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July 13, 2009
Volume 87, Number 28
pp. 2-4

July 13, 2009 Letters

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Chemical Safety: Mg3N2 Hazard

Both Steven Ley (C&EN, June 8, page 4) and Sheldon Crane (C&EN, April 13, page 2) have now reported that explosions can occur when using magnesium nitride to convert esters to amides following the procedure reported by Ley (Org. Lett. 2008, 10, 3623) or one similar to it.

Because I have worked for many years as part of a group whose purpose was to prevent such reactive chemical events from occurring on the laboratory scale all the way to commercial manufacture, four questions always come to mind when I read about these incidents. How much heat is released in the desired reactions? What potential temperatures and pressures could result? How fast are the reactions (that is, how fast is the heat released)? Can the heat released be removed quickly enough to prevent undesired consequences?

A quick Internet search reveals that magnesium nitride reacts rapidly and very exothermically with water to form magnesium hydroxide and ammonia. Thus, this chemistry presumably occurs in two steps. First, the magnesium nitride reacts with the methanol to form magnesium methoxide and ammonia. (Mg3N2 + 6 CH3OH → 3Mg(OCH3)2 + 2 NH3). Second, the ammonia reacts with the ester to form the amide and an alcohol. Using standard heats of formation, the reaction of magnesium nitride with water has a ΔH = –165 Kcal/mol of magnesium nitride.

Because the same types of bonds are broken and formed in the reaction with methanol, this is a good estimate for the heat of reaction of magnesium nitride with methanol. On the scale of 1.3 g of magnesium nitride used by Crane, that translates to –2,123 cal. Just over half of the methanol used will be consumed in the reaction to make magnesium methoxide. Assuming the reaction mixture has a heat capacity of 1 cal/g-ºC, and ignoring the heat capacity of the container, the vaporization of methanol, and the heat from the ester to amide reaction, one calculates an adiabatic temperature rise of 310 ºC. At just 25 ºC, assuming no solubility of the ammonia in the reaction mixture, and using the ideal gas law, one calculates a resultant pressure of 31.5 atmospheres in the closed 20-mL container. Ammonia will liquefy at about 9.9 atm and 25 ºC, but at 68 ºC, its vapor pressure is 31.2 atm. At 310 ºC, the ammonia will generate 61.6 atm of pressure. If the reaction of the magnesium nitride with methanol is rapid and the reaction of the resultant ammonia with the ester is slow, the potential temperatures and pressures are enough to burst a glass vial.

When exploring new or unfamiliar chemistry, one should always try to estimate the heats of reaction (using model reactions if needed), the maximum possible adiabatic temperature rise, and potential pressures generated using simple assumptions as needed. The results can be quite revealing about possible consequences.

I would suggest that this procedure is always a race between rates of consecutive reactions controlled by the time spent in an ice bath or water bath. A safer way to try running the reaction would be to add the magnesium nitride in small portions, letting the ammonia react between portions, or to dissolve/slurry the ester and magnesium nitride in an inert solvent such as toluene, diglyme, etc., and slowly add the methanol at a rate to control the reaction.

Gary Buske
Midland, Mich.

Solutions For 'Wallboard Woes'

I would like to submit an alternative hypothesis to the one suggested in the article regarding wallboard imported from China (C&EN, May 4, page 50). It appears there is "something stinky in Florida."

Instead of just looking at abiotic sources of sulfur-based molecules found in wallboard from China, why not look at possible biotic sources such as sulfur-reducing bacteria (for example, Desulfovibrio sp., Altermonas sp., Clostridium sp., Desulfotomaculum sp., and so on) or sulfur-oxidizing bacteria (for example, Thiobacillus sp., Beggiatoa sp., Thiotrix sp.)?

Finding these or related microorganisms would help explain the increased gaseous emissions during temperature and humidity increases. In a related manner, any hydrogen sulfide produced by sulfate-reducing bacteria would react with most metals spontaneously in an abiotic manner and explain the metal sulfide. Perhaps we are looking for a nail in a haystack when in fact we should be looking for the hammer. Please remember this idea is testable according to Koch's postulates.

If such reactions are found, it may be possible to tease out whether these microbes are endemic to the mine in China or the moist organic soils of Florida. I am certain a microbiologist could expedite this research, with the potential to isolate causative microbes.

I hope this hypothesis helps dissipate the odor around this investigation.

S. M. Armstrong
Halifax, Nova Scotia

It was acknowledged around the 1970s that upper-wall corrosion of underground concrete sewage collection conduits was aggravated by microbial action taking place in municipal wastewater flowing within partly filled conduits. The widespread and troublesome corrosion was attributed to gaseous hydrogen sulfide generated biologically from the aqueous sewage underflow within the conduits and the subsequent gaseous contact with the moist upper-wall surfaces.

Furthermore, it is known that landfilled calcium sulfate may undergo decomposition to produce malodorous gases, including hydrogen sulfide, during ongoing landfill exposure to the surrounding environment.

It has been proposed that the present inquiry into nontypical, imported wallboard also take into consideration the possibility of latent microbe populations that become active when exposed to increased temperature and humidity.

Walter Drobot
Tucson, Ariz.

"Wallboard Woes" was particularly interesting to me because I had seen the hubbub about it in Florida on a recent vacation. Is the problem of sulfide gas emission from drywall under warm, humid conditions the fault of contamination of such drywall with sulfate-reducing bacteria? Also, if other factors for growth of such bacteria, perhaps iron, are in the offending drywall, is this why the emission is limited to drywall from China? Are processing temperatures a factor?

Certainly there is plenty of available sulfate in drywall. The trick to eliminating the odor might involve killing the bacteria. At the factory, this could be as simple as making sure the processing temperature in the casting machines is high enough. Adding a disinfectant to the mix might work, too. This could be proven by inoculating some drywall with the bacteria and looking for sulfide.

I have had personal experience with these annoying beasts. When I come back from vacation to my home in Illinois, I find the water coming from my undersink filter has a sulfide odor; my suburb gets its water from Lake Michigan. When I change the cellulose particulate filter, I find a bacterial slime and use a bleach solution to clean the housing.

At Sohio in the 1970s, a friend of mine solved a problem with hydrogen sulfide generation in the holds of tankers sailing from Alaska to California by adding dilute hydrogen peroxide. Presumably, this killed the bacteria that were causing the odor.

Further data on these nuisance bacteria can be found at Wikipedia under "Sulfate-reducing bacteria."

Joseph P. Bartek
Wheaton, Ill.

Research At Big Pharma

In a remarkable stroke of editorial journalism, the April 20 issue of C&EN carried articles (pages 25 and 48) first decrying and then applauding the present state of scientific research in "big pharma." While appearing to be diametrically opposite in their assessment, facts appear to support both viewpoints.

As a neutral observer who was intimately involved with the broad industry from the early 1950s, I completely agree with the opinions of Jeffery Conn and others cited in the fine article "Breaking the Mold" by Lisa M. Jarvis. For example, she states, "Conn feels ... the old ways of finding drugs at big pharma aren't working," and "the only projects supported are those with a direct impact on the bottom line."

But in the book review of "Drug Truths: Dispelling the Myths about Pharma R&D," reviewer Jean-FranÇois Tremblay describes the author John LaMattina as a heroic defender of R&D in the major pharmaceutical firms. How can this apparent dichotomy be explained?

Basically, the answer is relatively simple. LaMattina details how dedicated, qualified, and altruistic the people are in big pharma R&D. But, bluntly put, those people are no longer in charge.

Let me explain. In the 1960s, the Pharmaceutical Manufacturers Association Board of Directors (PMA was later renamed the Pharmaceutical Research & Manufacturers of America) unceremoniously dumped the distinguished medical scientist and former editor of the Journal of the American Medical Association, Austin Smith, as its president and CEO in favor of a lobbyist-lawyer. That move was symbolic of a complete change in the fundamental culture of the industry.

In fact, soon afterward, I was talking with Farleigh Dickinson (of Becton, Dickinson & Co.) at a PMA annual meeting reception, and he looked around and commented to me: "I don't know these people anymore. Oh, I know their names and who they are, but they're not the kind of people we used to have running the drug companies."

And he was right. Formerly, at those meetings, I'd see people like Gifford Upjohn, Max Tishler, Harry Loynd, and other physicians, pharmacists, and scientists as the company CEOs. But now, the people there as company CEOs are lawyers, economists, bankers, and other business types who had come up through finance, marketing, advertising, and promotion.

So, the entire thrust of the industry had radically changed. The so-called full-line pharmaceutical companies had transformed into the "brand name" drug industry.

In his book, LaMattina—who is justifiably proud of the industry's R&D people—appears to be oblivious to the overall corporate change, a change that has inspired the fundamental research shift Jarvis describes and that has substantially tarnished the glowing image LaMattina still holds.

Edward G. Feldmann
Venice, Fla.

Methanol's Allure

Browsing through back issues, I encountered Jyllian Kemsley's excellent feature about the possibilities for methanol as a feedstock and fuel (C&EN, Dec. 3, 2007, page 55). I was especially intrigued by the inference in the last paragraph that potential methods for extracting carbon dioxide from the atmosphere have not been developed.

For several years, I have occasionally speculated on the feasibility of concentrating atmospheric CO2 for reaction with hydrogen. I dismissed distillation of liquefied air as energy and capital intensive, in addition to having a very low yield. I did this strictly on the basis of intuition, because my specialty is only remotely related to the necessary disciplines.

Another possibility, so simple that it may not have occurred to those with the resources and expertise to investigate it, has occurred to me. I pass it on for what it may be worth: The unfavorable ratio of the major components of air to CO2 can be almost inverted by the saturation of water with air followed by vacuum extraction.

Solubility data show the contamination of the retrieved gas with nitrogen, oxygen, and argon would total 7%. Water content would depend on the effectiveness of antimist screens, bath additives, and vapor condensers, if used. Additional purification could be done either before or after reaction with hydrogen, depending on whether oxygen or excess water would reduce yield and whether the Haber reaction could be avoided during hydrogenation. Investigators with the appropriate resources and expertise could determine whether either is necessary and feasible.

The process could be made continuous by using multiple chambers and whatever controls apply. The production rate would be increased with the use of efficient spargers, low temperature, and/or high pressure in the dissolution tank. High temperature and vacuum in the extraction tank would have a similar effect.

As always, the economics of the design and operation are likely not as simple as the concept, but large carbon credits or other subsidies seem justified.

G. H. Smith
Sandy Hook, Conn.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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