Microscale Elimination Reactions
Per Cent of Butenes Formed in Et and E2 Eliminations
Experiments for Organic Chemistry Using the Small Scale Approach Helmuth M. Gilow Rhodes College 2000 N. Parkway Memphis, TN 38112
A similar mixture of butenes (see table) is obtained from the acid-catalyzed dehydration of 2-butanol ( 1 3 )t h a t illustrates a n El elimination, and the base-catalyzed dehydrohalogenation of 2-bromobutane (3) that illustrates a n E2 elimination. From the ratio of butenes obtained, it is impossible for students to determine whether the elimination reactions are El, E2, or a combination of the two. If, instead, l-substituted butanes are used, the interpretation is straightforward. The acid-catalyzed dehydration of l-butanol fonns a mixture of alkenes consistent with a n El elimination, and the base-catalyzed dehydrobromination of l-bromobutane forms only l-butene that is wnsistent with a n El elimination. The acid-catalyzed dehydration of l-butanol with HzS04 or H2S04/H3P04,using any of the microscale procedures ( 1 3 )give a mixture of butenes similar to that obtained from the dehydration of 2-butanol (see table). (The only change in the procedure that needs to be made is that the reaction mixture is heated to 140-170 'C rather than 100120 T . ) These results are clearly consistent with the formation of a 1-butyl carbocation which rearranges to a 2' butyl carboeation followed by elimination. An El elimination is clearly demonstrated from the mixture of butenes observed. The E2 elimination of l-bromobutane can be carried out
Compound
Reagent
2-Butanol 2-Butanol l-Butanol l-Butanol 2-Bromobutane l-Bromobutane
HzS04M3P04 HzS04 HzSOdH3PO4 HzS04 KOHIEtOH KOC(CH3)3
I - %, trans-2- %, cis-2butene butene butene 5 62 33 5 53 42 10 56 34 56 32 12 10 49 41 100 0 0
best with 1M potassium tertbutoxide in tert-butyl alwhol using the apparatus described in Pavia et al. (3).Amixture of 0.16 mL of l-bromobutane and 3 mL of 1mL potassium tert-butoxide in tert-butyl alwhol (Aldrich Chem. Co.) in a 5-mL conical rial is slowly heated to 100-110 % and kept a t this temperature range until the necessary volume of gas is collected. Gas chromatographic analysis indicated only l-butene is formed which is only consistent with an E2 elirhation. An ethanolic potassium hydroxide solution, a s described in Pavia et al. (31, can be used for the E2 elimination of l-bromobutane; however, the solution should be slowly heated to 80-90 T and maintained a t this temperature range. Only a small volume of gas will be collected (2-3 mL); however, it is sufficiently large to identify the product by gas chromatography Literature Cited 1. Mayo, D. W;Pike, R.M.: Butcher, S.5. Miemsmle O~gonieLaboratory, 1st and 2nd John W~ley:NervYmk, 1986: Experiment 9. 2. Williamson. K L. Micmscale Organic Ezperimnfs: D. C. Heath: Lexington, MA, 1981: Chapter 12. 3. Pen.. D.L.:, Lamoman. 0.M.:&. G.I.: ~ ~R. G.~Intmdudlon ~ 1 to Orsanic . Lab~
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lkchnipuer o Miemamle Appmoch; Saundera College Publishin;:
phia, 1990; Experiment 14.
Philadel-
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Volume 69 Number 10 October 1992
A265
