GEORGE L. GILBERT DenlsOn Unlverslty Granv~lie.Ohlo 43023
The Nitration of Alkylbenzenes: A Lecture Demonstration Submitted by: M. Davis. L. W. Deady a n d T. G . Paproth, L a Trobe University Bundoora, Victoria 3083, Australia Ch~ck~ by:d Richard R. Doyle Denison University Granville, Ohio 43023 In our introductory organic chemistry course, hydrocarbons are the first examples of aromatic compounds to be discussed. This is no douht a standard practice. Thus, electrophilic substitution in alkyl benzenes represents the first encounter with the directing effects of substituents on an aromatic ring. This is a difficult concept to master, especially when such a marked effect can he oroduced hv the auite unexce~tional looking (at least to theheginner) alkyl gioups. It is therefore more convincing if the discussion can he based on some first-hand experimental data. We have devised a quick lecture demonstration to illustrate some aspects of the nitration of alkyl henzenes. The products are analyzed by glc and. bv fitting the recorder with a thick felt pen, the trace is v i s i b ~ ~all t o & a 250-seat theater. Toluene and t-hutylhenzene are each nitrated and the products extracted into ether.' This solution is injected into the glc. Under the conditions specified in the experimental section, the elution times are 5.5 min for the toluene products and 10 min for those from t-hutylbenzene. The order of elution is ortho, meta, para, and the nitrating mixture does not give rise to any dinitro products. Samole chromatoerams are illustrated and clearlv show (a) the remarkable o-p directing eflect of the methyl groups, and (h) thesteric hindrance to orthoattack provided by the bulky t-hutyl suhstituent. At a more advanced level it is also possible to discuss relative reactivities from these results; toluene is more reactive since some unreacted starting material is evident in the trace for the t-hutylhenzene. If it is desired to show a gradation of alkyl group effects, ethyl and isopropyl benzenes can he treated in the same way and show the expected intermediate behavior. ~
Gas chromatograms of the nitration products lrom toluene and t-butylbenrene.
Analysis A 2-m steel column filled with 10%Carhowax on Chnmosorh W operating at 120' is used. The injection pwt is at 200° and a cylinder pressure of 20 psi is used for the nitrogen carrier gas. A i-pl injection uf the samples gives satisfactory results with the flame ionization detrctrrr emoloved. and the ehmmatoerams illustrated were obtained
Chemical Kinetics: The Effect of Surface Area on Reaction Rate Submitted by:
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Experimental Reactions To 1 ml of nitrating acid (31conc. nitric, cnnc. sulfuric)is added 12 drops of the alkyl henzene, with constant shaking. The mixture turns red and then pale yellow. The 1-hutylhenzenemixture needs gentle warming for ahaut 1 min to produce appreciahle reaction in a short time. It is then poured into 20 ml water and extracted with 15 rnl diethyl ether. The ether extract is washed with 20 ml2% sodium earhonate solution and dried over calcium chloride. Time: * i n min. 1 In our course, this part of the demonstration has additional value in that is is carried out within a week of the time the students first learn about extraction in their laboratory course.
34 1 Journal of Chemical Education
M a r k S. Felice a n d M a r k B. Freilich P u r d u e University West Lafayette, Indiana 47907
Cheched by: Ronald E. DiStefano Northampton County Area Community Collegr Bethlehem, Pennsylvania I8017 Preparation A) Prepare 100 ml of a solution which is 2.5 X 1 0 ' M in 12, 2 X lo-' M in KI, and 4 M in HC1. To this add 1to 2 ml of 1% starch solution. To a 250-ml beaker add 1g of granulated tin (30 mesh). T o a second 250-ml beaker' add 1 p of mossy flaked tin." B) Prepare 100 ml of a solution which is 2 X lo-" M in KMnO, and 2.5 M in HySO,. To a 250-ml beaker' add 0.5 g of 40 mesh degreased iron filings. To a second 260-ml beaker' add 0.5 g of '11,;in. iron wire." C) Preoare 50 ml of 6 M HN0.k. To a 250-ml beaker1 add 0.3 g of purified copper powder. In a secoud 250-ml heaker,' place 0.3 g of copper wire.' ' A crystallizing dish of thr appropriate size would work equally well. T h e use ofequal massesofmetal is more important than the actual masses. The reacthms proceed most favorably when fiesh metal surfaces (unoxidiaedlare used.
D) Prepare a solution which is 3 M in HCI and 3% in H 2 0 ~ . (Slow evolution of gas will he noted due to the reaction hetween HCI and H20&huwever, the solution is good for a t least 1hr.) T o a 250-ml beaker' add 0.5 g of fine (0.009 in.) iron wire. To a second 250-ml beaker,' add 0.5 g of coarse ( % a in.) iron wire.' Demonstration In each case, add one half of the solution to each heaker a t approximately the same time and mix briefly. A color change demonstrates that the finer material reacts much more quickly due to the larger surface area. In cases B and D, it is important that the solutions he fresh (made within 1-2 hr of use). The demonstrations work very nicely using an overhead projector focused for maximum sharpness of the solid. Color changes on the projector can he seen from a t least 200 ft away. (The amounts of material may he scaled upwards for use without the overhead projector. Obviously, suitable care must betaken, especially in the reaction hetween nitric acid and copper.) Remarks In cases A, B, and D, the faster reaction is nearly instanta-
neous. (The oxidation of FeY+(aq) to Fe:'+ (aq) by H202 is fairly rapid under the given conditions.) The slower reaction may take up to 1hour to be noticeable. The above demonstrations represent true chemical processes whose rates of reaction are dependent upon the area of contact between two phases. In this way, these demonstrations are similar to the use of lycopodium powder and its explosive reaction with oxygen. The above demonstrations have the advantage, however, that the reactants are constantly in view. (A frequently cited example demonstrating the effect of surface area on reaction rate is the comparison between hurning a solid log and an equal mass of wood chips. However, it has been suggested that temperature effect may he in part responsible for the difference in reaction rate.) Acknowledgment We would like to thank Professor DiStephano for suggesting and testing the use of a crystallizing dish as an alternative to the 250-ml beaker and for his reminding us to mention the importance of using a metal with a clean, unoxidized surface.
Volume 55, Number 1. January 1978 / 35
