Explosion during Preparation of Benzenediazonium-2-carboxylate Hydrochloride
letters Need for Minimum Standards in Evaluating Doctoral Programs
T o the Editor: There are 176 PhD granting chemistry departments in the United States listed in the 1969 ACS Directory of Graduate Research.' In light of current demand for PhD chemists, this is a t least two times as many as needed. In a recent st,ndy2by the American Council on Education, 65 PhD granting departments of chemistry were rated by quality of graduate faculty. Of these departments, 6 were rated "distinguished," 22 rated "strong," 19 rated "good," and 18 rated "adequate plus." The clear implication is that many of the other 111 departments may be less than adequate. The Committee on Professional Training of the American Chemical Society annually publishes an approved list of chemistry departments which meet minimum standards a t the undergraduate level. HOW ever, in the 1970's one becomes a professional chemist only through training a t t,he graduate level. The American Medical Association does not "approve" undergraduate pre-medical programs but does approve medical schools at the graduate level. The Committee on Professional Training of the ACS should prepare a list of minimum standards to be used as criteria in evaluating doctoral programs in chemistry and publish annually a list of departments which meet such minimum standards. "Directory of Graduate Research," American Chemical Society, Washington, D.C., 1969, p. vii-viii.
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CARTTER. ALANM.. "An Assessment of Qualitv " in Graduate Education," American Council on Education, Washington, U.C., 1966, p. 62.
Stirrers from Mixmasters
T o the Editor: You described a simple, inexpensive laboratory stirring motor [J. CHEM.EDUC.,47, 726 (1970)l costing $14.00 plus five hours labor. We use portable Mixmasters available at local appliance dealers costing $8.00-10.00. We have found these devices to be reliable and serviceable. Although these Mixmasters are not sparkless, we have no trouble in this regard. Chucks are made by cutting off the mixer blades and fitting the stubs with rubber tubing. We obtain variable speeds by connecting the stirrers to Variacs. E. G. ZEY
T o the Editor: On November 10, 1970, two students were injuredone seriously-in our organic chemistry laboratory as a result of an explosion which occurred while handling benzenediazonium-2-carboxylate hydrochloride. This material had been prepared as described in Experiment 39 of the widely used laboratory manual, "Selected Experiments inorganic Chemistry," by M. P. Cava and M. J. Mitchell (W. A. Benjamin Inc., 1966). The authors acknowledge that the experiment is based on the work of I,. Friedman and F. M. Logullo of CaseWestern Reserve University. While recognizing the extreme sensitivity to shock of benzenediazonium-2-carboxylate, the authors make the following statement about benzenediazonium-2-carboxylate hydrochloride The instability of benzenediazonium carboxylate can also be sidestepped by isolating the compound in the form of its hydrochloride, o-carhoxybensenediazonium chloride. In contrast to the cmboxylate, the hydrochloride is quite insensitive to shock and elaborate safety precautions are not required.
After investigating this accident, we feel reasonably certain that the experimental procedure was accurately carried out by the student and that he did indeed obtain t,he desired diazonium chloride and not the undesired diazonium carboxylate, which could result if an insufficient amount of hydrochloric acid were used. The material was stored after isolation in a loosely covered container in a refrigerator from one laboratory period to the next (5 days). I n transferring the material from the storage container to the reaction vessel, it detonated with terrific force when touched witah a metal spatula. It might he possible that the hydrochloride has sufficient hvdrocen chloride vaoor . Dressure that on standinz " five days in the loosely covered container, it was partially converted to the diazonium carboxylate. However, according to Stiles and coworkers, both the diazonium halides and the diazonium carboxylates can cletonate when scraped against a hard surface.' As a result of the accident described here and the warning by Stiles and coworkers, we feel that the assertion in the C a m and Mitchell manual that t,he diazonium hydrochloride is "quite insensitive to shock and elaborate safety precautions are not required," is definitely open to question. We have discontinued the isolation of this diazonium salt, and it is our firm recommendation that other laboratories using this experiment do likewise, especially since a good method for generating benzyne from the same precurser is available which avoids the isolation of the hazardous diazonium salt.% Both Dr. Cava and W. A. Benjamin Publishers have encouraged publicizing the accident as a warning to prevent possible recurrence and have advised me that the experiment will be deleted from the next printing.
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1 STILES, M., MILLER,It. G., AND BURCKHARDT, U.,J. Arne. C h m . Soe., 85, 1795 (1963), footnote 30s. z F m ~L. ~A N D~ LUGOLLO, ~ ~ , F. M., J . OW. Chem., 34,3089 (1969).
Volume 48, Number 6, June 1971
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419
Equivalent Weights: Unnecessary Confusion
To the Editor: This is a short plea to eliminate one source of confusion from the General Chemistry course and textboolc-the concept of equivalent weights. Even a passing mention of the concept causes agony and frustration to most students, x h o are still struggling with The Mole. Is this necessary? I believe it can be argued that General Chemist,ry is regarded as "hard" by most students, not because the concepts involved are difficult, but because there are so many of them. Thus, a student's success in this course is dependent on how rapidly he can absorb an idea, hecause he has precious little time to mull it over before he must use it and relate it to hat follo~rs,in the next lecture or chapter of his text. "But general chemistry can be taught as an integrated whole!," you say. Perhaps so, but my experience indicates that no matter hov much you stress t,he inter-relat,edness of t,he concepts presented, the student studies them as separate topics, if only because his tests, not to mention his text, divide the material. Wit,h t,his in mind, and avoiding thc larger issue of which, and how many, major areas of chemistry should be included in a general course, several smaller topics bear scrutiny in terms of their contribut,ion to understanding versus the amount of effort they add t,o the course. Thc teaching of the equivalent weight concept is a particular sore point. Another, similar concept is that of normality in acid-base solution reactions. The case is less strong, but it resembles the problem with equivalent weights in all aspects. I believe equivalent 11-eights could be totally deleted from the General Chemistry course and no oneteachers or students-~vould mourn their passing. To my lmoviledge, very few pacticing chemists ever have recourse to the subject aft,er leaving their freshman course. It may be that the concept remains in t,heir memories-and thus in the freshman texts they \\-ritefor the reason that it, was a source of agony in their encounter with it. Since equivalent u-eight,s are a minor tool for the practicing chemist, much less for the student, the inclusion of the subject in the vast overview of a general course can be justified only on pedagogical grounds, i.e., its study deepens the understanding of the student in t,he broader areas of stoichiomet,ry and mass-balance relationships, in the narrower cont,ext of redox reactions. I n my experience, t,he reverse happens. The student,, who is just beginning to feel comfortable vith the mole concept and t,he balancing of simple equi~tions(and perhaps i s not comfortable yet with redox equations' and electrochemistry), is inevitably thrown into confusion by his encounter xvith equivalent ~veights,sometimes so badly that his previous understanding is shaken. The reason is simple; he quickly sees that a gram-equivalent veight of a compound is (someholv) simply related to it,s gram-molecular u~eight,but it is ve1.y difficult to get him to see that the gram-equivalent weight has meaning only in the cont,ext of a given reaction of the compound. He seems suddcnly to have been confronted ~vit,hsome sort of variable molecular weight! We want, of course, to teach the student that, for example, permanganate ions can be reduced to more than one oxidation stat,e. But this can be done 420
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Journal o f Chemical Educofion
without any reference to equivalent weights. Stoichiometry is one of the core concepts of the general course. Its essentially simple message is only confused by the introduction of this more "elegant" derivative. In ot,her xvords, I believe a student could comp1et)e his freshman ycar without ever having heard of equivalent wights, and be none the worse off, regardless of his future course of study. Indeed, this small lightening of the load would have a beneficial effect in its simplification of the total subject. R. D. WAUCHOPE
The Zeroth Law and the Thermodynamic Potentials
T o the Editov: I n the November issue of THIS JOURNAL [47, (1970)l there were two interesting articles treating t.hermodynamic p~tent~ials. 0 . Redlich (p. 740) calls attention to the fact that a zerot,h la~vshould be postulated for the introd~ct~ion of any generalized force (mechanical force, voltage, pressure, e t ~ . )not , only, as we do no\\--,for the introduction of temperature. B. A. Schaefer (p. 745) shows that a potential of any kind (mechanical force, pressure, temperature, etc.) may be operationally defined as the rate of change of a similar kind of energy with the change in an independent conjugate parameter, all other variables which can effect an energy change being held constant. Redlich emphasizes the dependence of a potential on equilibrium; Schaefer emphasizes the dependence of a potential on a change, i.e., on a process. The articles may seem to introduce a potential in two different ways which may appear to be incompatible. However, it appears to me that an introduction of a potential of a system should he made dependent on both the zeroth law and on a process. I n other words, an introduction of a potential of a system should be made dependent on a process obeying the condition of the zeroth law. A reversible process is just such a process, and it might therefore be called a " ~ e r o t ~ h process" or merely a "z-process." Accordingly, it might be appropriate to use the concept a "reversible pot,ential" in an analogous way as we use the concept a "reversible ivorlc." The concept an "irreversible potential" might also be of use, e.g., in chemical lcinet,ics, wherc we always have nonequilibrium conditions. We say that a rate of a reaction depends on temperature; in this case it appears appropriate to connect an irreversible process with an "irreversible temperature." When the kind of a processis evident from t,he letter used for the corresponding potential (T, p, p, etc.), we could mark the lett,er with z (e.g., T,, pa, a) if the process has obeyed the condition of the zeroth law, i.e., if t,he process has been a reversible process. The law appears to be one of the fundamentals of thermodynamics; it is strange that a number of authors of thermodynamic textbooks do not mention it. BJ~RN BERGTHORSSON
