1484
SIDNEY D. Ross [ OONTRIBUTION FROM
THE COXVERSE AND
Vol. 68
GIBBSMEMORIAL LABORATORIES OF HARVARD UNIVERSITY ]
The Rate of Oxidation of Thiodiglycol and Triethylamine by Hydrogen Peroxide BY SIDNEY D. Ross Sulfides' and tertiary amines2 are cleanly oxiTABLE I k (bimolecular) dized by aqueous hydrogen peroxide to give sulliters/molefoxides and amine oxides, respectively. Despite Buffer OH minute the formal simplicity of these reactions their mecha- H20 (not buffered) 0.153 nisms appear worth investigating because of Potassiuni acid phthalate-sodium the widespread occurrence of hydrogen peroxide potassium phthalate 6 ,129 as a product of natural oxidation-reduction proc- Sodium acetate-acetic acid 4.7 .137 esses. In order to study the effect of fiH on these Potassium acid phthalate-sodium reactions measurements were made of the rate potassium phthalate 4 ,131 of oxidation of thiodiglycol by hydrogen peroxide Hydrochloric acid 1 .273 in aqueous solution over the $H range 0 . 3 4 , and Hydrochloric acid 0.3 .946 of triethylamine by the same oxidizing agent in concentration of HzOz/initial concentration of unbuffered water and a t PH 1. thiodiglycol and z = fraction reacted. Experimental Results TABLE I1
TABLE I11
The thiodiglycol was obtained from the Carbide a = 0.0500 M , not buffered; a = 0.1027 X , b = 0.1357 b = 0.1406!!.ti -If, p H 6 and Carbon Chemicals Corporation. Only a con0.0500 N 0.0800 N stant boiling middle cut (b. p. 137' ( 5 mm.)) was SzOs-, % s203-, % Min. cc. Reaction Min. cc. Reaction used. The triethylamine was distilled several times and a middle cut boiling a t S9.5-89.8' 44.7 38 23.66 10.3 24.72 11.8 was used. All our kinetic measurements were 56 22.32 58.1 33.4 26.5 20.30 made a t 25 * 0.05'. Initial time was taken as 73 21.50 66.3 35.8 18.40 42.7 the time of addition of a solution of hydrogen 20.96 71.7 88 55.0 16.42 52.4 peroxide, adjusted to 25.0' in the thermostat, to 103 20.50 77.0 14.55 6 1 . 4 76.3 thiodiglycol or triethylamine in the appropriate 90.3 13.ij8 65.6 aqueous solution a t the same temperature. The 127 12.~16 73.5 aliquots, taken a t appropriate time intervals, were TABLE V TABLE IV analyzed directly for residual hydrogen peroxide by adding them to a solution containing 200 cc. a = 0.0822 M ,b = 0.0873 a = 0.0907 Jr, b = 0.1338 M , PH 4.7 111, p H 4 of water, 20 cc. of 1:2 sulfuric acid and 2 g. of po0.0999 N 0.0500 N tassium iodide. Three drops of 3y0 ammonium s203-, % SlOa-, % Min. cc. Reaction Min. cc. Reaction molybdate were then added and, after standing 10.6 7.70 12.6 7.00 24.66 11.7 for five minutes, the solution was titrated with 35.3 6.24 30.4 16.4 22.60 23.0 standard sodiurn thiosulfate to a starch end-point. 52.7 5.60 38.2 25.3 21.00 31.8 The oxidation of thiodiglycol in water by hy83.3 4.68 49.4 35.4 19.42 40 5 drogen peroxide is bimolecular with a rate con112 3 . 9 0 58.8 44.0 18.32 46 6 stant for the non-catalyzed reaction equal to 1.30 150 3.34 65.6 f 0.07 X 10-1 liters/mole-minute. As can be 201 2.85 71.6 seen from Table I, the reaction shows acid catalysis a t and below PH 1. This acceleration is not TABLE VI TABLEVI1 due simply to an increase in ionic strength a t the a = 0.0941 M,b = 0.1149 a = 0.0956 M , b = 0.117'9 lower pH's, for to buffer a t p H 4.7 the solution was M.PH 1 AI. PH 0.3 0.0999 N 0.0999 N made 025 ;ZLT in both sodium acetate and acetic SzOa-, % sod-, % acid and, consequently, this solution is of higher Min. cc. Reaction Min. cc. Reaction ionic strength than the run a t p H 1, where the 10.35 12.2 4.80 4.13 8.22 37.5 solution was 0.1 M i n hydrochloric acid. 8.60 13.2 5.70 (33.8 13.2 30.8 The experimental data for a typical run a t each 42 8 23.6 4.68 74.4 22.5 7.47 PH are given in Tables 11-VII. Here a and b 25.6 4.18 79.6 45.2 5.65 6 2 . 1 refer to thiodiglycol and hydrogen peroxide, re32.0 3.80 83.6 4.70 72.2 70.5 spectively. In all runs 5-cc. aliquots were taken. 42.4 3.30 88 8 89.1 4 . 2 1 77.4 T o indicate how closely the second order equation 56.5 2 9Q 92 1 is followed throughout the course of a single run The rate of oxidation of triethylamine by hyplots of log Q - z j Q ( 1 - z ) os. t for runs a t three PH's are presented in Fig. 1. Here Q = initial drogen peroxide in unbuffered aqueous solution liters/mole-minute. At pH1 no is 2.0 X (1) Gazdar and Smiles, J. Chem. Soc., 93, 1833 (1908). reaction had occurred after one hundred and (2) Dunst,in and Goulding, ibrd., 76, 1004 (1899).
Aug., 1046
OXIDATION RATEOF THIODIGLYCOL AND TRIETHYLAMINE WITH HzOz
twenty hours. The data for a typical run are given in Table VIII. TABLEVI11 0.08399 .AI hydrogrn peroxide, not buffered; 0.06643 ilf triethylairiiiic. 0.0971 N S20:$-,
Min.
CC.
70 Reactirm
52.7 105.1. 185.7 324.1 385.4 600 724 1348
7.85 7.40 6.88 6.00 5.85 5.05 4.70 3.43
11.7 18.3 26.0 38.7 40.9 52.6 57.7 76.3
1
0.50
0.411
/
G I
I
=: 0.311
9 4 I 00.20 t4 e
0.10
1485
i
k-
Discussion LIi----!dIn the oxidation of a tertiary amine by hydrogen peroxide, which is quite analogous to the 20 40 60 80 100 120 140 formation of sulfoxides, the direct product is a Minutes. base of the type (RBNOH)+OH-,from which an Fig. 1.-Second order plots of runs at three PH's: aniine oxide may be produced by d e h y d r a t i ~ n . ~ The action of hydrogen peroxide is, therefore, 0.1179 M HzO: and 0.0956 M thiodiglycol at pH 0.30; analogous to that of a methylating agent in the 0.1149 M Hz02 and 0.0941 M thiodiglycol a t pH 1.0; formation of quaternary ammonium (or tertiary 0.0873 M H:O2 and 0.0822 A I thiodiglycol at PH 4.7. sulfonium) salts, and its effectiveness is dependent upon the ease with which i t can donate the group l o ga t pH 1. The conjugate acid of triethylamine, K8NH+, is much less susceptible to oxidation than (OH)+ to the basic nitrogen or sulfur atom. In the oxidation of thiodiglycol the rate was triethylamine itself and as a result no reaction faster a t and below pH 1. The effect of acid on occurs a t p H 1. Acknowledgment.-The author wishes to exthe rate of oxidation is explained by supposing that the neutral molecule HOOH, which must press his indebtedness to Dr. Paul D. Bartlett for produce a hydroxyl ion in the oxidation can ac- helpful discussions relating t o the mechanism complish this oxidation a t a constant rate which is proposed above. less than the rate a t which its conjugate acid, Summary HOOH2+,cart do it. Only in strongly acid soluThe oxidation of thiodiglycol in water is bition is the:re enough of this species present to make a difference in rate. I t is faster because the molecular and shows acid catalysis a t and below other product of its oxidation is not the OH- ion, p H 1. The rate constant for the non-catalyzed irivolviiig a separation of charge in the critical reaction is 1.30 * 0.07 X 10-' liters/mole-minute step of the reaction, but a neutral water molecule. a t 25.0'. The rate constant for the oxidation of triethylIn the oxidation of triethylamine an additional factor becomes important. With thiodiglycol the amine by hydrogen peroxide in water is 2.0 X liters/mole minute a t 25'. A t p H 1 the shift in favor of HOOHz+ is not counteracted by a shift in fa.vor of (HOCHzCH2)2SH+. With tri- oxidation is too slow for convenient measureethylamifie which has a K B of 4.35 X a t ment. The mechanism of these reactions is briefly disthe ratio R3NHt/R3N is equal to 4.35 X cussed. ( 3 ) Meisenheimi:r, A n n . , 397, 273 (1913). ( 4 ) Rloore, .I (.Ile,n Soc., 91, 1383 (1907)
CAMBRIDGE, MASS.
RECEIVED MARCH20, 1946
