In the Laboratory
A Substitute for “Bromine in Carbon Tetrachloride”
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Joshua M. Daley and Robert G. Landolt* Department of Chemistry, Texas Wesleyan University, Fort Worth, TX 76105; *
[email protected] One of the most widely cited qualitative tests employed in organic chemistry (frequently taught in the first semester and utilized widely in qualitative organic analysis) involves addition of a dilute solution of bromine in carbon tetrachloride (Br2 in CCl4) to a compound, to test for carbon–carbon multiple bonds (1). Major advantages of this approach include (i) ease and rapidness of the procedure, (ii) the stability of the test solution over time, and (iii) the ability to distinguish alkenes from enols or phenols because the latter produced HBr as a byproduct, detectable owing to its insolubility in carbon tetrachloride. Significantly, academic use of Br2 in CCl4 effectively has been discontinued, since production of carbon tetrachloride has been phased out (2). In addition to being acutely and chronically toxic (3) and “reasonably anticipated to be a human carcinogen” (4), CCl4 is implicated as a stratospheric ozone-destroying substance and subject to regulation designed to limit environmental release (5). Suggested solvent substitutes, including dichloromethane (6) and chloroform (7), reportedly suffer disadvantages of reactivity with bromine. In addition to serving as a simple qualitative test, Br2 in CCl4 has provided a useful matrix for rationalizing free radical substitution reactivities of known compounds. Notable among these is the demonstration of “The Effect of Free Radical Stability on the Rate of Bromination of Hydrocarbons” (8) and the related experiment, “Bromination Selectivity of Hydrogen Atom Abstraction As a Function of Structure” (9). Key to both the demonstration and experiment is the impact of ambient conditions, heat, and visible light in inducing free radical chain substitution of a bromine atom for a hydrogen atom in a series of organic substrates that distinctively featured primary, secondary, tertiary, benzyl, and aromatic C⫺H bonds: R–H + Br •
R • + HBr
Substances tested in the experiment included (in decreasing C⫺H reactivity order based on 3⬚ benzyl, 2⬚ benzyl, 1⬚ benzyl, 3⬚ alkyl, 2⬚ alkyl, and 1⬚ alkyl): isopropylbenzene, ethylbenzene, toluene, methylcyclohexane, cyclohexane, and tert-butylbenzene. Based on its potential for use with free radical reactions (10), benzotrifluoride (BTF or α,α,α-trifluorotoluene) has been explored as a replacement solvent for CCl4 in an experiment also modified to emphasize a “problem-oriented” (11) approach. Rather than rationalize the behavior of known substances, students employ free radical halogenation reactivities to distinguish compounds in an “unlabeled bottle” experiment. Experimental Procedure and Results Students are provided with 1.0 M Br2 in BTF and a set of hydrocarbons (Table 1) as “unknowns” in containers coded by letter. The unknowns in quantities of 1.0 mL aliquots are placed individually into six test tubes and a seventh tube is charged with 1.0 mL of BTF to serve as a “blank”. In rapid succession, 1.0-mL premeasured aliquots of 1 M Br2 in BTF are added to the hydrocarbon samples and to the blank and mixed thoroughly. The reactivity of each individual “unknown” relative to the rest is determined by loss of bromine color under progressively energetic conditions. Typically, the neat hydrocarbons decolorize 1.0 M Br2 in BTF as shown in Table 1. Hazards Work should be conducted in a well-ventilated hood, and students should wear safety glasses and gloves. Bromine solutions should be dispensed to students from a buret, given the corrosive nature and volatility of the reagent (12). Although data on BTF reported to date (10, 13) indicate less unattractive toxicological properties compared to carbon tet-
Table 1. Reaction Conditions to Decolorize Bromine in BTF Hydrocarbon Isopropylbenzene
Type of Hydrogen 3⬚ benzyl
Conditions < 0.5 min, ambient temp
Ethylbenzene
2⬚ benzyl
< 1 min, ambient temp
Toluene
1⬚ benzyl
~ 6–11 min, ambient temp
Methylcyclohexane
3⬚ alkyl
10 min ambient temp and 7–13 min at 50 ⬚C (water bath)
Cyclohexane
2⬚ alkyl
10 min ambient temp, 10 min at 50 ⬚C, and 4–5 min irradiated (60-W bulb)
tert-Butylbenzene
1⬚ alkyl
no reaction even after heated and irradiated periods
NOTE: BTF is benzotrifluoride α,α,α-trifluorotoluene.
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In the Laboratory
rachloride and dichloromethane, exposure to BTF should be minimized. Volatility, flammability, and potential toxicity (see ref 12) of the hydrocarbons as well as their brominated products also warrant careful handling and proper disposal procedures for these materials Discussion Generally, free radical substitutions with bromine are faster when BTF is employed than with carbon tetrachloride, but the pattern of relative reactivities is the same and conveniently observed: 3⬚ benzyl > 2⬚ benzyl > 1⬚ benzyl > 3⬚ alkyl > 2⬚ alkyl > 1⬚ alkyl. Under conditions comparable to the classic “bromine in carbon tetrachloride” alkene test (6), Br2 in BTF is also a convenient matrix for distinguishing compounds with carbon–carbon π bonds from saturated substances, such as cyclohexene versus cyclohexane. BTF solutions of ketones (including acetone, cyclohexanone, acetophenone, and propiophenone) decolorize bromine in BTF comparable to the test employing CCl4, but solutions become cloudy, and HBr is detectable using wet, blue litmus. BTF boils at 102 ⬚C, has a flash point of 12 ⬚C, specific gravity 1.2, and is essentially insoluble in water. The cost (2003) of BTF is about $33 per liter (14). The CF3 group is relatively stable to acidic or basic conditions and deactivates the aromatic ring in electrophilic substitutions, directing primarily to the meta position (10). These factors lead to anticipation that BTF’s utility may extend, within limits, to other experiments requiring an inert, water-insoluble solvent. W
Supplemental Material
A table showing the free radical reactions under different solvent conditions is available in this issue of JCE Online. Acknowledgments Jack Gilbert made very helpful observations and suggestions in the preparation of this document. The project was made possible by a Welch Foundation Departmental Research Grant, number AZ0003. Literature Cited 1. Contemporary description: Shriner, R. L.; Hermann, C. K. F.; Morrill, T. C.; Curtin, D. Y.; Fuson, R. C. The Systematic Identification of Organic Compounds, 7th ed.; Wiley: New York, 1998; pp 276–279.
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2. The Accelerated Phaseout of Class I Ozone-Depleting Substances. U.S. Environmental Protection Agency Web site. http://www.epa.gov/ozone/title6/phaseout/accfact.html (accessed Sep 2004). 3. Center for Disease Control and Prevention/NIOSH. http:// www.cdc.gov/niosh/idlh/56235.html; Homepage. http:// www.cdc.gov/niosh/homepage.html (accessed Sep 2004). 4. Official Citation: Report on Carcinogens, 10th ed.; U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program, Dec 2002, http:// ehp.niehs.nih.gov/roc/tenth/profiles/s029carb.pdf (accessed Sep 2004). 5. Environmental Health Criteria, No. 208: Carbon tetrachloride, World Health Organization, CH-1211 Geneva 27, Switzerland, 1999; The International Programme on Chemical Safety (IPCS) Summary: http://www.who.int/pcs/ ehc/summaries/ehc_208.html; full text: http:// www.inchem.org/documents/ehc/ehc/ehc208.htm (accessed Sep 2004). 6. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Organic Laboratory Techniques, Small Scale Approach, 1st ed.; Saunders: New York, 1998; p 504, as well as ref 1. 7. Zhang, W.; Li, C. J. Org. Chem. 2000, 65, 5831–5833. 8. Doheny, A.; Loudon, G. M. J. Chem. Educ. 1980, 57, 507– 508. 9. Roberts, R. M.; Gilbert, J. C.; Martin, S. F. Experimental Organic Chemistry, A Miniscale Approach; Saunders: Ft. Worth, TX, 1998; pp 302–308. (Described on larger scale in earlier texts by the same authors; see their later textbook employing dichloromethane: Experimental Organic Chemistry: A Miniscale & Microscale Approach, 3rd ed.; Harcourt College Publishers: Ft. Worth, TX, 2002; pp 768–769). 10. Maul, J. J.; Ostrowski, P. J.; Ublacker, G. A.; Linclau, B.; Curran, D. P. Benzotrifluoride and Derivatives: Useful Solvents for Organic Synthesis and Fluorous Synthesis. In Topics in Current Chemistry; Springer-Verlag: Berlin, 1999; Vol. 206, pp 80–105. 11. Fife, W. K. J. Chem. Educ. 1968, 45, 416. 12. J. T. Baker, Bromine MSDS-Number: B3905. Effective Date: 11/02/01, Mallinckrodt Baker, Inc. Web site http:// www.jtbaker.com/msds/englishhtml/B3905.htm (accessed Sep 2004). 13. J. T. Baker, BTF MSDS-Number: B1700. Effective Date: 04/10/01, Mallinckrodt Baker, Inc. Web site http:// www.jtbaker.com/msds/englishhtml/B1700.htm (accessed Sep 2004). 14. Sigma Aldrich Online Catalog and Homepage, Cost for benzotrifluoride, product # 547948 www.sigma-aldrich.com/ suite7 (accessed Sep 2004).
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