In the Laboratory
A Puzzling Alcohol Dehydration Reaction Solved by GC–MS Analysis† Michael W. Pelter* and Rebecca M. Macudzinski Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323; *
[email protected] We have adapted the dehydration of 2-methyl-2-propanol (1) to a “puzzle” approach for use in our second-semester chemistry major organic laboratory (1, 2).1 The reaction of 1 with ~50% sulfuric acid at 100 °C yields isobutylene (2), which reacts further by a “puzzling” reaction (Scheme I). By coupling the GC/MS analysis of the product mixture with their knowledge of the mechanism of alcohol dehydration and alkene reactivity, students are able to identify the major products of this reaction. CH3 H3C
H2SO4
H3C C CH2
C OH H3C
CH3
1
2
?
Scheme I
Figure 1. Chromatogram obtained from the total ion current of the mass-selective detector.
Experimental Procedure To a 10-mL round-bottom flask containing 1 mL of water and a spin vane, add 1 mL of concentrated sulfuric acid. With stirring, cool the solution to 50 °C in an ice-water bath and slowly add 1 mL of 2-methyl-2-propanol. Attach a reflux condenser, and heat the mixture to a gentle reflux for 30 minutes (sand bath temperature2 ~100 °C). Cool the reaction mixture and transfer it to a 15-mL centrifuge tube. Remove the lower aqueous layer. Wash the remaining organic layer with an equal volume of water. Place the organic layer in a 10-mL Erlenmeyer flask containing ~0.1 g of anhydrous calcium chloride. Cap the flask and allow it to stand with occasional swirling for 10 minutes. Transfer the dried organic layer via Pasteur filter pipet to a tared, capped vial. Perform a GC–MS analysis and obtain an IR spectrum of the product mixture. Propose reasonable structures for each of the major products and give a mechanism that would account for their formation. GC Conditions. HP G1800A GCD system with electron ionization detector using an HP-5, 30-m × 0.25-mm column. Temperatures: inlet, 250 °C; detector, 280 °C; oven, initial 50 °C for 5 min then ramp 10 °C/min to 160 °C.3
Figure 2. Mass spectrum for the product with retention time 1.92 min.
Results Figure 1 shows the gas chromatogram of a typical product mixture. The retention times of the four major products are given in Figure 1 and the mass spectra of these components are shown in Figures 2–5. IR (neat): 3076, 2954, 1641, 1477, 1364, 1236, 1201, 892 cm{1.
† Presented at 211th ACS National Meeting, New Orleans, LA, March 1996.
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Figure 3. Mass spectrum for the product with retention time 2.00 min.
Journal of Chemical Education • Vol. 76 No. 6 June 1999 • JChemEd.chem.wisc.edu
In the Laboratory
Figure 4. Mass spectrum for the product with retention time 5.06 min.
Figure 5. Mass spectrum for the product with retention time 5.24 min.
Answer
The given ratio appears to violate what is taught about alkene formation. Or does it? On one hand, we teach that internal alkenes are thermodynamically more stable than terminal alkenes. On the other hand, we talk about steric hindrance. Examination of the carbocations shown in Scheme III leads to the following observations. Two sterically hindered methylene hydrogens lead to the more thermodynamically stable alkene. Abstraction of one of the six primary methyl hydrogens leads to the less thermodynamically stable product. Add to this the fact that the proton is abstracted by a weak base, water or HSO4{. The result is the path of least resistance: formation of the terminal alkene (2a), showing that the thermodynamics of the product does not influence the outcome of the reaction.
Under these reaction conditions, the “puzzling” product is a mixture of “diisobutylenes” (3 and 4) and “triisobutylenes” (5 and 6) (Scheme II)4 (3, 4). Data from the mass spectra show that the M+ peak for each compound is a multiple of 56, the molecular weight of isobutylene, and the IR spectrum indicates the presence of a double bond. In looking at the mechanism, “polymerization” can be rationalized (Scheme III). The intermediate carbocation can react with alkene to form a new tertiary carbocation, which can deprotonate to yield either 3 or 4. Alternatively, the carbocation can react with an alkene molecule to form a new carbocation which, upon deprotonation, yields 5 or 6. "Diisobutylenes" CH3
H2C C CH2
CH3
C CH3
C CH
C CH3
H3C
Discussion
CH3
H3C H3C
CH3 4
3 "Triisobutylenes" CH3
H2C C CH2 H3C
CH3
CH3
CH3
CH3
H3C
C CH2 C CH3
C CH
CH3
C CH2 C CH3
H3C
CH3
5
CH3
6
Scheme II H3C
H3C
H3C
+
H3C
H3C
H3C
+
C CH2
C CH3
C CH2
H3C C CH2 H3C
CH3 C CH3 CH3 -H+
OR 3 or 4
H3C 5 or 6
-H+
CH3
+
C CH2 H3C
CH3
C CH2 C CH3 CH3
In most of the published procedures, this experiment was completed by distillation and gas chromatographic analysis of the diisobutylenes (1, 2), although in one case this was coupled with qualitative tests and NMR analysis (1b, 2a). The students had to be told the identity of the products, which were then compared with known samples in the GC analysis. Our approach makes full use of the analytical power of GC–MS and allows the students to solve a real-life problem. During the past two years, students have consistently been able to solve at least two of the structures; many of them solved all four. As in all investigative experiments, individual instructors must decide how much additional information their students require.
CH3
Scheme III
While most organic texts discuss cationic polymerization using the polymerization of isobutylene as an example, few show the structures of the diisobutylenes and triisobutylenes (4). Out of these, only two give the ratio of isomers (4a,b).
Acknowledgments We would like to thank the students of CHM 266 Organic Chemistry Laboratory II for their assistance in perfecting this experiment, the reviewers for their helpful insight, and the National Science Foundation (DUE-9650826) for the funds to purchase an FT-IR. Notes 1. This is a 2-credit-hour laboratory, as is the first semester. 2. We have run this reaction in sand baths set at 150 and 200 °C, as well as 80–100°, with little variation of results. 3. Conditions should be optimized for each instrument. Starting at 40 °C with a 6–8-min isotherm followed by a slower ramp will give better baseline separation, but it also increases the run time.
JChemEd.chem.wisc.edu • Vol. 76 No. 6 June 1999 • Journal of Chemical Education
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In the Laboratory 4. The product composition for students’ samples ranges as follows: 3: 41–64%, 4: 17–23%, 5: 9–18%, 6: 11–23%. Several other peaks may be present in the gas chromatogram (Fig. 1). These account for less than 5% of the products mixture and are usually below the threshold of the instrument. The peaks around 5 minutes are triisobutylene isomers and the peaks around 10 minutes are tetraisobutylene isomers. The structures of 3 and 4 were confirmed by comparison to authentic samples.
Literature Cited 1. (a) Allen, M.; Joyner, C.; Kubler, P. G.; Wilcox, P. J. Chem. Educ. 1976, 53, 175. (b) Tremelling, M. J.; Hammond, C. N. J. Chem. Educ. 1982, 59, 697. 2. (a) Mohrig, J. R.; Morrill, T. C.; Hammond, C. N.; Neckers, D. C. Experimental Organic Chemistry; Freeman: New York, 1998; p
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91. (b) Zanger, M.; McKee, J. R. Small Scale Synthesis: A Laboratory Textbook of Organic Chemistry; Wm. C. Brown: Dubuque, 1995; p 157. (c) Wilcox, C. F. Experimental Organic Chemistry: A Small-Scale Approach, 2nd ed.; Prentice Hall: New York, 1994; Chapter 21. 3. Whitmore, F. C.; Church, J. M. J. Am. Chem. Soc. 1932, 54, 3710. 4. (a) Streitwieser, A.; Heathcock, C. H.; Kosower, E. M. Introduction to Organic Chemistry, 4th ed.; Macmillan: New York, 1992; p 1240. (b) E¯ge, S. N. Organic Chemistry, 3rd ed.; Heath: Lexington, MA, 1994; p 300. (c) Vollhardt, K. P. C.; Schore, N. Organic Chemistry, 2nd ed.; Freeman: New York, 1994; p 442. (d) Carey, F. A. Organic Chemistry, 3rd ed., McGraw-Hill: New York, 1996; p 249.
Journal of Chemical Education • Vol. 76 No. 6 June 1999 • JChemEd.chem.wisc.edu
