in the Chemical laboratory
CXXV. An Accident With Acetic Anhydride and Bromine W. 1.Burnett, Jr., Department of Chemistry Louisiana State University-Baton Rouge Baton Rouge, Louisiana 70803
This is a "wise-unwise" story of s laboratory accident which has enhanced our concern that graduate teaching assistants as well as undergraduate students familiarize themselves with the principles of laboratory safety. In the accident a graduate teaching assistant (G.T.A.) serving as a preparer of solutions received only two deep cuts on one arm in spite of the fact that two large student laboratories were filled with flying shards of glass. The task which led to two explrraionz and the inpries was the preparatim of two lilrlr of a zolurion containmx O 0 5 . V bromine m 90% acetic arrd. The following sequence of operations and events took place: GTA with the aid of his shopping list assembled reagents, glassware and miscellaneous equipment in large laboratory hood during a period not occupied by students (wise). He attempted calculations as to the amount of water required to make a 90 percent solution of acetic scid. He decided to prepare two solutions (A and B) with a volume of 1000 ml each containing 900 ml acetic anhydride, 100 ml water, and 8 grams of bromine (one version). Second version: GTA could not find glacial acetic acid in the stockroom and did not request same from the s t o e k m m clerk. He then elected to make acetic scid from acetic anhydride. The GTA acquired two used, 2-1, soft glass, brown bottles. which he cleaned before use. If he had considered the possihility of an exothermic reaction, he would not have selected soft glass bottles for the preparation of the solutions. To prepare solution A, GTA added 100 ml of water to one bottle followed hy 900 ml of seetie anhydride and then 8 grams of bromide. The bottle and its contents were shaken gently and transferred to adjoining Laboratory 2. The preparation of solution A in the hood of Laboratory 1 and transfer to Laboratory 2 required a period of 5-10 minutes (GTA's estimate). Solution B was then prepared in Laboratory 1 in the same way over the same period of time. The GTA elected to shake the battle of solution B a n d its contents much more vigorously than the bottle of solution A. He does not remember any layering. Solution B was left in the middle of the hood of Laboratory 1. A322 / Journal of ChemicalEducation
GTA then left Laboratory 1 on another task for a period of two or three minutes. On his return, he observed that Solution B was bubbling violently. Recognizing that something was wrong, he attempted to remove the plastic cap fmm the bottle. He wisely positioned himself off-center so that a hood partition afforded same protection to his body. He was not wearing safety goggles or gloves. At this moment, to quote the GTA verbatim, "BOOM." The explosion reverberated over the entire section of the building. He believes that he did remove the cap. However, the cap was found in the hood intact on a broken shard. The GTA left the laboratory with dispatch to seek help from the nearby stockmom and research laboratories. During emergency First Aid measures, to quote the GTA verbatim, "BOOM." Solution A exploded in Laboratory 2. There was a several minute interval between the two explosions. The student was then removed to the Campus Infirmary. It should he noted that following the fiat explosion, other graduate students in the immediate area had the presence of mind not to rush into the laboratories. Strong fumes recognized as bromine mixed with a n acrid odor filled the laboratories. Large fans were placed in the doorways to expel the fumes through windows. The GTA's shopping list, which was written on a paper towel, was found on a table across from the hood in Laboratory 1 so that an immediate reconstruction of the events leading to the explosions was possible. The clean-up proceeded without incident. Several published studies of the bmmination of acetic anhydride have provided leads as to the chemical events leading to the aecident.'.2.3 When one mole of bmmrne is addrd to one mole of acetic anhydride at %IT, o-bromoacrric ac~lland acetyI bmrnide are generated and no HBr is evolved. When a second male of hmmine is then added t o the reaction mixture, the acetyl bromide is converted to bromoacetyl bromide and HBr is evolved. Dissolved hydrogen bromide is then removed by heating the reaction mixture to 150°C for 30 minutes. With addition of water, the bmmoacetyl bromide is hydmlysed to n-bmmoaeetie aeid. The yield of manohmmoacetic aeid is a t least 98 percent of the theoretical. One mole of HBr is generated
with one mole of bromine. There appears to be no doubt that the bromination of acetic anhydride can lead to gas pressures which exceed the "boom paint" of old bottles. The internal pressure required to break a typical soft glass bottle is 5 atm or less. Several expedient experiments studying the bromination of acetic anhydride have been productive from the standpoint of our accident investigation. To insure that the investigator would not be "carried away" with the experiments, the stockroom clerk wisely cached all used, soft glass bottles in his custody. In the first experiment, 10 ml of water, 90 ml of acetic anhydride and 0.8 g of bromine were introduced into a 300 ml Pyrex round bottom flask. The flask was not stoppered. The temperature increased gradually from mom temperature (30'C) to a temperature of 3 8 T over a period of one hour with no visible reaction. The temperature then increased rapidly over a period of several minutes to 85°C with a slight to modest reflux action being observed on the walls of the vessel. At a temperature of X5', rhc bromine cdor rapidly niinpprared. After a iunher sharp rise in temptmture ro 11@(', thr contrnts hrynn to cad off rapidly. In the second experiment, 90 ml of ecetic anhydride and 0.8 g bromine were introduced into a 300 ml r.b. flask. A homogeneous solution formed with gentle swirling of the flask. The temperature held steady a t 30" for two hours. Upon the addition of 10 ml HzO (slowly), a violent exothermic reaction ensued in the flask (unstoppered) with the rapid disappearance of the bmmine color. Impracticable, but ingenuous, suggestions for larger scale experiments and demonstrations have been made by our organic students, who have followed the accident investigation with atypical zest. Their responses to our safety efforts have reinforced our view that a sound safety pmgram at the university level has much pedagogical value. The mishap of the GTA with acetic anhydride and bromine predictably has led to an "update" of an extensive 8et"f safety rules, suggestions and practices which is issued to each graduate teaching assistant and undergraduate student in our organic laboratories. In recognition of the fact that the development of a "safety gefulte" among our graduate teaching and research assistants is perhaps the most obvious, perhaps the most effective, yet possibly the most difficult way to establish a safety mode a t the university level, discussions on safety have been included in an orientation program5 for beginning graduate students. As far as possible, care is taken to make our safety programs congruous with the ego of the GTA.
LITERATURE CITED Schwerdle, A . and Sehwordle, M. L.. B.P. 882. 864 IL961l.(ToVinelendCh~mical Company). (2) Jolios, 2. E.. "Bmmine and its compound^? Academic Prer*., New YoA. 166, P. 358. (3) Sajus. T.. Bull. S o e Chim. Prance. 9. 2263 (19641. ( I ) Burnett. W. T., Jr.. Safety Rules. Sugrstions end Practicer for Chemistry Teaching Laboratories. Louisiana State University B m k Store. Baton Roueo.La.. Aup. 1,1973. (51 Carpenter. D. K . and Watkins, S.F.. 27th Southwest Rqional Meeting. ACS, sari Antonio. Texas. December 1-3.1971 (1)
Volume 52, Number 6, June 1975 / A323
