Vol. (i7
STAXLEY J. CRISTOI,
1494
after this evacuation show that the samples did not develop porosity and did not increase their
[CONTRIBUTION FROM
surface areas during degassing. PITTSBURGH,
PA.
RECEIVED MAY21, 1946
BUREAUOF ENTOMOLOGY AND PLANT QUARANTINE, AGRICULTURALRESEARCH ADMINISTRATION,U. S. DEPARTMENT OP AGRICULTURE]
THE
A Kinetic Study of the Dehydrochlorination of Substituted 2,2-Diphenylchloroethanes Related to DDT' B Y STANLEY
J.
CkISTOL
The purpose of this paper is to report kinetic data on the elimination of hydrogen chloride with ethanolic sodium hydroxide from compounds of type I with RI= H and Rz = H or C1, and with R1= RB = H, C1, Br, F, CH,, CH30 or t-butyl, and of one series of related compounds in which the trichloromethyl group has been replaced with dichloro- and monochloromethyl groups. This work also includes data on the o,P'-DDT tsonier, l-trichloro-2-o-chlorophenyl-2-~-chlorophenylethane. Rate constants were obtained a t YO. 11 and 30.37" in 92.6 weight per cent. ethanol with all the compounds studied. The effect of change of solvent to 8'7.0 and T(i.0 weight per f H2O (1) cent. ethanol on the rate of dehydrochlorination '-,a of p p'-DDT has also been studied. XI Measurement of Reaction Rates.-The folThese workers, as well as Neal3 and Gunther,* lowing procedure was used to follow the rates have demonstrated the quantitative nature of of reaction of the various chloroethanes with this reaction, and their work has indicated that sodium hydroxide : A solution containing the only one mole of chloride ion is produced per mole ethane, 0.005 to 0.01 hl, and sodium hydroxide, of trichloroethane at SO". Grummitt, Buck and 0.01 to 0.05 M, was prepared a t the desired reacJenkins6 have shown that a t elevated tempera- tion temperature by dissolving the required weight tures hydrolysis of the ethylene (11, RI = Rz = o f the ethane in "190-proof" ethanol in a voluC1) may occur with the formation of a substituted metric flask (adding the required amount of water diphenylacetic acid. in certain cases), equilibrating the solution in a In connection with an extended study in our thermostat, and then adding, by means of a laboratory of the insecticide D D T (I, R1 = Rt = pipet, a measured volume of standard ethanolic C1) and of analogs of DDT, a quantitative study sodium hydroxide. The solution was then made of the effect of modifications in molecular struc- up to volume, mixed well by shaking, and replaced ture upon chemical reactivity was undertaken. 111 the thermostat a t 20.11 or 30.3'7". Tempera-4lkaline dehydrochlorination was chosen as a ture control was constant within *0.03". Dursuitable reaction, as its course may be conven- ing the course of a run six or seven 10.00-or 20.00iently followed by titration for chloride ion and nil. aliquot samples were transferred to an Erlenthe reaction forms the basis for an analytical meyer flask at bath temperature, and the reaction method for the determination of DDT.3f4 This was then stopped by the addition of about 25 ml. reaction was also of interest in connection with cf 1.5 N nitric acid. The solutions were then work on the mode of action of DDT, since a t least treated with 5.00 ml. of standard 0.03 N silver two groups of workersbs7have suggested the possi- nitrate solution, and the precipitate was cobility that dehydrochlorination may be involved a v l a t e d with nitrobenzene. The excess silver in the toxic action of DDT against insects. ion was titrated with standard 0.02 N ammonium thiocyanate with the use of a microburet and (1) This research w w conducted under a transfer of funds, recommended by the Committee on Medical Research, from the Office of ferric sulfate as indicator in the customary VolScientific Research and Development t o the Bureau of Entomology hard procedure. and Plant Quarantine. Not copyrighted. In extremely rapid reactions the temperature of (2) Brand and Busse-Sundermann, Ber., 76, 1819 (1942). the room was maintained within 1" of the bath (3) Neal nnd co-workers, U.S. Pub. Xcallh Sdroicc, Suppl. to Pub. Health Repts. No. 177 (1944). temperature to minimize errors arising during (4) Gunther. Znd. Esp. Cham.. And. Ed., 17, 149 (1945). the period of transfer, which was approximately (5) Grummitt, Buck and Jenkins, THISJOURNAL, 67, 156 (1945). one minute. (6) Fleck and Hallu, W d . , 66, 2096 (1944). Sodium hydroxide was chosen as the basic re(7) Martin urd Wain, Nature. 164, 512 (1844). Brand and Busse-Sundermann2have reported a semiquantitative kinetic study of the dehydrochlorination of several 2,2-diaryltrichloroethanes with potassium hydroxide in 96% ethanol a t the boiling point of the solution. The reaction studied is represented by equation (1).
Sept. 1945
DEHYDROCHLORINATION OF SUBSTITUTED
agent rather than potassium hydroxide because of the higher solubility of sodium chloride in ethanol.* The titer of the standard sodium hydroxide solution used was checked periodically against standard hydrochloric acid. The solvent, commercial “190-proof” ethanol, was used without purification. I t had a density (20/4O corrected to vacuum) of 0.8109’ which is equivalent to 92.6 weight per cent. Calculation of the Rate Constants.-The reaction given in equation (1) was found to be of the second order-that is, first order with respect to both chloroethane and hydroxide concentrations-and use was made of the following equation connecting initial ethane concentration a, initial hydroxide concentration b, fraction cp of ethane consumed, and time t: d log 1-9
b-a
(2)
2.303
Values of log (1 - a cp/b)/(l - ‘p) corresponding to the various samples of a run were plotted as ordinates against corresponding values of time, t, and the best straight line was drawn by inspection through the points. The slope of this line was multiplied by the value of 2.303/(b - a ) for the run to give the rate constant k. This method does not give undue weight to the first analysis made, nor is the value of k affected by uncertainties involved in the time taken as the start of reaction. A typical treatment of the data is shown in Fig. 1, which is the plot of two runs with PIP’D D T a t 20.11O. Satisfactory values were obtained with the second-order rate expression for all the compounds studied. Table I presents the data and results for all the experiments in 92.6% ethanol.
0.6
I
c1
c1
Substituent
RS
cl
a
RI
Rc
C1
C1
C1
C1
Temp., OC.
20.11
Halide
M 0.00991 .00998 .01001 .01001 .00501 .00501 a,
30.37
.00495 .00497
20.11
.01006 .01009 .01022
30.37
.01002 Br
Br
C1
C1
20.11
30.37
.00503 ,00504 .00478 .00478
(8) Ferau and Mellon, I d . BR;. Chrm.. A d . Ed., 6, 845 (1984).
’ /
I 0.4
10 20 30 Time in minutes. Fig. 1.-Treatment of data for rate constant of # , # I D D T at 20.11”: 0-0, DDT = 0,0100 121, NaOH = 0.0475 M ; 0----0,D D T = 0.0100 M ,NaOH = 0.0238 M. 0
Medium Effects on the Rate Constants.-No salt effect was noted in studies with p,p’-DDT when the concentration of sodium hydroxide was varied from 0.012 to 0.048 fix. This is in agreement with theory for a second-order reaction in which one of the reactants is a neutral molecule. The effect of variation in solvent is shown in Table 11. Although part of this change may be due to the change in the hydroxide-ethoxide equilibrium, the major change in rate is undoubtedly due to the effect of the greater ionizing power of the solvent upon k. The mechanism of the elimination process here involved is clearly that denoted by Hughes and Ingoldg as Ez, and the decrease in k produced by increasing amounts of water is as predicted by them.
sH.
