Beer-Lambert Law

Abstract

The article " Vibration-Rotation Spectra " (J. Chem. Educ. 1993, 70, 1035) describes the preparation of gas-eous DBr, its related halides HCl, DCl, and HBr, and their IR spectra. The reagents involved are liquid Br 2 and D 8 -toluene catalyzed by FeCl 3 . These materials are toxic, corrosive, flammable, and expensive. It should be noted that there are two previous articles in this Jour-nal deals with the same subject (M. Bader, " Molecular Properties of Diatomic Molecules from IR Spectra " , J. Chem. Educ. 1969, 46, 206, and J. L. Hollenberg, " En-ergy State of Molecules—Resource Paper VIII, J. Chem. Educ. 1970, 47, 2). In 1969, trying to teach molecular spectroscopy, I discovered that DBr fell in the most sen-sitive region of the common Perkin-Elmer IR instru-ments of that day. The individual spectral lines were so well resolved that using a measuring microscope we could measure them, in principle, to 0.01 wave number. DBr then became a routine gas for undergraduate experiments. The synthesis I suggest, which is safer and simpler than the one proposed, is to place a small volume, ca. 1 mL, of liquid PBr 3 in a vertically clamped sidearm test tube (hood!) and cover with an equal volume of D 2 O without mixing so, that two layers result. The reagents are very slow to react. A slight tickle with a Bunsen burner will initiate a self-sustaining reaction that will generate co-pious amounts of DBr at a fairly constant rate. (The system must be open to the atmosphere some-where.) The gas can be dried over anhydrous CaBr 2 . HBr is generated using ordinary water. To our surprise we discovered that passing the wet DBr or HBr over CaCl 2 allowed sufficient exchange that significant amounts of the corresponding chlorides were obtained. One caution-ary note: since PBr 3 is exceedingly hygroscopic, when-ever we opened a fresh bottle we resealed it in ca. 2 mL portions in simple glass vials made from 6 mm Pyrex tubing having a small bulb blown on one end and leav-ing a 4–5 cm length of bare tubing. This vial is easy to crack without spilling and in this way students were al-ways guaranteed equal amounts of fresh reagent for their experiments. N. Ganapathisubramanian replies: I get the impression from Bader's letter that he com-pletely missed the thrust of my article (J. Chem. Educ. 1993, 70, 1035), which is the simultaneous generation of all the four gases of interest. I apologize for not hav-ing cited his 1969 note (J. Chem. Educ. 1969, 46, 206) in my article, but that note does not contain any experi-mental procedure. It is true that the procedure in my article calls for two toxic/dangerous substances (toluene–d 8 and bromine liquid). But that should not be a deterrent, in my opin-ion. It makes people trying the experiment be more con-scientious about safety, waste disposal, and pollution. In addition, they gain insight into an interesting reaction, namely a Lewis acid-catalyzed electrophilic aromatic substitution. Bader's procedure calls for PBr 3 that needs to be handled carefully. Bader's procedure is also valid, but I would like to stress once again that my procedure simultaneously leads to HCl, HBr, DCl, and DBr in a single experi-ment, which in my opinion is quite a desirable feature.