S. Tsuge, J. J. Leary, and T. 1. l s e n h o u r University of North Corolina Chopel Hill, 27514
1
CCI,-Reartions stration on
at High Temperatures
of steric effects
using atomic models
I n t h i s article a simple experiment is described which: 11 introduces t h e student to sealed tube reactions; 21 involves gas chromatographic analysis of t h e products; and, 31 demonstrates t h e use of a t o m i c models i n a steric hind r a n c e problem. Sealed t u b e reactions a r e becoming q u i t e commonplace in chemistry; therefore, a simple experim e n t using this technique c a n h e a very practical addition to an advanced laboratory course. Furthermore, t h e importance of t h e use of models i n chemistrv c a n n o t he &eremphasized. O n e of t h e most notable examples i s t h e extensive use m a d e of models b y Watson a n d Crick ( I ) in their Nobel prize winning elucidation of t h e general struct u r e of deoxyrihonucleic acid [DNA]. Experimental The sealed tube reaction to be performed proceeds very nicely using 0.1 ml of reagent grade CCI. sealed in an 8-cm length of 4-mm i.d. (l-mm wall) Pyrex tubing, a t 5W"C. Students should first familiarize themselves with the oxy-gas torch by sealing empty lengths of tuhing. Torch glasses should be worn whenever the torch is being used. The individual steps of tubing sealing are illustrated in Figure 1. This seal, often called an ampule seal, is most easily formed by heating the tubing about 2 em below its top with a well directed flame until the glass is quite soft. At this point the upper 2 em of the tube, which is still in the flame, is gently pulled away with a pair of tongs. Neat it is advisable for students to practice sealing a few tubes containing 0.1 ml of water. When the instructor is satisfied that these tubes have been properly sealed, they should be placed in individual safety shields. As shown in Figure 1, safety shields are easily fabricated from 15-cm lengths of l-em stainless &el tubing that have been crimped partly shut a t one end. The safety shields containing the sealed glass tubes are then placed in a 5 0 0 T furnace for a few minutes t o test the sealing technique.' A face shield should always be worn while handling hot tubes. After the practice run has been successful a new set of tubes containing 0.1 ml of CC14 is sealed and placed in the furnace. I t is advisable to place the furnace in a well ventilated area [e.g., a hood], because, if a tube has not been properly sealed, the hot CCL coming in contact with the oxygen in the air will produce phosgene [COC12],a poisonous gas.2 After 1 hr a t 5 0 0 T abaut 10% of the CClr has been converted into the products. The major products are heaachloroethane [CzCla] and Cia; the former was identified hy mass spectrometry. Minor amounts of hexachlorobenzene [C&la] are also farmed. The pressure in the reaction tubes after they have been removed from the furnace and allowed to cool to room temperature is appmximately atmospheric.3 Since the pressure is so law, the tubes are most easily opened by scoring with a file or diamond point and breaking (as you would an ordinary piece of glass tubing). [Remember t h a t all glass should be wrapped in a towel and pointed away from all persons when i t is being broken.] About xi, of the CCll is evaporated using either a vacuum pump or a water aspirator while heating to 40-50'C with a heat lamp. This removes almost all the dissolved CIS. Some of the physical properties of the products are
CCln CzCla CeCh
MolWt
MP("C)
BP("C)
153.8 236.8 284.8
-23 187 227
77 186 (at 777 mm Hg) 309-310 (at 725 mm Hg)
From the difference in boiling points it is clear that these compounds can be easily separated with a gas chromatograph. Al266
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Journal of Chemical Education
a b c d Figure 1 . Steps of tubing sealing.
most any liquid phase which will operate a t a t least 170'C can be used. At this point the experiment can be easily adjusted to the individual, the amount of time available, and the level of the course. Two possibilities using a gas chromatograph will be briefly described: 1) Qualitative analysis af the products. Prove that the major product is C&ls and that the minor product is CsCle, (The concentration of CsCls is abaut 100 times less than that of CzCls.) This type of analysis is easily done by comparing the retention times of the products on two columns of different polarities [e.g., SE-30 and OV-221, with the retention times of the knowns.&If the retention behavior is to be monitored using Kovat's indices (21, the series of normal hydrocarbons Clo through C ~ should O be available; and 2) Quantitative analysis of the products. Naphthalene is a very goad internal standard because it is easy to weigh and elutes between C2Cls and C6Cls on most columns. The relative molar sensitivities of the perchlaro compounds can either be determined experimentally or calculated using the method proposed by Sternberg, e t al., (3) if a flame ionization detector is used. Figure 2 shows a typical gas chromatogram of the CC14 reaction products using naphthalene as an internal standard.
'At 500°C all of the water is in the gas phase. It is instructive for students to calculate the approximate pressure inside the tube using both the ideal gas equation and the van der Wad's equation. It is surprising to find that the tubing used in this experiment can easily hold over 1M)atm pressure when properly sealed. 2This is an excellent example of the importance of a limiting reagent. Students might calculate the maximum number of moles of phosgene that could be produced if the tube remains sealed [Oz is the limiting reagent]. They could then compare this with the maximum number of moles of phosgene that could be produced if the tube was not properly sealed and the CCI. escaped into the atmosphere. Students should discuss this result in light of the solubility of Clr in CCI., and should also discuss why the CCln solution is greenish-yellow at the end of the reaction. Hexaehloroethane was purchased from K & K Labs. Inc., 121 Express Street, Engineers Hill. Plainview, New York 11803.
Figure 2. Chromatogram of CC4 reaction products: (mernai stanaara: naphthalene); instrument: Hewlett-Packard model 402; Column: 2 It X % in.. glass; ov-22 (10%) on Chromosorb W-H.P.. (80-100 mesh); carrier gas: helium (35 ml/mio): detector: flame ionization; conditions: 100200°C (temp. program 1O0C/min). Figure 3. >pace wing m o m s .
Discussion ol Steric Hindrance5 Compounds with formulas CmHzm+zand CmFm+a [m = 1,2, . . .] are known for very large values of m. Polyethylene and polytetrafluoroethylene [Teflon] are members of the two series, respectively. However, the series CmClzm+zhas never been reported for [m 2 4].6 Using a set of space filling atomic models (Fig. 3) [e.g., Fisher-Hershfelder type], it is easy to see that even in CzCI6 rotation about the C-C bond is inhibited. When the model of C3Cl8 is examined it is evident that almost no rotation around the C-C bonds is possible, and the chlorine atoms are all very closely packed. The model suezests that ~ e r c h l o.r o ~.r o ~ amav n e easilv decom~osea t elevated temperatures, where molecular motions increase. Experimental results also indicate that C&ls decomposes into CCll and CzCln at temperatures above 300°C (4). I t is interesting to notice that one of the products from
--
5The authors feel that this type of discussion of steric hindrance can be very instructive even if the experiment is not performed. ~~
Steric hindrance is so great in the perbromo- and periodo-alkanes that neither of these series has been reported above Cz. 6
CC4 at high temperatures is hexachlorobenzene (CsCle). The existence of this compound a t elevated temperatures must be explained by the thermal stability of aromatic compounds and the less sterically hindered conformation of the chlorine atoms in the molecule, which can he illustrated using models (Fig. 3). Acknowledgment The authors thank C. Robert McClure of Finnigan Corporation for supplying the GC-MS spectrum which identified hexachlombenzene. Work supported by Materials Research Center, University of North Carolina under contract number DAHC15-67-C-0223 with the Advanced Research Projects Agency. Literature Cited (11 Wataon.
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11 I ( w s l % F.and Kcrlman%A I M . A n n Cnem 3618 . 1 1 1 W 6 1 1 I . i!.rnsere. J . C l,alloua\ \V i., and .lono. I) T I. 'Ca. I ' h r n m a t o ~ n p h ) " Eoll.~r ilrennrr U el sl Aradcmx Plr*r. \ r w Y w k . 1961, p 271 4 1 l1dntr-. F H 'Oxnm,c Chl~.ncClmn.und3.' .lrhn U'I~PL an0 Son5 In:. Scr
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Volume 51. Number4, April 1974 / 267
