A Study of the E2 Reaction for the Microscale Organic Lab Pablck Flash, Fred Galle, and Mark Radil Kent State University-Ashtabula
Campus, Ashtabula, OH 44004
Most introductory chemistry lab texts offer a t least a few kinetics experiments (1-5). The most common experiment seems to be a study of the substitution reaction of alkyl halides (1-3).and the most common method of following the course of the reaction is by titration. The elimination reaction of alkyl halides and the competition between t h e E 2 and Sp~2mechanisms with alkyl halides is rarely studied (5). Since neither of the current microscale lab texts (6, 7) offered a kinetics experiment, we decided to adapt and expand a lab in reference (5) so that i t could he run a t the microscale level. The E2 elimination of HX from alkyl halides becomes useudo-first-order when run with a large excess of base. Assuming that all of the starting halidcis converted to a gaseous alkene, the final volume of gas produced, Vf, is directly proportional to the starting moles of alkyl halide. The gas volume durine the reaction a t time t, Vt, is proportionai to the moles of halide reacted and the change of volume, Vf - Vt, is proportional to the moles of unreacted halide. The standard first-order kinetic equation for the disappearance of halide in terms of the volume of gas produced is then written as
Figure 1. A sketch of Uw apparah
A plot of log (VrIVf - Vt) versus time then gives a straight line. T h e slope of the line times 2.303 gives k, the pseudofirst-order r i t e constant for the e1imi;ation ieactibn. The above discussion assumes 100% gas yield from a reaction. In the real world, most E2 eliminatibnssuffer from competition with the substitution reaction, and thus the gas yield is rarelv 100%. This will lead to errors in the value for k. but this (s the best we can do for this rough kinetics experiment. This notential fur error should be ~ o i n t e dout to the students: Experimental I n this exueriment the student will (1) determine the pseudo-fint-&der rate constant for the elimination of HBr from 2-bromoburane using KOH/water/ethanol solvent and measure the yield and approximate composition of the alkene products, (2) determine the yield of alkene(s) from 1bromobutane under the same conditions, and (3) examine the effect of changing solvent polarity on alkene yields for the two halides.
Apparatus The apparatus is easily constructed from standard microscale glassware, glass tube, rubber hose, and aquarium air hose. A sketch of the apparatus is shown in Figure 1. Both of the U tubes were made from 9-mm-0.d. tubing. The water reservoir was a 20-em Length of 18-mm-0.d. tube. Although a smaller diameter tuhe would have resulted in a greater drop in water level for each milliliter of gas evolved, we chose this size because it fit nicely with no. 3 rubber stoppers. One end of the U-shaped gas delivery tube was fitted with a short length of rubber tubing to serve as a stopper. A light touch of stopcock grease allowed this end to fit into the 7/10 joint at the top of the condenser. The other end of the delivery tube was led to one
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Journal of Chemical Education
hole of a two-hole rubber stopper at the top of the water reservoir. The second hole in the stopper was fitted with a glass tube and ruhber hose. A pinch rlnmp on this hose allowed lor fillingor dosing the reservoir and extractinga gas sample via a syringe needle inserted through the hose. The Klnetic Study (2-Bromobutane) The apparatus was assembled with the water reservoir unfilled. It was tested for air tightness by blowing gently into the outlet tuhe. The pinch clamp was then released. While water in a water bath was being heated, either on the hot plate or on a burner, 0.700-0.750 g (three-placeaccuracy) of potassium hydroxide was weighed into the 5-mL vial eontainine a soin vane. Exaetlv , 150 .ULof water was then added t o the vial, fkowed hy 2.5 mL of ahsolute erhnnoi. The vial was reattached to the rest of the apparatur and stirring was begun to divdolve the hydroxide. When the water bath reached YO-95°C. the condenser water was started and the water bath was positioned under the vial. The solution turned homogeneous as it boiled. The water reservoir was then filled by using a large syringe to inject the water from the outlet tube upward into the reservoir. Once the reservoir was filled, the pinch clamp was closed, and the top of the outlet U tube was held level with the water Level in the reservoir to adjust the air pressure and to allow excess water to drain. Once thepredsure wauadjustcd,tht.outlet U tube was hungover rherupuf the appararw to await the beginning ol the reaction. The delivery tube was carefully checked to make sure there were no air bubbles trapped within. About 0.10 mL of 2-bromobutane was drawn into a l-mL syringe, and the syringe plus liquid was accurately weighed to three decimal places. (You want about 0.1 g of halide to be delivered to the reaction.) The top of the U-shaped outlet tube was leveled with the water level in the reservoir, and the 2-bromohutane was injected
0
> 100 200 Time, in r
Time, in r Figure 2. Plotof log (V,/V,- V,) versus time in seconds.
300
Figure 3. P l d of tlw volume of water (1.0.. gas) collected versus time. through the septum in the Claisen adapter directly into the refluxing base solution. Timing was hegun at the instant of injection. A 1atm pressure in the water reservoir was maintained as the alkene was evolved by keeping the top of the gas delivery tube level with the reservoir surface. The water pushed out of the reservoir by the evolved gas was collected in a 10-mLgraduate, noting the volume of water collected at 30-8 intervals. Water collection was continued until the volume did not change for three consecutive readings. A second 10-mL graduate must be kept ready because more tban 10 mL was collected. When gas evolution ceased, the syringe was withdrawn from the Claisen head and reweighed to get an accurate mass of 2-hromohutane. The temperature of the water in the graduated cylinder was measured, and the barometric pressure was recorded to 1mm. The gaseous alkene mixtwe was analyzed by GLPC. A syringe was used to withdraw a 0.5-cc sample through the rubber tube at the top of the water reservoir, and the gas was injected onto a 4-ft, 20% DC 200 column at room temperature. Even with a helium flow rate of 20 mllmin, complete separation was not obtained. However, three peaks were observed in a ratio close to that expected for the hutenes. Complete separation of the hutenes can he accomplished using the conditions in ref 5. Because the kinetic study required considerable manipulation, it worked h a t with a team of two students. It could be done by one student who was well prepared. Gas Yields The KOHIwaterlethanol solution above was next used to dehydrohalogenate 1-bromobutane. A syringe containing slightly less tban 0.1 mL of the halide was weighedto iO.001g. The reservoir was refilled and the water level adjusted. The 1-hromobutanewas injected through the septum in the Claisen head. For this run, just the total volume of water evolved was measured. SolventlBase Effects The water bath was removed from the apparatus, and the 5-mL vial was disconnected. The solution in the vial was discarded, and the vial was cleaned and rinsed with absolute ethanol. The vial was reffied with 2.65 mL of a saturated solution of sodium ethoxide in absolute ethanol. A stir bar was added, and the vial was reconnected to the system. The solution was brought to reflux on the water bath. The dehydrohalogenation reaction was run on about 0.1 g of 2hromohutane. Just the total volume of water collected was noted for this reaction. Finally, the vial was cleaned again and refilled with the sodium ethoxide solution. About 0.1 g of 1-hromohutanewas added to the refluxing solution, and the final volume of water collected was again noted. Results and Discussion Duplicate runs on the 2-hromohutane in KOHIwaterlalcohol solvent eave plots . .- V,) versus time that . of loe- .(VdV, were linear for at least 5 min. T h e least-squares lines were
Oletln Yleld. for Ellmlnatlon of HBr from Bromobutaner a s Pwcent of the Theoretical Calwlated Yleld of Gas
Halida ?-brano 1-bramo
BaselSolvent KOHIwater athano1 67% 8.9%
ElOK1 elhand 77%
16%
plotted to the 5-min data point using ISI's Graphpack (8) program, and the slopes were determined (see Fig. 2.) T h e pseudo-first-order rate constants for the runs were 0.00173 8-1 and 0.00174 8-1 with r values for the lines of 0.9941 and 0.9964. T h e maximum gas yield occurred after 14 min. A plot of gas volume versus time showed no deviations near the origin (Fig. 3) so n o corrections needed t o he applied to the initial data. Using the known mass of starting halide, the theoretical maximum yield of alkene(s) was calculated for each run. Boyle's law, Charles's law, and Dalton's law corrections were applied to the theoretical yield a t STP to obtain the gas volume exnected under room conditions. A comnarison of gas yields for the dehydrohalogenation of the hro&obutanes in KOH/water/ethanol versuscthoxidelethanol showed that the olefin yield increased significantly in the less polar solventlstronger base situation (see table). The comparison of halide structure influence on the olefin yield was also equally gratifying. In both solvent mixtures the secondary halide gave more olefin product than the primary halide. For the KOHIwaterlalcohol solvent mixture, the secondary halide gave over seven times as much olefin as the primary halide. For the absolute ethanol solvent, the secondary halide gave almost a fivefold greater yield of olefin (see table). Concludon This experiment demonstrates verv nicelv a number of concepts discussed in class, and i t can be done in a typical 3h lab period. Students are able to do a kinetics experiment. plot the data, and get consistent results for the k value. he difference in tendency t o undergo elimination versus suhstitution for different classes of halides is dramatically demonstrated. T h e effect of solventfbase changes on the percent elimination vs. percent substitution for a halide is also shown. This experiment also offers significant advantages Volume 66
Number 11
November 1989
959
over the traditional approach. The small scale not only saves chemicals, it also saves time. More experiments can he done in one 3-h lah. There is no "violent eruption" associated with the large-scale reaction when the halide is added to the refluxing solvent. The lag time while the added halide
.. no expensive instruments, everyone in a cab section can do the lab the same day.
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Journal of Chemical Education
LRerature Cited D.L.; lam^,,. G. M.: Kri.. G. niquea: saundera: N ~ WYO& 19%.
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Philaddphi, 1976. ~ ~ J. A. ~ meorv b . and . fiactic. in tho olzanir Lobomtary; Heath: k i n s o n .
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York, 1986. willimm, K. L. ~ i ~ mnic ~~ ~~ ~ ~~ l~ ~ . ~ et ndh ~: ~1987. ~ t o ~~ . 8. M O I ~ UH.S -~ ~~, ~ ~ I S ~I S Oh~ ~ ~ W~U~h~ :i v~ ~ , d dISST. ~ h i ~ ,
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