Blowing things up with heterocycles, featuring Werner E. Bachmann
You may have seen the recent article by Jyllian Kemsley (C&EN) on how authorities dealt with what they called the “largest cache of homemade explosives ever found in the U.S.” (See also here and here.) George Djura Jakubec’s home was so infested with explosives and the chemicals used to make them that a firewall was built around the home so it could be burned down.
If you’ve ever looked at the structures of explosives (out of mere curiosity of course), you can’t help but appreciate the abundance of heterocycles. Browsing Agrawal and Hodgson’s 2007 book entitled Organic Chemistry of Explosives (Wiley) reveals hundreds of explosive heterocycles. Just for fun, let’s look at a couple, which will also give me a chance to touch on the accomplishments of one of my favorite chemists, Werner E. Bachmann from the University of Michigan.
HMTD, a peroxide
While Agrawal and Hodgson state that “no peroxide has found practical use as an explosive, a consequence of the weak oxygen-oxygen bond leading to poor thermal and chemical stability and a high sensitivity to impact,” this hasn’t stopped terrorists and folks who like to make bombs at home. I can thus imagine the concern of the hazmat team when they found six quart-sized jars of hexamethylenetriperoxidediamine (HMTD) in Jakubec’s stash. HMTD, made simply from hexamine and hydrogen peroxide in the presence of citric acid, is unstable and extremely dangerous to manufacture.
N-Nitroamines (Nitramines) and Werner E. Bachmann
Hexamine is also the starting material for two classic secondary explosives, RDX and HMX, shown above. Secondary explosives are those that are rather stable compounds, requiring a primary explosive to get the ball rolling.
RDX (cyclonite, hexogen, Royal Demolition eXplosive, or cyclotrimethylenetrinitramine) is the most important military high explosive in the U.S. It’s second to nitroglycerin among common explosives in strength and is a component of C-4 and Semtex. It played a huge role in World War II, as we’ll see in a minute.
The most common method of its manufacture is the Bachmann process, where hexamine is nitrated in the presence of ammonium nitrate. I must digress for a moment. I got to know of Werner Bachmann’s accomplishments when I was at the University of Michigan, where he was a professor from 1925 until his untimely death in 1951. The Department of Chemistry honored him annually with the Bachmann Lecture, where a distinguished organic chemist was invited to give a talk and attend a banquet. Having introduced a few of the speakers, I had the chance to comment on Bachmann’s achievements in the opening remarks.
It’s really worth reading a biographical memoir of Bachmann (pdf) by the famous heterocyclic chemist Robert C. Elderfield, also from the University of Michigan, published by the National Academy of Sciences.
In the summer of 1940 the National Defense Research Committee was established and in the fall of that year a group of organic chemists, most of whom were ignorant of the chemistry of explosives, met in Roger Adams’s residence in Urbana, Illinois, to discuss how they could best contribute to the budding war effort. Among the neophytes was Werner Bachmann and among the topics discussed was how best to manufacture and use a high explosive known in this country as Cyclonite and in England as RDX. The potential military value of RDX as the most powerful of the nonatomic high explosives had already been appreciated. However, in this country there was no knowledge at the time as to how it could be used safely and no practical process for its manufacture. It subsequently developed that the British had largely solved the problem of the use of the explosive and had developed a fairly satisfactory batch process for its manufacture. To Bachmann was assigned the problem of devising a more efficient and economical manufacturing process.
In some respects Bachmann’s achievement in discovering an efficient new method for preparing RDX testifies most eloquently to the amazing versatility and experimental technique of this extraordinary chemist. He had no previous experience in the chemistry of explosives, and was best known for his elegant syntheses of complicated molecules such as the sex hormones. When Bachmann first learned that his assignment under the NDRC program was to be the development of a practical method for the large-scale manufacture of the highly sensitive RDX, he records that his “heart sank.”
In January of 1941, J. C. Sheehan, who had just completed his doctoral thesis with Bachmann, began work on a novel approach to the synthesis of RDX. In the conventional British process for making RDX, hexamethylenetetramine is treated with 98-100 percent nitric acid, as is shown in equation (1).
C6H12N4 + 3 HNO3 –> C3H6O6N6 + 3 HCHO + NH3 (1)
This direct nitration method suffers from at least two serious disadvantages: large excesses of nitric acid must be employed for optimum yield, and one half of the equivalent of formaldehyde is lost, principally through oxidation by the nitric acid. Thus, the maximum amount of RDX possible is one mole from one mole of hexamethylenetetramine, and the actual yield is considerably less. In 1940 Bachmann learned that Ross and Schiessler at McGill University had obtained RDX from formaldehyde, ammonium nitrate, and acetic anhydride in the absence of nitric acid, but no details of their experiments were available. Although equation (2) is undoubtedly an oversimplification of the reaction, it occurred to Bachmann and Sheehan to attempt utilization of the by-products of the nitrolysis to obtain a second mole of RDX. In this way, if two moles of ammonium nitrate and six moles of acetic anhydride were present during the nitrolysis of hexamethylenetetramine, then two moles of RDX might be obtainable from one mole of hexamethylenetetramine.
C6H12N4 + 4 HNO3 + 2 NH4NO3 + 6 (CH3CO)2O –> RDX + 12 CH3CO2H (2)
Although exploratory experiments were discouraging and frequently led to spectacular “fume-offs,” from a few reactions a small amount of RDX was obtained. After each experiment, Bachmann, who personally spent long hours in the laboratory, would carefully and ingeniously design a variation in the experimental conditions until finally, after literally dozens of experiments, the reaction conditions which permitted control of the process were worked out and a consistent yield of RDX was obtained. A memorable occasion was the day on which Bachmann and Sheehan isolated more than 100 percent yield of RDX based upon one mole of hexamethylenetetramine to demonstrate conclusively that a “combination” process was prevailing. The enthusiastic encouragement given by Roger Adams and J. B. Conant was most heartening in the early phase of the work. It now became important to adapt the process for relatively large-scale use. A number of scaled-up reactions were carried out involving quantities as large as several kilograms. Owing to the hazardous nature of the reaction these experiments were conducted on Sunday mornings and at other times when the University buildings were sparsely occupied.
The RDX prepared by the new process was at first considerably more sensitive to impact than was RDX from the direct nitrolysis reaction. At one point an urgent telegram was received from the U.S. Bureau of Mines Laboratory in Bruceton, Pennsylvania, reporting that an RDX sample submitted to them for evaluation was highly sensitive and should be handled with extreme care. This sensitivity was later traced to the presence of impurities, in particular to a small amount of a high-melting substance termed HMX (HM—high melting), and to a lesser extent to an impurity termed BSX (BS—Bachmann and Sheehan).
In the early phase of this work the armed services showed little interest in RDX as a military explosive, but during the summer of 1941 Admiral Blandy, then Chief of the Bureau of Ordnance, after consultation with top scientists of the Office of Scientific Research and Development, recognized the potentialities of RDX for use in rockets, torpedoes, and aerial bombs. The greater explosive power as compared to TNT (RDX has 150 percent of the power of TNT on a weight basis; on a volume basis, which is important for certain applications, RDX is approximately twice as powerful by virtue of its greater density) offered tremendous potential advantages. In addition, its markedly greater brisance, or shattering power, made RDX the ideal explosive for use in shaped charges, the principle behind the bazooka.
Several companies undertook the development of the new combination process, but the efforts of Tennessee Eastman were the most successful. At Kingsport, Tennessee, the largest munitions plant in the world was constructed to produce RDX by the new process on ten continuous production lines. It has been reported (Scientists against Time by James Phinney Baxter, 3d) that RDX was produced in this way at the rate of 360 tons per day. The production of RDX by the direct nitration process would not only have been considerably more expensive but would have involved much larger quantities of critically short corrosion-resistant materials for handling the nitric acid. It has been estimated that the saving to the government in plant cost alone was over two hundred million dollars.
The contribution of RDX to Allied success in the Second World War can scarcely be overestimated. Although RDX was considered too sensitive to fire from conventional artillery, it found wide application in rocket heads, in the 12,000-pound “Tallboys,” in block-busters, and in the torpedoes which sank the “unsinkable” German battleship Tirpitz. Thus Bachmann, the very prototype of the unassuming scientist, was able to make an outstanding contribution to his country’s and to the free world’s victory by bringing to bear his extraordinary scientific prowess, his imagination, and his perfection in laboratory technique.
Simultaneously with and subsequent to his work on RDX he also carried on important investigations on the oxynitration of benzene as a route to picric acid and on various aspects of the penicillin problem. The strain created by these wartime researches and the effort devoted to them undoubtedly contributed to the serious undermining of his health.
In recognition of his services Bachmann was the recipient of the Naval Ordnance Award in 1945, and in 1948 he was granted the Presidential Certificate of Merit by the United States government and the King’s Medal by the British government.
The strain and effort of the war years finally took their toll and, with his health undermined, Werner Bachmann died of heart failure at the age of forty-nine on March 22, 1951.
Although Bachmann’s career was a relatively short one, he published over one hundred and fifty papers, featuring such accomplishments as the first total synthesis of a steroidal sex hormone (equilinen), the Gomberg-Bachmann reaction for the synthesis of biaryls from aryl diazonium ions, and some pivotal early work in the synthesis of penicillin. John C. Sheehan, a Ph.D. student of Bachmann’s who worked on the RDX project, went on to accomplish the first synthesis of penicillin V as a faculty member at MIT.
One more thing. If you enjoy watching explosions in slow motion, especially seeing shock waves, check it out (fast forward to 2:42 for the good stuff):