Steric Hindrance by Bromination of Alkylbenzenes Experimental Demonstration J a m e s H. cooleyl and Nagib M. Abobaker University of Idaho, Moscow, ID 83844 This experiment is novel i n t h a t t h e liquid reactants, bromine and the alkvlbenzene, are adsorbed separately on alumina, and the reaction occkrs when t h e two solidsare mixed and shaken (I).' Electrophilic aromatic substitution occurs to give ortho, para mixtures. Comparisons of products from four alkvlbenzenes-toluene, ethylbenzene, isopropylbenzene (cimene), and tert-bntylbenzene-provide a nice illustration of the influence of steric hindrance in organic chemistry ( 2 4 ) . Several other things make this a good experiment. First, the experiment provides a much safer way for the student to transfer bromine. Second, t h e reaction occurs very quickly, with a visual change and a very noticeable evolution of heat. Third, this experiment works quite well in t h e problem-solving format described earlier (5). Accordingly, students are not told what the outcome of the is., and thev must decide what data to collect ~ - emeriment ~~- r~~ ~ and how to interpret themein order to fmd out what has hapnened. Thev are told that the obiective is to decide whether the product is one compound or more, how much of each is present, and what the structure or structures are. ~
Experimental Safety Precautions Working with Bromine: Work should be done under the hood. Protective gloves should he worn. Students should be ready to neutralize any spilled reagent with sodium tbiosulfate &tion. Working with Mumina: Alumina should be transferred under the hood to avoid inhaling the fine powder. W o r k i n g with Methylene chloride : Methylen? chloridr w p m t i p~~trrninlly hnrmful ~f inhaled. hlcthylcnr rhlonde should Iw dealt w i t h undrr [he hwd. Presented beforethe ldaho Academy of Science. April 1993. 'Author to whom correspondenceshould be addressed. he reaction may occur on the alumina surface and be catalyzed by aluminum ion. Evidence on the mechanism of electrophilic aromatic substitution in the presence of alumina has not been reported, however. Evidence that alumina catalyzes the ionic addition of hydrogen halides to alkenes has been published ( 1 ) . 3Brockmann alumina was obtained from the Aidrich Chemical Company, Inc., 1001 West Saint Paul Avenue, Milwaukee,WI 53233. 4The sample size is about 1 g. A balance that weighs accurately to 1 mg is necessary for this experiment. OHAUS Model TS120S balances were used. SExcess bromine insures bromination of the alkylbenzene will be complete, and the excess is removed readily during the isolation procedcwe -... .
c ~ r o m ~inn emethy,ene cnlorloe reacts f~nherwllh tne prooLct or proo~ctsan0 most oe removed al lhfs polnl Tne m %lureshoJl0 oe keel away from strong lght soJrces a l l l excess brom ne 1s remove0 Metnvlene cn orme may oo I too v qorodsly and be osl I me pressure is reduced too much.if necessary, chill the sample in an ice bath for 10 min prior to attaching it to the rotary evaporator If a rotary evaoorator is not available. remove most of the solvent bv. sim~le . d SI a! on. Remove !he ast traces of so vent by attach ng !he sll Pot to an asp ralor an0 rolat ng tne f asd genlly lo spreao !he ltqd o on tne inside surface ~
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The procedure w a s adapted from t h a t described by Ranu, Sarker, and Chakraborty in 1992 (6). The bromine-alumina reagent was prepared by the instructor by adding 16 g (0.1 mole) of bromine to 8 4 g of activated, Brockmann I, neutral a l ~ m i n a .These ~ exact weights were placed on the label for student use. The reagent was kept a t 0 'C to decrease the bromine color i n the head space of t h e bottle. Procedure Using a plastic measuring spoon, transfer one quarter of a teaspoonful of the bromine-alumina reagent into a tared vial. Use a funnel or a folded weighing paper to assist in the transfer. Cap the vial. Weigh the sample and calculate the moles of bromine present.4Some bromine may be lost during the transfer. Allow for this loss by using only 95% of an equivalent number of moles of the alkylben~ene.~ Calculate the weight and volume of alkylbenzene needed. In the hood, transfer one quarter teaspoonful of alumina to a second vial using a second plastic spoon. Tare the vial and alumina. Use an Eppendorfpipet or a microliter syringe to measure the alkylbenzene, and add it to the alumina drop by drop. Determine the weight ~ h a n g e . ~ Stopper the vial and shake vigorously to form a uniform mixture. Pour the contents of the vial containing the alkylbenzene into the vial containing the bromine. Cap the vial and shake the reagents gently to mix. Loosen the cap to see if there has been a pressure build up. Shake intermittently for 15 min to complete the reaction. The student should observe: discharge of the bromine color no pressure build up the vial becomes warm Working in the hood, pour the contents into a small chromatography column, and elute t h e column with methylene chloride until 3 mL are collected. If the solution is yellow or orange, the presence of bromine is indicated. Wash with 1mL of 10% aqueous sodium sulfite solution to remove the excess of b r ~ m i n e Dm . ~ the methvlene chloride solution anhydroui calcium chloride. Rewith a few granules move the solvent carefullv usine a rotarv evaporator? The yield i s about 100 mg, "and tKe crude prokuct i s pure enough to be characterized.
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Results The following i s a summary of results and conclusions reached bv students. Our students turn to s~ectroscopvin tra this expehment with good success. The 1 ~ ~ s ~ e cshow peaks in the overtone-combination region (1660-2000 an1) and t h e out-of-plane bending region (810 and 740 cm-'1 t h a t indicate ortho-para mixtures for the products from toluene, ethylbenzene, and isopropylbenzene but only the para product from tert-butylbenzene. Proton NMR spectra bf a l l p o d u c t s show, by the integration of peaks, t h a t ring substitution h a s occurred. Also two methyl group peaks in a ratio of 35% a t 6 2.26 and 2.36 ppm are seen i n the spectrum of the product from toluene, while the spectrum from the ethylbenzene product shows two methylene quartets Volume 72 Number 5 May 1995
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a t 6 2.60 and 2.76 ppm in a ratio of 20:80. Likewise there are two methine seotets a t 6 2.94 and 3.44 oom in the soectrum of the product from isopropylbenzene in a ratio of 10:90.8 Methyl group peaks near 6 1.25 in the spectra of products from ethylbenzene and isopropylbenzene are seen as overlapping triplets and doublets, respectively. In contrast, the spectrum of the product from tert-butylbenzene shows a single methyl group peak a t 6 1.30 ppm. All students have elected to obtain and interoret the soectra of these prodncts. Mass spectrag of all reaction show Mi and M+2+~ e a k with s eaual intensities. and from these data many &dents concluhe that substitution by a single bromine has occurred. A majority of our students have drawn correct conclusions from these data. Some of the more careful and thorough students have reported other data. None of our students have chosen to determine a boiling point, but refractive indexes are freauently reported and these are close to the reported values fbr ortho-and para-bromoalkylbenzenes. some students have deduced that the oroduct is a n a w l halide from the observations that it is insoluble in wate; and sulfuric acid, and that it gives a positive Beilstein test. Some students have reported thin-layer chromatography using silica gel a s the stationary phase and hexane as the mobile phase gives a spot a t Rf0.65 for all the products." A faint spot also appears a t Rf0.35 in the products from toluene, ethylbenzene, and isopropylbenzene. Although these results reinforce the conclusion t h a t two products are present in three of the reaction mixtures, both spots are actually due to mixtures of two products each. The products with Rf0.35 are identified a s a mixture of dibromo compounds formed by alpha bromination of the side chain of the ortho-para mixture initially formed. If the procedure given above is followed, these impurities are formed in trace amounts. Their presence did not adversely affect results obtained by students. However, if the product mixture is left with a n excess of bromine in the methvlene chloride solution, the amount of dibromo compounds increases. Further observation of this bromination reaction indicates i t can occur rapidly if the product from the reaction on alumina is left in methylene chloride with a n excess bromine. As an additional test, a solution containing 10 mmol of p-bromotoluene and 1 mmol of bromine in 11 mL of methylene chloride was observed to remain red with ceiling fluorescent lighting. This same solution was decolorized in 1min when the hood lights, two 20-W fluorescent light bulbs four feet above the bench, were switched on. 'Ib identify the product with a n Rfof 0.35, the reaction mixtures were separated by column chromatography. For this separation, 2 g of 'Baker" Silica gel for flash chromatography, a 9 mm id chromatography column, and 10 mL of pentane were used with 100 mg of each reaction mixture. The monobromo compounds were eluted in the 2-5 mL fraction of pentane, and the dibromo compounds were eluted in the 7-10 mL fraction.
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Journal of Chemical Education
From the data that students choose to collect, most are able to conclude the identity and ratio of the monobromo products. Many make the observation that the amount of ortho product decreases as the size of the alkyl group increases and give the expected explanation that steric hindrance is involved. I n our program we divide the class into groups of four students and allow each group to work together to run the four reactions and obtain whatever data they decide are needed. If a mouo is dissatisfied with the data one of its members obtains; a small amount of time is made available for that moup to work together to repeat that reaction and collect betterdata. students are fre;to make copies of each spectrum, but they are expected to submit their reports independently. NMR spectra are obtained on a 200 MHz instrument in our program, and small samples are adequate to obtain good spectra. The reaction can be scaled up if larger samples are needed to get the necessary data. Alternately, and especially for those laboratories where spectrometers may not be available for student use, copies of spectra can be made available by contacting the author (JHC). I t is our belief that students learn more by having to decide what data to collect and how to interpret them on the day of the experiment. In subsequent periods we rehash the data students are expected to obtain and give our interpretation. Student comments following these sessions are that the discussion has resulted in a much clearer understanding of the experiment. We wish to acknowledge the help of Richard V Williams for pointing out the paper by Ranu, Sarker, and Chakraborty, and for many useful discussions. Also we acknowledge financial support for this work from the University Research Off~ce,University of Idaho. Literature Cited 1. h p , P. J.; Daus, K A ; Tubergen, M. W : Kepler, K D.:Wilson. Y P;Craig. S. L.; Baillargwn. M. M.:Breton, G.W . J.Am. Chrm. Snc 1993,115,3071-3079. 2. lagold, C. K. Struciurp a n d Mechanism in 01ganie Chemistry, 1953, Cornell Universit) Press: Ithaca. NY pp 36,400. 3. E l i 4 E. L. Stemxhemidw d C a r h o n Compoundr: MeGraw-Hill. Ine: New York. 1962. pp 222-223. 4. March,JAdoonced 01gnnle Cheml*?. 4th ad.: Wiley: New York,1992, p 511. 5. Coolev. J. H. J. Chsm. Educ 1991.67161. , . 503. 6. Ranu, B. C.: Sarker, D. C.: Chakrsbarty, R. Synthetic Communications 1992.22i81, 1095.
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'In contrast to tne report oy Ranb, Sarker, an0 Cnauaborty we nave conststenl q foJna !hat sopropy oenzene g ves bolh onno an0 para products, and we speculate that the alumina available in the two laboratories may have been different and may have caused this difference. in 'Mass SDectrometN is not available for runnino student samoles ,~ 0-r progrim so spectra of me m xldres are pro;loeo lor sl~denls to lnlerprel However. OecaJse st~denrsare expected lo aeclae what Oata are neeoeo. lhey mLsl rewest these speclra The "se of gc ms in this experiment should work well for those programs where this instrumentation is available. ' w e use Baker-flex,silica gel 1B-F plates. ~~
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