In the Laboratory
A Safe Simple Halogenation Experiment
W
Hilton M. Weiss* and Lara Ganz Department of Chemistry, Bard College, Annandale-on-Hudson, NY 12504
Most introductory organic chemistry courses begin with the chemistry of alkanes. Their relatively unreactive nature, however, makes it difficult to find appropriate laboratory experiments for these early weeks. Many halogenation experiments have been suggested (1), but they often suffer from the need for complicated techniques or equipment, long reaction times, or the presence of functional groups that diminish the relevance of the reaction. In addition, the organic products of some halogenations are volatile and dangerous. We have developed a simple halogenation experiment that can be performed quickly in an open vessel at room temperature with visible light. The product is a solid and can be isolated by a simple filtration, permitting the entire experiment to be completed within one hour. The reaction is also easily adapted to microscale. We use NMR in this experiment to identify the product and to introduce some simple NMR analysis. The reaction is shown below: CH3
CH
CH
CH3
CH3
CH3
Br2 light
Br CH3
C
CH
CH3
CH3
CH3
+
CH3
Br
Br
C
C
CH3
CH3
CH3
The bromination of 2,3-dimethylbutane is performed with a large excess of hydrocarbon in an open test tube. Because HBr is generated in the reaction, we have done these reactions in a well-ventilated hood under a 200-watt light bulb or outside in an open area under direct sunlight (and careful supervision). The high selectivity of the bromine atom insures substitution of the tertiary over the more prevalent primary hydrogen atoms. The question of monobromination versus dibromination of the alkane is left for the student to determine by NMR and melting point. The NMR pattern of an isopropyl group may be found by running a sample of the starting material. The dibrominated product has a single NMR peak (δ = 2.02 ppm) and a relatively high melting point deriving from its highly symmetrical structure and the resulting wellpacked crystal structure. The requirement for light in these brominations may be demonstrated by wrapping the reaction vessel with aluminum foil. The bromine color will remain for hours. The relative unreactivity of secondary hydrogens may be demonstrated by using 2,2-dimethylbutane, n-hexane, or cyclohexane as the alkane. Why the dibrominated product is formed remains W Supplementary materials for this article are available on JCE Online at http://JChemEd.chem.wisc.edu/Journal/issues/1999/ Apr/abs534.html.
*Email:
[email protected] .
534
a matter of some conjecture (2), and this open-ended question can stimulate student interest. The reaction may be monitored by gas chromatography, which will show the initial formation of the monobromo product followed by its rapid conversion to the dibromohexane. Kinetic modeling of this sequence indicates that the monobrominated alkane is approximately 103 times as reactive as the starting dimethylbutane. The high reactivity of the monobrominated intermediate may also be shown by the competitive bromination of a 2% solution of 2-bromo-2,3dimethylbutane in 3-methylpentane and GC analysis of the resulting alkyl halide mixture. Experimental Procedure WARNING: Bromine causes severe burns and should be dispensed by the instructor wearing chemical-resistant gloves. A 10% solution of sodium thiosulfate should be kept nearby in case of a spill or skin contact. Only after the reaction is complete (bromine color gone) are students allowed to contact the reaction vessel. Care must still be given to the remaining HBr fumes and the alkyl halides produced in the reaction. Into a wide 50-mL test tube or centrifuge tube is placed 10 mL of 2,3-dimethylbutane followed by 1.0 mL of bromine. Care is needed in handling bromine; we have the instructor dispense the bromine from a tilting dispenser. The reaction mixture is swirled briefly and the test tube is propped up in a 400-mL beaker pointed toward the light source. After a few minutes, copious amounts of “HBr” can be seen and the bromine color eventually fades to a light yellow. When the reaction is complete, the test tube should be loosely corked to diminish water condensation as ice is added to the surrounding beaker. Cooling the solution to 0 °C will precipitate a white solid, which can be collected by suction filtration. This should be done in the hood, since there will still be some HBr in the solution. The product should be air-dried for a few minutes on the filter paper before taking its melting point and its NMR spectrum. The reported melting point of 2,3-dibromo2,3-dimethylbutane is 169 °C (sealed tube) (3). Student yields are usually more than 50% and melting points are within a few degrees of the literature value when taken in sealed tubes. Literature Cited 1. Landgrebe, J. A. Theory and Practice in the Organic Laboratory; Heath: Lexington, MA, 1973. Roberts, R. M.; Gilbert, J. C.; Rodewald, L. B.; Wingrove, A.S. Modern Experimental Organic Chemistry, 4th ed.; Saunders: Philadelphia, 1985. Fieser, L. F.; Williamson, K. L. Organic Experiments, 7th ed.; D. C. Heath: Lexington, MA, 1992. 2. March, J. Advanced Organic Chemistry, 4th ed.; Wiley: New York, 1992, p 681 and references therein. 3. Shriner, R. L.; Fuson, R. C.; Curtin, D. V. The Systematic Identification of Organic Compounds: A Laboratory Manual, 4th ed.; Wiley: New York, 1956.
Journal of Chemical Education • Vol. 76 No. 4 April 1999 • JChemEd.chem.wisc.edu
