B. Cox, D. G. Kubler, and C. A. Wilson Furman University Greenville. South Carolina 29613
T h e hromination a n d nitration of benzene a r e classic examples used in undergraduate organic laboratory courses t o illustrate the nature of electrophilic aromatic substitution reactions ( 1 , ~ )~. ~ t h may be controlled to givehigh
yields of monosubstitution products, While both are deactivating the nitro group directs a second group to the m e t a position b u t t h e hromo group directs a second group t o t h e ortho-para positions. S t u d e n t s frequently come t o a n intuitive position t h a t while disnbstitution can occur with deactivating groups, i t is difficult, a n d , polysubstitution is rare. T h i s generalization is valid for nitrobenzene but n o t a t all for bromobenzene. In both instances, relatively small changes in experimental conditions cause significant changes in t h e yields of the pnssihle products. These variations provide s t u d e n t s with excellent learning opportunities.
Brominatlon of Benzene For many years our students made 14-dibromohenzene by a standard recipe. The yields were adequate and the procedure was interesting tostudents, in p a r t becauseof the beauty of the product. as it forms during recrystallization. On occasion, and in no pattern of occurrence. a student would fail totallv to obtain the desired of 1,4-dibromobenzene because of one or more reasons including the use of a large excess of bromine, m a largeexcess of thecat&& and/or the reaction system became too hot for good product control. Many variations of the experiment are possible to control the synthesis of bromobenzene, 1,2-dihromobenzene, 1,4-dihromohenzene. and 1.2.4.5-tetrabromobenzene. Our students have not found
and ir or nmr spectroscopy for product identification offer many opportunities for student involvement in useful ways.
Nitration of Benzene A common procedure for the nitration of benzene calls for the slow addition of benzene to a mixture of nitric and sulfuric acids with cooling. Students me cautioned to keep the temperature below 60°C to prevent the formation of significant amounts of rn-dinitrobenzene ( 1 ) . ourstudents ronsistently achieved 45-65% yields of ,,itrobenzene hut in no instance did a student ever obtain a yield as high as 15%. Students have always been somewhat frustrated by this experiment since their textbooks tell them nitrobenzene is readilv made in 95% yield Or better' The do not respectable yields is not due to their use of poor techniques but rather is due to an inherently poor procedure. By the simple device of reversing the order of addition our students were to yields f, 7545% and on occasions as high as 95%. Procedures are given in a few laboratory texts far this reverse addition but the reasons for it are not
Experiments with Electrophilic Aromatic Substitution Reactions discussed (44).When the benzene is added to the nitrating mixture there is throughout the reaction an excess of nitrating mixture which causes significant dinitration. On the other hand, when the nitrating mixture is added to the benzene, the benzene is in excess during the addition which diminishes the chances for disubstitution. We have also found that the reaction is much easier to control by the reverse addition except far the slight danger of handling the nitrating mixture in an addition
Experimental The procedures given for the bromination of benzene are not optimized. ~h~ usual precautionsfor handling bromine must be given tothestudents.
Preparation of ,,4-Dibromobenzene To 5 ml of benzene in a 125-ml Erlenmeyer flask add one small iron nail and cool the flask in ice water. With the flask in the hood add 6.2 mlof bromine and let stand for at least oneday or until thenext laboratory period. Add 50 ml of water to the flask and heat to boiling with vigorous swirline. Allow to settle and cool until the dibromide solidifies and then d&ant the liquid. Repeat the washing twice with hot water, adding to the second wash about 10 ml of 10% sodium hydroxide. Add 75 ml of ethanol, heat to boiling on a steam hath, and filter (fluted filter) into a clean Erlenmeyer which is heated on a steam hath. Add 30 ml of hot water while the alcohol solution is boiling and let stand undisturbed until crystallization is complete. Filter, air dry, weigh, and determine the mp of the product. (lit. (3) mp 88'C)
1,2,4,5-Tetrabromobenzene To 5 ml of benzene in a 125-ml Erlenmeyer flask add four or five iron nails. Heat the flask until the benzene just begins to boil, transfer the flask to the hwd, and add 15ml of bromine. Let the mixture stand until the next laboratory period. Wash the product as described for 1C-dibromobenzenebut as a final wash, wash the adhering water from the product with two small portions of alcohol. Recrystallize the product from the minimum amount of benzene filter, air dry, weigh, and take the mn (lit. (3) mo 181°C).
Literature 1 1 1 F..., L p . , , . ~ ~in omanic ~ ~ t chemibtry: i ~ ~ ~ 3rd. ~ ~~ d .D.C. . HI^^^ & c,,. H , , ~ ~ ~ , , , . 1957. p. 14s. 121 Adams, R..Johnson. J. R., and Wilmx. C. F.."1.ahoratw Experiments in O m n i r ChemiLw"5th Ed.. McMillan Co.,New York,1963, p. 291. I31 vwl, A. I.,"A Tertbaokof PcaetiraloreanicChemistry"3rd Ed..John wilcy&Sons. ~ n c .N. ~ W~ o r k 1956, . p. 54fi. 141 Cason,J.,and Rspopurf,H.,"~abaratory Textin OrpnicChemistry"2nd ~ d . . v w n ~ Lice~Hsll.lnc., Englewood Cliih. NJ. 1962. 151 R u k r u , K. M.. Gi1hert.J. C.. Rodewsid, L.B.. and Wingmvo. A. S.:'An tn ~ o d e r n~ ~ p e r i m e n tns lr p n i e chemietry,"
intnducticni 2nd ~ d .~. d t~ .i n e h a , tand win.t,,n.
lnc. ~ e ~wo r k ,974. .
w. E.. and Rabemon. G. R.,"Laboratory Pranice s i Or~anic Chemktry,"5thPd., MacMillan PuNishineCo. Inc, New York. 1974.Thir text dopi
(61 Jaahn. T . L., Truce. no,
".""
d i ~ ~ , t. h~o, n m ~ ~~~~.~ . mnr,hsn,~srnia~rli~inn h,,+ rsthe.sslir thr.+..rlnn, whuihs ~
eeid is added to the benzene.
Volume 54. Number 6, June 1977 1 379
