The Synthesis of 4,4'-Di-tert%utyl Biphenyl: A Sophomore Organic Chemistry Experiment A typical Friedel-Crafts alkylation experiment in organic laboratory manuals is the preparation of 1,4-di-tert-hutyl benzene. In this alkvlation, benzene is used as both solvent and reaeent. In recent vears it has become more evident that brwene i; 3 vtr? ha,drd~uscum~r.,l,nd.'.4i n reaulr, a c haw wplarcd bcn~cncaith thcsairr ioi\rnt ~,lucnci o uur scph.mc,rc urganic 13buratr.ry. One cannor essil\. replace bcnxne w t h t ,lurnc in rhli Frtrdel-Craft> alkylarwn experiment bemure g l q u ~ dand thus nm cssily purii~cda i t h u u ~a wruuni d~stilthe major product. 1-r?rr.bursl tolucnc. I; a h ~ g hb ~ i l ~ npjinr lation. Biphenyl would seem to he an attractive alternative to benzene for this alkylation reaction. It is inexpensive and does not pose a serious health h a ~ a r dThe . ~ product, 4,4'-di-tert-butyl biphenyl, is crystalline, and it is of some use as a transmetallatine aeent.s
Neckers4 has reported the alkylation of biphenyl with tert-hutyl chloride as an organic laboratory experiment. In his experiment the alkylation may be carried out using the alkyl chloride as solvent with poor yield (30%), or nitromethane may be used as solvent to give a better yield (50%). We feel that nitromethane is too flammable to he used routinely in an undergraduate organic laboratory and thus we set out to find a safer solvent, catalyst combination for this alkylation. One may expect the activation energy for the formation of chloromethyl cation to he higher than theactivation energy for the formation of tert-butyl cation. Chlorine is a net electron withdrawing group. Lewis acid
CH2Clz
eCHzC1 higher E, than Lewis acid
(CHhC-CI
(CH3)3C@
Thus, if a sufficiently inactive catalyst could he found, then tert-butyl cation could he prepared in dichloromethane solvent. Dichloromethane has been shown to he a reasonably nontoxic and nonflammable solvent.5 We tried several Lewis acid catalysts for the alkylation of biphenyl with tert-butyl chlaride in dichloromethane. We found anhydrous aluminum chloride to be too active. At least ten products were found hy gas chromatographic analysis. Anhydrous zinc chloride was not active enough; no reaction was observed. Anhydrous ferric chloride has the correct activity. It does not react with dichloromethane hut does react with tert-hutyl chloride. We have been able to prepare 4,4'-di-tert-hutyl hiphenyl in 70% yield using tert-hutyl chloride, anhydrous ferric chloride, and biphenyl in dichloromethane solvent. We have been using this experiment in our sophomore organic laboratory quite successfully. In our laboratory we have our students first prepare tert-butyl chloride from tert-hutyl alcohol and hydrochloric acid. We find that this sequence fits nicely into a four-hour laboratory period with about one-half hour discussion included.
Procedures Tert-butyl Chloride. In a 125-ml separatory funnel, place 60 ml of concentrated hydrochloric acid that has heen cwled to 0-5" and add 20 ml of tert-hutyl alcohol. Shake intermittently for 20 min, separate the tert-hutyl chloride layer, dry over CaClz and distill, collecting material with hp 49-52', This gives about 15 ml of tert-hutyl chlaride (65%yield). 4,4'-Di- tert-Butyl Biphenyl. In a 125-ml Erlenmeyer flask add 5 g (0.03 moll of biphenyl, 10 ml(0.09 mol) of tert-hutyl chloride and 25 ml of dichloromethane. After the biphenyl has dissolved add 0.20 g of anhydrous ferric chloride. Place a gas trap on the flask and gently swirl. It sometimes helps to speed the reaction by warming the flask in a warm water bath. After the gas evolution has stopped (about 20 m i d , pour the contents of the flask into a separatory funnel which contains 20 ml of cold 10%hydrochloric acid. Extract the organic layer several times with 10%hydrochloric acid and dry over anhydrous calcium chloride. Remove the solvent on a rotary evaporator and recrystallize the 4,4'-di-tert-hutyl from 95% ethanol. Yield
'
Repeated inhalation of low concentrations of benzene often results in severe or fatal anemia. See "Hazardous Materials;' National Fire Protection Association, Boston, Mass. OSHA tentatively lists benzene a s a category I carcinogen. See Federal Register, Oct. 4, 1977. Part VI. OSHA tentatively lists biphenyl a s a category Ill carcinogen. There is "no, or at most meager, evidence of carcinogenic risk." See FederalReuister, Oct. 4. 1977. Part VI. 3Freeman, P. K., ~utchinson,L. L., Tetrahedron Letters, 1849 (1976) Neckers, D. C., Duncan, M. B., Gainor, J., Grasse, P. B., J. CHEM.EDUC., 54, 690 (1977). "Dichloromethane is dangerous to the eyes but otherwise relatively non-toxic." Muir, G. D., "Hazards in the Chemical Laboratory," The Chemical Society, London. Address inquiries to author at Midland-Ross, Technical Center, P.O. Box 985, Toledo, OH 43696. Deane A. Homee University of Toledo Toledo, OH 43606
246
Journal of Chemical Education
