In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Colorful Chemical Demonstrations on the Extraction of Anionic Species from Water into Ether Mediated by Tricaprylylmethylammonium Chloride (Aliquat 336), a Liquid–Liquid Phase-Transfer Agent submitted by:
Anil Joseph Pezhathinal, Kerensa Rocke, Louis Susanto, Derek Handke, and Roch Chan-Yu-King* Division of Science and Physical Education, University of Science and Arts of Oklahoma, Chickasha, OK 73018; *
[email protected] Patrick Gordon Department of Chemistry, Simmons College, Boston, MA 02115-5898
checked by:
Richard L. Keiter Department of Chemistry, Eastern Illinois University, Charleston, IL 61920-3099 Eugene N. Losey Department of Chemistry, Elmhurst College, Elmhurst, IL 60126
Liquid–liquid phase transfer catalysis (LLPTC) is one of the most versatile methods used to achieve organic transformations (1). While a number of publications in this Journal (2) have dealt with LLPT-catalyzed reactions only a few undergraduate organic textbooks (3) cover its mechanism. In our view the importance of LLPTC in academic research and industrial applications is well supported by the fact that the annual production of chemicals (using LLPTC) can exceed 10 billion dollars (4). Thus, we have regularly included the topic of LLPTC in our lecture series and included at least one laboratory experiment (5) per semester to illustrate its synthetic utility. However, we found that students often have a difficult time writing the mechanism of LLPTC in its entirety during examinations. While several LLPTC mechanisms are known today (6), the authors of the above-mentioned textbooks cite the pioneering work of Charles Starks (7) and provide his mechanism for a displacement reaction shown in Scheme I where Q+ denotes the quaternary catalyst cation (e.g., tetralkylammonium or phosphonium ion); X−, the catalyst anion (e.g., chloride or bromide ion); M+Y−, the watersoluble reagent (e.g., NaCN); and RX, the substrate soluble in the organic phase (e.g., an alkyl halide). One of the key steps of this mechanism involves the transfer of the water-soluble anion and nucleophile (Y −) into the organic phase via a preliminary ionic association of Q+ and Y− in the aqueous phase (equilibrium 1) forming a more lipophilic pair (Q+Y−) that is then transported into the organic phase (equilibrium 2). The displacement reaction subsequently takes place (equilibrium 3) forming the products RY and the leaving group X−, which then pairs up with the tetraalkylammonium or phosphonium ion, regenerating the catalyst Q+X−. The latter species is subsequently transferred into the aqueous phase (equilibrium 4) establishing a catalytic cycle. An example of such a reaction (7) is the formation of nonanenitrile in 99% yield (2 h) when a solution of 1-chlorooctane in decane is treated with aqueous sodium cyawww.JCE.DivCHED.org
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nide in the presence of hexadecyltributylphosphonium bromide as the phase-transfer catalyst and the in situ generated hexadecyltributylphosphonium chloride as the cocatalyst (Scheme II). organic phase Qⴙ Xⴚ +
3
RY
4
+
RX
2
interphase 1
Mⴙ Yⴚ
Qⴙ Xⴚ +
Qⴙ Yⴚ
Mⴙ Xⴚ +
Qⴙ Yⴚ
aqueous phase Scheme I. Mechanism for a displacement reaction (7). Terms are defined in the text.
decane ⴙ
ⴚ
R4P Cl
+
3
R′CN
4
R′Cl
interphase
R4Pⴙ Brⴚ + Naⴙ CNⴚ
1
+
R4Pⴙ CNⴚ 2
Naⴙ Brⴚ + R4Pⴙ
water ⴙ where R4P = [CH3(CH2)14CH2] [CH3(CH2)2CH2]3Pⴙ R′ = CH3(CH2)6CH2
Scheme II. Reaction producing nonanenitrile in 99% yield.
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In the Classroom
Most organic chemistry students are familiar with the mechanism of a basic nucleophilic displacement reaction. In an effort to help them broaden their knowledge of mechanistic chemistry and to understand the preliminary steps (1 and 2 of Scheme I), we solicited the assistance of a few students enrolled in undergraduate research to provide a list of simple, colorful classroom demonstrations to illustrate the above steps by which water-soluble anionic species are extracted from water into ether with Aliquat 336, CH3N[(CH2)7CH3]3Cl, as the phase-transfer (PT) agent. Experimental Most chemicals used are commonly found in undergraduate chemical stockrooms or can be purchased either from Aldrich Chemical Co. (Milwaukee, WI 53201) or at any convenient store. The food color solutions are obtained from Adams Extract Co. (Austin, TX. 78760). The brine used is a saturated solution of NaCl in water (40% w兾w).
Procedure with Aliquat 336 To a vial containing a medium-size magnetic stir bar, add 10 mL of the colored aqueous solution containing the oxidizing agent or dye followed by the addition of ether (10 mL). Loosely cap the vial and vigorously stir for 2 min. Let
the mixture settle (2–3 min) and observe. Then add 5–6 drops of Aliquat 336 to the vial. Repeat the above stirring– settling process and observe.
Control Experiments with Brine All experiments described above are repeated using the above procedure with Aliquat 336 being substituted with 2– 3 mL of brine. Control Experiments with Base Sodium carbonate (1 M, 1 mL) or sodium hydroxide (1 M, 1 mL) is used instead of Aliquat 336 for solutions containing methyl red or bromocresol purple, respectively. Experiments follow the procedure above. Hazards Diethyl ether is flammable. Aliquat 336 is toxic and a severe irritant. Salts of chromate and dichromate ions are cancer suspect agents. They should be disposed of properly (8). Gloves must be used in the handling of oxidizing agents, dyes, and Aliquat 336. Chemical companies, from which the reagents are purchased, provide MSDS sheets that should be consulted before conducting these experiments.
Table 1. Compilation of the Students’ Data Entry No.
Water-Soluble, Colored Anionic Species (MY)
1
Na2CrO4
2
Blue or green food color
3
Gatorade containing “Blue 1” dye
Solution No. in Figure 1 1
N N
4.25
2 or 3
4
Initial Concentration of MY/(10−4 mol L−1)
1 drop in 100 mL
---
10 mL of the blue commercial drink is used as the aqueous phase
---
3.00
SO3Na OH
orange II
5
KMnO4
4
4.66
6
K2Cr2O7
---
4.00
5
4.66
---
5.25
6
4.50
7
2.80
7
NH2
NH2 N N
congo red
SO3Na
8
N N
SO3Na
H3C N
N N
SO3Na
H3C
9
methyl orange
H3C N
N N
COOH
H3C
10
HO
methyl red
CH3
Br
Br OH
bromocresol purple
O S O
1162
O
CH3
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In the Classroom
Results The experimental results show that Aliquat 336 is instrumental in the extraction of the colored species from water into ether; that is, no oxidizing agent or dye can be phase transferred into ether in the absence of a PT-catalyst (with or without brine) except for methyl red, which can be “salted out” of water with brine alone (see details given below). In the presence of a base but without the participation of Aliquat 336, methyl red and bromocresol purple cannot be phase transferred into ether. Table 1 (with representative solutions in Figure 1) is a compilation of most of the students’ research data. Upon addition of Aliquat 336 and subsequent stirring and settling, the following observations are made: • The less dense, upper ether layer (initially colorless) became colored while the reverse phenomenon is observed for the lower aqueous layer (entries 1, 2, and 3). • Most of the color is transferred into the organic phase and transfer of the residual color can be facilitated with the addition of brine (2 mL) (entries 4, 5, and 6). • Some dyes are essentially nontransferable even in the presence of Aliquat 336 unless brine (2 mL) is added (entries 7 and 8). • The dye is transferable only upon addition of 1 mL of 1 M NaOH (entry 10).
An aqueous, saturated solution of methyl red (10 mL) provided a red lower layer and a pale-yellow upper ether phase (entry 9; Figure 1, vial 6). Subsequent addition of brine (2 mL) with stirring revealed that methyl red can be completely “salted out” of water. This resulted in the appearance of a yellow top layer and a colorless lower layer. Further addition
of Na2CO3 (1 M, 2 mL) with stirring provided the reverse phenomenon. However, the subsequent addition of 5 drops of Aliquat 336 with stirring brought the yellow-colored species back to the top layer. Discussion The use of chemical demonstrations to reinforce classroom teaching and to facilitate students’ learning is well documented (9). One or more of these experiments can be selected, performed, and displayed on a screen in a large lecture hall with the aid of a document camera (Elmo visual presenter, EV-500 AF). Alternatively, if such a document camera is unavailable, the experiment(s) can be shown at closer range (e.g., in a smaller classroom setting) to chemistry students who are concurrently enrolled in a laboratory course as part of a prelab lecture. For example, we have performed experiments described in entries 1, 5, and 10 to students, enrolled in our second-semester organic chemistry lab. Following the demonstrations, the students were asked to write the steps corresponding to the extraction of the colored species from water into ether by Aliquat 336. We found that these classroom demonstrations, in conjunction with preexamination quizzes, have greatly helped the students’ understanding of the critical role played by the tetralkylammonium salt in the PT process and their ability to retain the mechanistic steps of the LLPT-reaction (Scheme II) has improved. We have also shown some of these experiments to students enrolled in advanced inorganic chemistry and prospective students during university-visitation day. In general, the audience enjoys viewing the transfer of the colored species and their surprise is clearly evident when the striking color change is observed upon the addition of hydroxide to bromocresol purple. This collection of demonstrations can also be used to help students review or to introduce them to various concepts, principles, or experimental procedures of undergraduate chemistry, which may include the following: • The use of brine (entries 4–8) to minimize the water solubility of the tetraalkylammonium–colored anion pair clearly demonstrates the “salting out” effect of a species. • The hard–soft acid–base principle (10) is illustrated in the experiments of entry 9 or 10. The conjugate bases of methyl red or bromocresol purple containing the hard oxygen anion center is associated with the hard quaternary ammonium ion of Aliquat 336 (11), which results in the formation of a lipophilic pair (Scheme III).
Figure 1. Mixture of ether and water containing the water-soluble colored anionic species.
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• A common procedure for the isolation of carboxylic acids or phenolic substances from other ether-soluble organic substances (12) is by extracting them into an aqueous layer under weakly basic condition for substrates containing a COOH group or under strongly basic condition for those substrates containing a phenolic group. An alternative method is found in the extraction of the conjugate base of methyl red or bromocresol purple by the tetralkylammonium ion into the ether, a step that constitutes the reverse phenomenon in that both compounds can be isolated in ether as tetraalkylammonium salts. This is also an appropriate place to qualitatively compare the Ka or pKa values of carboxylic acids versus phenolic substances.
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In the Classroom • This demonstration can be tied to a discussion on color changes observed when an indicator (aromatic organic acid, for example, bromocresol purple) is used for the detection of the end point in an acid–base titration experiment (13). Further discussion may include the topic of anionic-charge stabilization of the conjugate base via delocalization over the extended conjugated aromatic system. Also, students enrolled in organic chemistry are asked to write resonance structures. • The students enrolled in organic chemistry in our institution are asked to discuss what effect, if any, shortening the alkyl chain length of Aliquat 336 would have on the efficiency of the resulting PT agent, bearing in mind that tetramethylammonium chloride has a negligible solubility in organic solvents and that ammonium chloride cannot be
O
Br
• The fact that not all water-soluble species are transferable and that other factors must be considered, for example, the charge found on the color-bearing species; the volume, charge or volume–charge ratio of cation–anion pairs (16); the extent of hydrogen bonding of ions in the aqueous phase; the rate of stirring and the hydrophilicity versus lipophilicity of anionic species that affect the partition of the associated R4N+-colored anion pair. As an experiment to demonstrate what happens when PT is not operating, we have briefly shown to the students enrolled in advanced inorganic chemistry that the water-soluble, blue CuCl2⭈2H2O (10 mL of 0.5 M; 8.50 g兾100 mL) cannot be phase transferred by Aliquat 336 (5 drops) into ether (10 mL). This simple demonstration may be used as a basis for further research on methods of separation of colored water-soluble compounds via their selective extraction into ether.
OH CH3 O
S O
ether
ⴙ
N[(CH2)7CH3]3CH3
O
interphase O
CH3
Br
NaCl
Br
water OH
+
CH3 O
S O
ⴙ
N[(CH2)7CH3]3CH3
O O
CH3
Br Cl HO
Br
ⴙ
N[(CH2)7CH3]3CH3
OH
CH3
Br
CH3
Br
S O
O
S
OH O
O O
Naⴙ
O
CH3
Naⴙ O
CH3
Br
H2O
Br
+
OH O S O
O
CH3
Scheme III. Reaction between bromocresol purple and Aliquat 336.
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• The need to use a saturated solution of methyl red illustrates the well-known low water solubility of carboxylic acids having more than 5 carbons and their higher solubility in ether that can be improved with the addition of brine. The high water solubility of carboxylates is illustrated by the transfer of the conjugate base of methyl red into water. Students in organic chemistry might be reminded that a practical application of this finding is the isolation of soap molecules (in the form of carboxylic acids) from water by hydrochloric acid precipitation. • The fact that LLPTC reactions can be conducted at room temperature and that the organic solvent can be recovered by the sequence decantation-distillation makes LLPTC a “greener” and a more economical technology.
NaOH
1164
• The virtual absence of water in the organic phase allows the formation of carbanions from an acid–base reaction between an organic substrate bearing acidic hydrogens and the lipophilic tetralkylammonium hydroxide pair obtained by mixing NaOH (aq) with Aliquat 336. This precludes the need for anhydrous experimental conditions and the use of water sensitive bases such as NaH or NaNH2. • The common use of permanganate (MnO4−) as an oxidizing agent of organic substrates and the gradual fading of its purple color in water with concomitant formation of a brownish MnO2 (15) can be discussed.
CH3
Br
used as such a PT agent (14). That question can lend itself to further discussion on the pronounced rate enhancement generally observed in LLPT-catalyzed reactions, owing to the fact that once the water-soluble reagent (e.g., nucleophile) is transferred into the organic phase, it is essentially not hydrated. Therefore, the reaction activation energy is lowered. In the absence of a PT catalyst, reactions do not occur or are sluggish at best because the organic substrate and the water-soluble nucleophile are located in two immiscible layers.
The above concepts and principles with a suggested audience are listed in Table 2. Finally, it is worth reiterating that while the main objective of this article is to demonstrate the extractability of colored water-soluble species by Aliquat 336, a survey of the literature reveals that colorful LLPT-catalyzed reactions are known. These reactions can be used to nicely complement
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In the Classroom Table 2. List of Possible Demonstrations Illustrating Concepts and Principles Entry #
Possible Topics, Concepts, or Principles
Suggested Audience
1, 2, or 3
PT
Any audience
4, 6, 7, or 8
Salting out effect and PT
Organic, inorganic, or physical chemistry
4, 7, and 8
Azo dyes and their uses as staining agents in textile industries and microbiology
Organic chemistry
5
Oxidation, salting out effect, and PT
Organic or inorganic chemistry
9 and 10
Organic and Ka or pKa of carboxylic acids versus phenols; advanced inorganic chemistry Selective purification of carboxylic acids and phenols; Relationship between color, pH, and resonance effect; Solubility of carboxylic acids and carboxylates in ether and water
this collection of demonstrations may include its potential use to illustrate the preliminary steps of LLPT-catalyzed reactions and to introduce students to various undergraduate chemistry concepts, principles, or entertain prospective students or general public during a “college-visitation day” or “ACS-Chemistry Day”. Although the presentation of LLPT-catalyzed reactions is not the aim of this article, it is worthwhile to inform the students that reactions promoted by LLPTC are easy to perform and are often high-yielding reactions even when carried out at room temperature and that the major benefits afforded by this technology cannot be over emphasized. These benefits include low cost, large scale production of chemicals, enhanced pollution prevention, and a wide variety of synthetic applications. We recommend that teachers expose undergraduate students who plan to conduct research in academia and industry, to LLPTC and its importance in industrial applications. In addition, it is strongly recommended that, for subsequent editions and new organic textbooks, an in-depth coverage of this technology be included. Acknowledgments We are grateful to Lawrence K. Magrath for his comments and suggestions.
the above list of demonstrations. They include: • Acid–base reactions between hydroxide and aromatic substrates containing acidic hydrogens (e.g., indene, fluorene, or p-nitrophenylacetonitrile) in xylene with tetrabutylammonium hydrogen sulfate as the LLPT-catalyst (17). These reactions involve an initial colorless biphasic medium with tetrabutylammonium hydroxide that is being phase transferred from water into xylene. Subsequent acid–base reaction produces a brightly colored mixture indicating the formation of a resonance-stabilized aromatic carbanion. • The oxidation of cyclohexene with KMnO4 in benzene with either crown ether (18) or Aliquat 336 (19) as the LLPTcatalyst. It is found that the experimental procedure described in entry 5 can be used to oxidize cyclohexene. Thus, following the brine-assisted phase transfer of R4N+MnO4− into ether, cyclohexene (1 mL) is added. The deep purple color rapidly turned light brown within one minute of vigorous stirring (under identical conditions, without cyclohexene, the deep purple color of the upper phase did not fade away). This experiment obviates the use of the more toxic benzene. • The formation of azo dyes via the well-known coupling reaction between a heteroatom containing aromatic substrate and a water-soluble diazonium chloride salt (20). An interesting feature of these reactions is the use of sodium dodecyl sulfate (a “less traditional” PT agent) to extract a positively charged diazonium ion from water into chloroform.
Conclusion The nonexhaustive list of colorful and fairly safe experiments presented in this article can be used to acquaint students with the feasibility of extracting some soluble anionic reagents from water into an organic layer. Other values of www.JCE.DivCHED.org
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Literature Cited 1. Jones, R. A. Quaternary Ammonium Salts: Their Use in PhaseTransfer Catalyzed Reactions; Academic Press: San Diego, CA, 2001. 2. (a) Shabestary, N.; Khazaeli, S.; Hickman, R. J. Chem. Educ. 1998, 75, 1470. (b) Amsterdamsky, C. J. Chem. Educ. 1996, 73, 92. (c) Breuer, S. W. J. Chem Educ. 1991, 68, A58. 3. (a) Solomons T. W. G.; Fryhle, C. B. Organic Chemistry, 8th ed.; J. Wiley and Sons, Inc.: Hoboken, NJ, 2004; pp 526– 529. (b) Fessenden, R. J.; Fessenden, J. S. Organic Chemistry, 6th ed.; Brooks/Cole Publishing Co.: Pacific Grove, CA, 1998; p 780. (c) Carey, F. A. Organic Chemistry, 5th ed.; McGraw Hill: New York, 2003; pp 923–926. (d) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 6th ed.; Prentice-Hall Inc: Englewood Cliffs, NJ, 1992; pp 264–266. 4. Starks, C. M.; Liotta, C. L.; Halpern, M. Phase-Transfer Catalysis: Fundamentals, Applications, and Industrial Perspectives; Chapman and Hall: New York, 1994. 5. Addison, A. Techniques and Experiments for Organic Chemistry, 6th ed.; Allyn and Bacon: Newton, MA, 2003; pp 389– 397. 6. Phase-Transfer Catalysis. Mechanisms and Syntheses; Halpern, M. E., Ed.; American Chemical Society Series 659; American Chemical Society: Washington, DC, 1997. 7. Starks, C. M. J. Am. Chem. Soc. 1971, 93, 195. Starks, C. M.; Owens, R. M. J. Am. Chem. Soc. 1973, 95, 3613. 8. (a) Prudence Practice in the Laboratory: Handling and Disposal of Chemicals; National Academic Press: Washington, DC, 1995. (b) Hazardous Laboratory Chemicals Disposal Guide; CRC Press Inc.: Bocca Raton, FL. 1991. 9. Meyer, L. S.; Schmidt, S.; Nozawa, F.; Panee, D. J. Chem. Educ. 2003, 80, 431. 10. Huheey, J. E. Inorganic Chemistry, 3rd ed.; Harper and Row:
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In the Classroom New York, 1983; pp 312–315. Handcock, R. D.; Martell, A. E. J. Chem. Educ. 1996, 73, 654. 11. (a) Herriott, A. W.; Picker, D. Tetrahedron Lett. 1972, 4521. (b) Hideshima, T.; Morinaga, M.; Kimizuka, H. Bull. Chem. Soc. Jpn. 1981, 54, 85. (c) Phase-Transfer Catalysis. Mechanisms and Syntheses; Halpern, M. E., Ed.; American Chemical Society Series 659; American Chemical Society: Washington, DC, 1997; p 118. (d) Freedman, H. H.; Dubois, R. A. Tetrahedron Lett. 1975, 3251. 12. Morrison, R. T.; Boyd, R. N. Organic Chemistry, 6th ed.; Prentice-Hall: Englewood Cliffs, NJ, 1992; p 893. 13. Masterton, W. L.; Hurley, C. N. Chemistry, Principles and Re-
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actions, 5th ed.; Thomson: Belmont, CA, 2002; pp 386–388. 14. Jones, R. A. Quaternary Ammonium Salts: Their Use in PhaseTransfer Catalyzed Reactions; Academic Press: San Diego, CA, 2001; p 2. 15. Kitson, T. M.; Heyen, B. J. J. Chem Educ. 2003, 80, 892. 16. Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 3rd ed.; Wiley & Sons: New York, 1995; pp 131–135. 17. Hill, J. W.; Enice, L. J. J. Chem. Educ. 1985, 62, 608. 18. Doheny, A. J.; Ganem B. J. Chem. Educ. 1980, 57, 308. 19. Herriott, A. W. J. Chem. Educ. 1977, 54, 228. 20. Sommerfeld, H.; Blume, R. Prax. Naturwiss. Chem. 1993, 2, 39.
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