The Stepwise Nitration of Toluene A Multistep Microscale Synthesis Based on an Industrial Process Richard A. Russell, Robert W. Swltzer, and Robert W. Langmwe University College, University of N.S.W., Northcotl Drive, Campbell ACT. 2600, Australia The stepwise synthesis of 2,4,6-trinitrotoluene (symTNT) is a simple but important industrial example of electronhilic aromatic substitution that reflects a decreasine" reactivity accompanying the increasing degree of nitration.' Although 2,4,6-trinitrotoluene is itself a relatively stable c o m p o ~ n d its ~ - preparation ~ in the laboratory is, for safety reasons, best accomplished on a micro scale.5This limits the amount of material prepared without detracting from the industrial significance of the exercise. As there is no necessity to purifythe intermediate products, the experiment is accessible to students with only the basic skills in micro svntbesis6. and the nrenaration can be linked to modern &alyticalkethods 6 monitor the progressive nitration. The initial mono-nitration of toluene to afford MNT oil,' as described below, occurs under relatively mild conditions that mimic the industrial use of spent or diluted nitrating acid. The product of this reaction contains a mixture of the three isomeric mononitrotoluenes, the isomer distribution of which may be established by the use of HPLC and comrnercially available standards. Further nitration of the crude MNT, a t elevated temperature, affords a mixture of dinitrotoluenes readily shown by TLC to contain predominantly the 2.4. and 2.6-isomers accomnanied bv traces of the 2.3-. 3,4-, and ~,4,6:compounds~~ ~ h final k nitration step, wGch is carried out under more forcine conditions affords a crude product, which is contaminated i y small amounts of unsymmetrical isomers, in articular 2.3.4- and 2.4.5-trinitrotoluene. These latte; products are readily removed by conversion to water-soluble sulfonic acids upon treatment with sodium sulfite, a process again based on the industrial procedure, and capitalizing on the reactivity of the proximate nitro moms &wardsnucleonhilic diso1acement.-The final product is readily purified b; recrysthlization and may be characterised by either its IR or 'H-NMR spectrum.
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Experlmenlal Detail Preparation of Mononitrotoluenes Using a microsyringe, place 100 pL of toluene (86.4 mg, 938 wmol) in a 3-mL conical vial equipped with a magnetic stirring vane and an air condenser. Place the vial in a roomtemperature water bath and add 420 pL of nitrating acid 18 dropwise to the vigorously stirred toluene by means of a 1mL syringe. After the addition of the acid is complete, maintain stirring a t room temperature for a further 20 min and dilute the reaction with 1.2 mL of water. Remove the spin vane and wash with diethyl ether (1 mL); add the washings directly to the vial containing the quenched reaction mixture. Then, cap the vial, shake for 30 s, and remove the organic layer using a Pasteur filter pipet.6 Extract the aqueous phase again with ether, combine the extracts, and wash with water (2 X 1mL). Dry the resulting ethereal solution using a small
68
Journal of Chemical Education
column of dried magnesium sulfate (-4 em) contained in a cotton-wool-plugged Pasteur pipet,6 and wash the desiccant witha further 0.2 mL of ether. Collect the combinedsolution in a tared 3-mL vial, and remove the solvent (HOOD) under a slow stream of nitrogen while the vial is gently warmed. The residual oil (96mg, 75%),which contains a mixture of all three isomers of mononitrotoluene, is analyzed by HPLC (see below). Preparation of Dinitrotoluenes Equip the 3-mL vial containing the mononitrotoluenes from the nrevious sten with asnin vane and an air condenser and placP in a water bath at nn)m temperature. Add nitratina acid I (420 u1.J drouwise to the viaorouslv stirred mixture, raise the temperaiure of the bathto 65 OC, and maintain a t this temperature for 20 min. Cool the reaction mixture in ice and dilute with 1mL of water. Repeat the diethvl ether extraction nrocedure de~ ~which scribed pr&ously, this should produce a y e i l oil solidifies on standing and will be shown by TLC to contain predominantly a mixture of 2,4- and 2,6-d&itrotoluene. Preparation of 2.4.8-Trinitrotoluene. Place the preceding mixture of dinitrotoluenes in a 3-mL reaction vial equipped with a spin vane and an air condenser. Place the vial in a sand bath a t room temperature, and add nitrating acid 11(340wL) (CARE) dropwise to the vigorously stirred mixture. When all the acid had been added, elevate the temperature of the sand bath to 110 "C, and maintain the reaction mixture a t this temnerature for 90 min. Then cool the mixture to 0 OC and add to 2.5 mL of water contained in a 5-mL conical flask. Wash the reaction vial with a further 0.2 mL of water, which is then added to the quenched
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Although modern production of TNT is accomplished by a continuous process the batch process involving these three steps was for many years a major method of manufacture.The industrial termlnology MNT, ONT, and TNT appears to derive from this era. A review of this chemistry can be found in footnote 3. For a detailed list of hazards see footnote 4. irk-Othrner ~ n c y b e d i aof chemical ~echnolo~y. 2nd ed.; Wi-
. -. of Reactive Chemical Harards; Butter'Bretherick, L. Handbook
.lev: .,.. b w Yark. .~ ~,1965: V o I 11 n 61 - . 1. .~ ~~~~
worth: London.
1979. For safetyreasons the final product should collected forappropriate disposal. Mayo, D. W.: Pike. R. M.; Butcher, S. S. Microscale Organic Laboratory; Wiley: New York, 1986. More detailed analysis by HPLC is possible but in general the required reference samples are not available commerciaflyand are tedious to prepare. Nitrating acid I contains concentrated nitric acid 19%vlv,concentrated,sulfuricacid58%v/vand water 23%vlv.Nitrating acid II is a mixture of concentrated nitric acid 25%vlv and oleum (30% Sod 75%"l".
reaction mixture. Allow the resulting mixture to stand for 10 min to ensure complete precipitation of the product after which the aqueous phase is removed with a Pasteur pipet and discarded. Treat the remaining solid with 5% aqueous sodium sulfite (1 X 2 mL) for 5 min and aeain withdraw the aqueous phase: Wash the remaining solia repeatedly with water (0.5-mL portions) until the washings are colorless. Dissolve the crude product in dichloromethane (1 mL), and transfer the solution to a Craig tube via a short column of dried magnesium sulfate. ~ e n t i warm y the tube and remove the solvent (HOOD) under a slow stream of nitrogen to afford a solid which. when recrvstdlized from a minimum volume of methanol; produces &re 2,4,6-trinitrotoluene as white needles (78 mg, 65%),m.p. = 81-82 "C (liL9m.p. = 82 "C).
mL min-'. Monitor the eluent a t 254 nm, the order of elution being 2-nitro-, 3-nitro-, and 4-nitrotoluene. For quantitative results prepare standards using commercial samples of the isomeric nitrotoluenes to calibrate the detector. For best results, samoles of the crude ~ r o d u c(-1 t mg in 1mL of 4% ethyl acetate in petroleum) were first run through a disposable column of silica (-0.5 gJ in a Pasteur pipet.' TLC Analysis of Crude DNT
This analysis was performed using silica plates (Merck
Art. 5719) with 25% ethyl acetate in light petroleum as eluent. The RI values were 2,3,4-TNT (0.13); 2,3-DNT (0.25); 3,4-DNT (0.29); 2,4-DNT and 2,4,5-TNT (0.43); 2,6-DNT (0.49); 2,4,6,-TNT (0.61); 4-MNT (0.65); 3-MNT (0.67); 2MNT (0.68), respectively.
HPLC Analysis of Crude MNT Oil
HPLC can be accomplished using a standard bore 10-p silica column with hexane as the eluent, a t a flow rate of 1.5
Handbook of Chemistry andmyslcs, 55th ed.:
Weast. R. C.. ed.
C.R.C.: Cleveland. OH.
Volume 67
Number I
January 1990
69
