The Dimerization of isobutylene: A Student Experiment One of the highlights of teaching and learning organic chemistry is the enthusiastic response students have to learning and applying the thermodynamic basis for Markovnikov's rule and Saytzeffs rule.' However, when they learn that the dimerization of isobutylene produces a predominance of 2,4,4-trimethyl-I-pentene(with a terminal double bond) rather than 2,4,4-trimethyl-2-pentene (with an internal double hond), they often feel that thermodynamics has led them astray.? It has not, of course, but simplistic rules in applying thermodynamics has. After explaining the reasons for this fact a numher of times, it seemed a useful pedagogical device to have the students verify the experimental facts by reading the original literature on the dimerization of isobutylene." The students were able to quickly locate Whitmore's classic studies on isobutylene dimerization and thus verify the ronelunions which their texts had reoorted. The interestine" result of this readine was that it oroduced considerable skeoticism on their part as to the validity of the separation method used by Whitmore. Their reasoning presumed that since the work was performed over forty years ago that separation by distillation is unreliable compared to modern methods of separation. Students rarely appreciate the efficacy of separation which some chemists can achieve by fractional distillation and tend to think it is a crude method compared to modern separation techniques. Whitmore's method of dimerizing isobutylene was repeated on a small scale. The method involved heating t-butyl alcohol with aaueous sulfuric acid to ~ r o d u e eisobutvlene which then dimerizes. The oroduct was washed with bicarbonate, dried and analyzed by gle. The predominant isdmer was found to he 2,4,4-trimeihyl-1-pentene and in a ratio to the 2,4,4-trimethyl-2-pentene that was precisely that (4:l) reported by Whitmore. We also demonstrated that the pure isnmeric dimers rearranged under the reaction conditions to the equilibrium mixture. The separation of the two octanes was achieved hyglc on a Wilkens Aerograph A-700. Various columns and conditions were tried with a synthetic mixture (1:l)of the two isomers. The best separation was achieved with 15% OV-l on Chmmsorb P (Applied Science Laboratories, State College. Pa.) with an He flowrate of 55mllmin and a column temperature of 70°C. Calibration curves were made from glc analysis of synthetic mixtures of isomers having 4.06:1, 1:1.01, and k4.07 ratios of 2,4,4-trimethyl-l-pentene t o 2,4,4-trimethyl-2-pentene. The retention time far the l-isomer was 5.3 min and for the 2-isomer was 6.0 min. Both ~ e a k were s nicely symmetrical and sharp. ~~~
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I Morrison, R. T., and Boyd, R. N., "Organic Chemistry," 3rd ed., Allyn and Bacon, Boston, 1973, pp. 188 and 478. 'Roberts, J. D., Stewart, R., and Caserio, M. C., "Organic Chemistry," W. A. Benjamin, Inc. New York, 1971, p. 101. :' Whitmore, F. C., and Church, J. M., J. Arne?. Chem Soe. 54,3710 (1932) and cited references.
F u r m a n University Greenville, South Carolina 29613
M. Allen C. Jayner P. G. Kubler P. Wileox
Volume 53,Number 3. March 1976 1 175
