GEORGE L. GILBERT Denison University Granville. Ohio 43023
Free Radical Chlorination of Methane: A Demonstration SuehllrrEo BY:
Alfred R. Conklln and Alan Kramme Wilmington College Wilmingtan. OH 45177
Free radical reactions are very important and often discussed in chemistry. One of the first such reactions encountered by students of organic chemistry is the free radical chlorination of methane. This reaction serves to introduce the student to free radical reactions and chain reaction. In spite of its common occurrence. demonstrations of this reaction are uncommon. Althoueh total ~reuaration time for this demonstration is . . up 1,) 2 hr. mce the vquiprnc~~t and rwgentj are preportd, ~ 4 a .time i ~ fin dernunsrrati~nCXI IIC. '15 shurt 3.i 13 min. Td do this, I prepare the reactlcm t1a3kr with chlurine :n them the day hefirre. Alx,ur an huur hetm. I lass, I nttich a >hart ruhbrr tube to a gas tap in a hood near where the demonstration is to take place and insert the open end in a beaker of water. Gas is then allowed to bubble out slowly until the demonstration. For the demonstration I take a sample of natural gas (approx. 85% CH4) with a 1000-ml syringe by puncturing the rubber hose. This sample is injected into the reaction flask which is subsequently irradiated with UV light for about 5 to 7 min, while I discuss the workings of the gas chromatograph. A sample is then taken from the flask and injected. Peaks are observed as they are recorded. The students are generally told which peaks correspond to which components. Students are allowed to investigate further irradiation times and other parameters if they wish. This procedure could be used as a lab exercise if so desired. Although an oxygen-free atmosphere is needed, the demonstration can he carried out without extremely elaborate precautions to exclude 0 2 or air.
The accepted reaction mechanism involves a series of steps that ultimately produce mono-, di-, tri-, and tetrachloromethane. Step (1) involves the cleaving of a chlorine molecule into two chlorine free radicals, by heat or ultraviolet light. In step (2), a chlorine-free radical abstracts one of the hydrogen atoms from the methane molecule, resulting in the formation of a hydrogen chloride molecule and a methyl radical. Then, in step (31, the methyl radical reacts with a second chlorine molecule to form chloromethane and a chlorine free radical. Steps (2) and (3) are repeated over and over again, each time yielding a chloromethane molecule. Steps (41, (51, and (6) terminate the reaction hv consumine reactive intermediates and Surm~nga chlurumethanc m&cule i r m ,I methyl r:ldicnl nnd a ,.hlwrine radicnl, and r t h m e ni~,leculeirtm I \ W mt.111~1 radicals, and a chlorine molecule from two chlorine-free radicals. ~
~
These steps can he summarized as follows: Chain initiating step: CI-CI
haat
2 C1.
(1)
ultrsviolot light
Chain propagating steps: C1'
+ CH3-H +
CH3- CI-C1 Chain terminating steps: CHi CH$
-
+ C1.
+ CHa.
C1. + CI-
CH3. t H ~ C I CH&I
-
-
(2)
+ CI-
(3)
CH&
(4)
CH3-CH3
(5)
CI-CI
(6)
Steps (7) and (8)show that as the quantity of chloromethane begins to increase, i t too reacts with chlorine to form a dichloromethane molecule. CI CI
I
C1' + H - D H
I
H
Volume GO
-
H-C1
I
+ 'C-H
I
(7
H
Number 7 Julv 1983
597
c1
I H-C' I H
+ CI+1
-
CI
I
H+-t4.I ' d 1 tht needlr 11.11 just enough to close the stopcock, then completely remove it. T h e following method is used for laboratory-generated chlorine. In>ert a 5-ml syringe thrmyh tI~c,.
