An Early Laboratory Introduction to IR Spectroscopy Gaseous deuteroalkanes Loren L. Braun and Raymond L. Law Idaho State University, Pocatello, ID 83209 While i t seems desirable to introduce IR spectroscopy during the first few weeks of the organic course, viable lahoratory experiments demonstrating its utility are quite limited a t this point because students are generally acquainted with few, if any, functional groups. The preparation of a liquid deuteroalkane bv treatment of a Grianard reagent with deuterium oxide, and observation of the c - D stretching band has been one anoroach to the oroblem.' The product is isolated by distillat&n. We have found that the preparation of deuterated gaseous alkanes is a simpler and le& h&ardous experiment. Quantities used are small and distillation is avoided, while the products display simple but instructive spectra. Our experiment was facilitated by inexpensive gas cells constructed from the jackets of water-coaled condensers from laboratory stock. Conveniently, salt plates normally used for neat liouids ~ - - fit~over ~ the ends of the cells to act as windows. Construction of the cells is described below. Students (in nairs) in our laboratow are assigned the preparation of &her propane, 2-methylpropane, pn$ane-l-d. or 2-meth\bn,oane-1-d from the corresponding alkyl bromide via a ~ r i & i r d ; e a ~ e n t .~ e t r a h ~ d r o f u i arather n, than ether, ~~~
~
~
R-B~
Mg
THF
D,O
R-Mg-Br
R-D R-D
R=n-prop~l or hobutyl
is used as the solvent because i t is less readily entrained with the gaseous product. The gas is passed through a sulfuric acid and calcium chloride train before collection in a aas cell. The IR spectrum is obtained and comparisons are made. The spectra (2.5 min scans) of propane and propane-1-d are shown in Figure 1. The C-D stretching band (2180 cm-') is clearly displayed. Inspection also reveals that deuteration of the alkane has altered the appearance of the C-H stretching bands (2800-3000 cm-') and C-H bending bands (125Q1500 ~m-').~ We believe that this early acquaintance with relatively simple spectra, rather than a later initial encounter with a bewildering number of hands associated with a variety of functional groups, facilitates the study of IR spectroscopy.
Figure 2. Gas cell.
essentially a shortened jacket, made by cutting out the middle section and ioinine the two ends. The orocedure was m follows. A cut was mads5cm from each end of the jacket, yielding two tubulated seementj each havine one threaded end. Bv use of a glass-workkg torch, the cut ends of the two segme& were fused with a butt seal. Care was taken to keeo the newlv formed 10 cm tube straight. (The two windows of ;he cell must be parallel.) Cells were annealed to remove stress. The holes of the plastic screw-caps were bored to the inner diameter of the cells. Circular sodium chloride windows (2 mm thick with a 20 mm diameter) were purchased from Barnes Engineering Company3 a t $12 per pair, along with Teflon" gaskets a t $4 per dozen. A window, protected by a gasket on each side, is placed a t each end of the tube and secured by a screw cap. A picture of the finished product is shown in Figure 2. ~
Construction of Gas Cells Cells were fabricated from condenser cooling jackets of the screw-cap type (Kimble No. 18003-A). The finished cell was
'
Rosenblatt, D. H., and Davis, G. T., "Laboratory Course in Organic Chemistry," 2nd Ed., Allyn and Bacon, Inc., Boston, MA, 1973, p. 59 ff. Detailed analyses of the spectra of propane and propane-1-d are given by MeMurry, H. L., Thornton, V., and Condon, F. E., J Chem. Phys., 17, 918 (1949); and by Friedman, L., and Turkevich, J., J. Chem. Phys., 17.1012 (1949). 'Barnes Engineering Company, 30 Commerce Road, Stamford, CT 06904.
~~
~~
~~
~
Volume 58. Number 1. Januav 1981 1 79
R u b b e r policemen m a y be used t o c a p t h e cell tubulations.
Experimental
must be avoided. In our laboratory the IR spectra were obtained on aBeckman Acd a b 2 speetrophotome&r. Propane (or Propane-1-d). A 100-ml rb one~neekedflask is equipped with a Claisen connecting tube fitted with a dropping funnel and a condenser protected by a calcium chloride tube. In the flask is placed 0.96 g (0.040 mole) of dry magnesium turnings and 6 ml of tetrahydrofuran. From the dropping funnel is added about one-third of a 4.9 g (3.6 ml, 0.040 mole) quantity of l-bromopropane. The mixture sometimes starts to boil a t this paint, as the reaction begins. When spontaneous boiling fails to occur, the reaction is initiated by crushing one of the magnesium turnings with a stirring rod, addition of an iodine crystal, or heating on asteam bath. The remainder of the l-bromopropane is added as necessary to maintain moderate boiling When addition is c?mplete and boiling has subsided, the mixture is heated on a steam bath for 1 5 min to insure completion of the reaction, then cooled. The condenser is removed and the Claisen tube is connected by
80 I Journal of Chemical Education
tubing to a train consistingof the following items in series: (1)a50-ml sidearm flask (to trap sulfuric acid back-up) fitted with a one-holed stopper; () a bottle of sulfuric acid ( e l to 2 cm deep) fitted with a two-holed stopper having an inlet tube extending a few mm below the surface of the liquid; (3) a calcium chloride tube inserted in the remaining hole of the stoppered sulfuric acid bottle; () an open gas cell. For safety purposes the sulfuric acid bottle is placed in a beaker containing sand. In the dropping funnel is placed a solution of 1 ml of water (or deuterium oxide to prepare propane-l-d) in 9 ml of tetrahydrofuran. The aqueous solution is added slowly to the Grignard reagent. With the proper rate of addition a steady stream of gas is achieved. The rate of addition must be rapid enough to prevent back-up of sulfuric acid but not so rapid that pressure builds up in the system. When gas evolution is complete, the gas cell is capped and placed in the beam of the IR spectrophotometer. The standard gas cell holder for the instrument will readily accommodate this cell. In the absence of this holder, a foam pad can be used to cradle the cell in the bracket normallv used to s u o ~ o rother t cell holders. The cell is adiusted in the
