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The Reactions of P-Toluenesulfinic Acid with Mercaptans and Secondary Alkyl Sulfides

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The Reactions of P-Toluenesulfinic Acid with Mercaptans and Secondary Alkyl Sulfides AN ABSTRACT OF THE THESIS OF Gary Ronald Bray for the Master of Science in (Name) (Degree) Organic Chemistry (Major) Date thesis is presented _'..' °i`% Title The Reactions of p- Toluenesulfinic Acid with Mercaptans and Secondary Alkyl Sulfides Isopropyl, butyl or benzyl mercaptan all react very rapidly with 2- toluenesulfinic acid in acetic acid -0.56 M water -0.6 M sulfuric acid solution at 70 °. If the mer- captan is added rapidly as a concentrated solution three moles of mercaptan react per mole of sulfinic acid pre- sent. The reaction products in each case are two disul- fides, the first being the symmetrical derived from the mercaptan used, the second, the unsymmetrical 2 -tolyl alkyl disulfide. About equimolar amounts of the two disulfides were isolated in each case. Under the reaction conditions employed the mercaptan -sulfinic acid reaction clearly has the stoichiometry 3 RSH + ArSO2H ArSSR + RSSR + 2 H2O (1) These results are compared with those obtained under other conditions both in this laboratory and elsewhere, and some tentative conclusions about the details of the mechanism of the mercaptan -sulfinic acid reaction are outlined. Recently Kice and Bowers discovered a new reaction of sulfinic acids. This was their reaction with primary alkyl sulfides. For E- toluenesulfinic acid and a primary alkyl . sulfide this reaction has the stoichiometry shown below, 5 Ar SO2H + (RCH2)2S ----2 ArSO2SAr + ArSO2SCH2R (2) + RCHO + 3 H2O It was of interest to determine if this unusual cleavage of a sulfide could be extended to sec -alkyl sulfides. Studies in the present thesis show that both isopropyl. and sec -octyl sulfide react about as readily as primary alkyl sulfides with 2- toluenesulfinic acid. Acetone and 2- octanone have been identified as products of their respective reactions. The yields of 2 -tolyl 2- toluene- thiolsulfonate obtained in each case are in agreement with the stoichiometry of equation 2, but the expected sec- alkyl p- toluenethiolsulfonate has so far not been isolated. It is thought that this may well be due to its decompos- ing during the work -up procedure used. However, further study is needed before any definite conclusion can be drawn on this last point. THE REACTIONS OF p- TOLUENESULFINIC ACID WITH MERCAPTANS AND SECONDARY ALKYL SULFIDES by GARY RONALD BRAY A THESIS submitted to OREGON STATE UNIVERSITY in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE August 1963 APPROVED: e or .f Chemistry As ci -. Profe In Charge of Major -1.1M=M-Chairman of Department of Chemistry Dean of Graduate School / 1 -1,-, Date thesis is presented /'Y -t -)\ `-7 Typed by Ola Gara TABLE OF CONTENTS Page Introduction 1 Results 7 Reaction of Mercaptans with p- Toluenesulfinic Acid 7 p- Toluenesulfinic Acid -Butyl Mercaptan 8 p- Toluenesulfinic Acid -Isopropyl Mercaptan 8 p- Toluenesulfinic Acid -Benzyl Mercaptan 9 The Reaction of Secondary Alkyl Sulfides with p- Toluenesulfinic Acid 10 p- Toluenesulfinic Acid -Isopropyl Sulfide 11 p- Toluenesulfinic Acid -2 -Octyl Sulfide 12 Discussion 14 The Mercaptan -Sulfinic Acid Reaction 14 The Secondary Alkyl Sulfide -Sulfinic Acid Reaction 20 Experimental 23 Materials 23 Product Study of the Sulfinic Acid - Mercaptan Reaction General Procedure . 25 n -Butyl Mercaptan -p- Toluenesulfinic Acid . 27 Isopropyl Mercaptan -p- Toluenesulfinic Acid 29 Benzyl Mercaptan -p- Toluenesulfinic Acid . 30 Product Study of the Sulfinic Acid -Secondary Alkyl Sulfide Reaction General Procedure . 32 Isopropyl Sulfide -p- Toluenesulfinic Acid . 33 2 -Octyl Sulfide -p- Toluenesulfinic Acid . 33 Summary 34 Bibliography 36 THE REACTIONS OF 2- TOLUENESULFINIC ACID WITH MERCAPTANS AND SECONDARY ALKYL SULFIDES INTRODUCTION Sulfinic acids have been reported as early as 1860 by Kalle (13) and again in 1861 by Kolbe (13). Several ali- phatic and many aromatic sulfinic acids are known. The lower molecular weight sulfinic acids are unstable as the free acid and decompose rapidly, but these same acids are stable as salts. The aromatic sulfinic acids in general are more stable than the aliphatic acids. Some aromatic acids are stable enough to be stored for a considerable period of time as the free acid, provided they are kept in the dark at low temperatures. However, on standing at room temperatures these same acids will undergo dispro- portionation to sulfonic acid, water, and thiolsulfonate This reaction was first reported by R. Otto (6) in 1868. The reaction is as follows: 3 R -SO2H----,. R- SO2S -R + R -SO3H + H2O (1) The mechanism of this reaction was first investigated in some detail by Kice and Bowers. In the course of this work Kice and Bowers (6) discovered two previously unre- ported reactions of sulfinic acids, one a reaction with 2 disulfides, and the other with primary alkyl sulfides. The reaction of p -tolyl disulfide with 2- toluene- sulfinic acid is as follows: 4 ArSO2H + ArSSAr >3 ArSO2SAr + 2 H2O (2) Ar = p- CH3C6H4- In moist acetic acid containing moderate amounts of added sulfuric acid this reaction predominates strongly over the normal disproportionation of sulfinic acid. The dependence of the rate on the various reaction variables suggests a mechanism in which ion I, formed in the equilibrium shown in equation 3, undergoes rate -determining decomposition by reactions 4a and 4b. CD G > ArSO2 H + ArSSAr + H < ArSI -S-Ar + H20 (3) y I O S-Ar ArS-SAr yI Ar-S-SAr + ArP (4a) O SAr O `TJ ArSSAr + Ar-S-S-Ar Ar-S-SAr + Ar-S-S-SAr I O S-Ar (4b) I O Ar III Kice and Bowers suggested that the final products were then formed as follows. Both the sulfenium ion ArS-1 and ion III were thought to react with sulfinic acid to form 3 thiolsulfonate. ArSO + ArSO H ->ArSO SAr + H'" 2 2 (4c) III + ArSO2H ->ArSO2SAr + ArSSAr Thiosulfinates (ArS -SAr) are known to disproportionate O readily to thiolsulfonates and disulfides (14); it was suggested that this occurred here as well. 2 ArS -SAr -- >ArSO2SAr + ArSSAr (5) W O This mechanism would give a total stoichiometry as shown in equation 2, in agreement with the experimental findings. This raises the possibility that other divalent sul- fur compounds (X -S -Y) might form ions ArS(OPSXY with sul- finic acid and that these ions could decompose to various products. This was found to be true with primary alkyl sulfides. Again a reaction was found which strongly pre- dominated over the normal disproportionation reaction. This reaction was first order in sulfinic acid and sul- fide, and showed the same dependence as the disulfide re- action on water and strong acid concentration. During the course of the reaction one of the alkyl groups of the sulfide was cleaved and oxidized to the corresponding 4 aldehyde. The kinetic results show that the reaction in- volves a mechanism in which ion IV is formed in the equi- librium in equation 6 from a sulfide and sulfinic acid and then undergoes rate -determining un.imolecular decomposi- tion (eq. 6) . ejCH 2 -R ArSO2H + (RCH2) S ArS-S + H20 (6) 2 \CH2-R O IV CH2R final products ArS -S, rate after several CH2R steps 1 determining O It was originally suggested that this decomposition yield- ed an aldehyde as shown below. e H © H Ar-S-S-C-R Ar-S-S-C-R rate i H (7a) n determining H :0jC21 O-C-R H H © H i Ar-S-S-C-R H e > RCHO + Ar-S-S-C-R + H ( H H H O-C-RI H An alternative path is possible as shown in equation 7b. To decide between these two paths, the deuterated sulfide 5 (C6H5CD2)2S, was synthesized (9) and subjected to kinetic study in the sulfide- sulfinic acid reaction (9). The deuterated compound was found to react with.-- toluene- sulfinic acid at only about one -fifth the rate observed for benzyl sulfide. A kinetic isotope effect of this size requires that the carbon -deuterium bond be broken in the rate -determining step. 4\ +/CH2R rate (7b) ArS-S ArSOH + RCH = S-CH2R \H determining :O C-R -is FÌ This occurs in equation 7b but not in equation 7a. It is therefore evident that equation 7b is the rate -determining step of the sulfide -sulfinic acid reaction. It has also been shown (9) that the ion formed in this reaction (RCH = SCH2R) undergoes almost instantaneous hydrolysis to the aldehyde and mercaptan as shown in equation 8, with the mercaptan then reacting almost as rapidly with some of the sulfinic acid remaining. RCH = SCH2R + H20 RCHO + RCH2SH + H® (8) 2 This last result makes it important to learn what sorts of products are formed when mercaptans react with 6 sulfinic acids. Obviously, this knowledge is essential for a complete understanding of the sulfide- sulfinic acid reaction. Despite the ease with which these substances react with each other under the proper conditions, there is practically nothing in the literature pertaining to this reaction, and the reaction has apparently never been studied under the conditions employed for the sulfide - sulfinic acid reaction. Therefore, the reaction of 2.- toluenesulfinic acid with some representative mercaptans has been investigated in acetic acid, and the results form one part of this thesis. Also for more complete understanding of the sulfide - sulfinic acid reaction it was of interest to determine whether a secondary alkyl sulfide would undergo the same general type of reaction shown by the primary alkyl sul- fide. Therefore product studies of the reactions of 27 toluenesulfinic acid with isopropyl and 2 -octyl sulfides were conducted. These form the second part of the thesis. 7 RESULTS Reaction of Merca.tans with - Toluenesulfinic Acid In order that the results be as applicable to the sulfide- sulfinic acid reaction as possible, all the re- actions were carried out in acetic acid -0.56 M water con- taining 0.6 M sulfuric acid.
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