Mercaptan reaction mechanism

The earlier studies of aqueous solution of Cd2+ containing mercaptan were found to produce colloidal CdS by sonication [89] under nitrogen atmosphere. The H atoms produced during sonication attacked on molecules possessing a thiol moiety and lead to either H atoms extraction or H2S formation. The proportion of H2 to H2S was highly dependent on the type of mercaptan. Possible reaction mechanism was reported as follows [90],... 

In the investigation with n-butyl nitrate by Merrow, van Dolah and Cristol [41] already cited, neither the presence of the nitrate ion nor of mercaptan was detected. For this reason the following reaction mechanism was proposed for the hydrolysis of nitric acid alcohol esters by means of hydrosulphide ... 

Oxidation of Mercaptans by O Oxidation of methyl mercaptan in aqueous solution by 03 produces methane sulfonic acid as the major product. The reaction mechanisms are complex with the formation of dimethyl disulfide (CH3SSCH3), methyl methane thiolsulfonate (CH3S02SCH3), and methyl methanethiolsulfinate (CH3SOSCH3) as minor products. Continued ozonation could result in slow formation of sulfuric acid ... 

In the field of thiochemistry, the photocatalytic synthesis of mercaptans represents an interesting chemical route. Schoumacker et al. [60] performed the synthesis of propan-1-thiol by addition of H2S on propene in contact with illuminated Ti02 or CdS catalysts, according to a reaction mechanism implying photogenerated SH radicals. 

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

The mercaptan hardener is one of a very few hardeners which can cure the resin at low temperature even below the freezing point in a few minutes using basic catalysts such as amines. There are only very few studies of the curing reaction mechanism and the physical properties of the cured resin available 42 45). 

It was recognized very early on in the investigation of free radical reactions that mercaptans add to olefins in the same anti-Markownikoff manner as HBr, and there was some exploration of the scope of the reaction. However, little has been done in the way of examining the kinetics of the photoaddition, with the exception of one study by Graham and Soltys [153]. The reaction mechanism is not particularly complex, the main point of interest being the dimerization of the thiyl radicals to produce quite stable disulphides as the termination step. 

The alcohol BH, which is formed in this sequence, is a stronger nucleophile dian the mercaptane YH. This leads to a faster reaction than the first step in (5.60) leading to the following reaction mechanism... 

In the oxidation of mercaptans (thiols), LDH acts as both base and oxidant catalyst. The following reaction mechanism has been proposed (10) ... 

The elementary reactions of gas phase organic sulfur-centered radicals not discussed in Section 2 are the topic of Section 3. Brief discussions of reaction mechanisms are included when sufficient experimental information is available. Much of the material covered in Section 3 is related to the photodecomposition of sulfides, disulfides, and mercaptans or the thiyl-catalyzed reactions of unsaturated hydrocarbons. In many of these studies, kinetic and mechanistic information was obtained from indirect measurement techniques and/or relied on complex reaction mechanisms to interpret the experimental data. As a result, much of the mechanistic information in Section 3 is qualitative in nature. 

Scheme 22 Mechanism of radical chain reactions of the growth of styrene line along the edge of a dimer (left side) and of the growth of allyl mercaptan line across the dimer rows (right side) of a H-Si(l 0 0)-2 x 1 surface.

Absorption spectra of CdS colloid indicate the formation of quantum sized CdS particles. The particle size increased upon sonication, indicated by the red shift in the onset of absorption. The particle size was highly dependent on the mercaptan used, because of the absorption of the mercaptan on the particle acting as a capping agent and the rate of H2S produced. Study of mercaptan systems revealed that there was also a thermal process responsible for CdS formation. 25% of the total CdS produced sonochemically was formed via a thermal mechanism presumably in the hot shell around the compressed bubble. CdS colloid could be dissolved quite readily by sonicating solutions under air saturated conditions [89] by the following reaction,... 

If the oxidation which leads to the formation of Bindschedler s green takes place in the presence of hydrogen sulphide the process is varied by the entrance of sulphur. In principle, however, the mechanism is the same as before. The following formulae show that an addition of H—8H and a subsequent intramolecular addition of a mercaptan group are stages in the reaction ... 

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

The first stage of the synthesis involves the interaction of a nitro compound with sodium sulfide. When used alone, sodium sulfide is only slightly effective The reactions proceed slowly and the yields of mercaptanes are small. If elemental sulfur is added, the conversion accelerates markedly and the yield increases to 75-80%. The promoting effect of elemental sulfur can be easily explained by the radical-chain mechanism. The reaction starts with one-electron transfer from the nucleophile to the nitro compound further conversions resemble other chain ion-radical substitutions. 

Hydrogen sulfide, mercaptans, and hydrogen halides are also poisons for the reaction. This effect of the hydrogen halides probably results from the formation of di- and triisobutylene by a mechanism similar to that which occurs at ordinary temperatures. 

The best evidence for the photolytic decomposition of mercaptans and disulfides into free radicals involves photoinitiation of polymerization of olefins. Thus, photolysis of disulfides initiates the copolymerization of butadiene and styrene,154 as well as the polymerization of styrene207 and of acrylonitrile.19 Thiophenol and other thiols promote polymerization upon ultraviolet irradiation.19 Furthermore, the exchange of RS-groups between disulfides and thiols is greatly accelerated by light. Representative examples are benzothiazolyl disulfide and 2-mercapto-thiazole,90 tolyl disulfide and p-thiocresol, and benzyl disulfide and benzylmercaptan.91 The reaction probably has a free radical mechanism. Similar exchange reactions have been observed of RS-groups of pairs of disulfides have been observed.19... 

The half-life with respect to chemical transformation of CH3I in seawater at 20°C was determined to be 20 days, as compared to about 200 days in freshwater (reaction with H20 yielding CH3OH). In a case of a groundwater contamination with several alkyl bromides, Schwarzenbach et al. (1985) reported the formation of dialkyl sulfides under sulfate-reducing conditions in an aquifer. They postulated that in an initial reaction, primary alkyl bromides reacted with HS" by an SN2 mechanism to yield the corresponding mercaptans (thiols) ... 

At present the exact details of the mechanisms leading to each of the products are far from well understood (7.81. However, it seems likely that oxidation of the common naturally-occurring organo-sulfur compounds (dimethyl sulfide, methyl mercaptan and dimethyl disulfide) proceeds, at least in part, through formation of the methyl thiyl radical, CHjS (4.9-111. That is the main reason for carrying out direct studies on CH3S radical reactions where the individual reactions are isolated. 

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