Chemical Education Today
Letters
CLIP: Acetic Anhydride
The author replies:
On page 1176 in the September 2001 (1) issue there is a CLIP on acetic anhydride in which boric acid is described as a mild oxidizing agent. I do not question that heating a mixture of acetic anhydride and boric acid is a hazardous operation, but I do strongly question whether boric acid is an oxidizing agent of any kind, mild or otherwise. If boric acid is an oxidizing agent in this or any reaction, it must then be reduced. What are the products of the reduction of boric acid? No oxidation or reduction occurs in the reaction at all. The violent reaction is caused by the avidity of acetic anhydride for water. Acetic anhydride reacted with boric acid to form acetic acid and boric oxide (boric anhydride). A quick look at the heats of formation of the components will show that there is plenty of energy available. This may be considered a mere quibble, but I have been involved in a situation in which ferric oxide was labeled a strong oxidizer, due to the thermite reaction with aluminum metal. Our HazMat people (all nonchemists) insisted that ferric oxide must be labeled, stored, and handled with the same precautions as nitric acid, peroxide, etc. We do not want boric acid to be treated by (e.g.) HazMat as an oxidizing agent. (These same people thought that boric acid belonged with hydrochloric, phosphoric, and sulfuric acids.)
I appreciate Crutchfield’s comments, particularly because it is desirable to ensure to the extent possible that HazMat personnel do not overreact to any presumed hazards that a substance might possess. In fact, because of this concern, the CLIP in question specifically identifies boric acid as a mild oxidizer (1). (Consequently, I must disagree with Crutchfield’s worry that as a consequence of reading the CLIP on acetic anhydride some HazMat person might deduce that it is instead a strong oxidizer.) As to the products of the reduction of boric acid, I have no specific information; but we all, including Crutchfield, I presume, know that using heats of formation to predict that a chemical reaction will or will not occur often leads to error. The correct procedure is to calculate the change in free energy that would be associated with the process under the conditions that exist, which is different from using free energies of formation, of course. (Enough elementary thermo!) Hypothesizing, I offer hypoboric acid, BH3OH, or one of the boron suboxides, perhaps BO, or its dimer or tetramer as possible products from the reduction of boric acid. There are other candidates as well, of course. For further detailed information, carry out the tedious thermodynamic (and kinetic) exercises, or see the original German literature as cited in Chem. Eng. News 1973, 51 (34), 42 (2).
Literature Cited
Literature Cited
1. Young, J. A. J. Chem. Educ. 2001, 78, 1176. Charlie A. Crutchfield 2708 Pinelawn Drive La Crescenta, CA 91214-1430
[email protected] 1. Young, J. A. J. Chem. Educ. 2001, 78, 1176. 2. Chem. Eng. News 1973, 51 (34), 42 Jay A. Young 12916 Allerton Lane Silver Spring, MD 20904-3105
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Journal of Chemical Education • Vol. 79 No. 9 September 2002 • JChemEd.chem.wisc.edu
