5963
The 13Cspectra were determined with complete proton decoupling on the Varian DFS-60 spectrometer. le, Several hundred scans were required to observe the 13C satellites of the I3C spectrum which arise from molecules whose natural abundance is only 0.02 %.
l2
t
8
(16) F. J. Weigert and J. D. Roberts, J . Am. Chem. Soc., 89, 2769 (1967). (17) National Science Foundation Predoctoral Fellow 1965-1967.
Frank J. Weigert," John D. Roberts Contribution No. 3557 Gates and Crelliii Laboratories of Chemistry Culiforniu Institute of Technology Pasadena, California 91109 Receiced August 28, 1967
Base-Catalyzed Hemithioacetal Decomposition at a Diffusion-Controlled Rate' Sir : Measurements of the rate of hemithioacetal formation from acetaldehyde and thiols of differing acidity indicated that the rate of base-catalyzed hemithioacetal decomposition approaches the diffusi on-controlled limit as the leaving thiol becomes more acidic.2 The hydroxide ion catalyzed decomposition of the hemithioacetals formed by the condensation of acetaldehyde with benzenethiol, p-nitrobenzenethiol, and thioacetic acid has now been shown to proceed at a diffusioncontrolled rate. In addition, the reaction is subject to general base catalysis with a Bronsted slope near 0.8. The rate of hemithioacetal breakdown or formation was followed by ultraviolet spectroscopy with a stoppedflow mixing apparatus. Measurement of the equilibrium constants for thiol addition permitted calculation of both rate constants from rate measurements in either direction. The rate constants of the water and hydroxide ion catalyzed reactions were obtained over the range pH 0 to 6.5 from measurements in dilute hydrochloric acid or by extrapolation of rates obtained in buffer solutions to zero buffer concentration. Buffer catalysis was measured at two buffer ratios in most cases, and only the basic species was found to be catalytically active. The rate constants for the hydroxide ion catalyzed breakdown of the three hemithioacetals are approximately 1Olo M-' sec-', which is in the range expected for a diffusion-controlled reaction (Table I). The value of AH* for this reaction, from measurements at 2 , 25, 30, and 40°, is approximately 2 kcal/mole. Table I. Decomposition of Hemithioacetals Formed from Acetaldehyde and RSH at 25" and Ionic Strength 1.0 M
RSH CHaCOSH
Ke
kRIo,
ko=- x 10-10,* M-l sec-'
pK,
M-'
sec-1
Obsd
Calcd
3.20
29
0.25
0.83
0.83
0.98~
-
p-NOzCeHaSH
4.57
6.9
0.43
CBHjSH
6.33
29
0.010
K
=
[hemithioacetal]/[RSH][CH3CH0
* Based on hydroxide ion activity.
0.95 0.68
0.89 0.69
+
CH,CH(OH)2]. Ionic strength 0.003-0.010.
(1) Publication No. 525. Supported by grants from the National Science Foundation and the National Institute of Child Health and Human Development of the Public Health Service (HD-01247). (2) G. E. Lienhard and W. P. Jencks, J . Am. Chent. Soc., 88, 3982 (1966).
4
0
8
12
16
pKa + L o g p / q
Figure 1. Brpnsted plots for the decomposition of hemithioacetals formed from acetaldehyde and benzenethiol (IJ), thioacetic acid (O), and p-nitrobenzenethiol ( 0 ) . Statistical corrections have been made according to R. P. Bell and P. G. Evans, Proc. Roy. Soc. (London), A291, 297 (1966).
The rate constants, k B , for catalysis by weaker bases 0.03 (Figure 1). follow Bronsted slopes of 0.80 These results suggest that the reaction proceeds according to the mechanism of eq 1 and that diffusionB
BH+
+
I I
HOCSR
o=c
