Langmuir 2002, 18, 4819-4825
4819
Solvent Effects on the Heterogeneous Adsorption and Reactions of (2-Chloroethyl)ethyl Sulfide on Nanocrystalline Magnesium Oxide Richard M. Narske† Department of Chemistry, Augustana College, Rock Island, Illinois 61201
Kenneth J. Klabunde* Department of Chemistry, Kansas State University, Manhattan, Kansas 66505
Shawn Fultz Department of Chemistry, Kansas State University, Manhattan, Kansas 66505 Received February 22, 2002 The noncatalytic destructive adsorption of (2-chloroethyl)ethyl sulfide (2-CEES), a mimic of bis(2chloroethyl) sulfide (“HD” or Mustard Gas), on nanocrystalline magnesium oxide (AP-MgO) was studied in several solvents from pentane to methanol. The decomposition products formed in these reactions were vinylethyl sulfide and (2-hydroxyethyl)ethyl sulfide. Reactions in pentane allowed the highest reaction rates, while tetrahydrofuran (THF) and methanol gave results quite different from those for the hydrocarbon solvent. Reactions in methanol yielded (methoxyethyl)ethyl sulfide and not the vinylethyl sulfide and (2-hydroxyethyl)ethyl sulfide compounds. These studies showed that the MgO-2-CEES reaction chemistry is significantly affected by the solvent present and can be enhanced by choice of solvent and the addition of small amounts of water. Interestingly, the least polar, least reactive solvent (pentane) allowed the most rapid 2-CEES reactions, indicating that the solvent simply aided material transfer to the reactive surface sites without blocking these sites. Rate changes upon water addition, coupled with FTIR studies, indicate that isolated surface OH groups are important reactive sites. These results indicate that the use of certain “inert” solvents greatly aids material transfer, and thereby the reaction rates of the sorbent with the toxin are significantly enhanced.
Introduction Decomposition of chemical warfare agents has become an important topic of study over the past several years. Initial studies involved either hydrolysis by caustic solutions1 or incineration.2 These methods were adequate for agents that are at storage facilities, but they were inadequate for decontamination of agents in the field. Some recent studies have focused on the use of reactive sorbents. Wagner and Bartram,3 using MAS NMR technologies, reported in 1999 that certain zeolites, NaY and AgY, were able to react at room temperature with VX, O-ethyl S-2(diisopropylamino)ethyl methylphosphonothioate, and HD, bis(2-chloroethyl) sulfide, as well as the HD mimic CEPS, (2-chloroethyl)phenyl sulfide. Wagner and Bartram found in some cases the zeolites did not completely decontaminate these agents. For example, NaY zeolites produced in some cases highly toxic compounds as products, such as CH-TG (Scheme 1). AgY, however, formed the divinyl compound, DVHD (Scheme 3), and 1,4thioxane (Scheme 2) exclusively. Wagner et al.4,5 reported in 1999 and then in 2000 that nanosized particles of metal oxides, such as MgO and CaO, †
On leave at Kansas State University.
(1) Yang, Y. C. Acc. Chem. Res. 1999, 32, 109-115. (2) National Research Council. Review and Evaluation of Alternative Chemical Disposal Techniques; National Academy Press: Washington, DC, 1996. (3) Wagner, G. W.; Bartram, P. W. Langmuir 1999, 15, 8113-8118. (4) Wagner, G. W.; Bartram, P. W.; Koper, O. B.; Klabunde, K. J. J. Phys. Chem. B 1999, 103, 3225-3228.
Scheme 1
Scheme 2
were very effective in noncatalytic destructive adsorption of chemical agents, such as VX, GD (3,3-dimethyl-2-butyl methylphosphonofluoridate), and HD. These studies were at room temperature using the pure agent on a column of dry nanosized particles of oxide. It was found that the (5) Wagner, G. W.; Koper, O. B.; Lucas, E.; Decker, S.; Klabunde, K. J. J. Phys. Chem. B 2000, 104, 5118-5123.
10.1021/la020195j CCC: $22.00 © 2002 American Chemical Society Published on Web 05/15/2002
4820
Langmuir, Vol. 18, No. 12, 2002 Scheme 3
Narske et al. Surface Area Determination. Surface area measurements were accomplished by using Brunauer-Emmet-Teller (BET) methods using a Quantachrome NOVA 1200. The samples were first outgassed at 150 °C. Reactions in Various Solvents. A reaction protocol was established for all the solvent systems and the different MgO batches studied. At least two (usually more) reactions were investigated for each MgO sample investigated. The reaction protocol for this investigation follows: Reaction Protocol
Scheme 4
products formed in these reactions were the less toxic thioglycol (TG) and divinyl (DVHD) compounds (Scheme 3), and that these nanoparticles were more reactive than the zeolites. Wagner and co-workers also reported that these reactions were limited by the physical distribution of the agent through the column of oxide. However, when the agent was able to make contact with fresh nanoparticle oxide, the agent reacted very rapidly to form safer decomposition products. Indeed, it has been proposed6 that the reactivity of these oxides was enhanced because of the greater surface area and the unusual shape that the nanocrystals assume during their preparation, giving rise to a high surface concentration of reactive edges and corner defects. A serious drawback to the use of dry powder sorbents is the long half-lives for detoxification when large droplets of the chemical agents are present. When this is the case, physical mixing is slow and dependent on vapor transport,4,5 which is slow for low volatility toxins. Therefore, in some situations, it is very important that a better mixing process be employed. One way to envision doing this is to use an organic solvent that dissolves the organic toxin and brings it quickly into contact with the powder sorbent. However, solvents must be chosen that do not deactivate the sorbent and that do dissolve and distribute the organic toxin. Indeed, some early work7 has shown that with the presence of solvents the physical distribution limits are removed. In these studies it was found that solvents, such as pentane, change the reaction dynamics of these reactions, and the rates were vastly increased over those reported by Wagner et al.4 The purpose of this work was to study the effects of various solvents on the adsorption and reaction of (2chloroethyl)ethyl sulfide (2-CEES) with nanocrystalline magnesium oxide (AP-MgO) in order to obtain some idea about the rates and mechanism of these reactions. 2-CEES is a mimic of “mustard gas”, HD. The solvents that have been investigated at this time were pentane, tetrahydrofuran (THF), and methanol. The reaction products found were similar to those reported by Wagner4,5 (Scheme 4). Experimental Section Preparation of Magnesium Oxide. The preparations of APMgO (aerogel prepared) and CP-MgO (conventionally prepared) were accomplished according to the procedures reported in previous work.6-8 CM-MgO was used directly from the bottle (Fisher Certified magnesium oxide). (6) Klabunde, K. J.; Stark, J. V.; Koper, O.; Mohs, C.; Park, D. G.; Decker, S.; Jiang, Y.; Lagadic, I.; Zhang, D. J Phys. Chem. 1996, 100, 12142-12153. (7) Carnes, C. L. Ph.D. Dissertation, Kansas State University, 2001. (8) Lucas, E. M. Ph.D. Dissertation, Kansas State University, 2001.
solvent 10 mL internal standard, decane 15 µL 2-CEES 15 µL (0.129 mmol) Initial GC analysis was performed at this point to obtain the ratio of the internal standard, decane, to the 2-CEES. ∼150 mg of MgO, degassed at 150 °C under vacuum for 30 min. Analyze by GC at the following time (min) intervals: 0.1, 5, 10, 20, 30, 60, 120, 240, and 1440
Reactions at Various Temperatures in Pentane. Reactions were carried out at 3-5 °C and in refluxing pentane, 35-37 °C. The protocol used for reactions at room temperature was followed. Synthesis of Trimethylsilyl-Capped AP-MgO. Trimethylsilyl-capped AP-MgO was prepared by allowing 500 mg of APMgO to react with 300 mg of methoxytrimethylsilane in toluene at reflux for 7 h to produce a product containing 25% trimethylsilyl groups by weight. Reactions of Trimethylsilyl-Capped AP-MgO. Reactions of the trimethylsilyl-“capped” AP-MgO were carried out according to the same protocol used for reactions of AP-MgO at room temperature. Gas Chromatographic Analysis. Reactions were analyzed by gas chromatography using a Varian 3600 gas chromatograph equipped with a flame ionization detector (FID) and a 15 m × 0.53 mm i.d. Alltech Econwax capillary column coated with 1.25 µm of Carbowax W liquid. Analysis conditions were as follows: Gas Chromatography Conditions flow rate injector temp detector temp column program retention times (min)
3.1 mL/min 120 °C 200 °C 50 °C for 1 min, ramp to 125 °C at 20 °C/min, hold at 125 °C for 2 min (total analysis time 6.75 min) decane, 1.74; vinyl, 1.5; 2-CEES, 4.3; 2-hydroxy, 5.12; 2-methoxy, 3.78
Initial identification of products was done by analysis of standard mixtures on the Varian GC and by comparing retention times. Using Hewlett-Packard model HP5890 GC and model HP5972 GC/MS, an identical column and conditions accomplished further identification of starting materials, internal standard, and all possible products. Surface Analysis. Identification of the surface bonded products and surface changes was accomplished by infrared spectroscopy using a Nicolet Nexus 670 FTIR. Samples of MgO after the reaction were washed with four 2.5 mL portions of CH2Cl2. GC was used to check these washings for the presence of product or starting material. The solid samples were then analyzed for surface bonded products or changes by diffuse reflectance FTIR. Samples not involved in a reaction were analyzed directly by diffuse reflectance FTIR.
Results Reactions in Pentane. Several preparative batches of AP-MgO were studied in pentane in an attempt to determine reaction rates in relation to the surface area of the AP-MgO (Table 1). We found that the reaction rate in pentane is directly related to the surface area of the AP-MgO (but not CP- or CM-MgO as discussed later): the larger the surface area, the faster the rate of adsorption and decomposition of 2-CEES. One can see this correlation from Figure 1. For example, over 65% of the 2-CEES was consumed with batch 4 (581 m2/g surface area), and less than 25% disappeared with batch 6 (188 m2/g).
(2-Chloroethyl)ethyl Sulfide on Nanocrystalline MgO
Langmuir, Vol. 18, No. 12, 2002 4821
Table 1. Surface Areas of Different AP-MgO Preparation Batches batcha
surface area (m2/gram)
2 3 4 5 6 7 9 10
380 487 581 417 188 336 457 423
batcha
surface area (m2/gram)
12 13 14 15 18 CM CP
499 352 309 642 242 77.0 270
a Batches 2-18 are all AP-MgO prepared on different days. CM is commercially available MgO, and CP is conventionally prepared MgO.
Figure 3. Correlation of percent yield of ethylvinyl sulfide with surface area of MgO reactant.
Figure 1. Series of AP-MgO batches of varying surface areas and their heterogeneous reaction with 2-CEES in pentane.
Figure 2. Effect of water added on the reaction of AP-MgO with 2-CEES in pentane (batch 3). AP-MgO has a surface area of 487 m2/g, and between 10 and 15 µL of water could produce a monolayer on the MgO surface.
Reactions were also studied with the addition of water to the pentane reaction mixture. Figure 2 gives a graphical picture of these results. The addition of small amounts of water increased the rate of the reaction on the order of 1.2-1.7 times. Both samples studied showed decreased reactivity toward 2-CEES when water amounts exceeding 5 µL were added. Results after 24 h showed the same trend as those data plotted for the 4 h periods. The percent of 2-CEES
consumed ranged up to 98% for batch 15 with a surface area of 642 m2/g of MgO. The rate of product formation could be followed for the ethylvinyl compound, but the rate of formation of (2-hydroxyethyl)ethyl sulfide could not be monitored because it bonded directly to the surface of the MgO and could not be readily removed, even by washing the CH2Cl2. Indeed, the presence of the 2-hydroxy compound on the surface of the MgO was determined by hydrolyzing the MgO in the presence of some water over a period of several days. This resulting mixture was extracted and analyzed by GC and GC/MS. The rates of formation of the vinyl product over the initial 4 h period were slower than the rate of disappearance of 2-CEES. The 24 h yield of vinyl compound for these reactions ranged from a high of 18.6% to a low of 0%. A comparison of the percent yield of vinyl compound versus the surface area of some selected batches of APMgO is plotted in Figure 3. The ratio of elimination to hydrolysis varied with each reaction studied and ranged from 25/75 to 14/86. For one sample, the reactions involving addition of 1 and 5 µL of water gave elimination/hydrolysis ratios of 15/85 and 18/82, respectively. This ratio did not change much from that of the reaction without water, 14/86. The increase in the amount of vinyl compound did not match the amount of water added to the reaction, 1 µL ) 0.056 mmol, nor did the increase in the amount of 2-CEES reacted match the amount of water added. Results of studies using different concentrations showed that the reaction progressed toward completion more rapidly as the concentration of 2-CEES decreased (see Figure 4). With the lower concentration of 0.043 mmol and the addition of 1 µL of water, the reaction proceeded to completion (100% consumption of 2-CEES) in less than 2 h. Reactions of AP-MgO with 2-CEES in pentane at 3-5 and 35-37 °C were also studied. As expected, the lower temperature gave rates that were slower than that at room temperature by a factor of 1.3-1.5 times. Reactions carried out at 35-37 °C in pentane with no water present gave expected results: an increase in the rate of reaction of the 2-CEES with the AP-MgO by a factor of 1.5-1.7 times; however, when 1 µL of water was added, the reaction rate decreased by 25-30% when compared to that of the reaction without the water present at elevated temperature. When compared to that of the reaction with water present at room temperature, the reaction rate was also
4822
Langmuir, Vol. 18, No. 12, 2002
Figure 4. Reactions of Different Concentrations of 2-CEES, Batch 10: Effect of concentration of 2-CEES in pentane when allowed to react with the same mass of AP-MgO (batch 10 has a surface area of 423 m2/g).
Figure 5. Effect of temperature on the 2-CEES/AP-MgO reaction rate.
slower (see Figure 5). The amount of vinyl compound formed remained fairly constant at these temperatures. Studies by diffuse reflectance FT-IR indicated that the surface of the AP-MgO after being placed in pentane and then removed was similar to that of the MgO before placing it in the solvent. The surface of the MgO after being placed in pentane with 1 µL of water present, however, did exhibit differences, particularly in the 3750-3700 cm-1 range and the peak centered at ∼3600 cm-1 (see Figure 8). Changes after heating under vacuum at 150, 350, and 490 °C were observed by diffuse reflectance FT-IR also, and in particular the sharpness and intensity of the peak at 37503700 cm-1 compared to the peak at 3600 cm-1 was enhanced. The surface of the AP-MgO after reflux in pentane showed essentially no difference when compared to that of the fresh AP-MgO, whereas the surface of the AP-MgO after reflux in pentane with water present showed a definite change in the peaks at 3750-3700 and 3600 cm-1 (a decrease of the peak at 3750-3700 cm-1 and an increase and broadening of the peak centered at 3600 cm-1) (see Figures 8 and 9). Diffuse reflectance FTIR analysis of the residue MgO after reaction with 2-CEES showed a diminished 37503700 cm-1 peak, and the presence of a weak band at 1150-
Narske et al.
Figure 6. Comparison of commercial (CM-), conventionally prepared (CP-), and nanocrystalline aerogel prepared (AP-MgO) samples in the 2-CEES reaction. CM-MgO surface area ) 77 m2/g. CP-MgO ) 270 m2/g. AP-MgO ) 499 m2/g.
Figure 7. Comparison of AP-MgO surface before and after reaction with 2-CEES in pentane by diffuse reflectance FTIR.
1100 cm-1. This 1150 peak is very likely due to the MgO-C bond from the alkoxide product (see Figure 7). This peak was not diminished by repeated washings with methylene chloride, and as noted earlier, no products or 2-CEES was found in the washings according to GC analysis. Reactions involving CM-MgO and CP-MgO were compared to the AP-MgO reactions and were much slower, although CP-MgO reacts more rapidly with 2-CEES than the CM-MgO (see Figure 6). Reactions in Tetrahydrofuran. With THF the results were very different than those in pentane. The rate of consumption of 2-CEES and the rate of appearance of the vinyl product were experimentally the same. The only product formed in this solvent was the vinyl compound. The overall reaction rate was slower in THF than in pentane. Only 25-30% of the 2-CEES was consumed, and 25-30% of the vinyl compound formed after 4 h in THF compared to 45-50% in pentane. The 24 h data showed the same trend. The amount of 2-CEES consumed in THF after 24 h was 27-36% compared to 60-68% in pentane. Results in Methanol. The reaction of AP-MgO and 2-CEES in methanol gave only the substitution product,
(2-Chloroethyl)ethyl Sulfide on Nanocrystalline MgO
Langmuir, Vol. 18, No. 12, 2002 4823 Table 2. Comparison of the Rate of Consumption of 2-CEES for Some Representative Batches of AP-MgO in molecules/nm2‚min over the First 30 min of the Reaction time/ min B4, 581a B6, 188 B12, 499 B15, 642 CM, 77b CP, 220c 0.1 5 10 20 30
-1.7 -0.049 -0.027 -0.020 -0.013
-0.86 -0.038 -0.021 -0.016 -0.013
-1.7 -0.048 -0.026 -0.016 -0.012
-1.6 -0.044 -0.026 -0.015 -0.012
0 -0.009 -0.0047 -0.0024 -0.0014
0 -0.026 -0.015 -0.0094 -0.0053
a B4, 581 refers to AP-MgO batch 4, surface area ) 581 m2/g. Commercially available MgO, surface area ) 77 m2/g. c Conventionally available MgO, surface area ) 220 m2/g.
b
Figure 8. AP-MgO surface after addition of 1 µL of water at room temperature in pentane and before reaction with 2-CEES (batch 15).
Figure 9. AP-MgO surface after addition of 1 µL of water, refluxing in pentane, and before the reaction with 2-CEES.
(2-methoxyethyl)ethyl sulfide. After 4 h only 24% of the 2-CEES had reacted, and this gave a yield of 25% (2-
methoxyethyl)ethyl sulfide, so this was the only reaction product. The same solvolysis product was formed in methanol without the presence of AP-MgO but at a slower rate; after 4 h, 16% of the methoxy compound was formed and 18% of the 2-CEES had been consumed without the AP-MgO present compared to 23 and 25%, respectively, with AP-MgO. After 24 h, we observed 65% methoxy compound with AP-MgO present and 45% methoxy with no AP-MgO present. Discussion Pentane. Our intention in these studies was to gain a clearer insight into the mechanistic features of the surface reactions of AP-MgO with 2-CEES. Effects of surface area
as well as surface hydroxyl groups were important considerations. Pentane was initially chosen as a solvent that we considered to be inert but would facilitate material transfer of the 2-CEES to the MgO surface. Interestingly we found that the reactions proceeded faster in pentane than in more polar solvents, such as THF and methanol. The reaction rates showed a direct relationship between surface areas and reactivity for one type of MgO (see Figure 1). Interestingly, when the initial rates in molecules/nm2‚ min of nanocrystalline MgO were compared over the first 30 min of the reactions studied, it was found that the rates of all these reactions were essentially the same, indicating that for the AP-MgO reactions the only difference between batches of AP-MgO was the difference in surface areas, and not reactive sites/morphological features (Table 2). However, note that CP-MgO and CM-Mg, which are very different morphologically,9-14 exhibit far lower rates/nm2 (Table 2). It is clear that crystal shape as well as surface area is important. (Earlier work has shown that AP-MgO crystallites are polyhedral, CP-MgO exists as large hexagonal platelets, and CM-MgO is polycrystalline, mainly as cubes.)9,10 Actually, the initial rapid rate of these reactions slowed markedly after about 30 min, and this behavior has been observed earlier.4,9-14 It is likely that an array of site reactivity is involved, and the most active sites are consumed first (edges, corners, defects), followed by the less reactive sites (such as planar surfaces such as the 100 crystal face). An interesting aspect of these reactions was the addition of small amounts of water. In the studies of batches 3 and 4, small amounts of water, ranging from 1 to 100 µL, were added to the reaction mixtures. Below 5 µL the reaction rates were enhanced. Above 5 µL, the rates decreased steadily with increase in added water. Diffuse reflectance FTIR studies showed that the small amount of water gave a distinct increase in the IR peak at 3750-3700 cm-1. This peak is probably due to the non-H-bonded OH groups on the surface of the AP-MgO.15 This is a qualitative observation, but an increase in these non-H-bonded OH (9) Klabunde, K. J.; Decker, S.; Lucas, E.; Koper, O. How the Shape of Nanoparticles Affects Their Adsorption Properties. In Cluster and Nanostructure Interfaces; Jena, P., Khanna, S. N., Rao, B. K., Eds.; World Scientific Publishers: London, 2000, pp 577-582. (10) Klabunde, K. J., Ed. Nanoscale Materials in Chemistry; WileyInterscience: New York, 2001. Also, two chapters written for this book: Klabunde, K. J. Introduction to Nanotechnology. Chapter 1, pp 1-13; Klabunde, K. J.; Mulukutla, R. Chemical and Catalytic Aspects of Nanocrystals. Chapter 7, pp 223-261. (11) Jiang, Y.; Decker, S.; Mohs, C.; Klabunde, K. J. J. Catal. 1998, 180, 24-35. (12) Khaleel, A.; Kapoor, P. N.; Klabunde, K. J. Nanostruct. Mater. 1999, 11, 459-468. (13) Stark, J. V.; Park, D. G.; Lagadic, I.; Klabunde, K. J. Chem. Mater. 1996, 8, 1904-1912. (14) Itoh, H.; Utamapanya, S.; Stark, J. V.; Klabunde, K. J.; Schlup, J. R. Chem. Mater. 1993, 5, 71-77.
4824
Langmuir, Vol. 18, No. 12, 2002
groups would make more of these sites available for the reaction (see Figures 8 and 9). If the OH groups are so important to the reaction, decreasing the OH groups would slow the reaction rate. We observed this result when we studied AP-MgO that had been “capped” with trimethylsilyl groups. The reaction came almost to a complete standstill. Less than 3% of the 2-CEES had been consumed after 4 h. The results of this reaction gave strong evidence for the necessity of the OH groups in the reaction process. Something interesting was observed in the temperature studies with a small amount of water present in the reaction mixture. When we carried out this reaction at elevated temperatures in refluxing pentane with 1 µL of water present, the reaction rate decreased. This was opposite of what was observed at room temperature and was puzzling. Interestingly, the diffuse reflectance FTIR analysis of the surface after refluxing the AP-MgO in pentane with 1 µL indicated that the water was bound on the surface of the AP-MgO in an associated hydrogen bonded mode (see Figures 8 and 9). It appears that the added water first forms “isolated” OH groups likely on reactive open MgO sites that readily cause H2O dissociation even at 3 °C. However, upon warming to pentane reflux temperature, these isolated OH groups migrate such that they are able to hydrogen bond with other OH groups, thus becoming “associated”. These results give a good indication as to why the reaction rate slowed. The surface, after refluxing in pentane with water, was similar to the surface of the AP-MgO before degassing the AP-MgO. The peak at 3750 cm-1 was smaller than the same peak for the more reactive surfaces studied. These results, taken together, strongly suggest that isolated surface OH groups are the most reactive sites. The formation of the substitution product, (2-hydroxyethyl)ethyl sulfide (likely in the deprotonated alkoxide state), could not be followed by GC during these reactions, since it bonded very strongly to the surface; indeed the IR peak at 1150-1100 cm-1 indicated its presence (see Figure 7).15 Yates and co-workers16 reported similar results when working with Al2O3 reactions with “HD” mimics. We found that the other product, ethylvinyl sulfide, formed at a slower rate than the substitution product, (2-hydroxyethyl)ethyl sulfide, and that most of the 2-CEES that decomposed formed the substitution product. These results were different when compared to Wagner’s4 results with HD in which he reported a 50/50 ratio of elimination/ substitution. These results did, however, correspond very closely to the results Wagner5 reported for the reaction of 2-CEES with AP-CaO. The wide array of studies carried out allowed certain tentative conclusions. It is apparent that isolated hydroxyl groups are important, and addition of small amounts of water at room temperature allowed the formation of more isolated OH groups and enhanced the rate of 2-CEES reaction. However, too much water was detrimental, suggesting that nonhydroxylated surface sites are also important. More quantitative considerations also support the idea that isolated OH groups work in tandem with surface O2sites. Consider that the AP-MgO initially possesses three to four OH groups/nm2 (12 surface OH/nm2 would be a saturated monolayer).17 Upon addition of 5 µL water, the reaction rate and capacity were enhanced. However, upon (15) Smith, M. B. Organic Synthesis; McGraw-Hill: New York, 1994; pp 686-687. (16) Mawhinney, D. B.; Rossin, J. A.; Gerhart, K.; Yates, J. T. Langmuir 1999, 15, 4789-4795.
Narske et al. Scheme 5
addition of 20 µL, the reaction was substantially hindered. Consider that a 500 m2/g sample with an average crystallite size of 4 nm would have a surface-to-bulk ratio of about 1:2.3. In other words, about 30% of the MgO moieties would be on the surface and 70% would be in the bulk. Furthermore, it is known that three to four surface OH groups/nm2 are present on these MgO nanocrystals (12 would be a monolayer when all Mg ions possessed OH groups). Therefore, about one third of the Mg ions are hydroxylated. By calculating the moles of water in 5 µL and the moles of surface MgO present that are not hydroxylated, it can be shown that, by adding 5 µL water, another one third of the MgO moieties would be hydroxylated. It follows that, by adding more water, the surface would be completely hydroxylated, and Figure 2 shows that these larger amounts of water are detrimental. Furthermore, it was found that refluxing the pentane with these small amounts of added water appeared to cause the hydroxylated groups to associate (isolated OH groups were not observable by IR), and the water enhancement effect was mitigated. And finally, it was also found that, by capping surface OH groups with trimethylsilyl groups, the 2-CEES reaction was shut down. All this evidence suggests a MgO-2-CEES destructive adsorption reaction depicted in Scheme 5. Note the isolated OH groups working in tandem with nonhydroxylated MgO moieties (thus Mg2+ ion binds to the sulfur of 2-CEES) could lead to the bound substitution (alkoxide) product. For the vinyl product formation, it may be that isolated OH groups are capable of causing an E2 elimination as shown in Scheme 5. Reactions in Tetrahydrofuran. The results in THF were completely unexpected. As can be seen by the data reported above, the only product formed was the elimination product, ethylvinyl sulfide. We also found that the rates of the reactions in THF were much slower than were found in pentane. A completely different reaction dynamic was occurring in THF. It is likely that the THF forms a complex with the magnesium on the AP-MgO surface, similar in nature to those formed with other magnesium reactions in ether.15,18 The 2-CEES would be stopped from undergoing the substitution reaction by this complex but allowed to undergo the elimination reaction. It is known that the surface of the AP-MgO contains nucleophilic oxide (Lewis base) and electrophilic (Lewis acid) magnesium as well as basic (Bronsted base) OH groups. Therefore, it is likely that the THF blocks electrophilic magnesium sites and the sulfide thus could not form a complex on the surface and therefore could not react with the nucleophilic OH groups to form the 2-hydroxy compound (compare Schemes 5 and 6). In accordance with this idea, the only product formed was the elimination product, ethylvinyl sulfide, and only about 30-35% of the 2-CEES reacted (Scheme 6). (17) Fenelonov, V. B.; Mel’gunvo, M. S.; Mishakov, I. V.; Richards, R. M.; Chesnokov, V. V.; Volodin, A. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3937-3941. (18) Lambert, J. B.; Shurvell, H. F.; Lightner, D. A.; Cooks, R. G. Organic Structural Spectroscopy; Prentice-Hall: Upper Saddle River, NJ, 1998; pp 223-226.
(2-Chloroethyl)ethyl Sulfide on Nanocrystalline MgO
Langmuir, Vol. 18, No. 12, 2002 4825 Scheme 6
Scheme 7
Scheme 8
Reactions in Methanol. Reactions in methanol also gave results that were not expected. We found that the only product formed when 2-CEES was allowed to react with AP-MgO in methanol was the (2-methoxyethyl)ethyl sulfide. Reaction rates in methanol were not as great as those in pentane. Only 24-25% of the 2-CEES reacted to give a 24-25% yield of (2-methoxyethyl)ethyl sulfide. The amounts of 2-CEES consumed in pentane were two to three times as great over the same period of time. The results showed that in methanol the 2-CEES decomposed at the same rate as the formation of the 2-methoxy compound. However, it was noted that 2-CEES was also hydrolyzed to some extent in methanol alone (Scheme 7). A possible explanation as to why the AP-MgO increases the rate is shown in Scheme 8, which suggests a reason that only the methoxy compound is formed. Most if not
all reactive sites on the surface may be effectively blocked by the methanol. The surface then activates the complexed methanol for the reaction. This may be the reason the reaction has a faster rate with AP-MgO present. Acknowledgment. The support of the Army Research Office and the Edgewood CB center is acknowledged with gratitude. We thank Dr. B. Choudary for the silyl-capped AP-MgO samples. LA020195J
