)3CCl was detected as C3H635Cl+ (m/e 77).

and 155 K are assigned to desorption of (CH3)3CCl that is adsorbed to the ice surface. The low intensity of the desorption state at 155 K suggests that it may be associated with adsorption at defect sites at the ice surface. The (CH3)3CCl desorption yield at 155 K from reaction of (CH3)3COH on HCl‚6H2O(amorph) is quite low, ∼0.04 ML. The yield was estimated as follows. The TPD spectra of (CH3)3CCl from (CH3)3CCl adsorbed on ice were integrated with respect to time. The peak area of a saturated adlayer (e.g., the area of the peak in Figure 4b) was assigned to correspond to a coverage of 1 ML equivalent. Previous work has shown that the surface areas of ultrathin films of ice and HCl‚6H2O are approximately equal to each other and to the surface area of the underlying single-crystal metal surface.12 Because the macroscopic and microscopic surface areas of ice and HCl‚6H2O are roughly equivalent, the ratio of the (CH3)3CCl peak area from reaction of (CH3)3COH on HCl‚6H2O(amorph) to that from reversible (CH3)3CCl adsorption on ice is an approximate measure of the reaction yield. Methanol (CH3OH), in contrast to (CH3)3COH, does not react with adsorbed or absorbed HCl. Rather, methanol is reversibly taken up by HCl‚6H2O, ultimately evolving into the gas phase near 175 K. No reaction is observed with adsorbed HCl state, nor are the kinetics for HCl desorption significantly perturbed by coadsorbed methanol. When adsorbed on a pure ice-d2 film, methanol undergoes extensive H-D exchange with water. Methanol adsorption on DCl‚6D2O was not examined, but H-D exchange presumably occurs with this surface as well. Discussion The principal significance of the observation of (CH3)3CCl formation from (CH3)3COH on amorphous HCl‚6H2O, apart from the fact that it establishes a new class of

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chemical reactions at the surface of an icy solid, is that reactive chlorine is derived from the hexahydrate phase of water and hydrogen chloride. Thickness-dependent measurements demonstrate that low-temperature chloride substitution occurs at the (CH3)3COH/HCl‚6H2O(amorph) interface. However, what is required for reaction to occur is not adsorbed HCl, but rather that (CH3)3COH be adsorbed on an ice film that has been modified by the incorporation of HCl into the bulk. Bulk incorporation below 140 K in ultrahigh vacuum results in the formation of amorphous HCl‚6H2O, an ionic solid that crystallizes near 150 K. Given that chloride substitution occurs less readily on the crystalline HCl‚6H2O surface, the reactive sites are probably not associated with a perfect, crystalline lattice. Indeed, the reaction yield is quite low (∼0.04 ML), which may be indicative of chloride substitution at imperfections of the HCl‚6H2O surface (point defects, grain boundaries, etc.). Regardless of the origin of reactive chloride, the implication of the TPD results is clear: chloride that is located in the near-surface region of the amorphous HCl‚6H2O film is available for reaction with (CH3)COH at the surface. We now consider the pathways for (CH3)3CCl formation at 155 and 185 K. We begin with the lower-temperature state, which is observed only when (CH3)3COH is adsorbed on the amorphous hexahydrate. A key issue in the mechanism of (CH3)CCl formation on amorphous HCl‚ 6H2O is the rate-limiting step for evolution of the product into the gas phase. The thermal desorption of (CH3)3CCl from HCl‚6H2O was not investigated. However, (CH3)3CCl desorbs from ice at 155 K (Figure 4). The 155 K state accounts for a small fraction of the desorption yield from ice, to be sure, but the yield is approximately equal to that from (CH3)3COH reacting on HCl‚6H2O(amorph). Thus, (CH3)3CCl evolution at 155 K is probably limited by the desorption rate, with chloride displacement itself occurring at a lower and as yet unknown temperature. A pathway that is consistent with desorption-limited (CH3)3CCl evolution is:

(CH3)3COH(ad) + HCl‚6H2O(amorph) f (CH3)3CCl(ad) + 7H2O(s) (4) (CH3)3CCl(ad) f (CH3)3CCl(g)

(5)

where the subscript (ad) designates an adsorbed state. Note that the above pathway is not inconsistent with a surface reaction mechanism, because Cl- in HCl‚6H2O at the very near surface regions of the film is, in principle at least, available for reaction at the surface. The TPD spectra of (CH3)3COH adsorbed on amorphous and crystalline HCl‚6H2O are markedly different: there is essentially no (CH3)3CCl evolution below 180 K when (CH3)3COH is adsorbed on the crystalline hexahydrate. The difference in low-temperature yields from the amorphous and crystalline surfaces cannot be attributed to a surface area effect, because the adsorbate-accessible surface areas of the amorphous and crystalline films are approximately equal.12 The difference between the two films is more likely related to a difference in surface reactivities. Specifically, because amorphous HCl‚6H2O is a metastable phase, the chemical potential (and therefore the reactivity) of absorbed chloride near the surface of HCl‚6H2O(amorph) is greater than that near the surface of HCl‚6H2O(cryst). A second, related possibility is that the amorphous surface possesses more “defect” sites at which chloride substitution occurs. Both of these explanations are consistent with the observation that

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Scheme 1. Two Possible Mechanisms for Cl-1 Substitution in (CH3)3COH on HCl‚6H2O: (a) SN1 Substitution, Which Involves Heterolytic Dissociation of (CH3)3COH to Form the Carbocation (CH3)3C+, and (b) SN2 Substitution, Which Proceeds via a Bimolecular Reaction of Cl- Attack with the Adsorbed Alcohol; All Organic Reactants, Products, and Intermediates Are understood to Be in an Adsorbed Phase

adsorbed HCl completely suppresses reaction at 155 K, although the precise role of HCl in this regard is unclear. Hydrogen chloride might prevent chloride substitution in (CH3)3COH simply by adsorbing at and thereby blocking the reaction sites. Alternatively, adsorbed HCl may change surface reactivity through a more subtle effect, for instance by inducing a structural change in the surface and nearsurface regions of the film. Given that (CH3)3CCl desorbs from ice near 150 K, its evolution near 185 K in Figures 2 and 3 is strong evidence for a reaction-limited process. The product yield is independent of film thickness in this temperature region, but this observation is not unambiguous evidence for a reaction at the hexahydrate surface, because a significant fraction of the hexahydrate film sublimes below 185 K. It is possible that (CH3)3CCl is derived from a reaction at the crystalline hexahydrate surface, and that reaction is slower than on the amorphous surface because the chemical potential of chloride in the crystalline lattice is lower than that in the amorphous solid. A second possibility is that the reaction occurs in the gas phase near the surface, as (CH3)3CCl desorbs and HCl‚6H2O sublimes. We cannot at present distinguish between these and other pathways for (CH3)3CCl formation at 185 K. It is significant that methanol, unlike (CH3)3COH, does not react with absorbed chloride. The absence of a reaction between methanol and Cl- cannot be attributed to a lower methanol desorption energy, because adsorbed methanol persists beyond the temperature at which (CH3)3COH reacts. Rather, we assert that the difference between (CH3)3COH and CH3OH is indicative of a mechanism involving considerable charge separation in the intermediate or transition state leading to formation of the adsorbed alkyl chloride (Scheme 1a).19 In the limit of infinite charge separation, Scheme 1a implies an SN1 process proceeding via initial formation of the carbocation (CH3)3C+ followed by chloride addition. Because of inductive effects of the methyl groups, (CH3)3C+ is orders of magnitude more stable than CH3+, the carbocation derived from methanol. For this reason, the activation energy for Cl-1 substitution in methanol via an SN1-like mechanism

Graham and Roberts

is too great for SN1 substitution to occur. An SN1 or related mechanism is probably to be expected on a surface, where two-dimensional confinement of the reactants makes bimolecular displacement, like the SN2 process shown in Scheme 1b, implausible. The geometry of the SN2 transition state is such that simultaneous interaction of the entering and leaving groups (Cl- and OH-, respectively) with the ice surface could not occur. The conversion of (CH3)3COH to (CH3)3CCl on HCl‚6H2O has no immediate relevance for stratospheric PSC chemistry, because alcohols are present in the stratosphere in vanishingly small concentrations. Nevertheless, the results reported here may have implications for our understanding of how some stratospherically important reactions occur. For instance, the disproportionation of hydrogen chloride and chlorine nitrate on a PSC particle (eq 1) need not occur via a reaction between molecularly adsorbed HCl with adsorbed ClONO2. Adsorbed HCl, at least as it is formed on HCl‚6H2O, is unreactive toward (CH3)3COH, and we suspect it is unreactive toward ClONO2 as well. Rather, reactive HCl may originate from the bulk, as it does in the reaction of (CH3)3COH. Were that the case, the mechanism for Cl2 formation would have to be ionic, perhaps involving the migration of absorbed chloride to the PSC particle surface, followed by its attack at adsorbed chlorine nitrate. Such a sequence would lead to the formation of Cl2 and NO3- as reaction products. Ionic pathways for PSC-mediated reactions have been proposed previously.20-23 An alternative but closely related pathway suggested by Molina and colleagues posits a reaction between Cl- and ClONO2, which are dissolved in a liquidlike layer confined to the very near surface regions of the PSC particles.11 Possible nonionic pathways for Cl2 formation are discussed elsewhere.9,10 Conclusion A new chemical reaction is reported on the ice surface: the conversion of adsorbed 2-methyl-2-propanol [(CH3)3COH] to 2-chloro-2-methylpropane [(CH3)3CCl]. It has been firmly established that molecularly adsorbed HCl is unreactive toward (CH3)3COH. Rather, the source of chlorine is Cl-1 in the near-surface region of the amorphous, icy phase HCl‚6H2O. The lack of an analogous reaction for adsorbed methanol (CH3OH) implies that (CH3)3CCl formation proceeds via a carbocation intermediate. The work adds to a growing body of experimental evidence that ionic pathways are dominant in PSC chemistry. It also expands the number of reactions that are known to be promoted by ice. Acknowledgment. This work was supported by the National Science Foundation through grant no. CHE9527665. LA9906166 (19) Hartshorn, S. R. Aliphatic Nucleophilic Substitution; Cambridge University Press: London, 1973. (20) Nelson, C. M.; Okumura, M. J. Phys. Chem. 1992, 96, 61126115. (21) Chu, L. T.; Leu, M.-T.; Keyser, L. J. Phys. Chem. 1993, 97, 1279812804. (22) Haas, B. M.; Crellin, K. C.; Kawata, K. T.; Okumura, M. J. Phys. Chem. 1994, 98, 6740-6745. (23) Sodeau, J. R.; Horn, A. B.; Banham, S. F.; Koch, T. G. J. Phys. Chem. 1995, 99, 6258-6262.