JOURNAL OF CHEMICAL EDUCATION
SMALL-SCALE. EXPERIMENTS FOR THE ORGANIC CHEMICAL LABORATORY Oxidation of Para-Nit.rotoluene EDNA V. EASTCOTT and GEORGE F. WRIGHT University of Toronto, Toronto, Canada
T H E use of small-scale experiments in the elementary organic laboratory not only permits the introduction of. instructive procedures involving expensive chemicals and primitive apparatus but it also allows the use of reagents which might be dangerous on a larger scale. A typical experiment of the type which, for beginners, is hazardous on the large scale involves the use of chromic anhydride in acetic anhydride. However, the principle of "blocking" which Thiele (TRIELE,J., AND E. WINTER,Ann., 311,353 (1900)) employed to prevent the oxidation of a methyl group from proceeding past the aldehyde stage can profitably be demonstrated by the student using this reagent in the laboratory. I n the Household Economics Laboratory (second year) experience over four years has shown that less than 2y0 of the students have lost their reaction mixtures owing to sudden ebullition. I n no case was the student or
her neighbors personally endangered. The difficulty was traced to inadvertent introduction of water into the reacting system, and no losses have occurred since close attention mas given to this possible error. The oxidation procedure which was adopted involved the conversion of para-nitrotoluene to a mixture of pnitrobenzoic acid and p-nitrobenzal diacetate. The average yield of p-nitrohenzoic acid among a class of 50 students was 26%, with a high and low value of 55% and 8%, respectively. The melting point was not determined. The average yield of crude p-nitrobenzal diacetate was 39%, with a high and low value of 90% and 5%, respectively. The average value obtained for its melting point was 126' to 30°C. (low, l l Z O , high 200') with incomplete melting in some cases, owing to contamination with p-nitrobenzoic acid. The crude diacetate was purified by crystallization from
AUGUST, 1949
95% ethanol with an average recovery of 42% (low, 2 ; high 82%) and an average melting point of 126°C. (low, l l Z O , high 132"). That part of this pnitrobenzal diacetate which remained in the crystallization mother liquor was converted to the phenylhydrazone under conditions which illustrate the ease with which such a diacetate can he hydrolyzed. The melting point of this p-nitrobenzaldehyde phenylhydrazone varied from 125O to 178', although most values were close to the average, 148%. This compound exists in two forms, of which the yellow modification appearing a t low temperatures is converted t o the red form in boiling water. The average total time required to deliver the.three products to the instructing staff was eight hours extending over three three-hour periods. The student is expected to study the experiment and to calculate molar quantities before the laboratory period. The apparatus required for this experiment comprises a 600-ml. beaker, four Erlenmeyer flasks (25, 50, 125, and 250 ml. in capacity), a section of 10-mm. glass tubing, a 60-mm. glass funnel, corks, a Kirsch funnel (largest diameter, 40 mm.) and a 150- X 22-mm. sidearm test tube for suction filtration. Three feet of 3/16X l/~e-in. rubber tubing c6nnects this side-arm tube to the mouth, or to a water aspimtor if the latter facility is available.
for 15 minutes, then quickly replace the cold hath with one a t 10°C. Continue to spin the flask for 15 minutes longer while the bath gradually warms to 15%. Add small pieces of ice only if it tends to exceed this temperature. Observe color changes during the reaction period. A longer time at the lower temperature tends to increase the yield of diacetate and to decrease the yield of acid hut is not practicable for the laboratory period. After this half hour a t the several temperatures the mixture should he chilled once more in an ice-water hath. Ice (about 20 g.) and then water are added
PROCEDURE
Into a 125-ml. Erlenmeyer flask introduce 7.4 ml. of acetic anhydride, 7.6 me. of glacial acetic acid, 1.63 me. of eonc. sulfuric acid (d. 1.84), and 1g. of pnitrotoluene in that order. Now add about 25 glass beads in. diameter). Insert a rolled cork stopper loosely in the flask and place the flask in a 600-ml. beaker filled with fine ice and water as shown in the diagram. After the contents have cooled to the bath temperature add a t once 2 g. of chromic acid anhydride (since this chemical is so corrosive and hygroscopic it has been found convenient to devise a glass ladle 2.5 ml. in volume from which a predetermined weight can be measured quickly by emptying it after it has been filled level full. The stock bottle should have a wide mouth and should be kept closed). Now spin the flask on its inclmed axis
Remotion Flask in Position for Spinning in i c e
wetar
Bath
rapidly with swirling until the volume is finally 125 ml. After several minutes the suspension is filtered through paper and then is washed with water to remove most of the chromic salt. Insert the stem of the filter into a clean 250-ml. Erlenmeyer flask and pour 10 ml. of saturated aqueous sodium carbonate into the wet filter. The filtrate
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should be returned several times to the filter in order to complete solution of the p-nitrobenzoic acid. After five minutes transfer as much of the remaining solid as possible to the Hirsch funnel, set up for suction filtration. Then puncture the filter and wash the remainder of the precipitate into the Hirsch funnel with a minimum of water. Combine the suction filtrate with the aqueous carbonate solution already in the 250-ml. flask. Wash the contents of the Hirsch funnel with five Zml. portions of water, suck as dry as possible and then air dry the well-spread material in order to determine the yield and melting point of the crude pnitrobenzal diacetate. The aqueous alkaline solution should now be acidified by cautious addition of cold cone. hydrochloric acid'and then filtered by suction. Discard the clear filtrate. The collected precipitate on the lilter should be washed with water. This p-nitrobenzoic acid should be dried to constant weight for yield, but its melting point is too high (ea. 240°C.) to determine safely in a melting point apparatus containing fats and most oils (inflammable) or sulfuric acid which may become diluted with water, after which it is liable to sudden ebullition. The crude p-nitrobenzal diacetate is transferred to a 50-ml. Erlenmeyer flask equipped with a bored cork with a 10-mm. glass tube atleast 12 in. long. Add 95%
JOURNAL OF CHEMICAL EDUCATION
ethanol (15 ml. per gram of solid) and dissolve by boiling (FIRE HAZARD).NOW filter as hot and as rapidly as possible through a fluted filter paper in a funnel which has been previously warmed over the Bunsen flame. The filtrate is received in a warm 25ml. Erlenmeyer flask. When this has cooled, and crystallization is complete, filter off the purified pnitrobenzal diacetate by suction, dry in air, and determine weight for percentage yield and also the melting point. This diacetate ester of the hydrated aldehyde was formed by acid catalysis in an anhydrous medium; hence, of course, it can be hydrolyzed to the aldehyde by acid catalysis in aqueous medium. This may be demonstrated by transferring 10 ml. of the crystallization mother liquor back into the 50-ml. flask. Ten ml. of 3% aqueous hydrochloric acid and 0.4 g. of phenylhydrazine hydrochloride are now added and the whole boiled under reflux for five minutes. Then 10 ml. of water and a fresh boiling chip are added and reflux boiling is resumed for one to two minutes. The flask is then cooled to room temperature and the red precipitate is filtered off after 10 miuutes. If the p-nitrobenzaldehyde phenylhydrazone is yellow rather than red, it should be boiled in 5 ml. of water for a few minutes and then the hot slurry refiltered.
