the microtcole lobordory
mechanism of the reaction varies considerably in organic textbooks.' A much better understanding and appreciation of free radical halogenation can be gained by doing a laboratorv exneriment. However. most such exoerimentsZ emolov corrosive chemicals (r.g.. sulfur).l r h l o r i d e , ~ ~ .and ~ ~ generate .) HCiand S0,gaaes. Suifurylchlorid~isdifficult tu store and extremely rorrosive (wen to stainless steel) and may damage the plungers of microliter syringes, even when they are rinsed immediately after use. Furthermore, excess SO&ln in the reaction mixture cannot be decomposed without affecting the ratio of chlorinated products. A convenient small-scale technique far the free radical chlorination of 3-methylpentane, or any other suitable hydrocarbon or alkyl halide, is to bubble chlorine through 3-methylpentane for 10 s and irradiate the sample with a sun lamp until the yellow color is gone (less than 10 s), isolate the sample, and analyze gas chromatographically (Method A). This procedure must be carried out in a hood and requires the use of a very small amount of chlorine, which is readily available in most laboratories. The chlorination procedure takes less than 1min per student, and therefore even sizeablelaboratory sections may need only one hood. Alternatively, an acidified solution of "Chloror" can be used as the source of chlorine (Method B). I t is best to do this procedure in a hood also, hut the volume of gases released are very small. This method is almost as fast as Method A, and the results appear to he nearly the same. Bromination of a suitable alkane can also he carried out to demonstrate its selectivity. Chlorination. Method A. To a 5-mL conical vial add 1 mL of 3-methylpentane (or any other alkane or alkyl halide such as 2.3dimethylbutane, 2,4-dimethylpentane, 2,5dimethylhexane, heptane, l-chlorohutane, l-bromobutane, etc.). In a hood buhhle chlorine through the 3-methylpentane for approximately 10 s. Fit the conical vial with a calcium chloride tube, and irradiate with a sun lamp or an ordinary light bulb until the yellow color is gone. Remove the calcium chloride drying tube, and add 2 mL of 5% sodium bicarbonate to the conical vial, cap the vial, and shake it vigorously. Remove the major portion of organic layer with a pipet, and put it intoa sample vial. Add 100 me-anhvdrous calcium chloride, and analyze . gas chromatographically. Chlorination. MethodB. To a 5-mL conical vial add 1 mL of 3-methylpentane (or any other alkane or alkyl halide such as 2,3dimethylbutane, 2,4-dimethylpentane, 2,5dimethylhexane, heptane, l-ehlorobutane, l-bromobutane, etc.) and 1 mL of "Chlarox". In a hood add 0.5 mL of dil. hydrochlaric acid to the mixture, cap the vial, and irradiate with a sun lamo. Shake the vial vigonmsly in front of the lamp until all of the r t h r is gone. When the rrartion is complete, r lowly add 100 ma of anhydrous sodi~~~
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um carbonate. When the vigorous bubbling has subsided, cap the vial and shake vigorously. Remove the major portion of the organic layer with a pipet, and put in a sample vial. Add 100 mg anhydrous calcium chloride, and analyze gas chromatographically. Brorninotion. To a 5-mL conical vial add 1 mL of 3-methylpentane (or any alkane such as 2&dimethylhutane, 2,4-dimethylpentane, heptane, ete.) and 1mL of bromine in methylene chloride. (The bromine solution, 6 g bromine in 100 mL of methylene chloride, should he freshly prepared.) Fit the conical vial with a calcium chloride tube, and irradiate with a sun lamo under the hood until the hromine color.ir gone. Hemove the calcium chloride drying tube, and add 2 ml. of F c sodium hirarbunatr to the vial. Cap the vial, and shake it vigorously. Remove the major portion of the organic layer with a pipet, and put it into a sample vial. Add 100 mg anhydrous calcium chloride, and analyze gas chromatographically. Gas chromatographic analysis of the reaction mixture on a nonpolar methyl silicone column indicated 90-95s starting material, U% monosuhstituted products, and negligible amounts of polysubstitution. The order of illution of the monochlorinated products from 3-methylpentane and l-chlorobutane are reported in a numher of laboratory m a ~ u a l s . ~ , V hbromination e of 3-methylpentane forms one readily detected product, 3-hromo-3-methylpentaue. Gas chromatographic analysis of the monochlorinated products from 3-methylpentane on a 25-m methyl silicone capillary eolumn partially separates 3-(chloromethy1)pentaue from I-ehloro-3-methylpentane. In addition. 2-ehloro-3-methvlnentaneis oartiallv. s e k a t e d into two nesks. Because the two peaks have nearly the same mass spectra they ere assumed to he diascereo. mers. Students are expected to chlorinate 3methylpentane and 1-chlorabutane, brominate 3-methylpentane, and analyze the reaction mixtures gas chromatographically in a 3-h laboratom neriod ineludine a 20-min prelab. No stud& took more than 3 h, and most of them took considerably less. From the ratio of isomers determined gas chromatographically, students can determine the relative reactivity of the lo,2', and 3' hydrogens in the chlorination of 3-methylpentane and determine the relative reactivity of the I-, 2-, 3-, and 4-positions of 1chlorahutane and observe the effect of chlorine subatituent has on the stability of the radicals of l-ehlorobutane. The higher selectivity of hromination is also observed. Gas chromatographic analysis of the monochlorinated products from 3-methylpentane on a nonpolar 25-m capillary column begins to separate 2-chloro-3-methylpentane into two neaks. Students are challenged to study the murture of 2-chlom-3mrthylpentane carefully and speculate why ir is separated inu, two peaks.
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'Johnson. A. W. J. Chsm. Educ. 1990,67.299. (a) Williamson. K. L. Mi-cale Organic Experiments: Heath: Lexington. MA. 1987. Chapter 13. (b) Wilcox. Jr.. C. F. ExpsrlmsnteI Or~anIlc Chemishy A Small-Scale Approach; MacMillan: New York. 1988. Chapter 13. (c) Nimltz. J. S. Experimenk in Organic Chemistry from Micrp scale to Macmscale; Prentice-Hall: Engiewood CliHr. NJ. 1991. Section 12. ( m e are some typical laboratory manuals using the smell scale apprmch. There are numerous other labwatory manuals using a largescale approach.) A isrg~-smIeapprwch to the chlorination of alkanes with chlwine is found in Ault, A. T& niques sndExperimenk for hganic Chemishy. 5m 4.;Allyn and Bacon: Boston, 1987; Section 14.
Construction ot a Low-Cost Solution Cell for Infrared Analysis Anthony Wlnslon West Virginia University Morgantown. WV 26506 In the undergraduate organic laboratory sample -preparation is a problem with st"dent use of infrared. Although the KBr pellet techniaue is an excellent method for the preparation of samples, considerable experience is often necessary to obtain good transparent disks. Also, the dies for preparing the disks are expensive, and students are prone to strip the threads or cause other damage. A Nujal mull between salt plates is also a good and reasonably inexpensive technique, but much time is spent in grinding the material and preparing the mulls. Students are often not able to pick up the technique easily. Solution techniques, on the other hand, are simple to perform. I t is generally quite easy for a student ta prepare a solution of a compound, fill a solution cell, and run the spectrum. Emphasis can then be placed on the spectrum itself and its creation and interpretation, with less emphasis and time being spent on sample preparation. But solution cells are also subject to misuse, such as breakage or the failure to dry samples properly, and, since commercial cells are fairly expensive ($100 or more), there is some reluctance to allow students free access to such cells. To address this oroblem we have developrd a solutrm cell for use in the undergraduate oraanrc labursrory at a material COYI of about S1O.OO, the price of two salt plates. The cell consists of two salt plates cemented together, a spacer in between, and a slot for introducing the sample. Several designs were attempted, and the one described here is convenient and " eives excellent results. The pnmdure ~nrludesaaaing a groovr in one 01 the aait plates, polishing thr plates, cutting a spacer from a hrais shim, and CP. menting the plates together with the spacer. Construction of the cell is described below.
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Construcllon of the Cell Fabrication of a Wooden Jig The jig is used to hold the cell window and guide the saw hlade. A 25-mm-diameter cavity, 8 to 10 mm deep, is drilled into a small pieceofpine, about 5 X 6 X 2 em. Thedepth should be such that, when the cell window is inserted in theeavity, some wood, 3 or 4 mm perhaps, extends above the top face of the cell window. The block is sawed in half directly through the center of the cavity, and the two halves are joined together with wood screws. If the size of the cavity is just right, the cell may be placed in the cavity, and the screws tightened so that the jig firmly grim the cell and prevents it from moving during the sawing operation. Next, using the scroll saw hlsde, a guide groove is sawed in the wood parallel to the first cut and 4 mm off to the side. The depth should extend down to about 2 mm from the bottom of the cavity. This completes construction of the jig. Cuning the Groove in the Cell Window I t is particularly important to use the right hlade for sawing the groove. The scroll saw blade, (Rockwell International part No. 40-198, obtainable from The Gage Co., 3000 Liberty Ave, Pittsburgh, PA 15238, pkg. of 6 for $3.70) is a special saw blade designated as g w d for cutting leather. There is practcally no set to the teeth, a factor that was found to be especially desirable. Several other blades were tried, such as an ordinary coping saw blade, hut these all proved unsatisfactory due to the large set to the teeth, which caused the hlade to undercut the swface and cause chipping. With the Rwkwell blade, chipping problems were eliminated. The hlade may be held in a jeweler's saw, but a perfectly satisfactory groove can be cut by holding the blade by hand and applyinguniform pressure over the face of the salt plate. A standard unpolished 25-mm-diameter, 5-mm-thick NaCl disk (Harshaw, 6801 Cochran Road, Solon, OH 44139, Stock Number 02505, $5.00 each) is placed in the jig, the screws are tightened to hold the disk firmly, and a groove is sawed completely across the cell using the scroll saw blade and the guide groove in the jig. The groove in the cell window is made to a depth such that a 24-gauge needle will just fit completely into the groove. This can be determined while the cell window is still in the jig. A light touch on the saw is desirable, as too much pressure may cause chipping or fracture. only one of the windows needs a groove. The thickness of the saw blade is such that a needle lareer than a 24 zauze - - will not fit the groove. Pollshing the Cell Windows Both eell windows are oolished usinzstandard techniques using a medium grit with 154 water in alcohol for roughgrinding ~~~
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and various mixtures of water and alcohol for polishing. Water itself works well if polishing is carried out quickly.
Fabrication of the Window Spacer A 25-mm diameter circle is drawn on a piece of 0.004-in. (0.1-mm) brass shim (McMaster-Carr Supply Co., P.O. Box 440, New Brunswiek, NJ 08903-0440, $3.48 for a 6- x 60-in. roll, various thicknesses are available) and cut out with scissors. A rectangle, 17 X 10 mm, is drawn on the circle, startingat the edge ofthe circle and then cut out with scissors, figure. The edges of the various cuts are tapped lightly with a small mallet to make sure that the edges are flat and are not turned over in any way.
being careful not to let it drop. The cell may he dried by flushing with air using a syringe as an air pump. For dauhle-beam dispersive instruments two e l l s are required, the reference eell, and the sample cell. Although the cells are reasonably well matched, they are not perfectly matched, and so bands due to the solvent may be evident. Using the double beam Perkin Elmer 1310 snectroohotometer. a weak uncompensated dc14 b k d at 1540'cm-I is observed, indicating only a slight cell mismatch. With FT-IR instruments both the solvent background and the sample are run using the same sample cell; hence, unmatched cells are not a problem. However, if several cells are in use, students must recognize that, if cells are switched, a new background of pure solvent should be run in order to compensate perfectly for the spectrum of the solvent. In our hands the cells are matched well enough so that using different cells for background and sample gives good results.
Acknowledgment The contribution of Donald Feathers, Senior Scientific Instrument Technician, Department of Chemistry, to the design and construction of the jig and cell, is greatly appreciated.
Assembly of the CeN A small amount of a two-component fastdrying epoxy cement is mixed, and then, using a toothpick, a thin coating of epoxy is aoolied . . to one side of the snacer. The soacer is placed on a flat surface, and the cell window, thcone witnout the gnrove,rs placedon the spacer with pressure tu spread out the cement evenly and to squeeze out air huhhles. When the cement has set, 5-10 min, the window is turned over, some more epoxy is mixed, and another thin coating is applied to the other side of the soacer. The second window is placed on top oE the spacer so that the groove is parallel to the Long side of the window and is within the window. Pressure is applied as before until the glue sets. Since both ends of the groove are now open, the one under the spacer must he closed by forcing in a small amount of epoxy with a toothpick. The cell assembly is allowed to dry for an hour or two before use.
'The use of pure llqulds Is not recommended sin- me spacer thickness may cause lhe absorbances of some peaks to be so great that Ihey bottom out badly.
Construction and Use of an Inexpensive Microburet Mono M. Slngh, Zvl Szafran, and Ronald M. Pike Merrimack College Nonh Andover, MA 01845
The introduction of microscale laboratories in general chemistry requires the use of microscale titrations using microburets and pipeta. The cost of buying microburets for the large number of students enrolled in general chemistry may be prohibitive. In our general and analytical laboratories at Merrimack College, we have been using mierohurets made from readily available, inexUsing the Cell pensive, materials. This m&roburet has advantages over other micra-devices because Thecell is filled usineasvrinee fitted with it allows the formation of more uniform a q 2.1 nwdle or smallFr. he &Ishould be filled withadutiun' just prior to running r h ~ drops at its micro-tip, its accuracy is much greater, and the manipulations during spectrum; otheruide, excessive evaporation transfer of solution are much easier. may occur. After the spectrum has been run, The construction of the microburet is the cell is flushed with the solvent, usually shown in the figure. The followingmaterials carbon tetrachloride, and allowed to dry. are needed for its construction: a 1.00-mL The needle does not fit tightly enough to graduated pipet, a 34-cm-long piece of rubwithdraw the samole from the cell usine the syringe, and so, i i t h e cell is to be emptied (Continued on page A126) and dried, a good way is just to shake it once,
Volume 68
Number 5
May 1991
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